List of Cranial Nerves

Table

9.1 Electrical Transmission in the Nerve

The transmission of a nerve impulse along a neuron from one end to the other occurs as

a result of electrical changes across the membrane of the neuron. The membrane of an

unstimulated neuron is polarized-that is, there is a difference in electrical charge between

the outside and inside of the membrane. The inside is negative with respect to the

outside. The entire impulse asses through a neuron in about severi milliseconds faster

than a lightning strike. This occurs in 7 basic steps:

1. Polarization of the neuron’s membrane: Sodium is on the outside, and

potassium is on the inside: Cell membranes surround neurons just as any other

cell in the body has a membrane. When a neuron is not stimulated it’s just sitting

with no impulse to carry or transmit – its membrane is polarized. Being polarized

means that the electrical charge is on the outside of the membrane is positive

while the electrical charge on the inside of the membrane is negative. The outside

of the cell contains excess sodium ions (Na+); the inside of the cell contains

excess potassium ions (K+).

2. Resting potential gives the neuron a break: When the neuron is inactive and

polarized, it’s said to be at its resting potential. It remains this way until a stimulus

comes along.

3. Action potential: Sodium ions move inside the membrane: When a stimulus

reaches a resting neuron, the gated ion channels on the resting neuron’s

membrane open suddenly and allow the Na+ that was on the outside of the

membrane to go rushing into the cell. As this happens, the neuron goes from

being polarized to being depolarized.

Each neuron has a threshold level the point at which there’s no holding back. After the

stimulus goes above the threshold level, more gated ion channels open and allow more

Na+ inside the cell. This causes complete depolarization of the neuron and an action

potential is created. In this state, the neuron continues to open Na+ channels all along

the membrane. When this occurs, it’s an all-or-none phenomenon. “All-or-none” means

that if a stimulus doesn’t exceed the threshold level and cause all the gates to open, no

action potential results; however, after the threshold is crossed, there’s no turning back.

Complete depolarization occurs and the stimulus will be transmitted. When an impulse

travels down an axon covered by a myelin sheath, the impulse must move between the

uninsulated gaps called nodes of Ranvier that exist between each Schwann cell.

4. Repolarization: Potassium ions move outside, and sodium ions stay inside the

membrane: After the inside of the cell becomes flooded with Na+, the gated ion

channels on the inside of the membrane open to allow the K+ to move to the

outside of the membrane. With K+ moving to the outside, the membrane’s

repolarization restores electrical balance, although it’s opposite of the initial

polarized membrane that had Na+ on the outside and K+ on the inside. Just after

the K+ gates open, the Na+ gates close; otherwise, the membrane couldn’t

repolarize.

5. Hyperpolarization: More potassium ions are on the outside than there are sodium

ions on the inside: When the K+ gates finally close, the neuron has slightly more

K+ on the outside than it has Na+ on the inside. This causes the membrane

potential to drop slightly lower than the resting potential, and the membrane is said

to be hyperpolarized because it has a greater potential. (Because the membrane’s

potential is lower, it has more room to “grow.”). This period doesn’t last long. After

the impulse has traveled through the neuron, the action potential is over, and the

cell membrane returns to normal (that is, the resting potential).

6. Refractory period puts everything back to normal: Potassium returns inside,

sodium returns outside: The refractory period is when the Na+ and K+ are

returned to their original sides: Na+ on the outside and K+ on the inside. While

the neuron is busy returning everything to normal, it doesn’t respond to any

incoming stimuli. After the Na+/K+ pumps return the ions to their rightful side of

the neuron’s cell membrane, the neuron is back to its normal polarized state and

stays in the resting potential until another impulse comes along

Diagram

9.2 Neurotransmitters are the brain chemicals that communicate information throughout

the body and brain. They relay signals between nerve cells, called “neurons. There are

two kinds of neurotransmitters Inhibitory and Excitatory.

Excitatory neurotransmitters are not necessarily exciting – they are what stimulate the

brain. Those that calm the brain and help create balance are called inhibitory. Inhibitory

neurotransmitters balance mood and are easily depleted when the excitatory

neurotransmitters are overactive.

1. Inhibitory Neurotransmitters:

Serotonin is an inhibitory neurotransmitter – which means that it does not stimulate the

brain. Adequate amounts of serotonin are necessary for a stable mood and to balance

any excessive excitatory (stimulating) neurotransmitter firing in the brain. Serotonin also

regulates many other processes such as carbohydrate cravings, sleep cycle, pain control

and appropriate digestion, body temperature. Low serotonin levels are also associated

with decreased immune system function.

GABA (Gamma amino butyric acid) is an inhibitory neurotransmitter that is often referred

to as “nature’s VALIUM-like substance” is vey widely distributed in the neurons of the

cortex. GABA contributes to motor control, vision, and many other cortical functions. It

also regulates anxiety.

Dopamine is a special neurotransmitter because it is considered to be both excitatory and

inhibitory. Dopamine helps in controlling movement and posture. It also modulates mood

and plays a central role in positive reinforcement and drug dependency.

2. Excitatory Neurotransmitters

Norepinephrine is an excitatory neurotransmitter that is responsible for stimulatory

processes in the body. Norepinephrine helps to make epinephrine as well. This

neurotransmitter can cause anxiety at elevated excretion levels as well as some mood

dampening effects. Low levels of norepinephrine are associated with low energy.

Decreased focus ability and sleep cycle problems. Epinephrine is an excitatory

neurotransmitter that is reflective of stress. This neurotransmitter will often be elevated

when ADHD like symptoms are present. Long term stress or insomnia can cause

epinephrine levels to be depleted (low). Epinephrine also regulates heart rate and blood

pressure. Glutamate is a neurotransmitter that is associated with learning and memory.

9.3 Types of Nervous System

Diagram

Central Nervous System: The central nervous system is composed of the brain and

spinal cord. Brain and spinal cord serve as the main “processing center” for the entire

nervous system.

White matter and Grey matter: Both the spinal cord and the brain consist of white matter

i.e. bundles of axons each coated with a sheath of myelin and gray matter i.e. masses of

the cell bodies and dendrites synapses. Each covered with synapses.

In the spinal cord, the white matter is at the surface, the gray matter inside.

In the brain of mammals, this pattern is reversed. However, the brains of “lower”

vertebrates like fishes and amphibians have their white matter on the outside of their brain

as well as their spinal cord.

The Meninges: Both the spinal cord and brain are covered in three continuous sheets of

connective tissue, the meninges. From outside in, these are:

1. The Dura mater – pressed against the bony surface of the interior of the vertebrae

and the cranium,

2. The Arachnoid and
3. The Pia mater

Region between the arachanoid and pia mater is filled with cerebrospinal fluid (CSF).

Cerebrospinal Fluid (CSF): The cells of the central nervous system are bathed in a fluid,

called cerebrospinal fluid (CSF), that differs from that serving as the interstitial fluid (ISF)

of the cells in the rest of the body. Some properties of CSF are:

1. Cerebrospinal fluid leaves the capillaries in the choroid plexus of the brain.
2. It contains far less protein than “normal” because of the blood-brain barrier, a

system of tight junctions between the endothelial cells of the capillaries. (This

barrier creates problems in medicine as it prevents many therapeutic drugs from

reaching the brain.) 3. CSF flows uninterrupted throughout the central nervous 

system: through the central cerebrospinal canal of the spinal cord and through an

interconnected system of four ventricles in the brain.

3. CSF returns to the blood through lymphatic vessels draining the brain.

9.4 The Spinal Cord:

It extends from foramen magnum at the base of the skull to the level of first lumbar

vertebra. The cord is continuous with the medulla oblongata at the foramen magnum. It

conducts sensory information from the peripheral nervous system (both somatic and

autonomic) to the brain and conducts motor information from the brain to our various

effectors like skeletal muscles, cardiac muscle, smooth muscle and glands. It also serves

as a minor reflex center. 31 pairs of spinal nerves arise along the spinal cord. These are

“mixed” nerves because each contains both sensory and motor axons. However, within

the spinal column,

All the sensory axons pass into the dorsal root ganglion where their cell bodies are

located and then on into the spinal cord itself.

All the motor axons pass into the ventral roots before uniting with the sensory axons to

form the mixed nerves.

The two main functions of spinal cord are:

1. It connects a large part of the peripheral nervous system to the brain. Information

(nerve impulses) reaching the spinal cord through sensory neurons are transmitted

up into the brain. Signals arising in the motor areas of the brain travel back down

the cord and leave in the motor neurons.

2. Diagram

9.5 Reflex Action: When a receptor is stimulated it sends a signal to the central nervous

system, where the brain coordinates the response, but sometimes a very quick response

is needed, one that does not involve the brain: this is a reflex action. Reflex actions are

rapid and happen without us thinking.

Reflex arc: The structural and functional unit that carries our reflex action is called as

reflex arc. Reflex arc consists of a

1. A Receptor
2. A Sensory nerve (Afferent)
3. Interneuron
4. A Motor neuron (Efferent)
5. Effector

There are two types of reflexes

1. Un Conditional Reflexes: These are inborn reflexes. You don’t learn these

reflexes. For examples, no body taught you to withdraw your hand when you touch

a hot object.

2. Conditional Reflexes: These are not inherited. You learn them by doing the same

act several time. For example: Standing in attention when you heat national

Anthem is a conditional reflex.

Diagram

9.5.1 Reflex Arc

The Brain: The brain is one of the largest and most complex organ in the human body. It

is made up of more than 100 billion nerves that communicate in trillions of connections

called synapses. The brain is made up of many specialized areas that work together.

1. The cortex is the outermost layer of brain cells. Thinking and voluntary movements

begin in the cortex.

2. The brain stem is between the spinal cord and the rest of the brain. Basic

functions like breathing and sleep are controlled here. The basal ganglia are a

cluster of structures in the center of the brain. The basal ganglia coordinate

messages between multiple other brain areas.

3. The cerebellum is at the base and the back of the brain. The cerebellum is

responsible for coordination and balance.

The three lobes of the brain are:

1. The frontal lobe is responsible for solving problem, judgment and motor function.
2. The temporal lobe is involved in memory and hearing.
3. The occipital lobes contain the brain’s visual processing system.

The brain of an adult is 1.3 kg. It is divided into three parts: Forebrain, Midbrain and

Hindbrain.

1. Forebrain: Consists of Cerebrum, Basal nuclei, limbic system, reticular formation,

thalamus and hypothalamus.

2. Midbrain: consists of tectum, tegmentum, cerebral peduncle, corpus stratium,

substantia nigra and red nucleus.

3. Hindbrain: Consists of Cerebellum, pons and medulla oblongata.

Diagram

Medulla Oblongata: It is a conically continuation of the upper part of the spinal cord,

extending upto the lower margin of the pons. About three-fourth of the pyramidal tracts

cross at a level of medulla oblongata.

Functions: Carries out and regulates life sustaining functions such as breathing,

swallowing and heart rate

Pons: The pons is a portion of the brain located above the medulla oblongata and below

the midbrain. Although it is small, at roughly it is 2.5 centimeters long.

Functions: The trigeminal nerve is responsible for feeling in the face as well as controlling

the muscles that are responsible for biting, chewing, and swallowing. The abducens nerve

allows the eyes to look from side to side. The facial nerve controls facial expressions, and

the vestibularcochlear nerve allows sound to move from the ear to the brain. All of these

nerves start in the pons.

Thalamus: It is a large oval mass of grey matter found on third ventricle and extended

some distance behind it. . It works to correlate several important processes, including

consciousness, sleep, and sensory interpretation.

Basal Nuclei: A region located at the base of the brain composed of 4 clusters of

neurons, or nerve cells. This area of the brain is responsible for body movement and

coordination. The groups of neurons most prominently and consistently affected in

Huntington disease – the pallidum and striatum and are located in the basal nuclei.

The pallidum is composed of structures called the globus pallidus and the ventral pallidum

while the striatum consists of the caudate nucleus, putamen, and ventral striatum.

The basal nuclei are also called the basal ganglia. The term “basal” refers to the location

of these collections of neurons (nuclei or ganglia) deep within the brain, seemingly at its

very base.

The Limbic System: The limbic system is a complex set of structures that lies on both

sides of the thalamus, just under the cerebrum. It includes the hypothalamus, the

hippocampus, the amygdala, and several other nearby areas. It appears to be primarily

responsible for our emotional life, and has a lot to do with the formation of memories.

Diagram

Hypothalamus: The hypothalamus is a small part of the brain located just below the

thalamus on both sides of the third ventricle. (The ventricles are areas within the

cerebrum that are filled with cerebrospinal fluid, and connect to the fluid in the spine.) It

sits just inside the two tracts of the optic nerve, and just above (and intimately connected

with) the pituitary gland. The hypothalamus is one of the busiest parts of the brain, and is

mainly concerned with homeostasis. Homeostasis is the process of returning something to

some “set point.” It works like a thermostat: When your room gets too cold, the thermostat

conveys that information to the furnace and turns it on. As your room warms up and the

temperature gets beyond a certain point, it sends a signal that tells the furnace to turn off.

The hypothalamus is responsible for regulating your hunger, thirst, response to pain,

levels of pleasure, sexual satisfaction, anger and aggressive behavior, and more. It also

regulates the functioning of the autonomic nervous system, which in turn means it

regulates things like pulse, blood pressure, breathing, and arousal in response to

emotional circumstances. The hypothalamus receives inputs from a number of sources.

From the vagus nerve, it gets information about blood pressure and the distension of the

gut (that is, how full your stomach is). From the reticular formation in the brainstem, it

gets information about skin temperature. From the optic nerve, it gets information about

light and darkness. From unusual neurons lining the ventricles, it gets information about

the contents of the cerebrospinal fluid, including toxins that lead to vomiting. And from the

other parts of the limbic system and the olfactory (smell) nerves, it gets information that

helps regulate eating and sexuality. The hypothalamus also has some receptors of its

own, that provide information about ion balance and temperature of the blood. In one of

the more recent discoveries, it seems that there is a protein called leptin which is

released by fat cells when we overeat. The hypothalamus apparently senses the levels of

leptin in the bloodstream and responds by decreasing appetite. It would seem that some

people have a mutation in a gene which produces leptin, and their bodies can’t tell the

hypothalamus that they have had enough to eat. However, many overweight people do

not have this mutation, so there is still a lice off research to do! The hypothalamus sends

instructions to the rest of the body in two ways. The first is to the autonomic nervous

system. This allows the hypothalamus to have ultimate control of things like blood

pressure, heart rate, breathing, digestion, sweating, and all the sympathetic and

parasympathetic functions. The other way the hypothalamus controls things is via the

pituitary gland. It is neurally and chemically connected to the pituitary, which in turn

pumps hormones called releasing factors into the bloodstream. As you know, the pituitary

is the so-called “master gland,” and these hormones are vitally important in regulating

growth and metabolism.

Hippocampus: The hippocampus consists of two “horns” that curve back from the

amygdala. It appears to be very important in converting things that are “in your mind” at

the moment (in short-term memory) into things that you will remember for the long run

(long-term memory). If the hippocampus is damaged, a person cannot build new

memories, and lives instead in a strange world where everything they experience just

fades away, even while older memories from the time before the damage are untouched!

Amygdala: The amygdales are two almond-shaped masses of neurons on either side of

the thalamus at the lower end of the hippocampus. When it is stimulated electrically,

animals respond with aggression. And if the amygdala is removed, animals get very tame

and no longer respond to things that would have caused rage before. But there is more to

it than just anger: When removed, animals also become indifferent to stimuli that would

have otherwise have caused fear and even sexual responses.

The Cerebrum: The cerebrum, also known as the telencephalon, is the largest and most

highly developed part of the human brain. It encompasses about two-thirds of the brain

mass and lies over and around most of the structures of the brain. The outer portion

(1.5mm to 5mm) of the cerebrum is covered by a thin layer of gray tissue called the

cerebral cortex. The cerebrum is divided into right and left hemispheres that are

connected by the corpus callosum. Each hemisphere is in turn divided into four lobes.

1. Frontal Lobe- associated with reasoning, planning, parts of speech, movement,

emotions, and problem solving movement, orientation, recognition, perception of

stimuli

2. Parietal Lobe associated with
3. Occipital Lobe- associated with visual processing
4. Temporal Lobe- associated with perception and recognition of auditory stimuli,

memory, and speech A deep furrow divides the cerebrum into two halves, known

as the left and right hemispheres. The two hemispheres

Look mostly symmetrical yet it has been shown that each side functions slightly different

than the other. Sometimes the right hemisphere is associated with creativity and the left

hemispheres is associated with logic abilities. The corpus callosum is a bundle of axons

which connects these two hemispheres.

Nerve cells make up the gray surface of the cerebrum which is a little thicker than your

thumb. White nerve fibers underneath carry signals between the nerve cells and other

parts of the brain and body.

The neocortex occupies the bulk of the cerebrum. This is a six-layered structure of the

cerebral cortex which is only found in mammals. It is thought that the neocortex is a

recently evolved structure, and is associated with “higher” information processing by more

fully evolved animals (such as humans, primates, dolphins, etc).

Diagram

Functions:

Movement/motor functions of cerebrum

Simple body movements, whether we refer to walking or just shaking hands are

determined by the primary motor cortex, that is located in the frontal lobe. The neurons

link the cortex to the brain stem and then to the spinal cord, Innervating the skeletal

muscles (but not only). If damage occurs within the area of the motor cortex, motor

neuronal diseases might occur and people might lose their muscular tonus and power or

even get different levels of paralysis.

Sensorial processing

Sensorial processing takes place in the primary sensory areas located in the cerebral

cortex (where visual/auditory eustatory /olfactory information is analyzed and sent over

to the frontal lobe if necessary). Basically, due to this and ibrum function, we perceive the

world around us as it is, with all its action taking place near us, with all its smells its

dangers.

Olfaction processing

In the human cerebrum, the olfactory bulb (responsible for analyzing smells) is located

under the frontal lobe. The olfactory bulb is part of the limbic system asides structures like

the hippocampus, the anterior thalamic nucleus, the fornix etc. From a certain point of

view, the olfaction processing system is quite unique: the axons of olfactory bulb neurons

connect to the olfactory cortex directly instead of linking to the thalamus first.

Cerebrum function of learning and memorizing

One of the most important cerebrum functions is the one that allows learning and

memorizing the things around us. Memory, for example, is associated with the

hippocampus, while memories and the ability of memorizing different information are

linked to the basal ganglia. The language and the communication skills are allowed by

two complicated system in different parts of the left hemisphere. Speaking itself is linked

to the Broca’s area located in the frontal lobe, while processing and understanding words

and languages is linked to the Wernike’s area.

The Cerebellum: The word cerebellum means little brain. The cerebellum is the area of

the hindbrain that controls motor movement coordination, balance, equilibrium and muscle

tone. Like the cerebral cortex, the cerebellum is comprised of white matter and a thin,

outer layer of densely folded gray matter. The folded outer layer of the cerebellum

(cerebellar cortex) has smaller and more compact folds than those of the cerebral cortex.

The cerebellum contains hundreds of millions of neurons for processing data. It relays

information between body muscles and areas of the cerebral cortex that are involved in

motor control.

Function:

Maintenance of balance and posture: The cerebellum is important for making postural

adjustments in order to maintain balance. Through its input from vestibular receptors and

proprioceptors, it modulates commands to motor neurons to compensate for shifts in body

position or changes in load upon muscles. Patients with cerebellar damage suffer from

balance disorders, and they often develop stereotyped postural strategies to compensate

for this problem (e.g., a wide-based stance).

Coordination of voluntary movements: Most movements are composed of a number of

different muscle groups acting together in a temporally coordinated fashion. One major

function of the cerebellum is to coordinate the timing and force of these different muscle

groups to produce fluid limb or body movements.

Motor learning: The cerebellum is important for motor learning. The cerebellum plays a

major role in adapting and fine-tuning motor programs to make accurate movements

through a trial-and-error process (e.g., learning to hit a baseball).

Cognitive functions: Although the cerebellum is most understood in terms of its

contributions to motor control, it is also involved in certain cognitive functions, such as

language.

9.5.2 Physiology of Sleep: Normal sleep is divided into non-rapid eye movement

(NREM) and rapid eye movement (REM) sleep.

NREM sleep is further divided into progressively deeper stages of sleep: stage N1, stage

N2, and stage N3 (deep or delta-wave sleep). In adults, Stage N1 is considered a

transition between wake and sleep. It occurs upon falling asleep and during brief arousal

periods within sleep and usually accounts for 2-5% of total sleep time. Stage N2 occurs

throughout the sleep period and represents 45-55% of total sleep time. Stage N3 (delta

or slow wave sleep) occurs mostly in the first third of the night and constitutes 5-15% of

total sleep time. As NREM stages progress, stronger stimuli are required to result in an

awakening.

REM sleep represents 20-25% of total sleep time and occurs in 4-5 episodes throughout

the night Stage R sleep (REM sleep) has tonic and phasic components. The phasic

component is a sympathetically driven state characterized by rapid eye movements,

muscle twitches, and respiratory variability. Tonic REM is a parasympathetically driven

state with no eye movements. The REM period length and density of eye movements

increases throughout the sleep cycle. Typically, N3 sleep is present more in the first third

of the night, whereas REM sleep predominates in the last third of the night. This can be

helpful clinically as NREM parasomnias such as sleep walking typically occur in the first

third of the night with the presence of N3 sleep.

EEG (Electroencephalogram): Brain cells communicate with each other by producing

tiny electrical signals, called impulses. An EEG measures this activity. EEG is most often

used to diagnose epilepsy, which causes obvious abnormalities in EEG readings. It is

also used to diagnose sleep disorders, coma, encephalopathies, and brain death. EEG

used to be a first-line method of diagnosis for tumors, stroke and other focal brain

disorders, but this use has decreased with the advent of high-resolution anatomical

imaging techniques such as MRI and CT. Types of waves formed are:

1. Alpha waves: These waves are produced when an individual is relaxed especially

with the eyes closed or has just awakened from a restful sleep. The amplitude of

these waves is about 50 microvolts and rate 8-12 cycles per second.

2. Beta waves: These are produced during mental activity or arousal. They are found

in parietal and frontal regions of the scalp. They have a frequency of 13-31 per

second and are of 5-10 microvolts.

3. Delta waves: These are produced during the state of unconsciousness and deep

sleep. They occur at the rate of about 0.5-3.5 per second and with a potential of

20-200 microvolts.

4. Theta waves: These are formed during disappointment and frustration in young

children and adolescents. These are usually found over the parietal and temporal

regions of the brain. They occur at the rate of 4-7 per second and their potential is

10 microvolts.

Diagram

Chapter-10 Somatic Nervous System

The somatic nervous system (SNS), or voluntary nervous system is the part of the

peripheral nervous system associated with the voluntary control of body movements via

skeletal muscles.

The somatic nervous system consists of sensory nerves carrying afferent nerve fibers that

relay sensation from the body to the central nervous system (CNS). The other nerves in

the SNS are motor nerves carrying efferent nerve fibers that relay motor commands from

the CNS to stimulate muscle contraction.

The a- of afferent and the e- of efferent correspond to the prefixes ad- (to, toward) and

ex- (out of).

Diagram

1.(Brain) Precentral gyrus: the origin of nerve signals initiating movement. 2. (Cross

Section of Spinal Cord) Corticospinal tract: Mediator of message from brain to skeletal

muscles. 3. Axon: the messenger cell that carries the command to contract muscles. 4.

Neuromuscular junction: the messenger axon cell tells muscle cells to contract at this

intersection.

10.1 Structure

The efferent portion of the peripheral nervous system consists of the somatic nervous

system and the autonomic nervous system. The autonomic nervous system controls the

function of glands, smooth muscle, cardiac muscle, and the neurons of the Gl tract. It is

composed of two neurons in series that can either excite or inhibit the target organ. In

contrast, the somatic nervous system contains single neurons that excite skeletal

muscles. The movements controlled by the somatic nervous system can be voluntary or

involuntary (reflexes).

Motor Unit-

The axons of motor neurons are myelinated and have large diameters for fast conduction

of action potentials. As the axon approaches a skeletal muscle fiber (muscle cell) it

usually branches to form synapses with anywhere from three to one thousand muscle

fibers. However, each muscle fiber is usually innervated by only a single neuron. A motor

unit consists of a neuron and all of the muscle fibers it innervates. A single neuron

innervates fibers from only one muscle and the innervated muscle fibers are usually

spread throughout the muscle.

Diagram (neuromuscular junction)

The portion of the skeletal muscle fiber plasma membrane that synapses with the motor

neuron axon is called the motor end plate. Once an action potential arrives at the axon

terminal, the depolarization of the membrane opens voltage-gated calcium channels

(Fig.) An increase in intracellular calcium at the terminal causes release of acetylcholine

vesicles into the neuromuscular junction. The acetylcholine binds nicotinic channels at the

motor end plate which causes them to open and allow sodium to enter (Fig.) The sodium

entry triggers voltage-gated sodium channels near the motor end plate, initiating an action

potential which is propagated in all directions along the plasma membrane of the muscle

fiber.

Spinal Cord AnatomyThe cell bodies of the neurons that innervate skeletal muscle of the body are found in the

ventral horn of the spinal cord. The neurons that innervate the skeletal muscle of the head

are in the brainstem. In the body, sensory signals come into the spinal cord from the

dorsal root ganglia, which contain the cell bodies of sensory neurons.

Diagram (spinal cord structure)

These neurons can excite motor neurons in the spinal cord. Motor neuron axons travel

through tissues as nerves and synapse on skeletal muscle cells. Excitation of motor

neurons causes acetylcholine to be released at the neuromuscular junction causing

contraction of the muscle. The muscle relaxes when the motor neuron is no longer

excited.

10.2 Control of Movement

The spinal cord is not just a conduit connecting the peripheral nervous system and the

brain. Instead, movements such as walking as well as reflexes are organized by the

spinal cord. In situations when control by upper motor neurons in the brain is necessary,

they act on the lower motor neurons of the spinal cord to influence reflexes and voluntary

movements.

Diagram (Sensory Receptors in skeletal muscle)

Two types of lower motor neurons: The portion of a skeletal muscle that controls posture

and movement, the extrafusal muscle fibers, are innervated by alpha motor neurons. A

specialized type of skeletal muscle fiber, the intrafusal muscle cell, resides in the muscle

spindle in the interior of the muscle (Fig.) the intrafusal muscle fibers are innervated by

gamma motor neurons. During muscle contraction, alpha and gamma motor neurons are

coactivated. Stretching of the intrafusal fibers in the muscle spindle is sensed by stretch

receptors and sent via afferent sensory neurons to the spinal cord. This allows for

monitoring of the length of the muscle which helps control muscle tone.

Two types of muscle sensory receptors: In order for the body to be able to control

muscle contraction properly, there must be feedback about the contractile status of

individual muscles. The muscle spindle is an important muscle sensory receptor that

provides information about muscle length and the rate of change of muscle length. In

addition, Golgi tendon organs are encapsulated sensory receptors situated in tendons

near the junction with the muscle (Fig.) they detect changes in muscle tension instead of

changes in muscle length. Both types of sensory receptors send information to the spinal

cord and the brain that is usually subconscious.

Muscle Stretch Reflex: If a muscle spindle within a muscle is quickly stretched, the

muscle stretch reflex causes contraction of the muscle as well as nearby muscles. This is

what occurs when the patellar tendon is struck during a physical exam (Fig.). The

afferent sensory neuron relays the stretch signal from the muscle spindle to its cell body

in the dorsal root of the spinal cord.

Diagram (The circuitry of the muscle stretch reflex)

The sensory neuron synapses with the motor neuron in the spinal cord that controls that

muscle. In addition, the sensory neuron activates an inhibitory neuron which inhibits the

motor neuron (reducing its likelihood of firing an action potential) leading to the muscle on

the opposite side of the limb, causing it to relax (Fig.). The muscle spindle reflex is

important in allowing maintenance of the length of a certain muscle.

Golgi Tendon Reflex: If the Golgi tendon organs of a muscle are stretched and

stimulated, muscle contraction is inhibited through inhibition of the motor neuron leading

to that muscle. This is because the afferent neuron activates an inhibitory neuron which is

synapsing with the motor neuron (Fig.).

Diagram (the circuitry of the Golgi tendon reflex)

In addition, the opposing muscle is stimulated to contract through the interaction of an

excitatory interneuron with the afferent neuron (Fig.) The Golgi tendon reflex acts to

protect muscles and tendons from damage due to excessive tension. In addition, they

may play a role in equalizing the load across different parts of a muscle.

Withdrawal Reflexes: A withdrawal reflex occurs when a part of the body such as a

portion of a limb is subjected to a painful stimulus. The flexor reflex causes contraction of

one muscle and relaxation of the opposing muscle to move that portion of the body away

from the insult. A short time after the flexor reflex initiates, the crossed extensor reflex

initiates on the opposite side of the body. This reflex allows for the opposite side of the

body to support the body’s weight or to push the body out of the way of the painful

stimulus.

Locomotion: Walking and running require the legs to alternate between forward flexion

(the swing phase) and backward extension (the stance phase). The repetition of this

pattern is synchronized with the other leg so that the two legs remain in opposite phases.

In most animals, if the spinal cord is separated from the brain the four legs can still make

coordinated walking motions. This is accomplished through central pattern generators

which are oscillatory neural circuits of lower motor neurons in the spinal cord. Even

though these circuits control the basic movements of walking, they are greatly influenced

by the brain. For instance, running occurs when the brain causes the central pattern

generators to shorten the stance phase. In addition, posture and goaldirected locomotion

require input from the brain. However, the central pattern generators allow relatively

simple modifications by the brain to control a very complicated process such as

locomotion.

10.3 Parts of the Somatic Nervous System

The term somatic is drawn from the Greek word soma, which means “body,” which is

appropriate considering it is this system that transmits information back and forth between

the central nervous system (CNS) and the rest of the body.

The somatic nervous system contains two major types of neurons (nerve cells):

• Sensory neurons, also known as afferent neurons, are responsible for carrying

information from the body to the CNS.

• Motor neurons, also known as efferent neurons, are responsible for carrying

information from the brain and spinal cord to muscle fibers throughout the body.

The neurons that make up the somatic nervous system project outwards from the CNS

and connect directly to the muscles of the body, and carry signals from muscles and

sensory organs back to the central nervous system. The body of the neuron is located in

the CNS, and the axon (a portion of the neuron that carries nerve impulses away from the

cell body) then projects and terminates in the skin, sensory organs, or muscles.

Chapter-11 Autonomic Nervous System

Introduction:

The autonomic nervous system is a component of the peripheral nervous system that

regulates involuntary physiologic processes including heart rate, blood pressure,

respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions:

sympathetic, parasympathetic and enteric.

The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS)

contain both afferent and efferent fibers that provide sensory input and motor output,

respectively, to the central nervous system (CNS). Generally, the SNS and PNS motor

pathways consist of a two-neuron series: a preganglionic neuron with a cell body in the

CNS and a postganglionic neuron with a cell body in the periphery that innervates target

tissues. The enteric nervous system (ENS) is an extensive, web-like structure that is

capable of function independently of the remainder of the nervous system. It contains over

100 million neurons of over 15 morphologies, greater than the sum of all other peripheral

ganglia, and is chiefly responsible for the regulation of digestive processes.

Activation of the SNS leads to a state of overall elevated activity and attention: the “fight

or flight” response. In this process, blood pressure and heart rate increase, glycogenolysis

ensues, gastrointestinal peristalsis ceases, etc. The SNS innervates nearly every living

tissue in the body. The PNS promotes the “rest and digest” processes; heart rate and

blood pressure lower, gastrointestinal peristalsis/digestion restarts, etc. The PNS

innervates only the head, viscera and external genitalia, notably vacant in much of the

musculoskeletal system and skin, making it significantly smaller than the SNS. The ENS

is composed of reflex pathways that control the digestive functions of muscle

contraction/relaxation, secretion/absorption, and blood flow.

Presynaptic neurons of both the SNS and PNS utilize acetylcholine (Ach) as their

neurotransmitter. Postsynaptic sympathetic neurons generally produce norepinephrine

(NE) as their effector transmitter to act upon target tissues, while postsynaptic 

parasympathetic neurons use Ach throughout. Enteric neurons have been known to use

several major neurotransmitters such as Ach, nitrous oxide and serotonin, to name a few

Structure and Function

1) Sympathetic Nervous System:

Sympathetic neurons have cell bodies located in the intermediolateral columns, or lateral

horns, of the spinal cord. The presynaptic fibers exit the spinal cord through anterior roots

and enter the anterior rami of T1-L2 spinal nerves and onto the sympathetic trunks via

white rami communicantes. From here, the fibers may ascend or descend the sympathetic

trunk to a superior or inferior paravertebral ganglion, respectively, pass to adjacent

anterior spinal nerve rami via gray rami communicantes, or cross through the trunk

without synapsing and continue through an abdominopelvic splanchnic nerve to reach

prevertebral ganglia. Because of the central location of the sympathetic ganglia,

presynaptic fibers tend to be shorter than their postsynaptic counterparts.

Paravertebral ganglia exist as nodules throughout the sympathetic trunk, adjacent to the

spinal column, where pre- and postganglionic neurons synapse. While the numbers may

vary by individual, generally, there are three cervical, 12 thoracic, four lumbar, and five

sacral ganglia. Of these, only the cervical have names of superior, middle, and inferior

cervical ganglia. The inferior cervical ganglion may fuse with the first thoracic ganglion to

form the stellate ganglion. All nerves distal to the paravertebral ganglia are splanchnic

nerves. These convey afferent and efferent fibers between the CNS and the viscera.

Cardiopulmonary splanchnic nerves carry the postsynaptic fibers destined for the thoracic

cavity.

Nerves that will innervate the abdominal and pelvic viscera pass through the paravertebral

without synapsing, becoming abdominopelvic splanchnic nerves. These nerves include the

greater, lesser, least, and lumbar splanchnic nerves. The presynaptic nerves finally

synapse in prevertebral ganglia that are closer to their target organ. Prevertebral ganglia

are part of the nervous plexuses that surround the branches of the aorta. These include

the celiac, aorticorenal, and superior and inferior mesenteric ganglia. The celiac ganglion

receives input from the greater splanchnic nerve, the aorticorenal from the lesser and

least splanchnic nerves, and the superior and inferior mesenteric from the least and

lumbar splanchnic nerves. The celiac ganglion innervates organs derived from the foregut:

distal esophagus, stomach, proximal duodenum, pancreas, liver, biliary system, spleen,

and adrenal glands. The superior mesenteric ganglion innervates the derivatives of the

midgut: distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and

proximal transverse colon. Lastly, the inferior mesenteric ganglion provides sympathetic

innervation to the structures developed from the hindgut: distal transverse, descending,

and sigmoid colon; rectum and upper anal canal; as well as the bladder, external

genitalia, and gonads.

The two-neuron general rule for SNS and PNS circuits has several notable exceptions.

Sympathetic and parasympathetic postganglionic neurons that synapse onto the ENS are

functionally part of a three-or-more neuron chain. The presynaptic sympathetic fibers that

are destined for the adrenal medulla pass through the celiac ganglia and synapse directly

onto chromaffin cells. These unique cells function as postganglionic fibers that secrete

epinephrine directly into the venous system. Postganglionic sympathetic neurons release

NE that acts on adrenergic receptors in the target tissue. The subtype of the receptor,

alpha-1, alpha-2, beta-1, beta-2, or beta-3, and the tissues in which they express

influences the affinity of NE for the receptor.

As stated, the SNS enables the body to handle stressors via the “fight-or-flight” response.

This reaction primarily reg:date: blood vessels. Vessels are tonically innervated, and in

most cases, an increase in sympathetic signals leads to vasoconstriction and the opposite

of vasodilation. The exceptions include coronary vessels and those that supply the

skeletal muscles and external genitalia, for which the opposite reaction occurs. His

contradictory effect is mediated by the balance of alpha and beta receptor activity. In a

physiologic state, beta-receptor stimulation increases coronary vessel dilation, but there is

blunting of this effect by alpha-receptor-mediated vasoconstriction. In a pathologic state, 

such as in coronary artery disease, alpha-receptor activity is enhanced, and there is the

muting of beta-activity. Thus, the coronary arteries may constrict via sympathetic

stimulation. Sympathetic activation increases heart rate and contractile force, which,

however, increases metabolic demand and is thus detrimental to cardiac function in

compromised individuals.

The SNS is constantly active, even in non-stressful situations. In addition to the

aforementioned tonic stimulation of blood vessels, the SNS is active during the normal

respiratory cycle. Sympathetic activation complements the PNS by acting during

inspiration to dilate the airways allowing for an appropriate inflow of air. Additionally, the

SNS regulates immunity through the innervation of immune organs such as the spleen,

thymus, and lymph nodes. This influence may up- or down-regulate inflammation. Cells of

the adaptive immune system primarily express beta-2 receptors, while those of the innate

immune system express those as well as alpha-1 and alpha-2 adrenergic receptors.

Macrophages activate by alpha-2 stimulation and are suppressed by beta-2 adrenergic

receptor activation.

The majority of postganglionic sympathetic neurons are noradrenergic, and also release

one or more peptides such as neuropeptide Y or somatostatin. NE/neuropeptide Y

neurons innervate blood vessels of the heart, thus regulating blood flow while

NE/somatostatin neurons of the celiac and superior mesenteric ganglia supply the

submucosal ganglia of the intestine and are involved in the control of gastrointestinal

motility. The thinking is that these peptides serve to modulate the response of the

postsynaptic neuron to the primary neurotransmitter. Peptides also have associations with

cholinergic sympathetic postganglionic neurons. These neurons are most commonly found

innervating sweat glands and precapillary resistance vessels in skeletal muscle and

produce vasoactive intestinal polypeptide along with Ach. Calcitonin gene-related peptide,

a potent vasodilator, has also been discovered in paravertebral sympathetic neurons.

Sympathetic nervous system: Functions

• Promotes a fight-or-flight response, corresponds with arousal and energy

generation, and inhibits digestion

• Diverts blood flow away from the gastro-intestinal (GI) tract and skin via

vasoconstriction

• Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1200%

in the case of skeletal muscles)

• Dilates bronchioles of the lung through circulating epinephrine, which allows for

greater alveolar oxygen exchange

• Increases heart rate and the contractility of cardiac cells (myocytes), thereby

providing a mechanism for enhanced blood flow to skeletal muscles

• Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to

enter the eye and enhances

• Far vision
• Provides vasodilation for the coronary vessels of the heart
• Constricts all the intestinal sphincters and the urinary sphincter
• Inhibits peristalsis
• Stimulates orgasm

2) Parasympathetic Nervous System:

Parasympathetic fibers exit the CNS via cranial nerves (CN) III, VII, IX, and X, as well as

through the 52-4 nerve roots. There are four pairs of parasympathetic ganglia, and they

are all located in the head. CN III, via the ciliary ganglion, innervates the iris and ciliary

muscles of the eye. CN VII innervates the lacrimal, nasal, palatine, and pharyngeal glands

via the pterygopalatine ganglion, as well as the sublingual and submandibular glands via

the submandibular ganglion. CN IX innervates the parotid glands via the otic ganglion.

Every other presynaptic parasympathetic fiber synapses in a ganglion near or on the wall

of the target tissue; this leads to the presynaptic fibers being significantly longer than the

postsynaptic. The location of these ganglia gives the PNS its name: “para-“ means

adjacent to, hence, “parasympathetic.

The vagus nerve, CN X, makes up about 75% of the PNS and provides parasympathetie

input to most of the thoracic and abdominal viscera, with the sacral parasympathetic fibers

innervating the descending and sigmeid colon and rectum. The vagus nerve has four cell

bodies in the medulla oblongata. These include the following:

• Dorsal nucleus: provides parasympathetic output to the viscera Nucleus ambiguus:

produces motor fibers and preganglionic neurons that innervate the heart

• Nucleus solitarius: receives afferents of taste sensation and that from viscera, and

lastly

• Spinal trigeminal nucleus: receives information of touch, pain, and temperature of

the outer ear, the mucosa of the larynx, and part of the dura

Additionally, the vagus nerve conducts sensory information from baroreceptors of the

carotid sinus and the aortic arch to the medulla.

As mentioned in the introduction, the vagus nerve is responsible for the “rest and digest”

processes. The vagus nerve promotes cardiac relaxation in several aspects of function. It

decreases contractility in the atria and less-so in the ventricles. Primarily, it reduces

conduction speed through the atrioventricular node. It is by this mechanism that carotid

sinus massage acts to limit reentry in Wolff-Parkinson-White syndrome. The other key

function of the PNS centers around digestion. Parasympathetic fibers to the head promote

salivation, while those that synapse onto the ENS lead to increased peristaltic and

secretory activity. The vagus nerve also has a significant effect on the respiratory cycle. In

a nonpathological state, parasympathetic nerves fire during expiration, contracting and

stiffening airways to prevent collapse. This function has implicated the PNS in the onset

of postoperative acute respiratory distress syndrome.

Due to the expansive nature of the vagus nerve, it has been described as an ideal “early

warning system” for foreign invaders as well as for monitoring the body’s recovery. Up to

80% of vagal fibers are sensory and innervate nearly all major organs. Parasympathetic

ganglia have been found to express receptors for interleukin-1, a key cytokine in the

inflammatory immune response. This, in turn, activates the hypothalamic-pituitary-adrenal

axis and SNS, leading to the release of glucocorticoids and NE, respectively. Studies

have correlated inhibited vagal action through vagotomy and cholinergic inhibitors with

significantly reduced, if not eliminated, allergic, asthmatic, and inflammatory responses.

Postganglionic parasympathetic neurons release Ach that acts on muscarinic and nicotinic

receptors, each with various subunits: M1, M2, and M3, and N1 and N2, with “M” and “N”

standing for muscarine and nicotine, respectively. The postganglionic Ach receptors and

those on the adrenal medulla are N-type, while the parasympathetic effectors and sweat

glands are M-type. As in sympathetic neurons, several peptides, such as vasoactive

intestinal peptide (VIP), Neuropeptide Y (NPY), and calcitonin gene-related peptide

(CGRP) are expressed in, and released from, parasympathetic neurons.

Parasympathetic nervous system: Functions

The parasympathetic nervous system has been said to promote a “rest and digest”

response, promotes calming of the nerves return to regular function, and enhancing

digestion. Functions of nerves within the parasympathetic nervous system include:

• Dilating blood vessels leading to the Gl tract, increasing the blood flow.
• Constricting the bronchiolar diameter when the need for oxygen has diminished
• Dedicated cardiac branches of the vagus and thoracic spinal accessory nerves

impart parasympathetic control of the heart (myocardium)

• Constriction of the pupil and contraction of the ciliary muscles, facilitating

accommodation and allowing for closer vision

• Stimulating salivary gland secretion, and accelerates peristalsis, mediating

digestion of food and, indirectly, the absorption of nutrients

• Sexual. Nerves of the peripheral nervous system are involved in the erection of

genital tissues via the pelvic splanchnic nerves 2-4. They are also responsible for

stimulating sexual arousal.

Some typical actions of the sympathetic and parasympathetic nervous systems are:

Table

3) Enteric Nervous System (ENS):

The ENS is composed of two ganglionated plexuses: the myenteric (Auerbach) and the

submucosal (Meissner) The myenteric plexus sits in between the longitudinal and circular

mobschade of the Gf tract, while the submucosal plexus is present within the submucosa.

The ENS is self-contained, functioning through local reflex activity, but often receives

input from, and provides feedback self-contained NS The ENS may receive input from

postganglionic sympathetic neurons or preganglionic parashpathetic neurons The

submucosal plexus governs the movement of water and electrolytes across the intestinal

wall, while the myenteric plexus coordinates the contractility of the circular and

longitudinal muscle cells of the gut to produce peristalsis.

Motility is produced in the ENS through a reflex circuit involving the circular and

longitudinal muscles. Nicotinic excapses between interneurons mediate the reflex circuits.

When the circuit activates by the presence of a bolus, excitatory neurons in the circular

muscle and inhibitory neurons in the longitudinal muscle fire producing a narrow section of

bowel proximal to the bolus; this is known as the propulsive segment. Simultaneouslye

excitatory neurons in the longitudinal muscle and inhibitory neurons in the circular muscle

fire producing the “receiving segment” of the bowel in which the bolus will continue. This

process repeats with each subsequent section of the bowel. The ENS maintains several

similarities to the CNS. As in the CNS, enteric neurons can be bipolar, pseudounipolar,

and multipolar, between which neuromodulation via excitatory and inhibitory

Communication. Likewise, ENS neurons use over 30 neurotransmitters that are similar to

those of the CNS, with cholinergic and nitrergic transmitters being the most common.

While much of this discussion has focused on the efferent functions of the ANS, the

afferent fibers are responsible for numerous reflex activities that regulate everything from

heart rate to the immune system. Feedback from the ANS is usually processed at a

subconscious level to produce reflex actions in the visceral or somatic portions of the

body. The conscious sensation of the viscera is often interpreted as diffuse pain or

cramps that may correlate with hunger, fullness, or nausea. These sensations most

commonly result from sudden distention/contractions, chemical irritants, or pathological

conditions such as ischemia.

Enteric nervous system: Functions

The enteric nervous system is the intrinsic nervous system of the gastrointestinal system.

It has been described as “the Second Brain of the Human Body”. Its functions include:

• Sensing chemical and mechanical changes in the gut
• Regulating secretions in the gut
• Controlling peristalsis and some other movements

Chapter-12 Skeletal System

Bone, or osseous tissue, is a hard, dense connective tissue that forms most of the adult

skeleton, the support structure of the body. In the areas of the skeleton where bones

move (for example, the ribcage and joints), cartilage, a semi-rigid form of connective

tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the

body system composed of bones and cartilage and performs the following critical

functions for the human body:

• Supports the body
• Facilitates movement
• Protects internal organs
• Produces blood cells
• Stores and releases minerals and fat

Support, Movement, and Protection

The most apparent functions of the skeletal system are the gross functions-those visible

by observation. Simply by looking at a person, you can see how the bones support,

facilitate movement, and protect the human body.

Just as the steel beams of a building provide a scaffold to support its weight, the bones

and cartilage of your skeletal system compose the scaffold that supports the rest of your

body. Without the skeletal system, you would be a limp mass of organs, muscle, and

skin.

Bones also facilitate movement by serving as points of attachment for your muscles.

While some bones only serve as a support for the muscles, others also transmit the

forces produced when your muscles contract. From a mechanical point of view, bones act

as levers and joints serve as fulcrums. Unless a muscle spans a joint and contracts, a

bone is not going to mave. For information on the interaction of the skeletal and muscular

systems, that is, the musculoskeletal system, seek additional content

Bones also protect internal organs from injury by covering or surrounding them. For

example, your ribs protect your lungs and heart, the bones of your vertebral column

(spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain

Mineral Storage, Energy Storage, and Hematopoiesis

On a metabolic level, bone tissue performs several critical functions. For one, the bone

matrix acts as a reservoir for a number of minerals important to the functioning of the

body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue,

can be released back into the bloodstream to maintain levels needed to support

physiological processes. Calcium ions, for example, are essential for muscle contractions

and controlling the flow of other ions involved in the transmission of nerve impulses.

Bone also serves as a site for fat storage and blood cell production. The softer connective

tissue that fills the interior of most bone is referred to as bone marrow. There are two

types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose

tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of

energy. Red marrow is where hematopoiesis-the production of blood cells-takes place.

Red blood cells, white blood cells, and platelets are all produced in the red marrow.

Diagram

Figure: Head of Femur Showing Red and Yellow Marrow. The head of the femur contains

both yellow and red marrow. Yellow marrow stores fat. Red marrow is responsible for

hematopoiesis. (credit: modification of work by “stevenfruitsmaak”/Wikimedia Commons)

12.1 Functions of the Skeletal System:

• The main function of the skeletal system is that it provides a framework to the

body and provides shape.

• Along with the muscular system, the skeletal system helps in the movement of the

body parts of the body and locomotion of the body.

• The skeletal system is hard and so forms a protective layer for the softer, more

delicate organs from any form of injury. The rib cage protects the heart, lungs and

visceral organs, the brain is protected by the skull etc.

• It is the growth and development of bones that provides the height and width of an

individual.

• The centre of the bone consists of the bone marrow which produces blood cells

and therefore hemopoietic in nature.

12.2Parts of the Skeletal System:

The skeletal system is made up of bones and cartilage. There are two types of connective

tissues called tendons and ligaments that are also considered a part of the system.

Ligaments connect bones to bones whereas tendons connect bones to muscles.

The two main parts of the skeletal system, as mentioned above, are bones and cartilage.

12.2.1 Bones

There are 206 bones in the body which form more than 200 joints with each other. They

are classified into two broad categories based on location:

• Axial skeleton: These bones are found towards the midline of the body and include

the skull, the rib cage, and the vertebral column.

• The appendicular skeleton: These bones are found in the appendages such as

arms, legs fingers, and toes.

Bones can be classified into four types based on their shape:

• Long Bones -They are long and slender bones found generally in the limbs. Ex.

Humerus, femur.

• Short Bones: They are short bones which are smaller in size and are found in the

carpals and tarsals.

• Flat Bones: they are thin and flat in nature and not all of them are completely flat.

They provide surface area for muscle attachment. Ex: scapula, sternum

• Irregular Bones: These bones do not have specific shapes and therefore cannot be

put into any other group. Ex: vertebrae

12.2.1.1 Structure of a Bone

Each bone tissue is made up of two types of osseous tissue: compact bone and spongy

bone.

Compact bone is hard and compact in nature and always found towards the outside of the

bone whereas the spongy bone which is softer and more porous is found towards the

centre. The function of each bone determines the ratio in which these two types of tissues

exist within it.

The connective tissue that is found on the outside of the bone is known as the

periosteum. The periosteum is made up of cellular and fibrous tissue and plays a crucial

role in the attachment to muscles and joints as it is this layer which contains tendon and

ligament attachments. The endosteum is the connective tissue layer which lines the

marrow cavity.

The shaft of a bone is known as the diaphysis and the swollen end is called the

epiphysis. The epiphyseal line demarcates the two parts. It is the diaphysis which houses

the marrow cavity which is majorly composed of loose connective tissue and is

responsible for producing blood cells.

The cells that form bone matrix are known as osteoblasts and the mature cells of the

bone are called osteocytes. There is a special type of cells that help remove bone matrix

and are found during bone remodelling known as osteoclasts. These are gigantic cells

and are always found on the side of the bone where the matrix is being eaten away

during growth and remodelling.

The matrix in a bone tissue is made up of two components: the organic part that contains

fibres whereas the inorganic part consists of the minerals (hydroxyapatite).

12.2.2 Cartilage

Cartilage is the second component of the skeletal system. It is made up of fibres that are

embedded in connective tissue or ground substance. Cartilage consists of two types of

fibres: Collagen and elastin fibres. The cells that form cartilage are known as

chondroblasts and the mature cells of the cartilage are known as chondrocytes.

The chondrocytes lie in lacunae in the matrix. The outer layer of a cartilage is known as

the perichondrium. Unlike the bone, cartilage is avascular which means that it contains no

blood supply. However, the perichondrium contains blood supply. There are 3 types of

cartilages namely Hyaline cartilage, elastic cartilage, and fibrocartilage.

• Hyaline cartilage-the most abundant of the three cartilages and functions to help

surfaces slide over one another. Example: found in the respiratory system.

• Fibrocartilage- This cartilage is tough and its main function is to provide support

and strength to structures. Example: found in healing tissue during bone

repair(callus)

• Elastic cartilage- This cartilage is abundant in elastic fibres and functions to

maintain the shape of the area it is present in. Example: found in the middle ear.

Chapter- 13 Immune System

The immune system, which is made up of special cells, proteins, tissues, and organs,

defends people against germs and microorganisms every day. In most cases, the immune

system does a great job of keeping people healthy and preventing infections. But

sometimes problems with the immune system can lead to illness and infection.

13.1 Functions of Immune System:

The immune system is the body’s defense against infectious organisms and other

invaders. Through a series of steps called the immune response, the immune system

attacks organisms and substances that invade body systems and cause disease.

The immune system is made up of a network of cells, tissues, and organs that work

together to protect the body.

One of the important cells involved are white blood cells, also called leukocytes, which

come in two basic types that combine to seek out and destroy disease-causing organisms

or substances.

Leukocytes are produced or stored in many locations in the body, including the thymus,

spleen, and bone marrow. For this reason, they’re called the lymphoid organs. There are

also clumps of lymphoid tissue throughout the body, primarily as lymph nodes, that house

the leukocytes.

The leukocytes circulate through the body between the organs and nodes via lymphatic

vessels and blood vessels. In this way, the immune system works in a coordinated

manner to monitor the body for germs or substances that might cause problems.

The two basic types of leukocytes are:

1. Phagocytes, cells that chew up invading organisms
2. Lymphocytes, cells that allow the body to remember and recognize previous

invaders and help the body destroy them

A number of different cells are considered phagocytes. The most common type is the

neutrophil, which primarily fights bacteria. If doctors are worried about a bacterial

infection, they might order a blood test to see if a patient has an increased number of

neutrophils triggered by the infection. Other types of phagocytes have their own jobs to

make sure that the body responds appropriately to a specific type of invader.

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start

out in the bone marrow and either stay there and mature into B cells, or they leave for the

thymus gland, where they mature into T cells. B lymphocytes and T lymphocytes have

separate functions: B lymphocytes are like the body’s military intelligence system, seeking

out their targets and sending defenses to lock onto them. T cells are like the soldiers,

destroying the invaders that the intelligence system has identified.

13.2 How it works:

When antigens (foreign substances that invade the body) are detected, several types of

cells work together to recognize them and respond. These cells trigger the B lymphocytes

to produce antibodies, which are specialized proteins that lock onto specific antigens.

Once produced, these antibodies stay in a person’s body, so that if his or her immune

system encounters that antigen again, the antibodies are already there to do their job. So

if someone gets sick with a certain disease, like chickenpox, that person usually won’t get

sick from it again.

This is also how immunizations prevent certain diseases. An immunization introduces the

body to an antigen in a way that doesn’t make someone sick, but does allow the body to

produce antibodies that will then protect the person from future attack by the germ or

substance that produces that particular disease.

Although antibodies can recognize an antigen and lock onto it, they are not capable of

destroying it without help. That’s the job of the T cells, which are part of the system that

destroys antigens that have been tagged by antibodies or cells that have been infected or

somehow changed. (Some T cells are actually called “killer cells.”) T cells also are

involved in helping signal other cells (like phagocytes) to do their jobs.

Antibodies also can neutralize toxins (poisonous or damaging substances) produced by

different organisms. Lastly. Antibodies can activate a group of proteins called complement

that are also part of the immune system. Complement assists in killing bacteria, viruses,

or infected cells.

All of these specialized cells and parts of the immune system offer the body protection

against disease. This protection is called immunity.

Immunity

Humans have three types of immunity innate, adaptive, and passive:

Innate Immunity

Everyone is born with innate (or natural) immunity, a type of general protection. Many of

the germs that affect other species don’t harm us. For example, the viruses that cause

leukemia in cats or distemper in dogs don’t affect humans. Innate immunity works both

ways because some viruses that make humans ill- such as the virus that causes

HIV/AIDS – don’t make cats or dogs sick.

Innate immunity also includes the external barriers of the body, like the skin and mucous

membranes (like those that line the nose, throat, and gastrointestinal tract), which are the

first line of defense in preventing diseases from entering the body. If this outer defensive

wall is broken (as through a cut), the skin attempts to heal the break quickly and special

immune cells on the skin attack invading germs.

Adaptive Immunity

The second kind of protection is adaptive (or active) immunity, which develops throughout

our lives. Adaptive Immunity involves the lymphocytes and develops as people are

exposed to diseases or immunized against diseases through vaccination.

Passive Immunity

Passive immunity is “borrowed” from another source and it lasts for a short time. For

example, antibodies in a mother’s breast milk give a baby temporary immunity to diseases

the mother has been exposed to. This can help protect the baby against infection during

the early years of childhood.

Everyone’s immune system is different. Some people never seem to get infections,

whereas others seem to be sick all the time. As people get older, they usually become

immune to more germs as the immune system comes into contact with more and more of

them. That’s why adults and teens tend to get fewer colds than kids – their bodies have

learned to recognize and immediately attack many of the viruses that cause colds.

13.3 Problems of the Immune System

Disorders of the immune system fall into four main categories:

• Immunodeficiency disorders (primary or acquired)
• Autoimmune disorders (in which the body’s own immune system attacks its own

tissue as foreign matter)

• Allergic disorders (in which the immune system overreacts in response to an

antigen)

• Cancers of the immune system

13.4 Immunodeficiency Disorders

Immunodeficiencies happen when a part of the immune system is missing or not working

properly. Some people are born with an immunodeficiency (known as primary

immunodeficiencies), although symptoms of the disorder might not appear until later in 

life. Immunodeficiencies also can be acquired through infection or produced by drugs

(these are sometimes called secondary immunodeficiencies).

Immunodeficiencies can affect B lymphocytes, T lymphocytes, or phagocytes. Examples

of primary immunodeficiencies that can affect kids and teens are:

• IgA deficiency is the most common immunodeficiency disorder. IgA is an

immunoglobulin that is found primarily in the saliva and other body fluids that help

guard the entrances to the body. IgA deficiency is a disorder in which the body

doesn’t produce enough of the antibody IgA. People with IgA deficiency tend to

have allergies or get more colds and other respiratory infections, but the condition

is usually not severe.

• Severe combined immunodeficiency (SCID) is also known as the “bubble boy

disease” after a Texas boy with SCID who lived in a germ-free plastic bubble.

SCID is a serious immune system disorder that occurs because of a lack of both B

and T lymphocytes, which makes it almost impossible to fight infections.

• DiGeorge syndrome (thymic dysplasia), a birth defect in which kids are born

without a thymus gland, is an example of a primary T-lymphocyte disease. The

thymus gland is where T lymphocytes normally mature.

• Chediak-Higashi syndrome and chronic granulomatous disease (CGD) both

involve the inability of the neutrophils to function normally as phagocytes.

Acquired (or secondary) immunodeficiencies usually develop after someone has a

disease, although they can also be the result of malnutrition, burns, or other medical

problems. Certain medicines also can cause problems with the functioning of the immune

system.

Acquired (secondary) immunodeficiencies include:

• HIV (human immunodeficiency virus) infection/AIDS (acquired

immunodeficiency syndrome) is a disease that slowly and steadily destroys the

immune system. It is caused by HIV, a virus that wipes out certain types of

lymphocytes called T-helper cells. Without T-helper cells, the immune system is

unable to defend the body against normally harmless organisms, which can cause

life-threatening infections in people who have AIDS. Newborns can get HIV

infection from their mothers while in the uterus, during the birth process, or during

breastfeeding. People can get HIV infection by having unprotected sexual

intercourse with an infected person or from sharing contaminated needles for

drugs, steroids, or tattoos.

• Immunodeficiencies caused by medications. Some medicines suppress the

immune system. One of the drawbacks of chemotherapy treatment for cancer, for

example, is that it not only attacks cancer cells, but other fast-growing, healthy

cells, including those found in the bone marrow and other parts of the immune

system. In addition, people with autoirnmune disorders or who have had organ

transplants may need to take immunosuppressant medications, which also can

reduce the immune system’s ability to fight infections and can cause secondary

immunodeficiency.

13.5 Autoimmune Disorders

In autoimmune disorders, the immune system mistakenly attacks the body’s healthy

organs and tissues as though they were foreign invaders. Autoimmune diseases include:

• Lupus: a chronic disease marked by muscle and joint pain and inflammation (the

abnormal immune response also may involve attacks on the kidneys and other

organs)

• Juvenile idiopathic arthritis: a disease in which the body’s immune system acts

as though certain body parts (such as the joints of the knee, hand, and foot) are

foreign tissue and attacks them

• Scleroderma: a chronic autoimmune disease that can lead to inflammation and

damage of the skin, joints, and internal organs

• Ankylosing spondyli: a-disease that involves inflammation of the spine and joints,

causing stiffness and pain

• Juvenile dermatomyositis: a disorder marked by inflammation and damage of the

skin and muscles

13.6 Allergic Disorders

Allergic disorders happen when the immune system overreacts to exposure to antigens in

the environment. The substances that provoke such attacks are called allergens. The

immune response can cause symptoms such as swelling, watery eyes, and sneezing, and

even a life-threatening reaction called anaphylaxis. Medicines called antihistamines can

relieve most symptoms.

Allergic disorders include:

• Asthma, a respiratory disorder that can cause breathing problems, often involves

an allergic response by the lungs. If the lungs are oversensitive to certain

allergens (like pollen, molds, animal dander, or dust mites), breathing tubes can

become narrowed and swollen, making it hard for a person to breathe.

• Eczema is an itchy rash also known as atopic dermatitis. Although not necessarily

caused by an allergic reaction, eczema most often happens in kids and teens who

have allergies, hay fever, or asthma or who have a family history of these

conditions.

• Allergies of several types can affect kids and teens. Environmental allergies (to

dust mites, for example), seasonal allergies (such as hay fever), drug allergies

(reactions to specific medications or drugs), food allergies (such as to nuts), and

allergies to toxins (bee stings, for example) are the common conditions people

usually refer to as allergies.

13.7 Cancers of the Immune System

Cancer happens when cells grow out of control. This can include cells of the immune

system. Leukemia, which involves abnormal overgrowth of leukocytes, is the most

common childhood cancer. Lymphoma involves the lymphoid tissues and is also one of

the more common childhood cancers. With current treatments, most cases of both types

of cancer in kids and teens are curable.

Although immune system disorders usually can’t be prevented, you can help your child’s

immune system stay stronger and fight illnesses by staying informed about your child’s

condition and working closely with your doctor.