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Central Nervous System

The central nervous system (CNS) is the largest

part of the nervous system, and includes the

brain and spinal cord. The spinal cavity holds

and protects the spinal cord, while the head

contains and protects the brain. The CNS is

covered by the meninges, a three layered

protective coat. The brain is also protected bythe skull, and the spinal cord is also protected

by the vertebrae.


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Central Nervous System (CNS) is the part of the nervous

system that functions to coordinate the activity of all

parts of the bodies of multicellular organisms. In

vertebrates, the central nervous system is enclosed in the

meninges. The meninges (singular meninx) is the system

of membranes which envelops the central nervous

system. The meninges consist of three layers: the dura

mater, the arachnoid mater, and the pia mater. The

primary function of the meninges and of the

cerebrospinal fluid is to protect the central nervous

system. It contains the majority of the nervous system

and consists of the brain (in vertebrates which have

them), and the spinal cord.


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The meninges (singular meninx) is the system of

membranes which envelops the central nervoussystem. The meninges consist of three layers:

the dura mater, the arachnoid mater, and the pia

mater. The primary function of the meninges

and of the cerebrospinal fluid is to protect thecentral nervous system. The space between

these membranes is bathed with a spinal fluid

much like lymph, which serves as a protectivecushion for the delicate nerve tissue, and allows

some expansion space for the brain when its

blood supply is increased


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3 layers of meninges:

Dura mater - (also rarely called meninx fibrosa, or

pachymeninx) is a thick, durable membrane, closest to theskull. It consists of two layers, the periosteal layer, closest to

the calvaria and the inner meningeal layer. It contains larger

blood vessels which split into the capilliaries in the pia mater.

It is composed of dense fibrous tissue, and its inner surface iscovered by flattened cells like those present on the surfaces

of the pia mater and arachnoid. The dura mater is a sac which

envelops the arachnoid and has been modified to serve

several functions. The dura mater surrounds and supports thelarge venous channels (dural sinuses) carrying blood from the

brain toward the heart.


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The falx cerebri separates the hemispheres of the

cerebrum. The falx cerebelli separates the lobes of

the cerebellum.

The tentorium cerebelli separates the cerebrum from

the cerebellum.

The epidural space is a potential space between the

dura mater and the skull. If there is hemorrhaging in

the brain, blood may collect here. Adults are morelikely than children to bleed here as a result of closed

head injury.


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The subdural space is another potential

space. It is between the dura mater and

the middle layer of the meninges, thearachnoid mater. When bleeding occurs in

the cranium, blood may collect here and

push down on the lower layers of themeninges. If bleeding continues, brain

damage will result from this pressure.

Children are especially likely to havebleeding in the subdural space in cases of

head injury.


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Arachnoid mater - The middle element

of the meninges is the arachnoid

membrane, so named because of its

spider web-like appearance. It provides

a cushioning effect for the centralnervous system. The arachnoid mater

exists as a thin, transparent

membrane. It is composed of fibroustissue and, like the pia mater, is

covered by flat cells also thought to be

impermeable to fluid.


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subarachanoid space lies between the

arachnoid and pia mater. It is filled with

cerebrospinal fluid. All blood vessels

entering the brain, as well as cranialnerves pass through this space. The

term arachnoid refers to the spider

web like appearance of the blood

vessels within the space


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Pia mater - T e pia or pia mater is a very

delicate membrane. It is the meningeal

envelope which firmly adheres to the surface ofthe brain and spinal cord. As such it follows all

the minor contours of the brain (gyri and sulci).

It is a very thin membrane composed of fibroustissue covered on its outer surface by a sheet of

flat cells thought to be impermeable to fluid.

The pia mater is pierced by blood vessels which

travel to the brain and spinal cord, and its

capillaries are responsible for nourishing the

brain.


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Cerebrospinal fluid - is a clear liquidproduced within spaces in the brain

called ventricles. Like saliva it is a

filtrate of blood. It is also found inside

the subarachnoid space of the

meninges which surrounds both thebrain and the spinal cord.


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Choroid Plexus

All of the ventricles contain choroid

plexuses which produce cerebrospinal

fluid by allowing certain componentsof blood to enter the ventricles. The

choroid plexuses are formed by the

fusion of the pia mater, the most

internal layer of the meninges and the

ependyma, the lining of the ventricles.


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The Ventricles

These four spaces are filled

with cerebrospinal fluid and

protect the brain by

cushioning it and supporting

its weight.


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The two lateral ventricles extendacross a large area of the brain. The

anterior horns of these structures are

located in the frontal lobes. They

extend posteriorly into the parietal

lobes and their inferior horns arefound in the temporal lobes.


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The third ventricle lies between the two thalamic

bodies. The massa intermedia passes through it and

the hypothalamus forms its floor and part of its lateral

walls.

The fourth ventricle is located between the

cerebellum and the pons.The four ventricles are connected to one another.

The two foramina of Munro, which are also know as

the interventricular foramina, link the lateral

ventricles to the third ventricle.


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The Aqueduct of Sylvius which is also

called the cerebral aqueduct connectsthe third and fourth ventricles.

The fourth ventricle is connected to

the subarachnoid space via two lateral

foramina of Luschka and by one medialforamen of Magendie.


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ANATOMY OFTHE CNS

BRAIN

The center of the nervous system. The brain is

located in the head, protected by the skull andclose to the primary sensory apparatus of

vision, hearing, balance, taste, and smell.


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The frontal lobe is concerned with higher

intellectual functions, such as abstract thought

and reason, speech (Broca's area in the lefthemisphere only), olfaction, and emotion.

Voluntary movement is controlled in the

precentral gyrus (the primary motor area). The parietal lobe is dedicated to sensory

awareness, particularly in the postcentral gyrus

(the primary sensory area). It is also concerneswith abstract reasoning, language interpretation

and formation of a mental egocentric map of

the surrounding area.


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The brain can be subdivided into several distinct regions:

1. Brainstem consists of medulla oblongata, pons and midbrain.

Medulla oblongata - is the lower portion of the brainstem. It

deals with autonomic functions, such as breathing and blood pressure.

The cardiac center is the part of the medulla oblongata responsible for

controlling the heart rate.

Pons - relays sensory information between the cerebellum andcerebrum; aids in relaying other messages in the brain; controls arousal,

and regulates respiration (see respiratory centres). In some theories, the

pons has a role in dreaming.

Midbrain (mesencephalon) - The mesencephalon is considered

part of the brain stem. Its substantia nigra is closely associated withmotor system pathways of the basal ganglia.


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2. Cerebellum - is a region of the brain that plays

an important role in the integration of sensory

perception, coordination and motor control. In order

to coordinate motor control, there are many neural

pathways linking the cerebellum with the cerebral

motor cortex (which sends information to the muscles

causing them to move) and the spinocerebellar tract(which provides proprioceptive feedback on the

position of the body in space). The cerebellum

integrates these pathways, like a train conductor,using the constant feedback on body position to fine-

tune motor movements.


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Diencephalon - (or interbrain) is the region

of the brain that includes the thalamus,

hypothalamus, epithalamus, prethalamus

or subthalamus and pretectum. The

diencephalon is located at the midline of

the brain, above the mesencephalon of the

brain stem. The diencephalon contains the

zona limitans intrathalamica asmorphological boundary and signalling

center between the prethalamus and the

thalamus.


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Thalamus - plays an important role in

regulating states of sleep and wakefulness.

Thalamic nuclei have strong reciprocal

connections with the cerebral cortex,

forming thalamo-cortico-thalamic circuits

that are believed to be involved with

consciousness. The thalamus plays a major

role in regulating arousal, the level ofawareness, and activity. Damage to the

thalamus can lead to permanent coma.


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Epithalamus is a dorsal posterior

segment of the diencephalon (a

segment in the middle of the brain also

containing the hypothalamus and the

thalamus) which includes thehabenula, the stria medullaris and the

pineal body. Its function is the

connection between the limbic system

to other parts of the brain.


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Hypothalamus - is a small part of the brain

located just below the thalamus on both sides

of the third ventricle. Lesions of thehypothalamus interfere with several vegetative

functions and some so called motivated

behaviors like sexuality, combativeness, andhunger. The hypothalamus also plays a role in

emotion. Specifically, the lateral parts seem to

be involved with pleasure and rage, while themedial part is linked to aversion, displeasure,

and a tendency to uncontrollable and loud

laughing.


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Cerebrum - or top portion of the brain, is

divided by a deep crevice, called thelongitudinal sulcus. The longitudinal sulcus

separates the cerebrum in to the right and left

hemispheres. In the hemispheres you will find

the cerebral cortex, basal ganglia and the limbic

system. The two hemispheres are connected by

a bundle of nerve fibers called the corpus

callosum. The right hemisphere is responsible

for the left side of the body while the opposite

is true of the left hemisphere.


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PHYSIOLOGY OFTHE CNS

Medulla

The medulla is the control center for respiratory,

cardiovascular and digestive functions. Pons

The pons houses the control centers for

respiration and inhibitory functions. Here it

will interact with the cerebellum.


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Cerebellum


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cerebellum is the part of the brain that is

located posterior to the medulla oblongataand pons. It coordinates skeletal muscles to

produce smooth, graceful motions. The

cerebellum receives information from oureyes, ears, muscles, and joints about what

position our body is currently in

(proprioception). It also receives output

from the cerebral cortex about where these

parts should be. The main function of the

cerebellum is coordination.


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The Limbic System is a complex set of

structures found just beneath thecerebrum and on both sides of the

thalamus. It combines higher mental

functions, and primitive emotion, into onesystem. It is often referred to as the

emotional nervous system. It is not only

responsible for our emotional lives, butalso our higher mental functions, such as

learning and formation of memories.


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The Limbic system explains why some

things seem so pleasurable to us, such

as eating and why some medical

conditions are caused by mental stress,such as high blood pressure. There are

two significant structures within the

limbic system and several smaller

structures that are important as well.


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They are:

1. The Hippocampus

2. The Amygdala

3. The Thalamus

4. The Hypothalamus

5. The Fornix and Parahippocampus6. The Cingulate Gyrus


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Structures of the Limbic System

Hippocampus

The Hippocampus is found deep in the temporal

lobe, shaped like a seahorse. It consists of twohorns that curve back from the amygdala. It is

situated in the brain so as to make the

prefrontal area aware of our past experiences

stored in that area. The prefrontal area of thebrain consults this structure to use memories to

modify our behavior. The hippocampus is

responsible for memory.


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Amygdala

The Amygdala is a little almond shaped structure,

deep inside the anteroinferior region of the temporal

lobe, connects with the hippocampus, the septi

nuclei, the prefrontal area and the medial dorsal

nucleus of the thalamus. These connections make it

possible for the amygdala to play its important role onthe mediation and control of such activities and

feelings as love, friendship, affection, and expression

of mood. The amygdala is the center for identificationof danger and is fundamental for self preservation.

The amygdala is the nucleus responsible for fear.


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Thalamus

Lesions or stimulation of the medial, dorsal, and anterior

nuclei of the thalamus are associated with changes inemotional reactivity. However, the importance of these nuclei

on the regulation of emotional behavior is not due to the

thalamus itself, but to the connections of these nuclei with

other limbic system structures. The medial dorsal nucleusmakes connections with cortical zones of the prefrontal area

and with the hypothalamus. The anterior nuclei connect with

the mamillary bodies and through them, via fornix, with the

hippocampus and the cingulated gyrus, thus taking part inwhat is known as the Papez's circuit.


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Hypothalamus

The Hypothalamus is a small part of thebrain located just below the thalamus on

both sides of the third ventricle. Lesions of

the hypothalamus interfere with several

vegetative functions and some so called

motivated behaviors like sexuality,combativeness, and hunger. The

hypothalamus also plays a role in emotion.


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Specifically, the lateral parts seem to be

involved with pleasure and rage, while the

medial part is linked to aversion, displeasure,and a tendency to uncontrollable and loud

laughing. However, in general the hypothalamus

has more to do with the expression ofemotions. When the physical symptoms of

emotion appear, the threat they pose returns,

via the hypothalamus, to the limbic centers andthen the prefrontal nuclei, increasing anxiety.


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The Fornix and Parahippocampal

These small structures are important connecting pathways for

the limbic system.

The Cingulate Gyrus

The Cingulate Gyrus is located in the medial side of the brain

between the cingulated sulcus and the corpus callosum.There is still much to be learned about this gyrus, but it is

already known that its frontal part coordinates smells and

sights, with pleasant memories of previous emotions. The

region participates in the emotional reaction to pain and inthe regulation of aggressive behavior.


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