2 the central nervous system

1 DAWN V TO MY M.Pharm., Asst. Professor, Dept. of Pharmacology, ST.JOSEPH’S COLLEGE O F PHARMACY, CHERTHALA.

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THE CENTRAL NERVOUS SYSTEM (CNS)

The central nervous system consists of the brain and spinal cord. An adult brain weighs

between 1.3 to 1.4 kilograms (kg) and has a volume of about 1200 cc. The number of active

synapses in the brain determines the intelligence of a person. The brain’s four major regions

are the cerebrum, the diencephalon, the brainstem, and the cerebellum. Twelve pairs of cranial

nerves originate from the brain.

1. Cerebrum: It is the largest part of the brain situated on the diencephalon and brain

stem.

2. Diencephalon: It lies superior to the brain stem and consists of

a. Epithalamus

b. Thalamus

c. Subthalamus and

d. Hypothalamus.

3. Brain stem: It continues as spinal cord and consists of

a. Medulla oblongata

b. Pons and

c. Mid brain

4. Cerebellum (also known as the little brain): Situated posterior to brain stem.

The Brain’s Major Regions:

1. Prosencephalon (Forebrain).

2. Telencephalon: Cerebrum.

3. Diencephalon: Epithalamus, Thalamus, Hypothalamus.

4. Mesencephalon (Midbrain).

5. Mesencephalon: Cerebral Peduncles, Colliculi.

6. Rhombencephalon (Hindbrain).

7. Metencephalon: Pons, Cerebellum.

8. Myelencephalon: Medulla Oblongata.


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Blood flow:

Supply by: Internal carotid and vertebral arteries. Return by: Internal jugular veins (from

head to heart). Interruption in blood flow to brain for 30 sec imparts neuronal function, if it is

for 4 min it causes permanent damage to the brain, as brain doesn’t have glucose and oxygen

store. Low level of glucose in brain leads to mental confusion, dizziness, convulsions and loss

consciousness.

Blood Brain Barrier (BBB):

It is formed by tight junctions of the endothelial cells of brain capillaries surrounded by

thick basement membrane and astrocytes. It selectively passes nutrients from blood to the

neurons in the brain. Glucose doesn’t cross BBB normally, and hence facilitated by active

transport.

Brain ventricles:

Ventricles are fluid filled cavities inside the body. CSF filled cavities within the brain

are called brain ventricles. They are formed as cavities of expansions from the lumen (opening)

of the embryonic neural tube. It is continuous and connected with one another, as well as with

the central canal of the spinal cord. There are 4 brain ventricles:

1. Right and left lateral ventricles: It is situated in the cerebral hemisphere above the

Corpus callosum (Callosal commissure is a wide, flat bundle of neural fibres beneath

the cortex, which connects the left and right cerebral hemispheres and facilita tes

interhemispheric communication).

One lateral ventricle is situated in each cerebral hemisphere and consists of

anterior, posterior and inferior horns.

The transparent membrane, septum pellucidum connects both the ventricles and

it is lined by ciliated epithelium.

The inter-ventricular foramina/foramen of manroe – connects the two lateral

ventricles to the third ventricle.

2. Third ventricle:

It is the cavity below lateral ventricles with in the diencephalon, between the

thalamus.

The cerebral aqueduct connects the third ventricle with the fourth ventricle.

3. Fourth ventricle:

It is the cavity of the hind brain behind and below the third ventricle.

It is diamond or tent shaped with pons and medulla in the front and cerebellum

at the back.

It is continuous with the central canal of the spinal cord.

It communicates with the sub arachnoid space by median and lateral apertures.

CSF enters into the fourth ventricle via the foramina in the roof.

The floor of the fourth ventricle is called the rhomboid fossa.


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Cerebrospinal fluid (CSF):

It is a clear, colourless liquid that protects the brain and spinal cord from chemical and

physical injuries. It carries oxygen, glucose etc. from blood to brain, neurons and neuroglia.

CSF circulates through the cavities in the brain and spinal cord and around brain and spinal

cord in the subarachnoid space (the space between the arachnoid matter and pia matter)

Formation of CSF in the ventricles:

CSF formed in the choroid plexuses, which are networks of capillaries (microscop ic

blood vessels) in the walls of the ventricles (A plexus is a branching network of vessels or

nerves. The vessels may be veins, capillaries or lymphatic vessels). The ependymal cells that

cover the capillaries form cerebrospinal fluid from the blood plasma by filtration and secretion

process. The ependymal cells joined by tight junctions in the choroid capillaries forms the

blood-cerebrospinal fluid barrier permitting selective diffusion, thereby protecting the brain

and spinal cord from potentially harmful blood-born substances.

The composition of CSF:

Total volume of CSF is 80 to 150mL in an adult. It contains glucose, proteins, lactic

acid, urea, cations (Na+, K+, Ca2+ and Mg2+) and anions (Cl- and HCO3-). It also contains WBCs.

Functions of CSF: The CSF contributes to brain and spinal cord homeostasis in 3 ways.

1. Mechanical protection: It serves as shock absorbing medium that protects the delicate

tissues of the brain and spinal cord from impacts by not letting them hit the bony walls

of the cranium and vertebral cavities. The fluid also buoys the brain floating it in the

cranial cavity.

2. Chemical protection: Provides an optimal chemical environment for accurate neuronal

signalling. Ionic composition should be maintained in homeostasis as even slight

changes in them can affect the production of action potentials and postsynaptic

potentials.


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3. Circulation: CSF is a medium for exchange of nutrients and waste products between

the blood and nervous tissue.

Circulation of CSF:

CSF formed in the choroid plexuses, lined with ependymal cells, which form

cerebrospinal fluid from blood plasma by filtration and secretion. The CSF formed (i.e. 90%

of CSF produced) from lateral ventricle flows into the third ventricle through 2 narrow

openings, the interventricular foramina (foramina of monro). The CSF formed in the 3rd

ventricle flows through the cerebral aqueduct (aqueduct of sylvius) to 4th ventricle. The CSF

from 4th ventricle enters the sub-arachnoid space through 3 openings in the roof of 4th ventric le.

a. A median aperture (Foramen of Magendie) drains CSF from 4th ventricle into the

cisterna magna and

b. The paired lateral apertures (foramina of Luschka) drains CSF from 4th ventricle into

the cerebellopontine angle cistern.

Cisterns: The 3 openings in the subarachnoid space between the arachnoid and pia matter are

known as cisterns.

Sub arachnoid space forms inter communication pools to reinforce the protective effect

of CSF on vital centres of medulla. There are 2 main cisterns they are:

1. Cisterna magna (cerebromedullary): The cisterna magna is located between the

cerebellum and the dorsal surface of the medulla oblongata. Cerebrospinal fluid

produced in the fourth ventricle drains into the cisterna magna via the lateral

apertures and median aperture.

2. Cisterna pontine (cerebropontine) located between the pons and the medulla.

3. Interpeduncular cistern located between the cerebral peduncles.

CSF then circulates in the central canal of the spinal cord and in the subarachnoid space

around the surface of the brain and spinal cord.


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CSF is gradually reabsorbed into the blood by arachnoid villi as it is formed by the

choroid plexuses at a rate of 20 mL/hr. (480mL/day). The rate of formation and reabsorption

are the same and hence the pressure of CSF is maintained constant.

Hydrocephalus (condition associated with CSF): Tumours, inflammation or

developmental malformations may blocks the drainage of CSF from the ventricles into the

subarachnoid space leading to accumulation of excess CSF in the ventricle, which leads to

increased pressure condition called hydrocephalus (enlargement of the head. hydro=water,

cephal=head). It may occur after head injury, meningitis/subarachnoid haemorrhage.

Protective covering of the brain:

The cranium and cranial meninges protects the brain by enclosing it. The cranium is

formed by the cranial bones (skull bones). Membranous meninges: It is a protective fibrous

connective tissue membrane which cushion and surrounds the brain and spinal cord. They are

of 2 types:

1. Cranial meninges and

2. Spinal meninges.

The 3 meninges are:

1. The outer dura matter (the cranial dura matter has 2 layers and the spinal dura matter

has only one layer).

2. The middle arachnoid matter and

3. The inner pia matter.


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The space between dura matter and arachnoid matter is known as subdural space and

that between arachnoid matter and pia matter is knows as subarachnoid space (filled with

CSF).

Cranial meninges: It is formed by 3 dense regular connective tissues from the superficial to

deep; the cranial meninges are dura matter, arachnoid matter and the pia matter. Arachnoid and

pia – leptomeninges (literally thin meninges refers to the pia matter and arachnoid matter). The

arachnoid is connected to the pia by cob-web like strands, it is structurally continuous with the

pia, hence the name pia-arachnoid or leptomeninges.

Functions:

It separates the soft tissue of the brain from the bones of the cranium and act as a

cushion.

It encloses and protects blood vessels that supply the brain.

It contains and circulates cerebrospinal fluid.

Parts of the cranial meninges form some of the veins that drain blood from the brain.

Cranial dura matter (dura-tough/hard, matter-mother): The cranial meninges are continuous

with the spinal meninges.

Outer most layer of the meninges.

Strongest of the meninges.

Toughest membrane which is composed of 2 dense irregular fibrous layers.

1. Outer (endosteal) periosteal layer, the more superficial layer which attaches to

the periosteum of the cranial bones.

2. Inner meningeal layer lies deep to the periosteal layer.

The meningeal layer is usually fused to the periosteal layer.

Exceptions: In specific areas where the 2 layers separate to form large, blood-filled space called

dural venous sinuses.

Cranial dural septa:

The meningeal layer of the dura matter extends as flat partitions (septa) deep into the

cranial cavity at 4 locations called cranial dural septa.

These membranous partitions separate specific parts of the brain and provide additiona l

stabilization and support to the entire brain.

- Flax crebri (flax-sickle shaped): This separates the 2 hemispheres of cerebrum.

- Flax cerebelli: This separates the 2 hemispheres of cerebellum.

- Tentorium cerebellie (tentorium - tent like, triangular like):This separates the cerebrum

from the cerebellum.

- Diaphragm sellae (The diaphragm sellae or sellar diaphragm is the circular fold of dura

matter that almost completely roofs the fossa hypophyseos of the sella turcica of the

sphenoid bone within the skull).


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Epidural and extradural space – The space between dura matter and periosteum of the

vertebral cavity, contains blood vessels, areolar tissue and nerve endings leaving the spinal

cord.

Spinal dura-matter:

The cranial meninges are continuous with the spinal meninges.

It consists of only one layer in spinal cord.

It starts at the foramen magnum where the spinal cord attaches to the brain stem and

extends up to 2nd sacral vertebra.

It encloses filum terminale and fuses with the periosteum of coccyx.

It forms the epidural and extradural space.

The conus medullaris (Latin for "medullary cone") is the tapered lower end of the spinal

cord. It occurs near lumbar vertebral levels 1 (L1) and 2 (L2). After the spinal cord tapers out,

the spinal nerves continue to branch out diagonally, forming the cauda equina.

The pia mater that surrounds the spinal cord, however, projects directly downward,

forming a slender filament called the filum terminale, which connects the conus medullaris to


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the back of the coccyx. The tension that the filum terminale provides between the conus

medullaris and the coccyx stabilizes the entire spinal cord.

The filum terminale ("terminal thread") is a delicate strand of fibrous tissue, about 20

cm in length, proceeding downward from the apex of the conus medullaris. It gives longitud ina l

support to the spinal cord and consists of two parts:

The upper part, or filum terminale internum, is about 15 cm long and reaches as far as

the lower border of the second sacral vertebra. It is continuous above with the pia matter

and contained within a tubular sheath of the dura matter. In addition, it is surrounded

by the nerves forming the cauda equina, from which it can be easily recognized by its

bluish-white colour.

The lower part, or filum terminale externum, closely adheres to the dura matter. It

extends downward from the apex of the tubular sheath and is attached to the back of

the first segment of the coccyx in a structure sometimes referred to as the coccygeal

ligament.

The most inferior of the spinal nerves, the coccygeal nerve leaves the spinal cord at the

level of the conus medullaris, superior to the filum terminale. The central canal of the spinal

cord extends 5 to 6 cm beyond the conus medullaris, downward into the filum terminale.

Cranial arachnoid matter (arachno-spider, oid-similar)

It lies interior to the dura matter.

Partially composed of a delicate web of vascular collagen and elastic fibres, termed the

arachnoid trabeculae.

The space between the arachonid matter and the outer dura matter is the subdural space.

The space between the arachnoid matter and the inner most pia matter is the sub

arachnoid space which is filled with CSF.

It consists of the blood vessels of the brain.

The cranial nerves pass out through the meninges.

It contains small finger like projection to the dural venous sinuses called arachnoid villi

which absorbs the CSF.

Spinal arachnoid matter

Spinal sub arachnoid space continuous down from the brain and contains CSF.

It contains blood vessels of the spinal cord.

Spinal nerves pass out through the arachnoid matter.

Cranial pia matter

It is the innermost fibrous membrane of the cranial meninges.

It consists of thin layer of delicate vascular connective tissue that tightly adheres to the

brain.


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It follows every contour (each and every side) i.e. every sulci, irregularities of the brain

surface.

It forms a sheath for cranial nerves, blood vessels of the brain.

Folds ends in tuft of capillaries called choroid plexuses form tela-chorioidea

(Telachorioidea/Telachoroidea is a structure found in the ventricles of the brain).

Spinal pia matter

Denticulate ligament (small tooth) – triangular shaped membrane extensions of pia

matter suspends spinal cord to the middle of dural sheath.

THE CEREBRUM (THE SEAT OF INTELLIGENCE)

This is the largest part of the brain and it occupies the anterior and middle cranial fossae.

It consists of 85% of the brain mass.

It is divided by a deep cleft, the longitudinal cerebral fissure, into right and left cerebral

hemispheres, each containing one of the lateral ventricles.

The hemispheres are connected by a mass of white matter (nerve fibres) called the

corpus callosum.

Superficially it consists of grey matter called the cerebral cortex (cortex=rind/bark).

Inside the white matter of the brain, there are grey matter nuclei known as basal nuclei.

The cortical regions rolls and folds upon itself and the folds formed are called gyri

(circles) or convulsions. These convolutions greatly increase the surface area of the

cerebrum and intelligence.

The deepest grooves in between folds are known as fissures.

The shallower grooves between folds are known as sulci.

Singular for gyri is gyrus and for sulci is sulcus.

Functions of the cerebrum:

1. The sensory areas interpret sensory impulses.

2. The motor areas control muscular movements.

3. The association areas function in emotional and intellectual processes.

4. The basal ganglia coordinate gross, automatic muscle movements and regulate muscle

tone.

5. Limbic system functions in emotional aspects of behaviour related to survival.

There are 3 main varieties of activities associated with the cerebral cortex:

Mental activities involved in memory, intelligence, sense of responsibility, thinking,

reasoning, moral sense and learning are attributed to the higher centres. It provides the

ability to read, write, speak, to make calculations, compose music, to remember the

past, plan for the future and imagination.

Sensory perception, including the perception of pain, temperature, touch, sight, hearing,

taste and smell.

Initiation and control of skeletal (voluntary) muscle contraction.


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Fissures (deep grooves which separate major regions of brain):

The longitudinal fissures, separates the cerebrum into right and left cerebral

hemispheres.

The 2 hemisphere are interconnected by the corpus callosum (corpus=body,

callosum=hard) a broad band of white matter containing axons that extend between the

hemispheres.

The transverse fissure separates the cerebrum and the cerebellum.

Lobes of the cerebrum (5 lobes named for the skull bones overlying them)

Each cerebral hemisphere is subdivided into 5 functional areas called lobes, which is

named after the cranial bones that cover them.

o Frontal.

o Parietal.

o Temporal.

o Occipital.

o Insula: deep within the lateral sulcus.

The boundaries of the lobes are marked by deep sulci (fissures). These are the central, lateral

and parieto-occipital sulci.

The central sulcus bordered by 2 Gyri:

A. The precentralgyrus and

B. The postcentralgyrus separates the frontal lobe and parietal lobe.

o The precentralgyrus – anterior to central sulcus contains the motor areas

of cerebral cortex.

o The postcentralgyrus – posterior to central sulcus contains the primary

somatosensory areas of the cerebral cortex.

The lateral cerebral sulcus – separates frontal lobe and parietal lobe from the occipital

lobe.

The parieto-occipital sulcus – separates parietal lobe from the occipital lobe.

The 5th lobe known as insula lies within lateral cerebral sulcus, deep to the parietal,

frontal and temporal lobes.

Cerebral grey matter: The grey matter is formed by motor neuron, interneurons, cell bodies,

dendrites, axon terminals and unmyelinated axons. External sheets of grey matter is called

cortex covers the cerebrum and cerebellum and the innermost clusters of grey matter is called

cerebral nuclei/basal nuclei/basal ganglia. Folds in cortex – triples its size (surface area).

Cerebral white matter: Mainly consists of myelinated axons.

It lies deep into the grey matter of the cortex. Different areas of the cerebral cortex

communicate with each other and with the brainstem and spinal cord.

It consists of myelinated and unmyelinated axons in three types of tracts.


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1. Association tracts contain axons that conduct nerve impulses between gyri in the

same hemisphere.

2. Commissural tracts contain axons that conduct nerve impulses from gyri in one

cerebral hemisphere to corresponding gyri in the other cerebral hemisphere. The

three important commissural tracts are:

a. Corpus callosum (the largest commissure to cerebrum) connects the 2

hemispheres.

b. Anterior commissure and

c. Posterior commissure.

3. Projection tracts contain axons that conduct nerve impulses from the cerebrum to

lower parts of the CNS (thalamus, basal nuclei, brainstem and spinal cord) of from

lower parts of the CNS to cerebrum. E.g. internal capsule. It contains both ascending

and descending axons.

Basal nuclei/ basal ganglia (ganglion- a collection of neuronal cell bodies):

Mass of grey matter deep within the white matter of cerebral hemisphere with

connections to the cerebral cortex and thalamus.

The basal nuclei form part of the extrapyramidal tracts and are involved in initia t in g

muscle tone in slow and coordinated activities.

Basal ganglia lateral to thalamus is the lentiform nucleus (lens shaped) divided into 2

as:

a. Globus pallidus (globus=ball, pallidus=pale) closer to thalamus.

b. Putamen (putamen=shell) closer to cerebral cortex.

Caudate nucleus (caud=tail) ‘C’ shaped, large head, tail arches over the thalamus.

Caudate nucleus along with putamen and globus pallidus is known as corpus striatum.

Subthalamic nuclei interconnect the globus pallidus in each cerebral hemisphere.

Functions:

To help regulate initiation and termination of movements in coordination with cerebral

cortex.

Putamen precedes or anticipates body movement.

Caudate nucleus responsible for eye movement.

Globus pallidus helps to regulate the muscle tone required for specific body

movements.

It also controls subconscious contractions of skeletal muscles.

They initiate and coordinate cognitive processes such as attentions, memory, planning,

and may act with the limbic system to regulate emotional behaviours.

It estimates the passage of time.

It is associated with psychiatric disorders like:

o Obsessive compulsive disorder.

o Schizophrenia and


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o Chronic anxiety all due to the dysfunction in circuits between basal

ganglia and the limbic system.

Functional brain system:

Networks of neurons functioning together.

1. The limbic system – spreads widely in the forebrain.

2. The reticular formation – spans the brainstem.

The limbic system (the emotional brain) it is composed of:

1. Limbic lobe – rim of cerebral cortex on the medial surface of each hemisphere.

a. Allows us to shift between thoughts.

b. Interprets pain as unpleasant – cingulate gyrus (cingul=belt) lies above corpus

callosum. Parahippocampal gyrus lies in the temporal lobe below.

2. Dentate gyrus (dentate=tooth head) – lies between hippocampus (seahorse shape)

and parahippocampal gyrus.

3. Amygdala (amygda=almond shaped) stimulation of it produces fear and aggression.

It composed of groups of neurons located close to caudate nucleus.

4. Septal nuclei – lies in the septal area formed by regions under corpus callosum and

the paraterminal gyrus (cerebral gyrus).

5. Mammillary bodies of the hypothalamus – they are the 2 round masses near the

cerebral peduncles.

6. Anterior and medial nucleus – it participate in limbic circuits.

7. Olfactory bulbs – flattened bodies of olfactory pathway that rests on the cribrifo rm

plate.

8. Fornix, striaterminalis, striamedullaris, medial forebrain bundle and

mammillothalamic tract are interconnected by bundles of myelinated axons.

Functions:

It plays an important role in a range of emotions including pain, pleasure, docility,

affection, and anger.

It is also involved in olfactory (smelling) and memory.

It helps in experiencing intense pain or extreme pleasure.

Stimulation of limbic system area produces tameness and affection.

It encircles the upper part of the brainstem and corpus callosum.

It forms the inner borders of the cerebellum and the floor of diencephalon.

Hippocampus functions in memory.

Cerebral cortex areas and functions

Sensory, motor and integrative signals are processed in cerebral cortex by sensory areas, motor

areas and association areas.

Sensory areas/cortex: Cortical areas involved in conscious awareness of sensation.


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Located in parietal, temporal, and occipital lobes.

Posterior half of both cerebral hemispheres in regions behind central sulcus.

Primary sensory area connects with peripheral sensory receptors.

Secondary sensory areas and sensory association areas receive input from primary areas

and other brain regions. They integrate sensory experience to generate meaningful

patterns of recognition and awareness.

Important sensory areas of cerebral cortex:

a. Primary somatosensory area/cortex: Located along the postcentral gyrus. Involved with conscious awareness of general somatic senses. Spatial discrimination – precisely locates a stimulus. Receive sensory input from the opposite side of the body. It receives

impulses from touch, proprioception (joint and muscle position), pain, itching, tickle and thermal sensation.

b. Primary visual area: Primary visual cortex located deep within the calcarine sulcus on

the posterior and medial part of the occipital lobe. It receives impulses that convey

information for vision that originates on the retina.

c. Primary auditory area/cortex: Location – superior edge of the temporal lobe. It

interprets basic characteristics of sound such as pitch and rhythm.

d. Primary gustatory area: it receives impulses for taste.

e. Primary olfactory area: it receives impulses for smells.

Motor areas: anterior part of each hemisphere.

Important motor areas of cerebral cortex: Controls motor functions.

a. Primary motor area/cortex: Located in precentral gyrus – voluntary contraction of

specific muscles or group of muscles.


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b. Broca’s speech area: The planning and production of speech occurs in this area in the

frontal lobe. It controls the premotor regions that control the muscles of the larynx,

pharynx and mouth.

• Pyramidal cells – large neurons of primary motor cortex.

• Corticospinal tracts descend through brainstem and spinal cord. o Axons signal motor neurons to control skilled movements.

o Contralateral – pyramidal axons cross over to opposite side of the brain. • Specific pyramidal cells control specific areas of the body e.g. face and hand

muscles – controlled by many pyramidal cells.

Association areas: It consists of both motor and sensory areas on the lateral surfaces of the

occipital, parietal, temporal lobes and on frontal lobes anterior to the motor areas. Association

areas interconnected by association tracts.

Important association areas of cerebral cortex: Controls association functions.

a. Somatosensory association area: Lies posterior to the primary somatosensory cortex, it

integrates and interprets sensations, storage of memory of past sensory experiences

(touch, pressure etc.).

b. Visual association area: Located in the occipital lobe extends into: Temporal and

parietal lobes. It receives sensory impulses from primary visual areas and the thalamus.

It is essential for recognizing and evaluation of vision.

c. Facial recognition area:

d. Auditory association area: Lies posterior to the primary auditory cortex in the center of Wernicke’s area. Permits evaluation of different sounds. It differentiates between

speech, music or noise. Involved in recognizing and understanding speech. e. Orbitofrontal cortex:

f. Wernicke’s (posterior language) area: It interprets the meaning of speech by

recognizing spoken words.

g. Common integrative area: It integrates sensory interpretation from the association areas

and impulses from other areas, allowing formation of thoughts based on a variety of

sensory inputs.

h. Prefrontal cortex (frontal association area): Functional areas located on the medial side

of the frontal lobe.

• Regions anterior to the corpus callosum

o Involved in complex personal and social interactions. • Regions superior to the corpus callosum

o Involved in “mentalization. • General Interpretation Area located at the interface of the visual, auditory and

somatosensory association areas.

• Language Area – Surrounds the lateral sulcus in the left cerebral hemisphere. • Five parts have been identified.

o Broca’s area – speech production. o Wernicke’s area – speech comprehension.

o Lateral prefrontal cortex – conceptual analysis of spoken words. o Most of the lateral and inferior temporal lobe – Coordination of auditory

and visual aspects of language.


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o Parts of the insula – Initiation of word articulation. Recognition of rhymes and sound sequences.

i. Premotor area: It controls learned skilled movements and serves as a memory bank.

j. Frontal eye field area: It controls the voluntary scanning movements of the eyes.

k. Gustatory Cortex: Involved in the conscious awareness of taste stimuli.

Insula:

• Functions in language and the sense of balance. • Visceral function. Conscious perception of:

o Stomach upset. o Full bladder. o Some aspects of the sense of smell.

The Reticular formation

The reticular formation is a collection of neurones in the core of the brain stem (runs

through the central core of the medulla, pons and midbrain), surrounded by neurons which

conduct ascending and descending nerve impulses between the brain and the spinal cord. It has

a vast number of synaptic links with other parts of the brain, widespread connections ideal for

arousal of the brain as a whole.

Functions:

The reticular formation is involved in:

Coordination of skeletal muscle activity associated with voluntary motor movement

and the maintenance of balance.

Coordination of activity controlled by the autonomic nervous system, e.g.

cardiovascular, respiratory and gastrointestinal activity.

Selective awareness that functions through the reticular activating system (RAS) which

selectively blocks or passes sensory information to the cerebral cortex (Maintains

consciousness, alertness and also functions in sleep and arousal from sleep).


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The Diencephalon

• Forms the center core of the forebrain

• Surrounded by the cerebral hemispheres

• Composed of three paired structures:

o Thalamus, hypothalamus, and epithalamus.

• Border the third ventricle.

• Primarily composed of gray matter.

The Diencephalon – The Thalamus

• The thalamus consists of two masses of nerve cells and fibres situated within the

cerebral hemispheres just below the corpus callosum one on each side of the third

ventricle. It makes up 80% of the diencephalon.

• Send axons to regions of the cerebral cortex.

• Nuclei act as relay stations for incoming sensory messages. Sensory input from the skin,

viscera and special sense organs is transmitted to the thalamus before redistribution to

the cerebrum.

• Afferent impulses converge on the thalamus. Synapse in at least one of its nuclei. It is

the “gateway” to the cerebral cortex.

• Nuclei organize and amplify or tone down signals.

The Diencephalon – The Hypothalamus

It is composed of a number of groups of nerve cells. It is situated inferior and anterior

to the thalamus, immediately above the pituitary gland (pituitary gland projects inferiorly). The

hypothalamus is linked to the posterior lobe of the pituitary gland by nerve fibres and to the

anterior lobe by a complex system of blood vessels.


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Functions:

The hypothalamus is involved in:

• The hypothalamus controls the output of hormones from both lobes of the pituitary

gland.

• Control of the endocrine system.

• The control of the autonomic nervous system (main visceral control center of the body).

• The control of emotional responses (e.g. pleasure, fear, rage sexual behaviour includ ing

mating and child rearing).

• Regulation of body temperature.

• Regulation of hunger and thirst sensations.

• Control of behavior.

• Regulation of sleep-wake cycles: Biological clocks or circadian rhythms (e.g. sleeping

and waking cycles).

• Formation of memory.

The Diencephalon – The Epithalamus

• Forms part of the “roof” of the third ventricle

• Consists of a tiny group of nuclei

• Includes the pineal gland (pineal body)

o Secretes the hormone melatonin under influence of the hypothalamus.

Melatonin controls the circadian rhythm.

The Cerebellum

• no:2 in size ,1/2 neurons

• Butterfly shaped from posterior view.

• Located dorsal to the pons and medulla

o Smoothens and coordinates body movements

o Helps maintain equilibrium.

• Consists of two cerebellar hemispheres.

• Vermis – central constricted area.

• Hemispheres each subdivided into:

o Anterior lobe.

o Posterior lobe.


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• Cortex – gray matter Surface folded into ridges called folia slender parallel ridges of

grey matter.

o Separated by fissures.

o Internal white matter.

o Deep cerebellar nuclei – deeply situated in gray matter.

• Cerebellum must receive information.

o On equilibrium.

o On current movements of limbs, neck, and trunk from the cerebral cortex

• Arbour vitae tree of life – white matter tracts.

• Cerebellar Peduncles.

• Fibers to and from the cerebellum are ipsilateral.

o Run to and from the same side of the body.

• Thick tracts connecting the cerebellum to the brain stem- 3 pairs.

o Superior cerebellar peduncles from red nuclei and several nuclei of thalamus.

o Middle cerebellar peduncles-for voluntary movement.

o Inferior cerebellar peduncles sensory inner ear vestibular apparatus.


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THE BRAIN STEM – THE MIDBRAIN (MESENCEPHALON)

Midbrain

The midbrain is the area of the brain situated around the cerebral aqueduct between the

cerebrum above and the pons below. It consists of groups of cell bodies and nerve fibres (tracts)

which connect the cerebrum with lower parts of the brain and with the spinal cord. The cell

bodies act as relay stations for the ascending and descending nerve fibres.

Imbedded in the white matter of the midbrain. Two pigmented nuclei

Substantia nigra – neuronal cell bodies contain melanin.

Functionally linked to the basal nuclei.

o Red nucleus (right and left) – lies deep to the substantia nigra- rich blood supply,

iron pigmentation.

Largest nucleus of the reticular formation.


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• Includes the midbrain, pons, and medulla oblongata.

• Several general functions:

o Produces automatic behaviors necessary for survival.

o Passageway for all fiber tracts running between the cerebrum and spinal cord.

o Heavily involved with the innervation of the face and head.

10 of the 12 pairs of cranial nerves attach to it.

o cm, Lies between the diencephalon and the pons.

• Central cavity – the cerebral aqueduct.

• Cerebral peduncles located on the ventral surface of the brain.

o Contain pyramidal (corticospinal) tracts.

• Superior cerebellar peduncles.

o Connect midbrain to the cerebellum.

• Periaqueductal gray matter surrounds the cerebral aqueduct.

o Involved in two related functions.

Fright-and-flight reaction.

Mediates response to visceral pain.

• Posterior part – tectum with 4 rounded elevations.

• Corpora quadrigemina – the largest nuclei.

o Divided into the superior and inferior colliculi.

Superior colliculi – nuclei that act in visual reflexes.

Inferior colliculi – nuclei that act in reflexive response to sound.


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THE BRAIN STEM – THE PONS-BRIDGE

The pons is situated in front of the cerebellum 2.5 cm long, below the midbrain and above

the medulla oblongata (between). It consists mainly of nerve fibres which form a bridge

between the two hemispheres of the cerebellum and of fibres passing between the higher levels

of the brain and the spinal cord. There are groups of cells within the pons which act as relay

stations and some of these are associated with the cranial nerves. The anatomical structure of

the pons differs from that of the cerebrum in that the cell bodies (grey matter) lie deeply and

the nerve fibres are on the surface. It is anterior to cerebellum consists of 2 surfaces:

1. Anterior –convex in both directions.

2. Posterior –hidden by cerebellum.

It contains the nuclei of cranial nerves V (trigeminal), VI (abducens), VII (facial) and VIII

(Vestibulocochlear). Two general groups of cranial nerve nuclei present in pons are:

Motor nuclei: The pontine nuclei are the sites where the signals for voluntary movements

originating in the cerebral cortex relayed into the cerebellum.

Sensory nuclei: Pneumotaxic area and the apneustic area with the medullary rhythmic ity

areas help control breathing.


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THE BRAIN STEM – THE MEDULLA OBLONGATA

The medulla oblongata extends from the pons above till the foramen magnum and is

continuous with the spinal cord below. It is about 2.5 cm long and it lies just within the cranium

above the foramen magnum. Its anterior and posterior surfaces are marked by central fissures.

The outer aspect is composed of white matter (sensory/ascending and motor/descending) which

passes between the brain and the spinal cord. Some of the white matter forms bulges on the

anterior part of the medulla known as pyramids. The grey matter lies centrally. It acts as relay

stations for sensory nerves passing from the spinal cord to the cerebrum.

The medulla oblongata has several special features:

• Choroid plexus lies in the roof of the fourth ventricle.

• Decussation (crossing) of the pyramids: In the medulla motor nerves descending from

the motor area in the cerebrum to the spinal cord in the pyramidal (corticospinal) tracts

cross from one side to the other. The left hemisphere of the cerebrum controls the right

half of the body and vice versa. These tracts are the main pathway for impulses to

skeletal (voluntary) muscles.

• Sensory decussation: Some of the sensory nerves ascending to the cerebrum from the

spinal cord cross from one side to the other in the medulla. Others decussate at lower

levels, i.e. in the spinal cord itself.

• Cranial nerves VIII to XII attach to the medulla.


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• Olives – lateral to each pyramid, upper part has oval shaped swelling produced by

bundle of grey matter known as olive. Within the olive is the inferior olivary nucleus

(the neurons present here relay impulses from proprioceptors which monitors joint

and muscle position to the cerebellum).

• The right and left Gracile (slender) nucleus and Cuneate (wedge) nucleus are

associated with sensations of touch, conscious proprioception, pressure and

vibration

The core of the medulla contains the reticular formation and the nuclei that influence autonomic

functions.

The vital centres of medulla oblongata, consists of:

1. Cardiac centre

2. Respiratory centre

3. Vasomotor centre

4. Reflex centres of vomiting, coughing, sneezing and swallowing.

FUNCTIONS OF VITAL CENTERS OF MEDULLA OBLONGATA:

1. The cardiovascular centre controls the rate and force of cardiac contraction.

Sympathetic and parasympathetic nerve fibres originating in the medulla pass to the

heart. Sympathetic stimulation increases the rate and force of the heartbeat and

parasympathetic stimulation has the opposite effect.


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2. The respiratory centre controls the rate and depth of respiration. From this centre, nerve

impulses pass to the phrenic and intercostal nerves which stimulate contraction of the

diaphragm and intercostal muscles, thus initiating inspiration. The respiratory centre is

stimulated by excess carbon dioxide and, to a lesser extent, by deficiency of oxygen in

its blood supply and by nerve impulses from the chemoreceptors in the carotid bodies.

3. The vasomotor centre controls the diameter of the blood vessels, especially the small

arteries and arterioles. Stimulation may cause either constriction or dilatation of blood

vessels depending on the site. The sources of stimulation of the vasomotor centre are

the arterial baroreceptors, body temperature and emotions such as sexual excitement

and anger. Pain usually causes vasoconstriction although severe pain may cause

vasodilatation, a fall in blood pressure and fainting.

4. Reflex centres: When irritating substances are present in the stomach or respiratory

tract, stimulates the reflex centres which initiate the reflex actions of vomiting,

coughing and sneezing to expel the irritant.


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SPINAL CORD

Spinal cord forms the link between the brain and the body. It extends downward through the

vertebral canal protected by the spinal meninges. Begins at the foramen magnum and

terminates at the first and second lumbar vertebrae (L1/L2) interspace. It shows some

functional independence from the brain.

The spinal cord and spinal nerves serve two functions:

Pathway for sensory and motor impulses.

Responsible for reflexes (functional independence).

Structure of the Spinal Cord:

External anatomy:

Spinal cord is situated inside the vertebral canal protected by the spinal meninges. Typical adult

spinal cord length - ranges between 42 and 45 cm (16 to 18 inches) and diameter- 2cm.

Spinal cord starts from medulla oblongata and runs to the lower of 1st lumbar or superior of the

2nd lumbar vertebra (in New born till 3-4 lumbar, grows 4-5 years stops but vertebral canal

grows till adulthood). The spinal cord is shorter than the vertebral canal that houses it.


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Two enlargements in the spinal cord at:

• Cervical C4 to T1- nerves to upper limbs,

• Lumbar T9-T12 – lower limbs

Conus medullaris : It is the tapered inferior end (conical) of the spinal cord below lumbar

enlargement between the 1st and 2nd lumbar vertebrae, which marks the official “end” of the

spinal cord.

Cauda equina (horse tail): They are inferior to conus medularis, which forms collection of

nerve roots (lumbar, sacral, coccyx-groups of axons) that project and leave inferiorly from the

spinal cord.

Filum terminale: They are structures within the cauda equine consists of thin strands of pia

mater, which helps to anchor the conus medullaris to the coccyx.

The spinal cord is associated with 31 pairs of spinal nerves, which connect the CNS to:

• Receptors.

• Effectors (muscle and glands).

Regions of the Spinal Cord:

1. The cervical region is continuous with the medulla oblongata and contains neurons

whose axons form the cervical spinal nerves (8).

2. The thoracic region is attached to this region forms the thoracic spinal nerves (12).

3. The lumbar region contains the neurons for the lumbar spinal nerves (5).

4. The sacral region contains the neurons for the sacral spinal nerves (5).

5. The coccygeal region contains the one pair of coccygeal spinal nerves.

Spinal cord segment vary in size and shape

1. Cervical - large size, large amount of white matter.

2. Thoracic - small size, small amount of grey matter.

3. Lumbar - circular, large grey matter, less white.

4. Sacral - large grey matter.

5. Coccyx – small.

Tracts:

1. Corticospinal tract - motor descending.

2. Spinothalamic tract - sensory ascending.

Each side of the spinal cord contains:

• 8 cervical nerves (called C1–C8)

• 12 thoracic nerves (T1–T12)

• 5 lumbar nerves (L1–L5)

• 5 sacral nerves (S1–S5)


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• 1 coccygeal nerve (Co1).

Internal anatomy:

In cross section:

– Roughly cylindrical elongated lower part of CNS.

– Slightly flattened both posteriorly and anteriorly.

– External surface has two longitudinal depressions:

• The posterior (dorsal) median sulcus.

• The anterior (ventral) median fissure.

1. Grooves

• From white matter divide into right and left.

• Anterior median fissure – deep wide groove.

• Posterior median sulcus – shallow narrow.

2. Roots

• Bundle of axon connect nerve to spinal nuclei.

• Posterior root – sensory.

• Anterior root – motor.

• Posterior root ganglion-swelling at the posterior nerve.

o Root lets - connecting to spinal cord.

Location and Distribution of Gray Matter:

In the spinal cord, it is centrally located

• Its shape resembles a letter H or a butterfly.

• Surrounded by white matter, dendrites, cell bodies, unmyelinated axons and neuroglia.

• Central canal filled with CSF.


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• The gray matter may be subdivided into the following components:

o Anterior horns.

o Lateral horns.

o Posterior horns.

o The gray commissure - cross bar which connects 2 halves horns.

o Nuclei - clusters of cell bodies form functional groups.

Sensory and motor nuclei.

Lateral only in thoracic area autonomic motor.

Location and Distribution of White Matter:

• The white matter of the spinal cord is external to the gray matter.

• Three regions. Composed of tracts – (bundles of axon).

Ascending sensory tract.

Descending motor tract.

o Posterior (white column) funiculus:

Lies between the posterior gray horns and the posterior median sulcus.

o Lateral (white column) funiculus.

o Anterior (white column) funiculus.

Between the anterior gray horns and the anterior median fissure.

• The anterior funiculi are interconnected by the white commissure.

Tracts of the Spinal Cord:

1. Ascending tracts conduct sensory impulses to the brain.

2. Descending tracts conduct motor impulses from the brain to motor neurons reaching

muscles and glands.

Major ascending (sensory) spinal cord tracts:

Fasciculus gracilis and fasciculus cuneatus.

Spinothalamic tracts.

o Lateral and anterior.

Spinocerebellar tracts.

o Posterior and anterior.

Major descending (motor) spinal cord tracts:

Corticospinal tracts.

o Lateral and anterior.

Reticulospinal tracts.

o Lateral, anterior and medial.

Rubrospinal tract.


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REFLUX ARCS:

Reflexes are automatic, subconscious responses to stimuli within or outside the body. The

nervous pathway through, which the impulses travel to make a reflex action forms the reflex

arc. The 2 types are:

1. Simple reflex arc (sensory – motor).

2. Most common reflex arc (sensory – association – motor).

The reflex arc consists of receptors, sensory neurons, interneurons, motor neurons and effector

muscle or glands.


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Reflex Behavior:

Example is the knee-jerk reflex.

Simple monosynaptic reflex.

Helps maintain an upright posture.

Example is a withdrawal reflex (flexor reflex).

Prevents or limits tissue damage.


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Example crossed extensor reflex.

Crossing of sensory impulses within the reflex centre to produce an opposite effect.

Important questions of CNS.

1. Write a short note on meninges (5 Marks).

2. Write about the formation, composition and functions of CSF (10 Marks).

3. Compare and contrast graded potential and action potential in nervous tissue (5 Marks).

4. Structural and functional difference between autonomic and somatic nervous system (5

marks).

5. Write the anatomy of spinal cord (5 marks).

6. Cranial nerves and their functions (5 marks).

7. Describe briefly on secretion, circulation and functions of CSF.

8. Parasympathetic innervations and its functions.

9. Name the parts of CNS.

10. Reflex arch.

11. BBB.

12. Neurohumoral transmission in ANS (5 marks).

13. CSF.

14. With the help of a neat diagram explain the histology of nervous tissue.

15. Summarize the functions of medulla oblongata.

16. CNS disorders.

17. Which divisions of the autonomic nervous system cause miosis and mydriasis and

explain how.

18. Write the composition and functions of cerebrospinal fluid.

19. Describe the outflow and functions of ANS with reference to its sympathetic division.

20. Enumerate the difference between sympathetic and parasympathetic nervous system.


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21. Define nerve plexus. Name the principle plexuses.

22. What is reflex action? Explain the structure involved in its production.

23. Explain the neuronal transmission of impulse (10 Marks).

24. Explain the neuromuscular transmission (10 Marks).

Education

2 the central nervous system