Nervous System by Chaman Lal Karotia (CK)

CNS NOTES BY: Chaman Lal, CNPS, AKUH, Karachi.

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NERVOUS SYSTEM: CENTRAL NERVOUS SYSTEM

When you have completed this chapter you should be able to:

Describe the difference between the CNS and PNS.

Locate and identify the forebrain, midbrain and hindbrain.

Identify the major gyri and sulci of the brain.

Describe the main parts of the forebrain and describe its function.

Describe the main parts of the midbrain and describe its function.

Describe the main parts of the hindbrain and describe its function.

Describe the main parts of the spinal cord.

Mind stimulating Questions to read and learn more.

The central nervous system (CNS) consists of the brain and spinal cord and the peripheral nervous system (PNS) consists of the spinal nerves and ganglia.

Brain:- The brain occupies the cranial cavity (1400gms ≈ 3 lbs) and can be divided into three main parts. The forebrain, midbrain and hindbrain. The midbrain and hindbrain are collectively known as the brain stem and contain the nuclei from which the cranial nerves originate.

Name Description Important named parts

Forebrain Prosencephalon

Largest part of the brain.

Telencephalon (Cerebral Cortex & cerebrum). Basal Ganglia Diencephalon (thalamus, hypothalamus, pineal body)

Midbrain Mesencephalon

(brain stem) 1.5 cm in length.

Quadrigeminal bodies - cerebral peduncles.

Hindbrain Rhombencephalon

(brain stem)

Pons. Medulla oblongata. Cerebellum.


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Forebrain:

Telencephalon (Cerebrum):

The cerebrum is the largest part of the brain and is divided into left and right hemispheres by a

longitudinal fissure that runs along the median sagittal plane. Inferiorly the hemispheres are connected

together by a band of white matter called the corpus collosum. The outer layer of the cerebrum is

composed of grey matter and called the cerebral cortex. It is responsible for the analysis of sensory input,

memory, learning and cognitive thought. Each hemisphere is greatly folded forming gyri (folds) and sulci

(grooves) which increases the surface area of the cerebral cortex. Although the exact location of the sulci

and gyri varies between different individuals, there are a number of large gyri and deep sulci which can be

identified as constant landmarks. The main ones have been listed below;

Name Description

Longitudinal fissure

A large fissure running from back to front along the median sagittal plane; it divides the cerebrum into left and right cerebral hemispheres.

Central sulcus Descending downwards and forwards from the top of the hemisphere. It divides the frontal and parietal lobes.

Parietal-occipital sulcus

Descending downwards and forwards mainly inside the longitudinal fissure, it divides the parietal and occipital lobes.

Precentral gyrus This is found at the posterior border of the frontal lobe, in front of the central sulcus. It descends downwards and forwards from the top of the hemisphere. Forms the primary motor area (cortex).

Postcentral gyrus

This is found at the anterior border of the parietal lobe, behind the central sulcus. It descends downwards and forwards from the top of the hemisphere. Forms the primary sensory area (cortex).

Lateral sulcus Found on the lateral side of the brain it ascends almost horizontally from the front of the brain to the angular gyrus and separates the temporal lobe from the frontal lobe above.

Each hemisphere can be further divided into lobes, their names of which correlate with the surrounding

bones that protect them.


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DIFFERENT LOBES AND THEIR FUNCTIONS

Lobe of the Cerebrum

Description Function

Frontal lobe

The largest lobe found at the front of the brain undercover of the frontal bone. It contains the precentral gyrus posteriorly. It is separated from the parietal lobe posteriorly by the central sulcus and from the temporal lobe inferiorly by the lateral sulcus.

The primary motor area (cortex).

Motor association area (motor control).

Broca's area - motor speech (production)

Cognitive thought and memory. Personality Primary olfactory cortex.

Temporal lobe

Found at the side of the brain undercover of the temporal bone. It is separated above from the frontal lobe by the lateral sulcus.

Primary auditory area (hearing). Auditory association area

(hearing). Wernicke area (speech

comprehension). Special senses (hearing,

smelling). Learning and memory (retrieval). Emotions

Parietal lobe

Found at the top of the brain undercover of the parietal bone. Anteriorly it contains the postcentral gyrus and is separated from the frontal lobe by the central sulcus. Posteriorly it is separated from the occipital lobe by the parietal-occipital sulcus.

Primary sensory area (cortex). Sensory association area

(general senses). Body orientation. Primary gustatory cortex (taste).

Occipital lobe

Found at the back of the brain undercover of the occipital bone.

Primary visual area (cortex). Visual association area (vision) -

visual interpretation.

Insula The smallest lobe of the brain found deep in the cerebrum between the lips of the lateral sulcus.

Special senses (taste, hearing). Visceral sensation.


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Diencephalon:-

The diencephalon consists of two thalami, two hypothalami and a single pineal body.

Thalamus:-

The thalami are the largest parts of the

diencephalon and are located in the centre of the

brain in the outer walls of the third ventricle. They

are often connected to each other across the third

ventricle by a small interthalamic adhesion. The

thalamus receives sensory and motor input as well

as influences mood. It receives mostly sensory input including auditory and visual input and relays the

signals to the cerebral cortex.

Sensory Nuclei of the Thalamus Sensory input from

Medial geniculate nucleus Auditory

Lateral geniculate nucleus Visual

Ventral posterior nucleus Other sensory input

Pineal body:-

This is a small pine-cone shaped gland projecting from the posterior of the third ventricle by a mid

line stalk. Its role is not fully understood but it is thought to be involved in the sleep-wake cycle and the

onset of puberty.

Hypothalamus:-

The hypothalamus is located at the very bottom of the diencephalon below the thalamus and

behind the optic chiasma. It is very important and is often referred to as the 'master gland' as it controls a

large number of bodily functions, one of the most important being that of homeostasis. Homeostasis is

the maintenance of the bodies’ physiology, i.e. the maintenance of blood pressure, body temperature,

weight and the chemical composition of the body's fluids. Other regulatory roles of the hypothalamus are


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control of our mood and emotions, autonomic functions, food and water intake, sleep wake cycle and

endocrine function.

Name Description Function

Mammillary bodies

A pair of small white bodies protruding from the front of the hypothalamus.

Emotional responses to smells.

Infundibulum

A stalk which connects the hypothalamus with the pituitary gland (hypophysis).

Through its connection the hypothalamus regulates the function of the pituitary gland.

Mesencephalon (midbrain):-

The smallest part of the brainstem measuring 1.5 cms it consists of the tectum, tegmentum,

cerebral peduncles and the substantia nigra. It is responsible for the visual and gustatory response as well

as the coordination of movement.

Name Description

Tectum

Roof of the midbrain, consisting of four nuclei which form 4 mounds, collectively know as quadrigeminal bodies, on the dorsal surface of the brain stem.

The 2 superior nuclei are called the superior colliculi the 2 inferior nuclei are called the inferior colliculi. The superior colliculi control the visual response and the inferior colliculi

control the auditory response.

Tegmentum Is the floor of the midbrain and consists of ascending tracts from the spinal cord to the

brain. It controls motor functions.

Substantia nigra

A pigmented lamina located between the tegmentum and cerebral peduncles which helps to coordinate movement.

Cerebral peduncles

Located inferior to the tegmentum and consist of descending (motor) tracts from the cerebrum to the spinal cord and cerebellum.

Cranial nerve nuclei

The nuclei of the trochlear and oculomotor cranial nerves are located in the midbrain.

Brainstem:-

The midbrain and hindbrain are collectively known as the brain stem. It is the lowest part of the

brain and is continuous inferiorly with the cervical spinal cord at the foramen magnum. Its fibres connect

the peripheral nervous system (spinal nerves and cranial nerves) to the central nervous system (brain and

spinal cord). The brain stem is extremely important because it contains the nuclei from which most


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cranial nerves originate as well as the vital centers necessary for survival; breathing, digestion, heart rate,

blood pressure and for consciousness (being awake and alert).

Reticular Formation:-

The reticular formation is a series of important nuclei that span the brainstem and receive the

majority of the sensory information from the body and the motor signals from the cerebrum. The nuclei

also play an integral role in the maintenance of the conscious state.

Rhombencephalon (hindbrain):-

The hindbrain consists of the pons superiorly, the cerebellum posteriorly and the medulla oblongata inferiorly. The medulla oblongata is continuous inferiorly with the spinal cord.

Pons:-

Located in front of the cerebellum, the pons is only 2.5 cm in length and bulges anteriorly. It consists of descending fibres travelling to the spinal cord and ascending fibres to the cerebellum. It also contains the nuclei of four of the cranial nerves and the respiratory centre which controls expiration.


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Name Description

Pontine nuclei Located anteriorly in the pons they connect the cerebrum to the cerebellum and

coordinate voluntary movement.

Respiratory centre

Controls respiratory (expiration) movements.


The nuclei of the following cranial nerves are located in the posterior part of the pons;

Trigeminal (V) Abducens (VI) Facial (VII) Vestibular cochlear (VIII)

Medulla Oblongata:-

The medulla oblongata is only 3 cm in length and is the most inferior portion of the brainstem being continuous with the spinal cord inferiorly. It consists of the pyramids and olives and contains ascending and descending nerve tracts, several nuclei and importantly the 'vital centres', which regulate heart rate, respiration and blood vessel diameter. It also contains some non-vital centres involved in swallowing, vomiting, sneezing and coughing.

Name Description Function

Pyramids

Two enlargements on the anterior surface of the length of the medulla; they taper towards the spinal cord. Here the descending nerve tract fibres (corticospinal fibres) cross over to the other side to form the 'pyramidal decussation'.

Conscious voluntary movements.

Olive

Two protrusions found on the anterolateral side of the medulla just lateral to the pyramids. It consists of an olivary complex of nuclei.

Balance Coordination of sound from the ear.

Vital Centres

The medulla is the centre for several important regulatory reflexes;

Cardiac centres Respiratory centres Vasomotor centres

Heart rate. Respiratory (inspiration). Blood vessel diameter.


The nuclei of the following cranial nerves are located in the medulla oblongata;

Glossopharyngeal (IX) Vagus (X) Accessory (XI) Hypoglossal (XII)


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Cerebellum:-

The cerebellum is the lobe of the brain situated in the posterior cranial fossa. Its surface is folded

into folia and consists of two hemispheres connected in the mid line by the vermis. It is separated from

the pons and medulla oblongata anteriorly by the fourth ventricle. The cerebellum is responsible for

coordination of movement and sends information to the thalamus and cortex.

Grey and white matter: The brain and spinal cord contain both grey and white matter.

In the Brain:-

The grey matter can be found in the cerebral cortex, the basal ganglia and the limbic system. It is

made up of the cell bodies, dendrites and synapses of the neurons and is grouped into functionally

important nuclei.

The white matter is made up of the myelinated fibres (axons) which connect the different parts of

the brain to each other as well as to the spinal cord.

In the Spinal Cord:-

The spinal cord is oval in cross section and consists of white and grey matter. The grey matter lies

centrally and is arranged into ventral, dorsal and lateral grey horns (anterior and posterior horns). It

consists of neurons and neurites, neuroglia and blood vessels. It appears grey because of the abundance

of neuronal cell bodies.

The white matter surrounds the grey mater and is white in colour due to the presence of myelin,

which insulates the nerve fibres.

Ventricles:-

Inside the brain are four interconnected cavities filled with cerebral spinal fluid; two lateral

ventricles, a single third ventricle and a single fourth ventricle. The two lateral ventricles are the largest

ventricles and lie one in each cerebral hemisphere. They are approximately C-shaped (wish bone), each

communicating with the thin mid line third ventricle via an intraventricular foramen. The third ventricle

communicates inferiorly with the fourth ventricle via the cerebral aqueduct and descends in the mid


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plane through the midbrain. The fourth ventricle is a small, triangular chamber found between the pons

in front and the cerebellum behind. Inferiorly it narrows to form the central canal which descends though

the medulla oblongata and spinal cord.

Each ventricle contains a choroid plexus which secretes cerebral spinal fluid (CSF) into the

ventricles. The third ventricle contains the thalamus and hypothalamus in its lateral walls and the

infundibulum, tuber cinereum and the mammillary bodies in its floor. The corpus collosum forms the roof

of the lateral and third ventricles.

Cerebral Spinal Fluid (CSF):-

The CSF is a clear fluid produced by the choroid plexuses of the ventricles. It circulates within the ventricles as well as in the subarachnoid space between the pia mater and arachnoid mater surrounding the brain and spinal cord. CSF baths the brain and spinal cord in a chemically stable environment and provides it with nutrients. It also allows the brain to be buoyant and protects the brain from jolting into the cranium.

Meninges: These are membranous layers surrounding the brain and spinal cord.

In the Brain: Around the brain these three layers are collectively known as the meningies;

Outermost layer:-

The outermost layer, the dura mater is dense and consists of two layers, a periosteal layer and a meningeal layer. The periosteal layer adheres to the internal surface of the cranium and for the majority of its course lies directly touching the meningeal layer. At certain locations the periosteal and meningeal layers are pulled away from each other to create a space, a dural sinus. The sinuses are filled with venous blood from the brain via the cerebral veins. They drain blood into the internal jugular vein.

Dural Sinuses

Superior sagittal sinus

Cavernous sinus

Inferior petrosal sinus

Superior petrosal sinus

Transverse sinus

Sigmoid sinus

Occipital sinus


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The meningeal layer folds inwards to form two double thickness sheets which help to hold the brain in place. The falx cerebri is the fold of dura which projects vertically into the longitudinal fissure between the cerebral hemispheres. The tentorum cerebelli is the fold of dura which projects horizontally between the cerebellum below and the cerebrum above.

Middle layer:-

The middle layer, the arachnoid mater, is thin and transparent and lines the inner surface of the dura mater. It possesses arachnoid trabeculae (granulations) which project into the pia mater and villi which project into the dura mater

Innermost layer:-

The innermost layer, the pia mater lies directly on the surface of the brain. This layer is very thin and transparent and closely follows all of the gyri and sulci. Between the pia mater and the arachnoid mater is the subarachnoid space in which the CSF is circulated. CSF is returned to the blood via the arachnoid trabeculae (granulations).


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In summary:

Name Location Description

Pia Mater Innermost

layer.

Thin and transparent. Invests the surface of the brain and spinal cord.

Arachnoid Mater Middle layer.

Thin and transparent. Project villi through the dura into the venous sinuses to

absorb CSF.

Dura Mater Outermost

layer.

Thick and fibrous. Made up of two layers; periosteal and meningeal. Contains the venous sinuses. Folds inwards to form the falx cerebri and tentorum

cerebelli.

In the Spine:-

The spinal cord meninges are a continuation of the cranial meninges and are connected to

the foramen magnum. Like the cranial meninges the fibrous dura mater is the thick outermost layer and is

connected posteriorly to the posterior longitudinal ligament. The arachnoid mater lines the inner surface

of the dura mater and the pia mater lies directly on the spinal cord itself. The pia mater attaches to the

dura mater via the denticulate ligament.

DENTICULATE LIGAMENT:-

The spinal cord terminates at the level of L2, but the dura continues to the level of S2 creating a cistern into which the lower spinal roots hang.


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SELF WRITTEN TEST QUESTIONS

Complete the following questions before you go onto the next section:

In how many parts chronologically brain is divided?

Cranium or Vault (Skull) is made up of how many bones, Names?

What are the sutures? What is their role at birth and later on? Name any two?

Describe the important structures of the midbrain?

What are choroid plexuses? And briefly describe their functions?

Name three structures related to the third ventricle?

Describe the functions of Cerebral Aqueduct?

What are the sinuses, what are their functions, names any two sinuses?

What are the supporting cells of the CNS?

What are cranial nerves?

Spinal cord:-

The spinal cord is continuous with the medulla oblongata at the foramen magnum, and

descends in the vertebral canal. It consists of 31 segments corresponding to the 31 spinal nerves;

8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

At the level of the second lumbar vertebrae the spinal cord terminates by tapering to a

conus medullaris (it’s the terminal portion of the spinal cord). From the conus medullaris there

extended a long thin filament of nervous tissue called the filum terminale. The vertebral canal

below the second lumbar vertebra is filled with the nerve roots from the lumbosacral spine; this

bunch of nerve roots resembles a horse’s tail and so is known as the cauda equina (literally

means horse tail). The spinal cord ends up at this 2nd lumber vertebrae level. Hence lumber

puncture is done between the 3rd and 4th lumber vertebrae, to draw the cerebrospinal fluid for

laboratory check up. At this level only meninges with CSF is present and there is no danger of

spinal cord puncture.


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Along its course the spinal cord has two enlargements, the cervical enlargement and the

lumbosacral enlargement, in the cervical and lumbar regions respectively. These swellings are

due to the large spinal nerves which emerge from these parts of the cord to supply the upper and

lower limbs.

The spinal cord is made up of a column of grey matter (contains cell bodies) surrounded by

a cylinder of white matter (myelinated neurons). The neurons of the grey matter are arranged

into ventral, dorsal and lateral horns. The fibres of the white matter travel longitudinally along

the spinal cord in designated columns.


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Self Assessment Questions:

Q.1 The ___________ is the outermost meninx, and forms supportive and protective partitions

between some portions of the brain.

Q.2 The embryonic hindbrain gives rise to what structures in the brain?

Q.3 The portion of the brain involved in understanding speech and using words is located in the

_____________________________ of the cerebrum.

Q.4 When a person thinks and solves problems, which area of the cerebrum is involved?

Q.5 The _________________ is a portion of the brain that maintains homeostasis by linking activities

of the endocrine and nervous systems together.

Q.6 The connection between the third and fourth ventricle is called the _____________________.

Q.7 The vital centers are located within the ____________________________ area of the brain.

Q.8 Why Central Nervous System is called “Central Nervous System”?

_____________________________________________________________________________

Q.9 Write down the names of three layers of brain in order (from in to out)?

i) _________________ ii) __________________ iii) ___________________

Q.10 Write down anatomical location & main function of the followings;

I) Foramen Monro

II) Broca’s Area

III) Wernicke’s Area

Iv) Cerebrospinal Fluid

V) Occipital Lobe

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Nervous System by Chaman Lal Karotia (CK)

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