Central nervous system (Brain)

February 22, 2019

A)Brain

It is highly specialized delicate organ of the human body.
It weighs about 1.3-1.5kg depending on the gender (in case of human).
It is protected by a bony brain case or cranium.
It is composed of soft nervous tissues covered by three membranes together known as the meninges.

i)Outer membrane duramater

ii)Inner membrane piamater

iii)Middle membrane arachnoid

The space between these layers is filled with cerebrospinal fluid. It performs various functions as follows:

This fluid acts as cushion and shock absorber between brain and cranial nerves.
It provides a medium for exchange of food materials, wastes, respiratory gases and other materials.
It keeps the brain and spinal cord moist maintaining uniform pressures around these delicate structures.

Structure of human Brain

a)Fore-brain (Prosencephalon)

This part is greatly developed and distinguished into two parts. They are: Cerebrum and Thalamus.

1.Cerebrum

It is the largest part of the brain.
It is divided into right and left cerebral hemispheres.
Both hemispheres are joined together by a broad, curved, thick band of nerve fibres called corpus callosum.
Outer layer of cerebrum is known as cerebral cortex which forms the grey matter of brain.
Cortex has numerous, folds like convolutions that greatly increase surface area.
Ridges of these convolutions are called gyri and depressions between them are called sulci.
The cavity of cerebrum is called lateral ventricle.
Each hemisphere is divided by sulci into four lobes– frontal, parietal, temporal and occipital lobes.
Cerebral cortex has different areas for various functions.
They are motor area (movement), sensory area (heat, cold, pain, tough, light, and pressure), auditory area (hearing), visual area (seeing), olfactory area (taste and smell) and speech area.

Functions of Cerebrum

It is the dominant coordinating centre of the brain and is separate areas control different motor and sensory activities.
It is the main centre that governs all mental activities (intelligence, memory, reason, will, feelings and emotions).
It is the site of consciousness, the interpreter of sensations and the originator of voluntary acts.
It also acts as a control on many reflex actions that originate involuntarily (weeping, laughing, etc.)

2.Thalamus

It lies between cerebrum and midbrain.
Its cavity is called third ventricle or diocoel.
It consists of two rounded masses of grey mater into ventricle.
The pituitary gland is directly attached to the hypothalamus by a stalk, the infundibulum.
Thalamus serves as a relay centre for sensory and motor impulses from spinal cord and brain stem to various parts of the cerebrum.
It regulates emotions, perceptions of heat, cold and pain.

b) Midbrain (Mesencephalon)

It is the middle small constricted area of the brain between the cerebrum above and pons variolii below.
It basically connects the fore brain with hind brain.
It consists of crura cerebri and corpora quadrigemina.
Crura cerebri are two thick cylindrical band of nerve fibres (white matter).
These connect the cerebrum with the hind brain and spinal cord and transmit motor impulses to limb muscles from cerebrum.
Mid brain also possesses two pairs of round elevations called the corpora quadrigemina.
It consists of group of nerve cells- grey matter scattered in the white matter.
It cannot be easily distinguished as it is covered by cerebrum.
The ventricle of mid brain is called iter.
It acts as centre for auditory and visual reflexes.

c) Hind brain (Rhombencephalon)

i) Cerebellum

It is the second largest part of brain.
It lies at posterior region of the brain.
It has two cerebellar hemispheres similar to cerebrum.
The outer cerebellar cortex is made up of grey matter, which is not convoluted but is transversed by numerous furrows (sulci).
Central part is made up of white matter.
It is divided partially into three lobes central part called vermis and two lateral lobes called cerebellar hemispheres.
The function of cerebellum is to coordinate muscular movement, equilibrium and control the posture.
It also controls reflex actions of skeletal muscle activities.

ii) Pons varolii

It is situated in-front of cerebellum and above the medulla oblongata.
Its fibres connect the two halves of the cerebellum and join the medulla oblongata with the mid brain.
It coordinates muscles movements on two sides of the body.

iii) Medulla oblongata

It lies between the pons varolii and the spinal cord.
Its cavity is called fourth ventricle or metacoel.
It consists of white matter which is continuous with the spinal cord.
It is the centre for all the vital activities such as cardiac, respiratory and vasomotor.
It controls complex activities such as heart action, respiration, sneezing, coughing, etc.
Mid brain, pons and medulla oblongata together form the brain stem.

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I am Shailesh Koirala