There are various organ systems functioning in the system is the body. These functions are regulated, controlled and co-ordinated for proper functioning by a system called nervous system. Functional unit of nervous system is the neuron. Neurons have the characteristics of irritability and conductivity.
· Irritability means the ability to initiate nerve impulses in response to any stimuli
· Conductivity means the ability to transmit nerve impulses to any stimuli from target organs to the brain and vice- versa.
Functions of nervous system
The functions of the nervous system are as follows-
(1) The impulses or stimuli are carried from the tissues and organs to the nerve centres. After being interpreted and analyzed, messages are carried from the nerve centres to the desired tissues and organs.
(2) It links various organs and system and co-ordinates all their activities.
(3) It stimulates and inhibits the activities of various muscles, glands and visceral organs according to the kind of information received.
(4) It helps to maintain steady state of the body (homeostatic condition) by coordinating various activities.
Parts of nervous system
The nervous system is composed of three sub- divisions-
(a) Central nervous system (CNS)- it includes brain and spinal cord.
(b) Peripheral nervous system (PNS) – it includes the nerves arising from brain and spinal cord such as cranial nerves and spinal nerves.
(c) Automatic nervous system (ANS) - it consists sympathetic and parasympathetic nervous system. It includes nerves and ganglia. It controls and co-ordinates such organs which are under the involuntary control.
Main parts of human brain (lateral view)
fig : Mid- sagital section of human brain showing corpus callosum and other parts
Central Nervous System (CNS)
The central nervous system consists of two parts:
i. Brain
ii. Spinal cord
A) Brain: Brain is highly specialized delicate organ of the human body weighting about 1.2 kg. it ia protected by a bony brain case or Cranium. It is composed of soft nervous tissues covered bt three membranes together as the meninges-
i. Outer membrane duramater
ii. Inner membrane piamater
iii. Middle membrane arachnoid
The space between these layers is filled with cerebro-spinal fluid. It perform following functions-
· This fluid acts as cushion and shock absorber between brain and cranial bones.
· 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 os human brain- human brain is divided into three parts-
1. Fore-brain (Prosencephalon)
2. Mid-brain (Mesocephalon)
3. Hind-brain (Rhombencephalon)
1. Fore-brain
This part is greatly developed and distinguished into two parts –
(a) Cerebrum (b) Thalamus
a. Cerebrum _ it is the largest part of the brain. It divides 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. It forms gray matter of brain. Cortex has numerous, fold 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 moral area (movement), sensory area (heat, cold, pain, touch, light and pressure), auditory area (hearing), visual area (seeing), olfactory area (taste and smell) and speech area.
Function of cerebrum – cerebrum functions as follows –
i. It is the main centre that governs all mental activities (intelligence, memory, reason, will, feelings and emotions).
ii. It is the site of consciousness, the interpreter of sensations and the originator of voluntary acts.
iii. It is also acts as a control on many reflex actions that originates involuntarily (weeping, laughing, etc.)
b) Thalamus – it lies in between cerebrum and mid brain. Its cavity is called third ventricle or dicoel. Thalamus consists of two rounded masses of grey matter into ventricle. The pituitary gland is directly attached to the hypothalamus by a stalk, the infundibulum.
Functions – 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 perceptions of heat, cold and pain.
2) Mid-brain-
Mid-brain basically connects the fore-brain with hind-brain. It posses two parts of round elevations called the corpora quadrigemina. Mid-brain 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.
Function- mid-brain acts as centre for auditory and visual reflexes.
3) Hind-brains
It consists of cerebellum, pons varoli and medulla oblongata
a) Cerebellum – it is the second largest part of brain. I lies at posterior region of the brain. It has two cerebellar hemispheres similar to cerebrum. The outer cerebellar but is transversed by numerous furrows (sulci). Central part is made up of whit matter. Cerebellum is divided partially into three lobes-
i) central part called vermis.
ii) two lateral lobes called cerebellar hemispheres.
Functions
· It co-ordinates muscular body movement, equilibrium and controls the posture.
· It controls reflex actions of skeletal muscle activities.
(b) 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 co-ordinates muscles movements on two sides of the body.
c) Medulla oblongata- it lies between the pons varolii and the spinal cord. Its cavity is called 4th ventricle or metacoel. It consists of white matter. It is continuous with the spinal cord.
Functions – medulla oblongata is the centre for the activities such as cardiac, respiratory and vasomotor. It controls complex activities such as heart actions, respiration, sneezing, coughing, etc.
Mid-brain. Pons and medulla oblongata together form the brain stem.
Ventricles of human brain
Ventricles are the cavity within the brain. These consists cerebrospinal fluid. Ventricles are 4 in numbers
i. Right and left lateral ventricles
ii. Third ventricles
iii. Fourth ventricles
1. Right and left lateral ventricles – these cavities lie within the cerebral hemispheres, one on each side of the median planes just below the corpus. They communicate with the third ventricle by foramen of Monro.
2. Third ventricle – it is a cavity situated below the lateral ventricle between the two parts of the thalamus. It communicates with the fourth ventricle by a canal, cerebral adeqeduct or iter.
3. Fourth ventricle – this ventricle is a lonzenge-shaped cavity situated below and behind the third ventricle, between the cerebellum and pons varolii. It is continuous below the central canal of spinal cord.
Cerebro-spinal fluid (CSF)
CSF is a clear, slightly alkaline fluid. It consists of water, mineral salts glucose, and small amountsof albumin, and few traces of urea and creatinine.
CSF is secreted by anterior and posterior chriod plexus in the ventricles and absorbed by arrachnoid villi which are projected into the venous sinus. The total volume of CSF is 150ml. the rate of formation is 20ml/hour. CSF can be obtained by lumbar puncture.
Functions of CSF-
· It supports and protects the brain and spinal cord from external injury.
