Nervous system co-ordinates the functions of different organs of the body. With the help of external stimulus, the system analyses and produces sensations accordingly. It retains the memory of the past. It conducts the messages and information between the different organs of the body. The sense organs like eyes, ears, smell, taste etc. are also associated with the nervous system. The unit of nervous system is neurons and they constitute an integrated network in the body. The nervous system in man consists of,
(1) Central nervous system (CNS) : It contains brain and spinal cord.
(2) Peripheral nervous system : The nerves that branch off the brain and spinal cord.
(3) Autonomic nervous system : This contains nerves and coming arising from the ventral part of the spinal cord.
Brain and spinal cord is covered by meninges. This is membranous in structure. The different parts of nervous system are discussed below :
The Brain
The brain can be divided into three parts
(a) Fore brain
(b) Mid brain
(c) Hind brain
(a) Fore brain : Cerebral hemisphere, olfactory lobe and diencephalon are the parts of the fore brain.
(b) Mid brain : Mid brain has four optic lobes and cerebral peduncle.
(c) Hind brain : The main parts of hind brain are cerebellum and medulle oblongata.
(1) Both the cerebral hemisphere are joined together by a strip of nerve fibre internally. The cerebral cortex is made up of grey matter. It contains nerve cells in it.
(2) Hyphothalamus contains nerve centres for cold, heat, touch, hunger, emotions etc.
(3) Muscle tone is controlled by optic lobes.
(4) Balance of the body, posture etc is maintained by cerebellum.
(5) Medulla oblongata has the centres for respiration, circulation etc.
(6) End part of medulla controls involuntary activities. Sneezing and coughing centres are also located in medulla.
(7) The centres for cold, heat, touch etc. are in the somasthetic area of parietal lobe.
Spinal Cord
In spinal cord white matter surrounds the grey matter. The nuclei of spinal are embedded in the grey matter. In the centre the spinal cord has a central canal. Dorsal septum joins Dorsal sulcus of spinal cord with central canal. All reflex actions are controlled by spinal cord. Nerve impulses are conducted by spinal cord from peripheral organs to the brain. Spinal nerves control the motor actions. There are 31 pairs of spinal nerves in man. These are
Cervical nerves—8 pairs
Thoracic nerves—12 pairs
Lumbar nerves—5 pairs
Sacrat nerves—5 pairs
Coccygeal nerves—5 pairs
All these nerves are mixed
NERVE CELLS
Most impotant unit of Nervous system is neurons. Neurons together form nerve fibres. Nerve fibres are of two types.
(a) Myellinated Nerve fibres and
(b) Non-Myellinated Nerve fibres.
The first type is present in the white matter. The second type is of very small fibres.
Myellinated fibres are covered with a high electric resistant sheath. It acts as electric insulator. The nerve impulse in these fibres travels 20 times faster than non-myellinated fibres.
The outer surface of non-myellinated fibres has positive change while the internal surface has negative charge. Nervous stimulation travels forward which produces the nerve impulses.
The nerve impulses travel as electrochemical transmission.
Important terms of Nervous system :
1. Action Potential : The membrane potential of an active neuron.
2. Acetylecholin : A neuro transmitter present in peripheral and central nervous system.
Axon : The part of a neuron that conducts impulses from the cell body to the synapse.
Central Canal : Small tube in the spinal cord that is continuous with the ventricles of the brain.
Central Nervous System (CNS) : The brain and spinal cord.
Cerebellum : Portion of the brain that co-ordinates skeletal muscles to produce motions.
Cerebrospinal Fluid : A fluid found in the ventricles of the brain.
Cerebrum : The main part of the brain consisting of two large masses or cerebral hemispheres.
Dendrite : The part of a neuron that sends impulses toward the cell body.
Genglion : A knot or bundle of neuron cell bodies usually outside the central nervous system.
Hypothalamus : A part of the brain involved in control of heart rate, body temperature, water balance and glandular secretions of the stomach and pituitary gland.
Interneuron : Neuron, conveying message between parts of the central nervous system.
Limbic System : Pathways linking the hypothalamus to some areas of the cerebral cortex; governs learning and memory and various emotions.
Medulla Oblongata : A part of the brain stem controlling heartbeat, blood pressure, breathing and other vital functions.
Meninges (sing., meninx) : Protective membranous coverings about the central nervous system.
Midbrain : The part of the brain located below the thalamus and above the pons.
Myelin Sheath : A white, fatty material—that forms a covering for nerve fibres.
Nerve : A bundle of long axons outside the central nervous system.
Neuron : A nerve cell made up of a cell body and an axon.
Neurotransmitter : A chemical stored at the ends of axons that is responsible for transmission across a synapse.
Norepinephrine (NE) : Neurotransmitter active in the peripheral and central nervous systems; a hormone secreted by adrenal medulla.
Parasympathetic System : A division of the autonomic system that is active under normal conditions.
Peripheral Nervous System (PNS) : The nerves of the central nervous system.
Ons : A part of the brain stem above the medulla oblongata and below the midbrain.
Reflex : An automatic, involuntary response of an organism to a stimulus.
Resting Potential : The membrane potential of an inactive neuron.
Spinal Cord : Part of the central nervous system.
Sympathetic System : A division of the autonomic system that is active in excited condition.
Thalamus : It is a part of the brain that serves as the integrating centre for sensory input.
Ventricle : A cavity in an organ, such as a ventricle in the brain.
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