NERVOUS SYSTEM – TYPES, FUNCTIONS AND STRUCTURES

What is the nervous system? The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body

Components of the nervous system in humans

Every organ is the human body is connected to nerves. The nervous system is made up of nerve cells (neurons) which transmit impulses from one part of the body to another.

It consists of the following:

1. The Central Nervous System (CNS) is a concentrated mass of interconnected nerve cells which make up the brain and the spinal cord.
2. The peripheral nervous system is made up of nerves which link the CNS to the receptors and the effectors.
3. Sensory nerves link the sensory cells (receptors) to the central nervous system and transmit nerve impulses from a sense organ to the CNS.

Structure and Functions of Neurons

A nerve cell consists of a cell body (centron) where the nucleus is located, and projections called dendrites arise. One of the projections is drawn out into an axon i.e. the longest process. Each axon contains axoplasm which is continuous with the cytoplasm in the cell body.

The axon is enclosed in a fatty myelin sheath which is secreted by Schwarm cell. The myelin sheath is interrupted at approximately 1 mm intervals by constrictions known as nodes of Ranvier. The myelin sheath is enclosed by a thin membrane called the neurilemma, which is part of the Schwann cell in contact with axon. The myelin sheath and nodes of Ranvier enhance transmission of the impulse.

There are three types of neurons:

Sensory neurone

Also known as afferent neurone. Transmits impulses from sensory cells to the CNS. The cell body of a sensory nerve cell is located at some distance along the length of the axon outside the CNS.

Motor neurone

Known as efferent or effector neurone  Transmit impulses from the CNS to the effectors(muscles and glands) Its cell body is located inside the CNS.

Intermediate or connector neurone

Also called relay neurone Found inside the CNS. The connect sensory and motor neurons with each other and with other nerve cells in the CNS.

Functions of the neurone

1. The nerve impulse is electrical in nature.
2. Its transmission depends on differences in electrical potential between the inside and the outside of the axion.
3. The outside is positive while the inside is negative.
4. The stimulus triggers a change that affects the permeability of neurone membrane.
5. The result is a change in the composition of ions on either side of the membrane.
6. The outside becomes negative as the inside becomes positive due to sodium ions rushing in.
7. The above constitutes a nervous impulse which is transmitted along the sensory neurone to the CNS.
8. The speed of transmission is very high.
9. Certain mammalian axions transmit impulses at the rate of 100m/s.
10. The dendrites of neurons do not connect directly to each other, but they leave a small gap called synapse.
11. The transmission of an impulse from one cell to the next takes place through synapse.
12. Synaptic knobs are structures found at the ends of dendrites.
13. Thus the dendrites of one nerve cell make contact with the dendrites of the adjacent nerve cell through the synapses.
14. Impulses are transmitted in the form of a chemical transmitter substance which crosses the gap between one dendrite and the next.
15. The transmitter substance is found within synaptic vesicles.
16. The chemical substance is either acetylcholine or noradrnaline.
17. The synaptic vesicles burst and release the transmitter substance when an impulse arrives at the synaptic knob.
18. Impulses in motor neurone s are trans mitted to effectors.
19. The space between motor end dendrite and muscle is known as neuro-muscular Junction.
20. Synaptic vesicles in the ends of the dendrites release the transmitter substance across the neural muscular junction.