COORDINATION: NERVOUS SYSTEM OF MAN

 

Q : Describe the basic elements or components of nervous system of man.

Ans:- NERVOUS SYSTEM OF MAN

          Nervous system of man is complex and advanced but it is built on the same basic pattern as the nervous system of other animals. It consists of receptors, neurons, brain, spinal cord, nerves and effectors. The brain of man is a unique structure.

RECEPTORS IN MAN

                  Receptors or sense organs are the structures which receive information (stimuli) about the external and internal environment.

                   In man 11 distinct Sensations or sense are recognized, I,e., sense for touch, smell, taste, hearing, sight, warmth cold, equilibrium, pain, awareness of position and movement, and visceral sensation such as hunger, nausea, sexual sensations etc. In man there is an extra-sensory perception which is a unique kind of sensation that enables him to predict, forsee or imagine things. These receptors can be grouped according to the nature or Imagine of the stimulus into three main groups: nature of the stimulus into the

 

i)                      Chemical Receptors. These receptors are only stimulated by chemical stimuli. e.g., receptors for taste located on the tongue and for smell located in the nose and nasal passage.

ii)                   Mechanical Receptors. These receptors are only stimulated by mechanical stimuli. e.g., receptors for touch, pressure, pain temperature, sound and motion.

iii)                   Photoreceptors OF eyes. They are stimulated by a certain range of wavelengths constituting the visible light.

Each sense organ 1s specialized to receive a specific type of stimulus and only a small portion of the total stimulus is received by the sense organs. For example eyes of man are stimulated by electro-magnetic radiations between the short-wave violet to the long-wave red. Eyes cannot be stimulated by ultra-violet and infra-red light however their presence can be detected by special instruments. Similarly ear of man can detect sounds ranging in frequency from about 20 to 20000 cycles per second. Ears are most sensitive to sounds between 1000 and 2000 cycles per second. The range of frequency of sounds weves used by bats is from 30,000 to 70,000 vibrations per second.

NEURONS

A neuron

The structural and functional unit of the nervous system of man are the neurons or nerve cells. A typical neuron consists of:

i)                     Cell body. It contains nucleus, cytoplasm and inclusians.

ii)                   Dendrites. These are cytoplasmic extensions or fibers usually present on either side of the cell body. They conduct nerve impulses.

  lii)          Axon. It is a single long fiber that conducts nerve impulses away from the cell body.

                              There are three main types types of neurons:

i)                     Sensory neurons;

ii)                    Motor neurons;

iii)                 Associative neurons;

Sensory Neurons

These are the neurons which carry nerve impulses from receptors to brain or spinal cord. In the sensory neurons, the single, elongated dendrite is called a dendron because it carries nerve impulses from the receptors to the cell body. The shorter fiber is called axon because it conducts impulses away from the cell body. Both dendrite and axon arise from the cell body.

Motor neurons

 These are the neurons which carry nerve impulses from brain and spinal cord to the effectors in all parts of the body. In the motor neuron there is a long axon and a number of small processes given out from the cell body are the dendrites.

Associative Neurons

Kinds of nerve cells

Summary diagrams showing the mechanisms involved in chemical transmission at a neuronal synapse, 
(a)to(b) time sequence

These are the neurons which occur exclusively in the spinal cord and brain. They serve as intermediate links between numerous sensory and motor neurons. The cell body of these neurons usually lies in the brain or spinal cord while the axon or dendron extends for a considerable length to reach on organ.

Myelin and Cellular Sheaths

In certain cases the axon and dendrites are surrounded by an insulating covering of fatty material called the myelin sheath. The myelin sheath may again be surrounded by a cellular sheath and both sheaths are interrupted at: regular intervals by constrictions or nodes. The sheath prevents the indiscriminate distribution of nerve impulse to adjacent nerve fiber and serves as a means of increasing the rate of conduction of nerve impulse.

The human nervous system is composed of nearly ten to twelve billion nerve cells. Neurons lack the ability to divide to form new cells. Howere many axons and dendrites can regenerate if cell bodies are not damaged.

EFFECTORS

Effectors are the parts of the body which respond according to the nerve impulses received from the brain. In man muscles and glands are the effectors which respond by contraction and secretion of hormones respectively.