The function of nervous system is to coordinate the activities of our body. It is the control system for all our actions, thinking and behaviour. The nervous system helps all other systems of our body to work together.The nervous system is like a manager inside our body. Its job is to control and coordinate the parts of our body so that they work together, doing their job at the right time.
Our nervous system coordinates muscles so that we can do things which need thinking like reading, writing, cycling or dancing. The nervous system also coordinates things which we don't have to think about, like heart beat and breathing.
The human nervous system receives information from the surroundings, processes it, interprets it and then responds accordingly. The nervous system also passes information from one internal system to another. For example, as soon as we put food in our mouth, it immediately causes the release of saliva from the salivary glands.
The units which make up the nervous system are called nerve cells or neurons. So, neuron is the structural and functional unit of the nervous system. We can now say that nervous system is made of special cells called neurons. Neuron is the largest cell in the body (which looks like an electric wire). Neurons contain the same basic parts as any other animal cell but their structure is specially adapted to be able to carry messages over large distances in the body quickly. The neurons carry messages in the form of electrical signals called electrical impulses or nerve impulses.
A neuron (or nerve cell) has three components:
(ii) Dendrites, and
The cell body of a neuron is like a typical animal cell which contains cytoplasm and a nucleus. A number of long and thin fibres are stretching out from the cell body of a neuron. They are called nerve fibres.
The shorter fibres on the body of a neuron are called dendrites. The longest fibre on the cell body of a neuron is called axon. The axon has an insulating and protective sheath (or cover) of myelin around it (Myelin is made of fat and protein). It is clear that both dendrites and axon arise from the cell body of a neuron.
The messages which the neurons transmit in the nervous system are in the form of electrical impulses called nerve impulses (or just impulses). The dendrites pick up the nerve impulses (or messages) from receptors. They pass the impulses to the cell body and then along the axon. The axon passes the impulse (or message) to another neuron through a junction called synapse. Neurons are of three types: sensory neurons, motor neurons and relay neurons.
(i) Sensory neurons transmit impulses from the sensory cells (or receptors) towards the central nervous system (spinal cord and brain).
(ii) Motor neurons transmit impulses from the central nervous system (spinal cord and brain) towards the muscle cells (or effectors).
(iii) Relay neurons occur in the central nervous system (brain and spinal cord) where they serve as links between other neurons.
We will now explain how nerve impulses (or messages) are transferred from one neuron to another in the nervous system. Any two neurons in the nervous system do not join to one another completely. There is always a very, very small gap between the two neurons (where they join).
This gap is called a synapse. The nerve impulses are carried over this small gap between a pair of neurons by means of a chemical substance called neurotransmitter substance. We can now say that: A microscopic gap between a pair of adjacent neurons over which nerve impulses pass when going from one neuron to the next is called a synapse.
Thus, synapses connect neurons (though it looks surprising that even gaps can connect two things!). We will now understand the conduction of electrical nerve impulses through synapse with the help of a diagram.
Suppose there are two neurous (or nerve cells) A and B near each other. Let A be a sensory neuron which is directly connected to the receptor. There is an extremely small, microscopic gap between the end of the axon of neuron A and the dendrite of the next neuron B which is called a synapse We will now explain how the electrical impulse travels through the gap (synapse) between the two neurons.
The receptor in a sense organ is in touch with the dendrites of sensory neuron. When a stimulus acts on the receptor, a chemical reaction is set off which produces an electrical impulse in it. This impulse travels from the dendrite of sensory neuron A to its cell body and then along its axon.
At the end of axon of sensory neuron A, the electrical impulse releases tiny amount of a chemical substance into the synapse (or gap). This chemical substance crosses the gap (or synapse) and starts a similar electrical impulse in the dendrite of the next neuron B. From the dendrite, this electrical impulse is carried to the cell body and then to the end of axon of the second neuron.
It can then be transferred to a third neuron in a similar way. This process goes on till the electrical impulse reaches the relay neurons in spinal cord and brain. The relay neurons and motor neurons connect in a similar way to bring electrical impulses from the brain and spinal cord to the effectors like muscles and glands.
Synapses actually act like one-way valves. This is because the chemical substance is present on only one side of the gap. Due to this, the nerve impulses (or messages) through a particular set of neurons can go across only from one side (which contains the chemical substance). In this way, synapses ensure that nerve impulses travel in only one direction (through a particular set of neurons).