After reading this article you will learn about:- 1. Elementary Structures of Neurons 2. Functions of Neurons 3. Three Types 4. Nerve Current.
Elementary Structures of Neurons:
The nervous system is made up of cells. A nerve-cell with all its filaments is called a neuron. Its filaments are really a part of the cell. It has a tough coat called cell-wall, and contains a white jellylike called ‘protoplasm’.
It has a cell-body containing a thick substance called ‘nucleus’, and often within this substance nucleus smaller bodies called nucleoli, and its branches or filaments radiating from it. Two types of branches are connected with each nerve- cell. One is the receiving type and is called ‘dendrite’.
The other is the sending-type, and is called ‘axon’. An axon transmits the nerve current away from the cell-body. A dendrite transmits the nerve current towards the cell-body. An axon has a myelin or medullary sheath which insulates it. A dendrite has no medullary sheath. An axon is long while a dendrite is short.
The grey matter of the brain is composed of cell-bodies and dendrites, while the white Matter is made tip of the axons. There are millions of neurons in the nervous system. They are the elementary structural units, which are separate and distinct.
Each neuron consists of the nerve-cell and dendrites and axon. The dendrites look like the branches of a tree. The axon looks like a long slender thread without branches and ends in an end-brush.
The junction of two neurons is called the ‘synapse’. Here the axon of one neuron breaks up into an end-brush of fine branches, which interlace with the dendrites of the other neuron. The dendrite in a synapse is a receiving organ, while the axon end-brush is a stimulating organ, and not a receiving organ.
At the synapse the axon end-brush of one neuron stimulates the dendrites of another neuron, but there is no continuity between the two processes. Nerve-currents are conducted from the ends of the sensory axon to the dendrites of the motor cells.
They are not conducted from the latter to the former. There is inhibition of nerve-currents in the synapse. They flow freely through the nerves. But there is a greater delay in the passage of nerve-currents through the synapse. So it obstructs the flow of nervous energy.
The fully developed nerve-fibres have a complex structure. The central strand is called the axis cylinder. It constitutes the true nerve and carries the nerve-impulse from one point to another. It has a relatively thick covering called the myelin or medullary sheath. It has another membranous sheath called the neurilemma.
When a sensory nerve is stimulated, it quickly transmits a nerve-impulse to the spinal cord or the brain. When a motor neuron in the cord or the brain is stimulated, it quickly transmits a nerve- impulse to a muscle which makes a movement. A nerve-impulse or current is an electro-chemical wave which can move a nerve-centre or a muscle to action.
Functions of Neurons:
The neurons consist of the cell-bodies and nerve-fibres. Both cell-bodies and nerve-fibres possess irritability and conductivity. A slight simulus can make it active. This is called ‘irritability’. A neuron can send nerve-currents from one part of the organism to another. This is called conductivity. The cell-bodies have also the power of either reinforcing or inhibiting the nerve-impulses sent to them.
They can strengthen the impulses received or arrest them. Sometimes their action is automatic; they send out nervous excitation along the nerve-fibres without any external stimulation. They interconnect the neurons with one another. The cell-bodies supply nutrition to the nerve-fibres. The function of inhibition formerly ascribed to the cell-bodies is now ascribed to the synapses or junctions of neurons.
Three Types of Neurons:
There are three kinds of neurons. The first is the sensory neurons, the second is the motor neuron, and the third is the central neuron. The third type is often called the association or correlation neurons. The sensory neuron connects a sense-organ with a sensory centre. The motor neuron connects a motor centre with a muscle. The central neuron connects a sensory neuron with a motor neuron. It is the coordinating neuron.
The neurons are of three kinds according to their functions. The sensory neurons conduct nerve currents from the sense-organs to the sensory centres. The motor neurons which terminate in muscles carry nerve currents from the motor centres to the muscles. The central neurons connect sensory neurons with motor neurons.
The nerve current is an electro-chemical wave which is transmitted through a nerve-fibre. Its nature is not precisely known. It cannot be discharged by a weak stimulus. A stimulus must have minimum intensity to excite the nerve impulse. But its greater intensity does not excite a more intense nerve current.
This is called the all or-none-law. If a nerve-fibre responds at all, it responds completely. But this does not mean that a more intense stimulus does not produce a more intense sensation. In fact, a more intense stimulus does produce a more intense sensation.
It is due-to the facts:
(1) That it increases the frequency of response in each nerve-fibre and,
(2) That it stimulates more nerve-endings at the same time.
All nerve currents are not of the same size. A nerve current is large in large nerve-fibres, and small in small nerve-fibres. The size of the nerve current also depends upon the condition of the nerve-fibre, fresh and fatigued. But a nerve-fibre always responds according to the all-or none-law. Either it does not respond, or responds completely. A nerve impulse travels along a nerve-fibre from about 1 to 100 metres per second.