The neuron is the basic element of the nervous system. The neuron or the nerve ceil is the information transmitting and information-processing element of the nervous system. Neurons come in many different shapes, sizes and varieties according to the specialized jobs they perform. Some of them are very small in size. They are so small that we will require an electronic microscope to detect them. Some of the neurons are very long in structure. Their length in some cases may be more than one meter.
Whatever may be their sizes and shapes, neurons usually have a common form. We shall consider the structure and functions of neurons having a common form. Neurons in general have the following structures:
(i) cell body or soma, (ii) dendrites, (iii) axon, and (iv) terminal buttons or axon terminals.
Cell Body or Soma:
The enlarged “head” of the neuron is called “cell body” or soma. The soma or the cell body is enclosed by the cell membrane and contains the nucleus of the cell. Soma is the head side of the neuron. It uses oxygen and nutrients to generate energy to carry out the work of the cell. Its shape varies considerably in different kinds of neurons.
Dendron is the Greek word for tree and the dendrites of a neuron look very much like trees. Dendrites are extended from the cell body. There are several extensions from the cell body of the neuron. Dendrites receive the neural impulses from the receptors or from several adjoining neurons. In no case, other parts of a neuron excepting dendrites receive the impulse.
There are two types of extensions from the cell body. The many shorter extensions from the ceil body are called dendrites. The longer single- branched extension is called the axon. Each neuron has one axon that extends trunk-like from the cell body. The point in the axon nearest the cell body is called the axon hillock. Axons are very thin and long. Like tree trunks, axons may have some branches also. These branches are called axon collaterals.
Axon has two coverings, which of course may not be found in the axons of all neurons. The outer boundary of the neuron is called membrane. The membrane serves as a barrier for the neuron. In the axons of some of the neurons, there is a fatty white sheath called myelin sheath. Axons, which have myelin sheath, are called as myelinated axons and which do not have it are called unmyelinated axons. Myelin sheath is found only around the axon, and not around any other parts of the neuron. Myelin sheath is made up of a series of specialized ceils of fat and protein. It is not of equal thickness around the axon. Its function is to insulate the message while traveling along the axon; it also serves to increase the velocity with which the electrical impulses travel through the axons.
Neurilemma is another covering found in axons of neurons exclusively outside the brain and the spinal cord. It is a very thin covering, which takes part in regeneration. Therefore, if a neuron outside the brain and the spinal cord is damaged, it can be regenerated. But the neurons of the brain and spinal cord cannot be regenerated, as they do not have neurilemma in their axons. They are highly specialized cells and if damaged, they are damaged forever.
Axons end in smaller branching structures called axon terminals or terminal buttons. These are swollen bulb-like structures located at the end of the axon through which stimulations pass to near by glands, muscles, and other neurons. There are some bag-like structures called as synaptic vesicles at the tips of axon terminals. These have the capability of transmitting information to another neuron. Thus, the axon terminal end of the neuron is the delivering side of the neuron. Dendrites receive and axon terminals deliver. Neurons transmit information in only one direction: from the dendrites through the cell body to the axon to the terminal buttons. This is known as the /aw of forward induction. We shall discuss the process of neural transmission in detail in a later section.