Mention the parts and functions of a neuron.

NEURONS are the building blocks of the nervous system. They consists of three basic parts: a Cell body, an Axon and one or more Dendrites. Dendrites carry information toward the cell body while axons carry information away from it. Thus neurons are one way channels of communication.

In many neurons, the axon is covered by a sheath of fatty material known as myelin. The myelin sheath is actually protected by another basic set of building blocks within the nervous system, glial cells. Glial cells outnumber neurons by about ten to one. They send out particles that wrap around axons. The myelin sheath so produced is interrupted by small gaps. Both the sheath and the gaps in it play an important role in the neuron’s ability to transmit information.

Near its end, the axon divides into several small branches. These in turn, end in round structures known as axon terminals that closely approach, but do not actually touch, other cells. The regions at which the axon terminals closely approach other cells is known as the synapse.

Basic functions

When a neuron is at rest, there is a tiny negative charge (-70 volts) across its membrane. That is, the inside of the cell has a slight negative charge relative to the outside. This electrical charge is due to the fact that several types of ions (positively and negatively charged particles) exist in different concentrations outside and inside the cell. Specifically, there are more negatively charged particles inside, so the interior of the cell acquires a tiny negative electric charge relative to the outside. This is called resting potential and it does not occur by accident, neurons work to maintain it, actively pumping some types of ions back outside if they enter and retaining others in greater concentrations than are present outside the cell.

When the neuron is stimulated, either directly by light, heat, or pressure or by messages from other neurons, the situation changes radically. If the stimulation is of sufficient magnitude- if it exceeds the threshold of the neuron in question- complex biochemical changes occur in the cell membrane. As a result of these changes, some types of positively charged ions can enter more readily than before. This influx of positive ions reduces and then totally eliminates the resting potential. Indeed, so great is the influx of positively charged ions that for a brief period of time, the interior of the cell may become positive relative to the outside.

After a very brief period (1 or 2 milliseconds), the membrane returns to its original state. The neuron then actively pumps the positive ions that entered back outside and allows other ions, which flood outside to re-enter. As a result, the resting potential is gradually restored; and the cell becomes ready to fire once again. Together these swings in electric charge from negative to positive and back again are termed the action potential.
And it is the passage of this electrical disturbance along the cell membrane that constitutes the basic signal within our nervous system.

The action potential travels through cell membrane often from dendrites to axons and finally reaches the axon terminals located at the end of the axon. Within the axon terminals are many structures known as synaptic vesicles. Arrival of the action potential causes these vesicles to approach the cell membrane, where they fuse with the membrane and then empty their contents into the synapse. These chemicals thus released are called neurotransmitters. These chemicals travel across the synapse until they reach special receptor cites in the membranes of the other cell and the process continues.