Overview of the nervous system

The nervous system as a whole is composed of two components: the central nervous system(CNS) and the peripheral nervous system(PNS). The CNS is composed of the brain and spinal cord, while the PNS is composed of two major divisions, the sensory division(or afferent division) and the motor division(or efferent division). The sensory division is responsible for informing the CNS about what is going on within and outside the body. The motor division is responsible for sending information from CNS to the various parts of the body in response to the signals coming in from the sensory division. The motor division is composed of two parts, the autonomic nervous system and the somatic nervous system. Under is the schema of these relationships.

Structure and function of the nervous system

The neuron is the structural unit of the nervous system. We first review the anatomy of the neuron and then look at how it functions – allowing electrical impulses to be transmitted throughout the body.

Neuron

Individual nerve fibers(nerve cells), depicted in the figure below, are called neurons. A typical neuron is composed of three regions:

• The cell body, or soma
• The dendrites
• The axon

The cell body contains the nucleus. Radiating out from the cell body are the cell processes; the dendrites and the axon. On the side toward the axon, the cell body tapers into a cone-shaped region region known as the axon hillock. The axon hillock has an important role in impulse conduction.

Most neurons contain many dendrites. These are the neuron’s receivers. Most impulses, or action potentials, coming into the neuron from sensory stimuli or from adjacent neurons typically enter the neuron via the dendrites. These processes then carry the impulses toward the cell body.

In contrast, most neurons have only one axon. The axon is the neuron’s transmitter and conducts impulses away from the cell body. Near its end, an axon splits into numerous end branches. The tips of these branches are dilated into tiny bulbs known as axon terminals or synaptic knobs. These terminals or knobs house numerous vesicles(sacs) filled with chemicals known as neurotransmitters that are used for communication between a neuron and another cell. The structure of the neuron allows nerve impulses to enter the neuron through the dendrites, and to a lesser extent through the cell body, and to travel through the cell body and axon hillock, down the axon, and out through the end branches to the axon terminals. We next explain in more detail how this happens, including how these impulses travel from one neuron to another and from a motor neuron to muscle fibers.