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Like other system in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia. These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system.
The system is composed of specialized cells, termed nerve cells or neurons that communicate with each other and with other cells in the body. A neuron has three parts:
• the cell body, containing the nucleus;
• dendrites, hair-like structures surrounding the cell body, which conduct incoming signals;
• the axon (or nerve fiber), varying in length from a millimeter to a meter, which conduct outgoing signals emitted by the neuron. Axons are encased in a fat-like sheath, called myelin, which acts like an insulator and speeds impulse transmission.
Typically a given neuron is connected to many thousands of neurons. The specific point of contact between the axon of one cell and a dendrite of another is called a synapse. Messages passed to and from the brain take the form of electrical impulses, produced by a chemical change that progresses along the axon. At the synapse, the release of neurotransmitters and this, in turn, drives the impulse to the next neuron. These impulses travel very fast along these chain of neurons – up to 250 miles per hour. This contrasts with other system, such as the endocrine system, which may take many hours to respond with hormones.
The nerve cell bodies are generally located in groups. Within the brain and spinal cord, the collections of neurons are called nuclei and constitute the gray matter, so-called because of their colour. Outside the brain and spinal cord the groups are called ganglia. The remaining areas of the nervous system are tracts of axons, the white matter, so-called because of white myelin sheath.
The nerves of the body are organized into two major systems:
- the central nervous system (CNS), consisting of the brain and spinal cord;
- the peripheral nervous system (PNS), the vast network of spinal and cranial nerves linking the body to the brain and spinal cord.