Introduction

The nervous system has two distinct parts: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves outside the brain and spinal cord).

The basic unit of the nervous system is the nerve cell (neuron). Nerve cells consist of a large cell body and nerve fibers--one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. Normally, nerves transmit impulses electrically in one direction--from the impulse-sending axon of one nerve cell to the impulse-receiving dendrites of the next nerve cell. At contact points between nerve cells (synapses), the axon secretes tiny amounts of chemical messengers called neurotransmitters. Neurotransmitters trigger the receptors on the next nerve cell's dendrites to start up a new electrical current. Different types of nerves use different neurotransmitters to convey impulses across the synapses.

The nervous system is an extraordinarily complex communication system that can send and receive voluminous amounts of information simultaneously. However, the system is vulnerable to diseases and injuries. For example, nerves can degenerate, causing Alzheimer's disease or Parkinson's disease. Bacteria or viruses can infect the brain or spinal cord, causing encephalitis or meningitis. A blockage in the blood supply to the brain can cause a stroke. Injuries or tumors can cause structural damage to the brain or spinal cord.