Behavior and mental processes result from activities in the body's nervous system and other physiological systems.

Structure. The basic unit of the nervous system is a cell known as the neuron (Figure 1). It is estimated that the nervous system contains over 11 billion neurons. The neuron, which is covered by a cell membrane, consists of

  • dendrites, branched appendages that carry information to the cell body

  • a cell body (soma), which contains the nucleus

  • an axon, which conveys information away from the cell body

Figure 1

A Neuron

Some axons are covered with a myelin sheath (interspersed with spaces called nodes of Ranvier), which aids in neural transmission. Neurons are surrounded by glial cells, which nourish the neurons and hold them in place; these cells are the basis of the myelin sheaths. Axon terminals are branched and contain terminal buttons, tiny swellings that in turn contain synaptic vesicles (Figure 2). Synaptic vesicles are filled with chemicals called neurotransmitters, which assist in transmission of information to other neurons.

Types of neurons. There are three types of neurons:

  • Sensory neurons are located in the body's sense organs (for example, the eye, ear, or nose) and send information from these organs to the brain.

  • Motor neurons convey information from the nervous system to the body's organs, glands, and muscles.

  • Interneurons (association neurons) transmit information from one neuron to another within the nervous system.