The human body contains a dozen billion nerve cells (neurons) interwoven into what we call the brain and spinal cord. The basic unit of the nervous system is the  neuron, which consists of a cell body and branched extensions, or processes. Dendrites are those processes conducting nerve (electrical) impulses to the cell body, and axons are those processes conducting nerve impulses away from it. Sensory neurons conduct nerve impulses to or toward the central nervous system; motor neurons conduct nerve impulses to the muscles, glands, and organs; and connector neurons conduct impulses from sensory to motor neurons.

Whereas the sensory neuron typically has long dendrites and a short axon, the motor neuron has short dendrit and a long axon; connective neurons generally have short processes. The junction between the terminal endings of the axon of one neuron and the dendrites of another is referred to as a synapse.

Nerves are made up of nerve fibers, a nerve fiber referring to a dendrite or axon extending from the brain or spinal cord out into the body proper, sometimes for many inches. Myelin, a white fatty substance, envelops most of these fibers. Some nerves are pure motor (composed solely of the axons of motor neurons); some are "pure sensory" solely of sensory dendrites; and some nerves are" mixed".

Stimulus, Response, and The Nerve Impulse

The response to a stimulus may be voluntary or involuntary. The former response is under conscious control and generally involves the more highly developed areas of the brain, and the latter-the so-called reflex-is automatic and executed unconsciously. A reflex in its simplest form calls into play a sensory neuron, a motor neuron, a receptor and an effector. On stimulation the receptor initiates a nerve impulse that travel via the sensory neuron and motor neuron (in this order) to the effector (muscle or gland), causing the effector to respond.