Neurons do exist throughout the body, performing a variety of functions. Most neurons fall into three classifications: sensory, motor, or interneuron.
Sensory neurons are spread throughout organs, including the skin, muscles, and joints. These specialized neurons allow for feelings of pain, pressure, and temperature changes. The senses of sight, taste, hearing, and smell also depend on the functioning of neurons in the associated systems such as the auditory and olfactory systems. The retina contains neurons that function to interpret light thus allowing for sight.
Motor neurons are found in cell in the heart, intestinal system, diaphragm, and glands. They stimulate these cells to fire into action as the heart beats, a breath is taken, or food moves through the digestive system.
The third major category of neurons is the interneurons. These neurons are specialized to provide for communication between the sensory and motor neurons. Interneurons are also able to communicate each other.