The function of the nervous system is to coordinate the activity of each individual cell and organ with that of all other cells and organs so that the horse acts as an integrated unit. The system is necessarily extremely complex. The basic unit is the neuron, comprising a cell body and processes (in some cases, extremely long ones) along which electrical impulses are transmitted. This is done by the movement of electrically charged ions (or mineral particles), particularly sodium, potassium, calcium and chloride, across the cell membranes. Most neurons have many branches so chat they can each communicate with many other cells. Between the individual cells are synapses across which electrical charges can pass in only one direction. Anatomically the nervous system can be divided into the central nervous system and the peripheral nervous system. The spinal cord running along the back, protected by The neural arches of the vertebrae, forms the central nervous system. At the head, the spinal cord expands into the brain which accommodates various specialized functions and coordinates activity from the whole body. Running from the spinal cord are forty-two pairs of spinal nerves which form the basis of the peripheral nervous system, one pair for each body segment. The spinal nerves run out between the vertebral arches. Functionally, the nervous system can be divided into somatic and visceral (or autonomic) nervous systems. The somatic nervous system relates to organs and tissues other than the gut and deals with voluntary muscle control and the sense organs. The visceral nervous system relates to involuntary control of large organs such as the heart and the digestive system, and the functioning of glands. Both these systems are present in the central and peripheral nervous systems.