Neurons exist in many shapes and sizes. Their structure, like that of other cells in the body or in nature, illustrates that structure often determines function. There are three basic structural and functional classifications of neurons.
The structural classification of a neurons depends upon the number of dendrites extending from the cell body. Multipolar neurons have several dendrites; the majority of neurons in the spinal chord and brain are multipolar. Bipolar neurons have only two processes: a single dendrite and an axon. Bipolar neurons are found in the sense organs-and in the retina of the eye and in olfactory cells. Unipolar neurons lack dendrites and have a single axon, and are also sensory neurons.
Multipolar neurons have many processes and serve principally as motor neurons. Motor neurons, efferent because they conduct impulses away from the central nervous system (the brain and spinal cord) regulate the function of muscles and glands. Afferent neural pathways that send signals to the (CNS) are generally composed of unipolar neurons. Unipolar neurons also serve as sensory neurons—their filamentous dendritic processes exposed and elaborated into or connected to sensory receptor cells. Interneurons are neurons that connect neurons along a neural pathway.