The billions of neurons in the nervous system are quite variable in form. They can be grouped by structure or by function.

Structural Classification

Neurons are classified as anaxonic, bipolar, unipolar, or multipolar on the basis of the relationship of the dendrites to the cell body and the axon:

1. Anaxonic neurons are small and have no anatomical clues to distinguish dendrites from axons; all the cell processes look alike (Figure 12-4a). Anaxonic neurons are located in the brain and in special sense organs. Their functions are poorly understood.
2. Bipolar neurons have two distinct processes—one dendritic process that branches extensively at its distal tip, and one axon, with the cell body between them (Figure 12-4b). Bipolar neurons are rare but occur in special sense organs, where they relay information about sight, smell, or hearing from the receptor cells to other neurons. They are smaller than unipolar or multipolar neurons; the largest measure less than 30 mm from end to end.
3. In a unipolar neuron, or pseudounipolar neuron, the dendrites and axon are continuous--basically, fused--and the cell body lies off to one side (Figure 12-4c). In such a neuron, the initial segment lies where the dendrites converge. The rest of the process, which carries action potentials, is usually considered to be an axon. Most sensory neurons of the peripheral nervous system are unipolar. Their axons may extend a meter or more, ending at synapses in the central nervous system.
4. Multipolar neurons have two or more dendrites and a single axon. Multipolar neurons (Figure 12-4d) are the most common type of neuron in the CNS. For example, all the motor neurons that control skeletal muscles are multipolar neurons. Their axons can be as long as those of unipolar neurons.

Functional Classification

Alternatively, neurons are categorized by function as (1) sensory neurons, (2) motor neurons, and (3) interneurons. Their relationships are diagrammed in Figure 12-5.

Sensory Neurons Sensory neurons, or afferent neurons, form the afferent division of the PNS. They deliver information from sensory receptors to the CNS. The cell bodies of sensory neurons are located in peripheral sensory ganglia. (A ganglion is a collection of neuron cell bodies in the PNS.) Sensory neurons are unipolar neurons with processes, known as afferent fibers, that extend between a sensory receptor and the spinal cord or brain. Sensory neurons collect information concerning the external or internal environment. The human body has about 10 million sensory neurons. Somatic sensory neurons monitor the outside world and our position within it. Visceral sensory neurons monitor internal conditions and the status of other organ systems.

Sensory receptors may be the processes of specialized sensory neurons or cells monitored by sensory neurons. Receptors are broadly categorized as follows:

1. Exteroceptors  provide information about the external environment in the form of touch, temperature, or pressure sensations and the more complex senses of sight, smell, and hearing.
2. Proprioceptors  monitor the position and movement of skeletal muscles and joints.
3. Interoceptors  monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and provide sensations of taste, deep pressure, and pain.

Motor Neurons Motor neurons, or efferent neurons, form the efferent division of the PNS. These neurons carry instructions from the CNS to peripheral effectors in a peripheral tissue, organ, or organ system. Your body has about half a million motor neurons. Axons traveling away from the CNS are called efferent fibers. As we learned earlier, the two major efferent systems are the somatic nervous system (SNS) and the autonomic (visceral) nervous system (ANS). The somatic nervous system includes all the somatic motor neurons that innervate skeletal muscles. You have conscious control over the activity of the SNS. The cell body of a somatic motor neuron lies in the CNS, and its axon extends into the periphery to innervate skeletal muscle fibers at neuromuscular junctions.

You do not have conscious control over the activities of the ANS. Visceral motor neurons innervate all peripheral effectors other than skeletal muscles. Thus, the ANS innervates smooth muscle, cardiac muscle, glands, and adipose tissue throughout the body. The axons of visceral motor neurons in the CNS innervate a second set of visceral motor neurons in peripheral autonomic ganglia. The neurons whose cell bodies are located in those ganglia innervate and control peripheral effectors.

To get from the CNS to a visceral effector, such as a smooth muscle cell, the signal must travel along one axon, be relayed across a synapse, and then travel along a second axon to its final destination. The axons extending from the CNS to an autonomic ganglion are called preganglionic fibers. Axons connecting the ganglion cells with the peripheral effectors are known as postganglionic fibers.

Interneurons The 20 billion or so interneurons, or association neurons, outnumber all other types of neurons combined. These neurons are located entirely within the brain and spinal cord. They are responsible for the distribution of sensory information and the coordination of motor activity. For example, one or more interneurons are usually situated between sensory neurons and motor neurons. The more complex the response to a given stimulus, the greater the number of interneurons involved. Interneurons are also involved with all higher functions, such as memory, planning, and learning.