Structural Classification of Neurons PDF

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Structural Classification of Neurons...

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Structural Classification of Neurons 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 (Figure 12–3): ■ Anaxonic (an-aks-ON-ik; an-, without) neurons are small and have numerous dendrites, but no obvious axons. They do have axons, yet they are not readily visible, even when viewed with a microscope. Anaxonic neurons are located in the brain and in special sense organs. Their functions are poorly understood. ■ Bipolar neurons have two distinct processes—one dendrite (that branches extensively into dendritic branches at its distal tip) and one axon—with the cell body between the two. Bipolar neurons are rare. They occur in special sense organs, where they relay information about sight, smell, or hearing from receptor cells to other neurons. Bipolar neurons are relatively small: The largest measure less than 30 μm from end to end. ■ In a unipolar neuron, or pseudounipolar neuron, the dendrites and axon are continuous—basically, fused—and the cell body lies off to one side. In such a neuron, the initial segment lies where the dendrites converge. The rest of the process, which carries action potentials, is usually considered 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. The longest axons of unipolar neurons carry sensations from the tips of the toes to the spinal cord. ■ Multipolar neurons have two or more dendrites and a single axon. They are the most common neurons in the CNS. All the motor neurons that control skeletal muscles, for example, are multipolar neurons. The longest axons of multipolar neurons carry motor commands from the spinal cord to small muscles that move the toes. Functional Classification of Neurons Alternatively, we can categorize neurons by function as (1) sensory neurons, (2) motor neurons, or (3) interneurons. Sensory Neurons. Sensory neurons, or afferent neurons, form the afferent division of the PNS. 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 whose processes, known as afferent. fibers, extend between a sensory receptor and the CNS (they deliver information from sensory receptors to the spinal cord or brain). The human body’s 10 million or so sensory neurons collect information about the external or internal environment. 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 (see Figure 12–1). Sensory receptors are either the processes of specialized sensory neurons or cells monitored by sensory neurons. We can broadly categorize these receptors into three groups:

■■ Interoceptors (intero-, inside) monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems, and provide sensations of distension (stretch), deep pressure, and pain. ■■ Exteroceptors (extero-, outside) provide information about the external environment in the form of touch, temperature, or pressure sensations and the more complex senses of taste, smell, sight, equilibrium (balance), and hearing. ■■ Proprioceptors (pro . -pre . -o . -SEP-terz) monitor the position and movement of skeletal muscles and joints....