Functional Classification of Neurons PDF

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Functional Classification of Neurons...

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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. Motor Neurons. Motor neurons, or efferent neurons, form the efferent division of the PNS. The half a million motor neurons carry instructions from the CNS to peripheral effectors in a peripheral tissue, organ, or organ system. Axons of these neurons that

travel away from the CNS are called efferent fibers. As noted earlier, the two major efferent systems are the somatic nervous system (SNS) and the autonomic (visceral) nervous system (ANS) (see Figure 12–1). 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 runs within a peripheral nerve 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—that is, 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, a 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, and axons connecting the ganglion cells with the peripheral effectors are known as postganglionic fibers. Interneurons. Interneurons are located between sensory and motor neurons. The 20 billion or so interneurons (association neurons) outnumber all other types of neurons combined. Most are located within the brain and spinal cord, but some are in autonomic ganglia. The main function of interneurons is integration—they distribute sensory information and coordinate motor activity. The more complex the response to a given stimulus, the more interneurons are involved. Interneurons also play a part in all higher functions, such as memory, planning, and learning. We now turn our attention to the neuroglia, cells that support and protect the neurons....