The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. The Latin root “effere” means “to carry.” Adding the prefix “ef-” suggests the meaning “to carry away,” whereas adding the prefix “af-” suggests “to carry toward or inward.”įigure 15.6 Comparison of Somatic and Visceral Reflexes The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Sensory neurons receiving input from the periphery-with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord-project into the CNS to initiate the reflex ( Figure 15.6). The other part of a reflex, the afferent branch, is often the same between the two systems. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion-followed by the postganglionic fiber projecting to a target effector. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. Whereas the basic circuit is a reflex arc, there are differences in the structure of those reflexes for the somatic and autonomic systems. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Somatic responses are solely based on skeletal muscle contraction. The main difference between the somatic and autonomic systems is in what target tissues are effectors. The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. Describe the effects of drugs that affect autonomic function.Determine the effect of the autonomic nervous system on the regulation of the various organ systems on the basis of the signaling molecules involved.Differentiate between short and long reflexes.Explain the differences in sympathetic and parasympathetic reflexes.Compare the structure of somatic and autonomic reflex arcs.By the end of this section, you will be able to:
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