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By the end of this section, you will be able to:
  • Compare the structure of somatic and autonomic reflex arcs
  • Explain the differences in sympathetic and parasympathetic reflexes
  • Differentiate between short and long reflexes
  • Determine the effect of the autonomic nervous system on the regulation of the various organ systems on the basis of the signaling molecules involved
  • Describe the effects of drugs that affect autonomic function

The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system. The main difference between the somatic and autonomic systems is in what target tissues are effectors. Somatic responses are solely based on skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc    , there are differences in the structure of those reflexes for the somatic and autonomic systems.

The structure of reflexes

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    . 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. 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 other part of a reflex, the afferent branch    , is often the same between the two systems. 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 ( [link] ). 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.”

Comparison of somatic and visceral reflexes

The top panel in this figure shows the autonomic efferent pathway. The spinal cord is shown on the left, and a myelinated axon is shown going from the spinal cord to the central neuron. An unmyelinated axon is shown going from the central neuron to the target effector. The bottom panel shows the somatic efferent pathway. The spinal cord is shown on the left, and a myelinated axon is shown going from the spinal cord to the target effector director. In both cases, magnified images show the synapses.
The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle. Visceral reflexes involve a projection from the central neuron to a ganglion, followed by a second projection from the ganglion to the target effector.

Afferent branch

The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system. For example, there is a specific type of mechanoreceptor, called a baroreceptor    , in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes. The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.

Questions & Answers

the diagram of the respiratory system
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Anatomy is the study of internal and external structures and the relationship among body parts. (the study of structure).
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