The Autonomic Nervous System and Visceral Reflexes

The autonomic nervous system (ANS) can be defined as a motor nervous system that controls glands, cardiac muscle, and smooth muscle.

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The autonomic nervous system (ANS) is also called the visceral motor system to distinguish it from the somatic motor system that controls the skeletal muscles.

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The autonomic nervous system (ANS) is responsible for the body’s visceral reflexes.

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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.

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A visceral reflex arc includes (1) receptors (nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli), (2) afferent neurons leading to (3) an integrating center and interneurons in the CNS, (4) efferent neurons carrying motor signals away from the CNS, and finally (5) an effector that carries out the end response.

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The autonomic nervous system has two subsystems: sympathetic and parasympathetic.

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The sympathetic division has a calming effect on many body functions. It is associated with reduced energy expenditure and normal bodily maintenance, including such functions as digestion and waste elimination. This is often called the resting- and-digesting state.

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The parasympathetic division adapts the body in many ways for physical activity—it increases alertness, heart rate, blood pressure, pulmonary airflow, blood glucose concentration, and blood flow to cardiac and skeletal muscle, but at the same time, it reduces blood flow to the skin and digestive tract. Cannon referred to extreme sympathetic responses as the fight-or-flight reaction because it comes into play when an animal must attack, defend itself, or flee from danger.

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The balance between sympathetic tone and parasympathetic tone shifts in accordance with the body’s changing needs.

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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.

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