The autonomic nervous system (ANS) controls visceral (organ) functions. Visceral motor neurons arise in the hypothalamus and synapse with preganglionic neurons. Preganglionic neurons arise in autonomic nuclei of the central nervous system (CNS), in the brainstem or spinal cord. These neurons exit through cranial or spinal nerves to synapse with peripheral postganglionic neurons in autonomic ganglia. Reflexes can either be integrated in the CNS (long reflexes) or bypass the CNS and be integrated in autonomic ganglia (short reflexes).
The ANS has two divisions: sympathetic and parasympathetic. The sympathetic division of the ANS passes out thoracic and lumbar spinal nerves. The parasympathetic division of the ANS surrounds the sympathetic system (cranial and sacral nerves). Most organs are innervated by both divisions of the ANS (dual innervation) to provide tighter homeostatic control.
The activities of the sympathetic nervous system increase heart rate and inhibit digestion. Sympathetic nervous activity is a response to threats, so is called a “fight-or-flight” response. Most parasympathetic activities are the opposite of sympathetic activities (“rest-and-digest”).
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