The autonomic nervous system can be divided anatomically into the sympathetic and parasympathetic divisions, with each division having central (brain and spinal cord) and peripheral components consisting of preganglionic and postganglionic nerves, as well as an enteric nervous system.
Most visceral organs are innervated by both divisions; the effects that stimulation of each division has on a particular organ usually oppose each other.
Transmission across synapses occurs via chemicals called neurotransmitters, the two most important being acetylcholine and norepinephrine.
Neurotransmitters exert their effects by binding to postsynaptic receptors. Acetylcholine binds to nicotinic and muscarinic receptors; norepinephrine binds to a and p adrenergic receptors. The intensity of response of an effector organ depends in large part on the concentration of neurotransmitters in the synapse.
Each innervated organ usually receives continuous but varying input from the autonomic nervous system. This input generally modulates rather than initiates organ activity. Alterations of autonomic nervous system activity can be en masse (fight or flight) or discrete. The enteric system is comprised of afferent nerves, interneurones, and efferent nerves, all contained within effector organs, mainly of the gastrointestinal tract.
The enteric system regulates many functions of these organs without the need of extrinsic innervation; however, the enteric system is both innervated and influenced by the sympathetic and parasympathetic divisions. Autonomic ganglia appear to be not just relay sites but also sites at which integration of inputs can take place. Also, chemicals other than acetylcholine and norepinephrine appear to be present within the autonomic nervous system and to modulate autonomic activity.
Essential Medical Physiology, Third Edition
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