Autonomic Nervous System Overview

Autonomic Nervous System

The autonomic nervous system (ANS) controls involuntary functions in the body.  There are three main components of the ANS:

• Parasympathetic Nervous System (PNS)

• Sympathetic Nervous System (SNS)

• Enteric Nervous System

Most organs of the body are innervated by the PNS and the SNS

The somatic nervous system controls voluntary activity and innervates striated muscle cells and is not a part of the ANS.

Parasympathetic Nervous System

The PNS generally has long presynaptic nerves that synapse near the effector organs.  PNS neurons originate from cranial nerves (CN III, VII, IX, and X) and in the sacral region of the spinal cord.  The PNS generally controls "rest and digest" functions such as digestion, lacrimation, and salivation.  PNS is more targeted in its signaling than the SNS.  PNS tone causes a slowing of heart rate.  The PNS is also responsible for bladder contraction and relaxation of the bladder sphincter as well as erections among many other functions.

Sympathetic Nervous System

The SNS, in contrast to the PNS, generally has short presynaptic nerves and long postsynaptic nerves.  SNS nerves originate in the thoracic and lumbar spinal regions.  The SNS controls "fight or flight" responses and acts as more of a broad response than a targeted per organ response as in the PNS.  Pupil dilation, bronchial dilation, tachycardia, sphincter contraction, ejaculation, and vasodilation in muscles arterioles are all examples of activation of the sympathetic nervous system.

Enteric Nervous System

The enteric nervous system is a neural complex of the GI system that received modulation from the PNS and the SNS.  

Neurotransmitters and Receptors

The parasympathetic nervous system has a long preganglionic nerve and short postganglionic nerve. The sympathetic nervous system has a short preganglionic nerve and long postganglionic nerve. The somatic nervous system does not have a ganglion.