Autonomic Nervous System

Overview

The Autonomic Nervous System (ANS) is the portion of the peripheral nervous system that regulates subconscious control of visceral organs. The ANS is split into two anatomically distinct subsystems known as the sympathetic nervous system and parasympathetic nervous systems that tend to exert reciprocal regulatory functions on innervated organs. Here we talk about basic themes of the autonomic nervous system and connect to more specific features of the sympathetic and parasympathetic nervous systems, their component neurons, and and the basic classes of autonomic neurotransmitter receptors.

Basic Control

The overarching control of the ANS is carried out by the central nervous system in a variety of centers. The majority of control is likely carried out by the hypothalamus although additional important loci exist within the spinal cord and brain stem. Although functionality of the ANS is not dependent on higher cortical centers, the cortex can also modulate ANS output.

Basic Physiological Anatomy

All autonomic pathways from the CNS to the innervated organ are composed of two-neuron systems. This stands in contrast to skeletal motor pathways in which a single neuron composes the entire pathway.
The cell body of the first neuron, termed the "Preganglionic Neuron", resides within the CNS. Its axons synapse on the cell body of the second neuron, termed the "Postganglionic Neuron", within autonomic ganglia located outside the CNS. The axons of the Post-ganglionic Neuron then directly innervate the target organ.
However, the locations of the pre- and post-ganglionic neurons are distinct between the sympathetic and parasympathetic systems. Sympathetic ganglia tend to lie close to the CNS and thus are composed of short preganglionic neurons and long postganglionic neurons that travel the bulk of the distance to target organs. In contrast, PNS ganglia tend to lie adjacent or directly within the target organs; consequently, preganglionic neurons are long whereas postganglionic neurons are often microscopic in length.

All autonomic ganglia, whether of the sympathetic or parasympathetic, are functionally similar. All preganglionic neurons secrete acetylcholine (Ach) which binds nicotinic receptors on the surface of the postganglionic neurons. Binding of Ach on the nicotinic receptor is excitatory at the ganglion and results in firing of the postganglionic neurons.
However, the neurotransmitters secreted by the post-ganglionic neurons of the sympathetic and parasympathetic systems differ. All parasympathetic postganglionic fibers secrete acetylcholine. Additionally, the receptor on all parasympathetic target organs is the muscarinic receptor.
In contrast, the vast majority of sympathetic post-ganglionic fibers secrete norepinephrine and their target organs display a wide variety of different norephinephrine receptors that include Alpha1, Alpha2, Beta1, and Beta2 Receptors. It should be pointed out that a small minority of postganglionic sympathetic fibers, specifically to the sweat glands, do secrete acetylcholine and in this situation the acetylcholine receptor on the target organ is the muscarinic receptor.

Basic Functionality

The ANS serves to control a wide variety of visceral functions in the absence of conscious control. Nearly all major organs are innervated by autonomic fibers and in most cases by both sympathetic and parasympathetic pathways. The physiological effect that sympathetic and parasympathetic systems exert on any particular organ does not follow a clear system; however, in nearly all cases the sympathetic and parasympathetic pathways exert opposing physiological changes in the organ. We will briefly outline these opposing functions here; however, they are discussed in substantially more detail in the respective sympathetic nervous system and parasympathetic nervous system sections.
We should also point out that the sympathetic and parasympathetic systems are not simply "off" in the resting state; rather, these pathways exert baseline "tone" at all times. Consequently, control of an organ is not solely achieved by simply activating one or the other pathway, but is rather accomplished by increasing the activity of one while inhibiting the baseline "tone" of the other.

Specific Functionality

Here we outline briefly some of the general opposing effects of the sympathetic and parasympathetic pathways on specific target organs. These effects are discussed in more detail on the respective sympathetic nervous system and parasympathetic nervous system pages.

Sympathetic: Inhibits secretion from most glands by causing vasoconstriction; however, increases secretion from sweat glands
Parasympathetic: Promotes secretion from nasal, lacrimal, salivary, intestinal glands.

Sympathetic: Promotes vasoconstriction to increase total peripheral resistance, promotes venoconstriction to enhance preload on the heart
Parasympathetic: Almost no effect on the vasculature

Subtopics

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