Autonomic Nervous System Overview

Study Notes

The ANS controls involuntary functions, such as smooth and cardiac muscles in visceral organs
It has two divisions: sympathetic and parasympathetic
The hypothalamus (part of the limbic system) is the primary control center of the ANS, though influenced by the cingulate gyrus.
The posterior hypothalamus controls the sympathetic nervous system
The anterior hypothalamus controls the parasympathetic nervous system
These hypothalamic centers act via motor centers in the brainstem and spinal cord

Students should understand the arrangement and higher control centers of the sympathetic and parasympathetic systems
Describe the actions of both systems on different organs
Learn where acetylcholine, norepinephrine and epinephrine are secreted
Learn about the adrenal medulla
Learn about adrenergic and cholinergic receptors

Preganglionic nerves exit the CNS from the thoracic and lumbar segments. This outflow is also called "thoracolumbar"
Preganglionic fibers are myelinated
Postganglionic neuron cell bodies are in paravertebral ganglia, separate from target tissues
Postganglionic fibers are unmyelinated and longer than preganglionic fibers
A high degree of divergence exists (preganglionic-to-postganglionic ratio approximately 1:10). This leads to generalized/widespread effects when activated

Preganglionic nerves exit the CNS from the cranium and sacral segments
This outflow is also called "craniosacral"
Preganglionic fibers are myelinated
Postganglionic neuron cell bodies are in parasympathetic ganglia close to or embedded within target tissues
Postganglionic fibers are unmyelinated and shorter than preganglionic fibers
There is little divergence (preganglionic-to-postganglionic ratio approximately 1:3) . This results in more localized effects when activated

Sympathetic: Active during stress (e.g., exercise, fear, anxiety, pain), preparing the body for "fight or flight" . Increases energy production and accelerates metabolism.
Parasympathetic: Active during rest and digestion, saving body energy. Promotes vegetative processes (nutritive, body-building, restorative anabolic functions, tissue repair), digestion, rest, sleep

Acetylcholine (ACh): Secreted by all preganglionic nerves, all postganglionic parasympathetic nerves, and some postganglionic sympathetic fibers (e.g. sweat glands , blood vessels in skeletal muscles)
Norepinephrine (NE): Secreted by most postganglionic sympathetic nerves
Epinephrine (EP): Secreted primarily by the adrenal medulla (80%)

Considered an integral part of the sympathetic nervous system (SNS)
80% of its secretion is epinephrine, and 20% norepinephrine
Epinephrine in the bloodstream originates from the adrenal medulla; postganglionic sympathetic nerves cannot synthesize it.

Organ	Sympathetic Effect	Parasympathetic Effect
Pupil	Dilation	Constriction
Heart	Increased rate, increased force of contraction	Decreased rate
Blood Pressure	Raised	Little or no effect
Lung (Bronchioles)	Dilation	Constriction

Nicotinic: Located in all ganglia (synapses between pre- and postganglionic neurons of both sympathetic and parasympathetic divisions)
Muscarinic: Found on all effector cells innervated by postganglionic parasympathetic fibers, and some postganglionic sympathetic nerves (sweat glands, blood vessels in skeletal muscle)

Alpha (α): Found in iris radial muscles (causing pupil dilation), blood vessels (especially arterioles), and gastrointestinal tract.
Beta (β): Found in the heart, bronchioles, skeletal muscle, and gastrointestinal tract (α and β are mostly stimulated) .Important for the impact on heart rate, breathing, blood vessels, and digestion.

Alpha receptors primarily cause vasoconstriction and increased blood pressure, pupils dilation.
Beta receptors primarily cause increased heart rate and force of contraction, vasodilation and relaxation in many organs.