Autonomic Nervous System
Autonomic nervous system (ANS): system of neurons that innervates smooth and cardiac muscle and glands; involuntary division of the PNS.
I. Overview.
A. ANS.
- versus the somatic nervous system (SNS): have fibers that differ in effectors, efferent pathways, responses of the target organs.
1. Effectors: SNS stimulates skeletal muscle; ANS stimulates cardiac and smooth muscle, and some glands.
2. Efferent Pathways: SNS cell bodies of motor neurons are in the CNS, their axons give rise to spinal nerves, rami, associated plexuses, and travel to effector organs that they serve.
- in the ANS, two motor neuron chain; cell body of the first motor neuron, preganglionic neuron, in the CNS; cell body of postganglionic neuron in ganglion outside CNS, postaganglionic axon extends to the effector organ.
3. Effects on target organs: all somatic motor neurons release ACH at the neuromuscular junction; all preganglionic autonomic motor neurons release ACH at axon terminals; postganglionic motor neurons release norepinephrine or ACH at synapse with effector organs.
B. Divisions of the ANS.
- sympathetic and parasympathetic.
- serve same visceral organs, cause essentially opposite effects - dual innervation.
1. Parasympathetic division: "resting and digesting"; keeps energy use low while maintaining vital functions; fibers originate in the brain and sacral spinal cord; long preganglionic fibers, short postganglionic fibers (ganglia are close to or on the effector organs innervated).
2. Sympathetic Division: "fight or flight" takes over during stressful or emotional situations; fibers originate in the thoracolumbar region of SC; short preganglionic fibers, and long post ganglionic fibers; ganglia are close to the spinal cord.
II. Parasympathetic Division
- preganglionic axons extend from CNS nearly all the ways to structure being innervated; they synapse with the postganglionic neurons very close to or within the target organ.
A. Cranial outflow.
- some cranial nerves have parasympathetic fibers.
- preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus nerves.
- cell bodies in motor nuclei in the brain steam.
B. Sacral flow
- arises form neurons located in the lateral gray matter of spinal cord segments.
- axons of those preganglionic neurons run in ventral roots of spinal cord to ventral rami, then branch to form pelvic splanchnic nerves.
- postganglionic fibers are found in the intramural ganglia of innervated organs (large intestines, bladder).
III. Sympathetic Division.
- besides internal organs, innervates certain visceral structures in the skin (sweat glands, pili muscles).
A. General
- all preganglionic fibers arise from cell bodies located in lateral horns of spinal cord segments.
- leave spinal cord via a ventral root, enters a paravertebral ganglion of sympathetic chain, three things can happen:
1. Synapses with a postganglionic neuron at that paravertebral ganglion.
2. Ascends or descends sympathetic chain to synapse with postganglionic neuron in another paravertebral ganglion.
3. Can pass through paravertebral ganglion without synapsing; synapses with postganglionic neurons in prevertebral ganglion associated with the aorta; postganglionic fibers travel to target organs.
IV. ANS physiology.
A. Nervous tissue and their receptors.
- ACH and norepinephrine (NE) are major neurotransmitters released by ANS neurons.
- all preganglionic axons of the ANS release ACH.
- parasympathetic system postganglionic axons release ACH.
- sympathetic postganglionic axons release NE.
- remember that ACH and NE effects on effectors are not consistent with either excitation or inhibition; response depends on NT receptors present in the effectors.
B. NT receptors.
a. Cholinergic receptors --bind ACH.
- two types, nicotinic and muscarinic.
b. Adrenergic receptors -- bind NE.
- two major classes are alpha (usually stimulatory), and beta (usually inhibitory).
V. Interactions of ANS divisions/ Unique roles of each division.
- recall that visceral organs innervated by both the parasympathetic and sympathetic divisions of the ANS, dual innervation, usually antagonistic; however some structures exclusively innervated by one division or the other.
A. Sympathetic.
- vascular system is mostly innervated by the sympathetic fibers, keeps blood vessels in a state of partial constriction, vasomotor tone.
- adrenal medulla, sweat glands, arrector pilli muscles only receive sympathetic input.
- sympathetic division mediated reflexes help regulate body temperature.
- release of renin from kidneys, increases blood pressure.
- metabolic effects: increases basal metabolic rate, blood glucose, mental alertness.
B. Parasympathetic.
- parasympathetic division effects dominate heart and all smooth muscle of digestive, urinary and respiratory tracts.
- prevents unnecessary heart acceleration
- determines normal activity levels of the gastrointestinal and urinary tracts.
C. Localized versus diffuse effects.
- pattern of parasympathetic division innervation is very specific -- ganglia located in the walls of effector organs.
- pattern of sympathetic innervation is more diffuse, fibers going to many different effectors originate in the same ganglia, therefore, sympathetic effects are more diffuse and the parasympathetic effects are more specific.
- ACH: NT of all postganglionic parasympathetic neurons quickly destroyed, parasympathetic effects are short lived.
- NE: NT of most postganglionic sympathetic neurons, inactivated slowly; acts via second messengers; effects are more diffuse and long-lasting.