Chapter 15 - Autonomic Nervous System

Somatic nervous system

innervates skeletal mm. (voluntary), excitation (m. contracts), neurotransmitter acetylcholine, reflexes (patellar reflex, stretch reflex, withdrawal reflex)

Autonomic nervous system

innervates cardiac m., smooth m., and glands (involuntary), excitation or inhibition, neurotransmitter acetylcholine or norepinephrine

Preganglionic neuron secretes

acetylcholine

Postganglionic neuron secretes

acetylcholine or norepinephrine

Parasympathetic NS anatomy

originates from cranial and sacral region of spinal cord, ganglia are near the organs, long preganglionic neuron (ACh), short postganglionic neuron (ACh), vagus n. provides 75% of activity

Sympathetic NS anatomy

originates from the thoracic and lumbar regions of spinal cord (sympathetic trunk), short preganglionic neuron (ACh), long postganglionic neuron (ACh or NE)

Splanchic n.

does not synapse in sympathetic chain but rather celiac ganglia and superior and inferior mesenteric ganglia, function is to reduce blood flow and energy use by digestive organs

Adrenal medulla

modified sympathetic ganglion, pre-ganglionic fibers synapse on neuroendocrine cells, secretes epinephrine (adrenaline, 75%) and norepinephrine (25%), same action as sympathetic but affects most cells and is longer lasting

Sympathetic NS function

fight, flight, or freeze response (heightened mental alertness, increased metabolic rate, reduced digestive and urinary function, increased respiratory rate and dilation of passageways, increased HR and BP, increased sweating

Parasympathetic NS function

controls housekeeping body activities (SLUDGE), decreased metabolic rate, decreased HR and BP

SLUDGE

Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis

Adrenergic

neurons that release NE

NE is broken down by

MAO in the synaptic cleft and COMT (low levels) in surrounding areas

longer effect of NE in bloodstream due to

no presence of MAO and COMT

alpha 1

excitatory, causes release of intracellular calcium in endoplasmic reticulum (smooth muscle's version of sacroplasmic reticulum), stimulates smooth muscle contraction (blood vessels, digestive and urinary sphincters)

alpha 2

inhibitory, when sympathetic division is active, it binds to the NE released at parasympathetic junctions and inhibits activity

beta 1

excitatory, increases metabolic activity in skeletal muscle, increases HR and force of contraction

beta 2

inhibitory, relaxation of smooth muscle along respiratory tract to make breathing easier

Pre-ganglionic neurotransmitter

ACh

Receptor on post-ganglionic neuron

Nicotinic

Parasympathetic post-ganglionic neurotransmitter

ACh

Sympathetic post-ganglionic neurotransmitter

NE

Parasympathetic receptor on organ

Muscarinic

Sympathetic receptor on organ

a1, a2, b1, b2

Cholinergic

neurons that release ACh

ACh inactivated by

AChE, in the synaptic cleft

Nicotinic receptors

bind ACh secreted by preganglionic neurons, ACh causes excitation by opening Na+ chemically gated channels

Muscarinic receptors

excitatory or inhibitory, doesn't directly open ion channel (depends on which enzyme is activated/inactivated), stimulates saliva in PSNS, stimulates sweating in SNS

Sympathetic NS impact

widespread, many post-gang neurons result in amplification

Parasympathetic NS impact

limited to visceral organs (few post-gang organs)

Dual-intervention

some organs receive instructions from both sympathetic and parasympathetic systems (heart, though arterioles are only sympathetic)

Control on ANS

regulated by hypothalamus, which may coordinate with other areas of the brain (such as the medulla oblongata to regulate cardiac activities)

Agonists

bind to receptors and stimulate them

Antagonists

bind to receptors and block them

Nicotine

binds to nicotinic receptor, increases BP due to its action in sympathetic NS

Ganglionic blocking agents

binds to nicotinic receptor, blocks the effect of ACh in both parasympathetic and sympathetic divisions

Muscarinic agents

binds to muscarinic receptor, activates both parasympathetic division and sweating

Muscarinic blocking agents

binds to muscarinic receptor, blocks parasympathetic effects (ex. atropine)

Adrenergic agents

binds to alpha and beta receptors, sympathomimetic

Alpha and beta adrenergic blocking agents

binds to alpha and beta receptors, treats hypertension (ex. metroprolol to block beta 1)