430
UNIT THREE
Terminating Autonomic
Neurotransmitter Actions
The enzyme acetylcholinesterase rapidly decomposes the
acetylcholine that cholinergic f bers release. (Recall that this
decomposition also occurs at the neuromuscular junctions oF
skeletal muscle, chapter 9, p. 293.) Thus, acetylcholine usu-
ally aFFects the postsynaptic membrane For only a Fraction oF
a second.
Much oF the norepinephrine released From adrenergic
f bers is removed From the synapse by active transport back
into the nerve endings. The enzyme monoamine oxidase, in
The two major types oF adrenergic receptors are
alpha
and
beta.
Exciting them elicits diFFerent responses in the
eFFector organs. ±or example, stimulation oF the alpha
receptors in vascular smooth muscle causes vasoconstric-
tion, whereas stimulation oF the beta receptors in bronchial
smooth muscle causes relaxation leading to bronchodila-
tion. ±urthermore, although norepinephrine has a somewhat
stronger eFFect on alpha receptors, both oF these mediators
can stimulate both types oF receptors. Consequently, the way
each oF these adrenergic substances infl
uences eFFector cells
depends on the relative numbers oF alpha and beta receptors
in the cell membranes.
TABLE
11.10
|
Ef
ects oF Autonomic Stimulation on Various Visceral Ef
ectors
Ef
ector Location
Response to Sympathetic Stimulation
Response to Parasympathetic Stimulation
Integumentary system
Apocrine glands
Increased secretion
No action
Eccrine glands
Increased secretion (cholinergic ef
ect)
No action
Special senses
Iris oF eye
Dilation
Constriction
Tear gland
Slightly increased secretion
Greatly increased secretion
Endocrine system
Adrenal cortex
Increased secretion
No action
Adrenal medulla
Increased secretion
No action
Digestive system
Muscle oF gallbladder wall
Relaxation
Contraction
Muscle oF intestinal wall
Decreased peristaltic action
Increased peristaltic action
Muscle oF internal anal sphincter
Contraction
Relaxation
Pancreatic glands
Reduced secretion
Greatly increased secretion
Salivary glands
Reduced secretion
Greatly increased secretion
Respiratory system
Muscles in walls oF bronchioles
Dilation
Constriction
Cardiovascular system
Blood vessels supplying muscles
Constriction (alpha adrenergic)
Dilation (beta adrenergic)
No action
Blood vessels supplying skin
Constriction
No action
Blood vessels supplying heart (coronary arteries)
Constriction (alpha adrenergic)
Dilation (beta adrenergic)
No action
Muscles in wall oF heart
Increased contraction rate
Decreased contraction rate
Urinary system
Muscle oF bladder wall
Relaxation
Contraction
Muscle oF internal urethral sphincter
Contraction
Relaxation
Reproductive systems
Blood vessels to penis and clitoris
No action
Dilation leading to erection oF penis and
clitoris
Muscles associated with internal reproductive organs
Male ejaculation, Female orgasm
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