Download Copyright © 2006 Pearson Education, Inc., publishing as Benjamin

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Contraction of Smooth Muscle
Whole sheets of smooth muscle exhibit slow,
synchronized contraction
How: They contract in unison, reflecting their
electrical coupling with gap junctions
Skeletal muscle cells are electrically isolated from
one another with each stimulated by its own
neuromuscular junction
In smooth muscle, gap junctions transmit action
potentials from cell to cell
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Contraction of Smooth Muscle
Some smooth muscle cells:
Act as pacemakers and set the contractile pace for
whole sheets of muscle
Are self-excitatory and depolarize without external
stimuli
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Contraction Mechanism
Actin and myosin interact according to the sliding filament
mechanism
The final trigger for contractions is a rise in intracellular
Ca2+
Sliding process is energized by ATP
Ca2+ is released from the SR and from the extracellular
space
Ca2+ interacts with the regulatory protein calmodulin (a
cytoplasmic Ca++ binding protein)
Calmodulin interacts with myosin kinase (myosin light
chain kinase) which phosphorylates the myosin heads
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Role of Calcium Ion
Ca2+ binds to calmodulin and activates it
Activated calmodulin activates the kinase enzyme
Activated kinase transfers phosphate from ATP to
myosin cross bridges
Phosphorylated cross bridges interact with actin to
produce shortening
Smooth muscle relaxes when intracellular Ca2+
levels drop
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Special Features of Smooth Muscle
Contraction
Unique characteristics of smooth muscle include:
Slow acting ATPases compared to skeletal muscle
Slow, prolonged contractile activity
Smooth muscle has low energy requirements
ATP from aerobic pathways
Response to stretch
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Regulation of Contraction
Action potentials are generated by neurotransmitter
binding coupled to an increase in Ca++ in the
sarcoplasm
Different neurotransmitter than found in skeletal
muscles’ acetylcholine (norepinephrine)
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Hormones and Local Factors Affecting Contraction
Depolarized spontaneously or by chemical stimuli
without an action potential
E.g. hormones, lack of O2, histamine, excess CO2,
low pH
E.g. gastrin affects stomach muscles
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Response to Stretch
Smooth muscle exhibits a phenomenon called
stress-relaxation response in which:
Stretching provokes contraction, but only briefly
and then it adapts to new length and relaxes
E.g. adaptation prevents expulsion of contents in
the gut
E.g. urinary bladder to fill prior to release
Allows time for digestion/absorption of nutrients
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Hyperplasia
Certain smooth muscles can divide and increase
their numbers by undergoing hyperplasia
This is shown by estrogen’s effect on the uterus
At puberty, estrogen stimulates the synthesis of
more smooth muscle, causing the uterus to grow to
adult size
During pregnancy, estrogen stimulates uterine
growth to accommodate the increasing size of the
growing fetus
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Smooth Muscle
Varies in:
Fiber arrangement and organization
Responsiveness to various stimuli
Innervation
Single Unit vs. Multi Unit
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Types of Smooth Muscle: Single Unit
The cells of single-unit smooth muscle, commonly
called visceral muscle:
Contract rhythmically as a unit
Are electrically coupled to one another via gap
junctions
Often exhibit spontaneous action potentials
Are arranged in opposing sheets and exhibit stressrelaxation response
Contractile force is proportional to [Ca++]
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Smooth Muscle: Multiunit
Multiunit smooth muscles are found:
In large airways to the lungs
In large arteries
In arrector pili muscles attached to hair follicles
In the internal eye muscles (iris and ciliary process)
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Types of Smooth Muscle: Multiunit
Their characteristics include:
Rare gap junctions
Infrequent spontaneous depolarizations
Structurally independent muscle fibers
A rich nerve supply, which, with a number of muscle fibers,
forms motor units
Responds to neural stimulation with graded contractions
involving recruitment
Innervated by the autonomic (involuntary) nervous system
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Table 9.3.1
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Table 9.3.2
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Table 9.3.3
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Table 9.3.4