Page 83 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Smooth Muscle pendent and, in many cases, spontaneous (my-
ogenic tonus). The second type, multi-unit
Smooth muscle (SmM) consists of multiple SmM, contracts primarily due to stimuli from
layers of spindle-shaped cells. It is involved in the autonomic nervous system (neurogenic
the function of many organs (stomach, in- tonus). This occurs in structures such as the
testine, gall bladder, urinary bladder, uterus, arterioles, spermatic ducts, iris, ciliary body,
bronchi, eyes, etc.) and the blood vessels, and the muscles at the roots of the hair. Since
where it plays an important role in circulatory these SmM cells generally are not connected
control. SmM contains a special type of F- by gap junctions, stimulation remains local-
Nerve and Muscle, Physical Work and no sarcomeres (nonstriated). It is there- degree of depolarization (e.g., through stretch
actin-tropomyosin and myosin II filaments
ized, as in the motor units of the skeletal
muscle.
(! p. 60), but lacks troponin and myofibrils.
Furthermore, it has no distinct tubular system
Smooth muscle tonus is regulated by the
or pacemaker cells) as well as by transmitter
fore called smooth muscle because of this lack
substances (e.g., acetylcholine or noradrena-
of striation (see p. 59 A for further differences
line) and numerous hormones (e.g., estrogens,
in the muscle types). SmM filaments form a
progesterone and oxytocin in the uterus and
loose contractile apparatus arranged approxi-
histamine, angiotensin II, adiuretin, serotonin
mately longitudinally within the cell and at-
tached to discoid plaques (see B for model),
and bradykinin in vascular muscle). An in-
which also provide a mechanical means for
directly or indirectly increases the cytosolic
shorten much more than striated muscle.
concentration to more than 10
mol/L.
– 6
Ca
2+
2 cell–cell binding of SmM. Smooth muscle can crease in tonus will occur if any of these factors
The membrane potential of the SmM cells of
influx comes mainly from extracellu-
2+
The Ca
many organs (e.g., the intestine) is not con- lar sources, but a small portion comes from in-
stant, but fluctuates rhythmically at a low tracellular stores (! B1). Ca 2+ ions bind to cal-
frequency (3 to 15 min ) and amplitude (10 to modulin (CM) (! B2), and Ca -CM promotes
2+
– 1
20 mV), producing slow waves. These waves contraction in the following manner.
trigger a burst of action potentials (spikes) Regulation at myosin II (! B3): The Ca -CM
2+
when they exceed a certain threshold poten- complex activates myosin light chain kinase
tial. The longer the slow wave remains above (MLCK), which phosphorylates myosin’s regu-
the threshold potential, the greater the num- latory light chain (RLC) in a certain position,
ber and frequency of the action potentials it thereby enabling the myosin head to interact
produces. A relatively sluggish contraction oc- with actin (! B6).
curs around 150 ms after a spike (! p. 59 A, left Regulation at the actin level (! B4). The
panel). Tetanus occurs at relatively low spike Ca -CM complex also binds with caldesmon
2+
frequencies ( ! p. 66). Hence, SmM is con- (CDM), which then detaches from the actin–
stantly in a state of a more or less strong con- tropomyosin complex, thus making it available
traction (tonus or tone). The action potential of for filament sliding (! B6). Phosphorylation of
SmM cells of some organs has a plateau similar CDM by protein kinase C (PK-C) also seems to
to that of the cardiac action potential be able to induce filament sliding (! B5).
(! p. 59 A, middle panel). Factors that lead to a reduction of tonus are:
There are two types of smooth muscles reduction of the cytosolic Ca 2+ concentration
(! A). The cells of single-unit SmM are electri- to less than 10 – 6 mol/L (! B7 ), phosphatase
cally coupled with each other by gap junctions activity (! B8), and PK-C if it phosphorylates
(! pp. 18 and 50). Stimuli are passed along another position on the RLC (! B9).
from cell to cell in organs such as the stomach, When length–force curves are recorded for
intestine, gallbladder, urinary bladder, ureter, smooth muscle, the curve shows that muscle
uterus, and some types of blood vessels. force decreases continuously while muscle
Stimuli are generated autonomously from length remains constant. This property of a
within the SmM, partly by pacemaker cells). In muscle is called plasticity.
70 other words, the stimulus is innervation-inde-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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