Muscle
287
nels in the plasma membrane results in an increased fl ow of
calcium into the cell. Because of the small cell size, the enter-
ing calcium does not have far to diffuse to reach binding sites
within the cell.
Removal of calcium from the cytosol to bring about
relaxation is achieved by the active transport of calcium back
into the sarcoplasmic reticulum as well as out of the cell across
the plasma membrane. The rate of calcium removal in smooth
muscle is much slower than in skeletal muscle, with the result
that a single twitch lasts several seconds in smooth muscle
compared to a fraction of a second in skeletal muscle.
The degree of activation also differs between muscle
types. In skeletal muscle a single action potential releases suffi -
cient calcium to saturate all troponin sites on the thin fi laments,
whereas only a portion of the cross-bridges are activated in a
smooth muscle fi
ber in response to most stimuli. Therefore,
the tension generated by a smooth muscle cell can be
graded
by varying cytosolic calcium concentration. The greater the
increase in calcium concentration, the greater the number of
cross-bridges activated, and the greater the tension.
In some smooth muscles, the cytosolic calcium con-
centration is suffi cient to maintain a low level of basal cross-
bridge activity in the absence of external stimuli. This activity
is known as
smooth muscle tone.
Factors that alter the cyto-
solic calcium concentration also vary the intensity of smooth
muscle tone.
As in our description of skeletal muscle, we have
approached the question of excitation-contraction coupling
in smooth muscle by fi rst describing the coupling (the changes
in cytosolic calcium). Now we must back up a step and ask
what constitutes the excitation that elicits these changes in
calcium concentration.
Membrane Activation
In contrast to skeletal muscle, in which membrane activation
is dependent on a single input—the somatic neurons to the
muscle—many inputs to a smooth muscle plasma membrane
can alter the contractile activity of the muscle (
Table 9–5
).
Some of these increase contraction, while others inhibit it.
Moreover, at any one time, the smooth muscle plasma mem-
brane may be receiving multiple inputs, with the contractile
state of the muscle dependent on the relative intensity of the
various inhibitory and excitatory stimuli. All these inputs
infl uence contractile activity by altering cytosolic calcium con-
centration as described in the previous section.
Some smooth muscles contract in response to membrane
depolarization, whereas others can contract in the absence
of any membrane potential change. Interestingly, in smooth
muscles in which action potentials occur, calcium ions, rather
than sodium ions, carry a positive charge into the cell during
the rising phase of the action potential—that is, depolariza-
tion of the membrane opens voltage-gated calcium channels,
producing calcium-mediated rather than sodium-mediated
action potentials.
Another important point applies to electrical activity and
cytosolic calcium concentration in smooth muscle. Unlike the
situation in skeletal muscle, in smooth muscle cytosolic cal-
cium concentration can be increased (or decreased) by graded
depolarizations (or hyperpolarizations) in membrane poten-
tial, which increase or decrease the number of open calcium
channels.
Spontaneous Electrical Activity
Some types of smooth muscle cells generate action poten-
tials spontaneously in the absence of any neural or hormonal
input. The plasma membranes of such cells do not maintain
a constant resting potential. Instead, they gradually depo-
larize until they reach the threshold potential and produce
an action potential. Following repolarization, the membrane
again begins to depolarize (
Figure 9–36a
), so that a sequence
of action potentials occurs, producing a rhythmic state of con-
tractile activity. The membrane potential change occurring
during the spontaneous depolarization to threshold is known
as a
pacemaker potential.
Other smooth muscle pacemaker cells have a slightly
different pattern of activity. The membrane potential drifts
up and down due to regular variation in ion fl
ux across the
membrane. These periodic fl uctuations are called
slow waves
(
Figure 9–36b
). When an excitatory input is superimposed,
as when food enters a segment of the gastrointestinal tract,
slow waves are depolarized above threshold, and action poten-
tials lead to smooth muscle contraction.
Pacemaker cells are found throughout the gastrointesti-
nal tract, and thus gut smooth muscle tends to contract rhyth-
mically even in the absence of neural input. Some cardiac
muscle fi bers and a few neurons in the central nervous system
also have pacemaker potentials and can spontaneously gener-
ate action potentials in the absence of external stimuli.
Nerves and Hormones
The contractile activity of smooth muscles is infl uenced by
neurotransmitters released by autonomic nerve endings. Unlike
skeletal muscle fi bers, smooth muscle cells do not have a spe-
cialized motor end-plate region. As the axon of a postgangli-
onic autonomic neuron enters the region of smooth muscle
Table 9–5
Inputs Infl uencing Smooth Muscle
Contractile Activity
1. Spontaneous electrical activity in the plasma membrane of
the muscle cell
2. Neurotransmitters released by autonomic neurons
3. Hormones
4. Locally induced changes in the chemical composition
(paracrine agents, acidity, oxygen, osmolarity, and ion
concentrations) of the extracellular fl uid surrounding the cell
5. Stretch
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