Page 200 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 185

hormone produces contraction only of strength. For example, stimulation of β ‐
2
adrenergic receptors on airway smooth
organs with receptors for that specific hor-
VetBooks.ir mone. Calcium entering from the outside muscle in the lungs produces relaxation
via the channels may stimulate the release
and an increase in airway diameter. In gen-
of further calcium from the sarcoplasmic eral, this type of relaxation is due to either
reticulum, which further strengthens the a reduction in the number or availability
contractions. of calcium channels in the cell membrane
An understanding of the role of calcium or a reduction in the phosphorylation of
channels in the outer cell membrane of myosin within the myofilaments of smooth
smooth muscle cells led to the develop- muscle cells.
ment of a group of drugs termed calcium
channel blockers. These agents are
capable of binding to and inactivating Action Potentials and Slow Waves
these calcium channels, and this tends to
reduce the strength of contractions. Not all smooth muscle cells have action
Smooth muscle cells in the walls of many potentials on their cell membranes during
arterial blood vessels have these types of contraction and relaxation. Cells with
channels, and this type of drug has been predominantly ligand‐gated calcium chan-
proved effective in lowering blood pres- nels in their cell membranes may undergo
sure in both humans and animals. a contraction–relaxation cycle without
When cytosolic Ca increases within action potentials. The cell membrane may
2+
smooth muscles, calcium ions bind to a depolarize slightly because of the entry of
regulatory protein (calmodulin) (Fig. 9‐9). calcium, but no action potential is seen.
Unlike skeletal muscle, whose regulatory Smooth muscle cells that do have action
proteins are associated with the actin fila- potentials during contraction have both
ments, the calmodulin in smooth muscle is sodium and calcium electrically gated
associated with the heads of myosin mole- channels in their cell membrane.
cules in myosin filaments. The binding of Action potentials in skeletal muscle
calcium to calmodulin activates a kinase, occur after the binding of ACh to its cell
also associated with myosin, and this kinase membrane receptors. In smooth muscle,
phosphorylates other sites on myosin. action potentials can be elicited by a variety
This phosphorylation ultimately results in of stimuli. Some smooth muscle is similar
contraction, so myosin phosphorylation to skeletal muscle in that action potentials
is a key regulatory step (Fig. 9‐9). This is in occur only after the binding of ligands to
contrast to skeletal muscle, whose myosin cell membrane receptors, but action poten-
does not require phosphorylation. For tials may also occur in smooth muscle in
relaxation, the phosphorylation of myosin response to mechanical stretch or during
is reversed by other intracellular enzymes slow‐wave electrical activity. The smooth
that are always present and active (Fig. 9‐9). muscle that responds to stretch is believed
This reversal and relaxation can occur to have membrane channels that are subject
when the cytosolic calcium concentration to mechanical stimulation.
is reduced. Calcium is removed from the The visceral or single‐unit smooth
cytosol by the transport back into the muscle in some organs (the gastrointestinal
sarcoplasmic reticulum or out of the cell. tract being the classic example) exhibit a
Most of this transport is done by an active unique type of membrane electrical activity
transport system (Ca ‐ATPase) on the termed slow waves. These are waves, or
2+
membrane of the endoplasmic reticulum periods, of spontaneous fluctuations in the
and the outer cell membrane. resting membrane potential that spread
Unlike skeletal muscle, some smooth throughout a body of smooth muscle and
muscle responds to certain stimuli by typically occur at some regular rhythm
relaxation or a reduction in contraction (Fig. 9‐10). Slow waves alone do not cause