Page 198 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 183

Like skeletal muscle, smooth muscle
regulated (e.g., gastrointestinal tract, blood cells can hypertrophy to increase the size
vessels, reproductive tract). About 99% of
VetBooks.ir the smooth muscle of the animal body is of organs. However, unlike skeletal muscle,
smooth muscle cells can also divide mitot-
of the visceral type, which is also called
single‐unit or unitary smooth muscle. ically to increase the number of cells. For
Cells of this type of smooth muscle are example, the increase in size of the uterine
joined with gap junctions to provide wall during pregnancy to several times its
mechanical and electrical connections nonpregnant volume is due in considerable
between cells. The electrical connections measure to an increase in the amount of
permit the propagation of action potentials smooth muscle in the wall. Some of the
directly from cell to cell. The spreading of increase is due to an increase in size of
action potentials and the contraction that individual muscle fibers, but there is also
is elicited allow a group of muscle cells to an increase in the number of cells. This
act together as a single unit. This type of hypertrophy and hyperplasia in the uterus
activity is appropriate for generalized are under the influence of reproductive
activities of an entire organ, such as the hormones. Late in pregnancy there is
contraction of the stomach. also a change in the structure of the cell
About 1% of the smooth muscle of the membranes of uterine smooth muscle
animal body is multiunit smooth muscle cells. An increase in gap junctions between
(e.g., iris and ciliary body of the eye and cells in late pregnancy sets the stage for
pilomotor fibers that erect the hair in the the mechanical and electrical coupling
skin). In this type of smooth muscle, the between smooth muscle cells that will be
contraction of each individual smooth necessary for normal parturition. In gen-
muscle cell is more dependent on its auto- eral, the structure and function of smooth
nomic innervation. Gap junctions are not muscle are more subject to modification by
prevalent between these cells. external factors, such as hormones, than
are those of skeletal muscle.
Structure
Stress–Relaxation
The smooth muscle cell is a fusiform
(spindle‐shaped) contractile unit with a Smooth muscle exhibits a special property
central nucleus. Size of smooth muscle called stress–relaxation, or plasticity.
fibers varies considerably. Most cells are This is the ability to adjust to stretching
50 to 250 μm long and 5 to 10 μm in great- without increasing the final tension or the
est diameter. The major portion of the cell pressure exerted on the contents within a
consists of sarcoplasm. No cross‐striations, hollow viscus surrounded by smooth mus-
myofibrils, or sarcolemma are easily cle. As the muscle stretches, the tension
visible with the light microscope (see increases at first, but then in a few seconds
Fig. 1‐10). Filaments are present as actin or a few minutes the smooth muscle relaxes
and myosin molecules, but there are no again to its original tension, even though it
orderly arrangements to form striations. is still elongated.
Interactions between actin and myosin This stress–relaxation occurs in the
filaments (i.e., sliding filaments) are stomach when it is filling with food, in the
believed to be the basis for smooth muscle intestines as the processed food moves
contractions, but the characteristic short- along, in the blood vessels when the blood
ening of sarcomeres is not seen because volume increases, in the urinary bladder as
there is no distinct organization of the it increases its volume of urine, and in the
filaments. A sarcoplasmic reticulum, which uterus as pregnancy develops.
accumulates calcium, is present, but it is Plasticity allows expansion of stretch
not as extensive or as highly organized as within physiologic limits without an
in skeletal muscle. increase in pressure and without pain; the