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enough for it to be effective), the speed of application (a
strong stimulus applied quickly and quickly pulled away
may not have time enough to take effect even though it is
quite strong), the weight of the load (one can pick up a
waste basket with one hand but not a din-ing room table),
and, finally, the temperature (-muscles operate- best at
normal body temperature 37°C or 98.6°F in humans). A
stimulus strong enough to elicit a re-sponse in an individual
muscle cell will produce maxi-mal contraction. The
contraction either occurs or it does not. This is known as
the all-or-none law.
Muscle Tone
Tone is defined as a property of muscle in which a steady
or constant state of partial contraction is maintained in a
muscle. Some muscle cells in a particular muscle will
always be contracting while other muscle cells are at rest.
Then those at rest will contract, while those that were
contracting will go into relaxation. This allows us, for ex-
ample, to maintain body posture for long periods of time
without showing any evidence of tiring. This is accom-
plished because nerve stimuli alternate between various
groups of muscle cells, thus allowing all to have peri-ods of
rest. Tone results in skeletal muscles exhibiting a certain
degree of firmness as they maintain a slight and steady pull
on attached bones. Tone maintains pressure on abdominal
contents, maintains blood pressure in ar-teries and veins,
and assists in digestion in the stomach and intestines.
There are two types of contraction. When lifting a
weight, muscles become shorter and thicker. In this type of
contraction, tone or tension remains the same and is
referred to as isotonic contraction. When we push
against a wall or attempt to lift a huge boulder, the muscles
in-volved remain at a constant length while the tension
against the muscle increases, and this is known as iso-
metric contraction. From this fact, a whole series of ex-
ercises have been developed called isometric exercises (like
locking fingers of opposite hands and pulling to de-velop
the biceps). These exercises help develop tone or firmness
in muscles.
The Anatomy of Smooth
Muscle
Smooth muscle is found in hollow structures of the body
like the intestines, blood vessels, and urinary bladder. It
cannot be controlled at will because it is under the
Chapter 9control of the autonomic nervous system and also may be
hormonally stimulated. Each smooth muscle cell contains a
single large nucleus and because its fiber is more delicate
than skeletal muscle, cross-striation of the myosin and actin
arrangements is not visible. The cells connect by fibrils
extending from one cell to another closely adjoining cell. In
hollow structures like the small intestine, the smooth
muscle is arranged in two layers, an outer longitudinal layer
and an inner circular layer. Contraction of these two layers,
with the circular layer contracting first, results in reducing
both the length of the tube and the circumference of the
tube. This contrac-tion pushes whatever is in the tube in a
forward direc-tion, for example, digested food or chyme in
the intestine or blood in the arteries and veins. Smooth
muscle cells produce a slower contraction than skeletal
muscle, but smooth muscle contraction allows greater
extensibility of the muscle.The actin and myosin fibers are not so regularly ar-
ranged in smooth muscle as in striated muscle. There-fore,
contraction occurs in a similar way but without the regular
rearrangement of the fibrils. The fibrils do slide together
and rhythmically shorten the cell, but a slow wave of
contraction passes over the entire muscle mass as the nerve
impulse reaches a cell and gets transmitted to the remainder
of the cells or fibers.The Anatomy of Cardiac
Muscle
Cardiac muscle cannot be influenced at will because it,
like smooth muscle, is under the control of the au-tonomic
nervous system. It is uninucleated, similar to smooth
muscle; however, it is striated like skeletal mus-cle.
Cardiac muscle also has another unique quality. If one
muscle cell is stimulated, all the muscle cells or fibers are
stimulated so all the muscle cells contract to-gether. Also,
the muscle cell that contracts the fastest will control the
speed of other muscle cells, causing them all to contract at
the faster rate.
The rapid rhythm of cardiac muscle is the result of a
special property of this type of cell to receive an impulse,
contract, immediately relax, and then receive another
impulse. These events all occur about 75 times a min-ute.
However, the period of an individual contraction is slower
in cardiac (about 0.8 second) as opposed to skel-etal
muscle, which is much faster (about 0.09 second).
If rapid, uncontrolled contraction of individual cells in
the heart occurs, this is called fibrillation. This results