The Muscular System
whole muscle. Stimulation of a single motor unit causes
weak but steady contractions in a broad area of the mus-cle
rather than a strong contraction at one tiny specific point.
Muscles controlling very fine movements (like mus-
cles that move the eye) are characterized by the presence of
only a few muscle fibers in each motor unit. Another way
to state this would be the ratio of nerve fibers to muscle
cells is high. For example, each motor unit pres-ent in the
ocular muscle contains about 10 muscle cells. However,
gross movements (like lifting an object with your hand)
will contain a motor unit with 200 or more muscle cells. On
the average, a single motor nerve fiber innervates about 150
muscle cells.
Muscle cells possess four properties: excitability,
conductivity, contractility, and elasticity. Muscle fibers can
be excited by a stimulus. In our bodies, this stimu-lus is a
nerve cell. In the laboratory, we can stimulate and excite a
muscle with an electrical charge. Besides the property of
excitability, all protoplasm in the mus-cle cell possesses the
property of conductivity, which allows a response to travel
throughout the cell. The type of response will depend on
the type of tissue that is excited. In muscle cells, the
response is a contrac-tion. Elasticity then allows the muscle
cell to return to its original shape after contraction. Muscle
contrac-tion is caused by the interactions of three factors:
neu-roelectrical factors, chemical interactions, and energy
sources.
Potassium ions (K+) greater inside cellSodium ions (Na+) greater outside cell^
Inside of cell is negatively charged and
outside is positively charged electrically
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Neuroelectrical Factors
Surrounding the muscle fiber’s membrane or sarco-lemma
are ions. Refer to Figure 9-2 for the ionic and electrical
distribution. The ionic distribution is such that there is a
greater concentration of potassium ions (K^1 ) inside the cell
than outside the cell, whereas there is a greater
concentration of sodium ions (Na^1 ) outside the cell
membrane than inside the cell. These ions are all pos-
itively charged. Because of an uneven distribution of these
ions, there is an electrical distribution around the muscle
cell. The inside of the cell is negatively charged and the
outside of the cell is positively charged electrically. This
situation is known as the muscle cell’s resting potential.
As the nerve impulse reaches the neuromuscular
junction where the axon terminals of the nerve cell are in
close proximity to the muscle and its numerous cells, it
triggers the axon terminals to release a neurotrans-mitter
substance called acetylcholine (ah-seh-till-KOH-leen).
This chemical substance affects the muscle cell membrane.
It causes the sodium ions (which were kept outside during
the resting potential) to rush inside the muscle cell. This
rapid influx of sodium ions creates an electrical
potential that travels in both directions along the muscle
cell at a rate of 5 meters per second. This in-flux of Na^1
causes the inside of the cell to go from be-ing electrically
negative to being positive. This is a signal to the muscle
cell to generate its own impulse called the action-
potential. This is the signal to contract. MeanwhileNerve cell’s axon endings (^)
(^) Neuromuscular junction
K+
(^)
Na+ Na+ Na+ Na+ (^)
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 (^)
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 (^)
K+^ K+^ K+^ K+^
K+ K+ Na
+
K+^ K+^
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Na+^ Na+^ Na+^ Na+^
K+
Figure 9- 2 Ionic and neuroelectrical factors affecting the skeletal muscle cell.
Muscle cell’s sarcolemma(^) ®
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