Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 344

the terminal button of the NMJ. This facilitates the release of the chemical messenger Acetylcholine
(ACH) that crosses the space to the motor end plate. This causes an ionic shift, which results in the
propagation of the action potential down the basement membrane of the muscle fiber and then down
the T-tubules of the muscle cell (Figure 15.2). The action potential activates the voltage-gated dihydro-
pyridine receptors in the T-tubule. This change in the T-tubules triggers the opening of Ca 2þ release
channels (ryanodine receptors) on the SR. Ca 2þ leaves the SR through the ryanodine receptors, enters
the cytoplasm, and binds to troponin, one of the proteins on the thin filaments.
Troponin has three polypeptide units; one binds to tropomyosin, one binds to actin, and a third one
binds to Ca 2þ . Under resting conditions, tropomyosin is bound to actin and it blocks the myosin-binding
site on the actin protein, preventing the formation of cross-bridges (Figure 15.3). However, when free
Ca 2þ rises in the cytoplasm of a myofiber, it binds to troponin and tropomyosin is pulled away
from the myosin-binding site on actin, leaving it open for cross-bridge formation. Once cross-bridges
are formed, the ATPase located on the myosin head increases its activity and hydrolyzes ATP. This
causes the cross-bridge to break, and Ca 2þ then dissociates from its binding site on tropomyosin.
When Ca 2þ is removed, tropomyosin slides back into the blocking position and the muscle relaxes.
Thus, troponin and tropomyosin are referred to as regulatory proteins in muscle contraction
(Figure 15.3).

15.2.3 Force Generation and Transmission in Skeletal Muscle
One of the main functions of skeletal muscle is to generate and transmit force. Force, or muscle tension,
is directly related to the number of actin and myosin cross-bridges that are formed and the frequency of
stimulation. A single action potential results in a single muscle contraction referred to as “twitch.” As the
frequency of stimulation increases, the resultant twitch tension (Figure 15.4a) increases with increasing
stimulation frequency (Figure 15.4b) until a force plateau results (Figure 15.4c). Force is produced at
each attached cross-bridge, so the total force development is proportional to the number of attached
cross-bridges. The number of cross-bridges that can be formed depends upon the degree of overlap
between the thin and thick filaments (Figure 15.5). When a sarcomere is overstretched or compressed,

FIGURE 15.2 The T-tubules and SR in relationship to the myofibrils. (Copyright Brooks Cole — Thomson

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Learning, 2001.)