Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 343

FIGURE 15.1 (a) Cross-sectional view of whole muscle and the attached tendon. (b) Enlarged view of myofibrils
within a muscle fiber. (c) View of the thick filament (Myosin). (d) View of the thin filament (Actin). (e) Cytoskeletal
components of a myofibril showing cross-bridge arrangement. a-e (Copyright Brooks

Cole — Thomson Learning.)

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15.2.2 Cellular Processes Initiating Muscle Contractions
To initiate a contraction, myofibers must receive stimulation from motor neurons located in the
ventral horn of the spinal cord. Motor neurons are considered the final common pathway where skeletal
muscle activity can be governed only by input from these neurons. When activated, an action potential is
propagated down the nerve axon and terminates on the neuromuscular junction (NMJ). The NMJ is a
space where the action potential cannot cross from the nerve axon to the muscle fibers it innervates.
Thus, a chemical messenger is used to transmit the signal from the nerve axon to the muscle fibers.
As the signal is transmitted down the nerve axon, voltage-gated channels open to release Ca 2þ into