ABSTRACT
Actin attaches to the Z-lines at the end of each sarcomere, while myosin attaches to the M-lines in the center of the sarcomeres.12 The A (dark) band includes the region where actin and myosin overlap. The I (light) band only contains actin. The H zone contains only myosin. During muscle contraction, the H zone and I band shorten.7,12
The process of muscle contraction begins when an alphamotor neuron depolarizes, releasing acetylcholine (Ach) into the neuromuscular junction, which binds to receptors on the muscle cell membrane (sarcolemma), causing the muscle cell to depolarize. Depolarization is rapidly transmitted into the interior of the muscle cell via the transverse tubules, which signals the sarcoplasmic reticulum to release calcium (Ca++) inside the muscle cell (Figure 14.1). The Ca++ binds to troponin causing a shift in the tropomyosin position, uncovering the actin lament’s myosin-binding sites. A myosin head, with an attached adenosine triphosphate (ATP) molecule, then binds to the actin lament. When this occurs, the ATP is hydrolyzed to adenosine diphosphate (ADP) by the enzyme ATPase. This chemical reaction releases energy, which is used to ex the myosin head, which slides the actin lament across the myosin lament (sliding-lament theory), causing muscle contraction. The ADP is released from the myosin lament, allowing a new ATP molecule to attach to the myosin head, and the
14.1 Muscle Microanatomy and Physiology .............................................................................................................................117 14.2 Metabolic Pathways in Exercise .......................................................................................................................................118 14.3 Exercise Concepts .............................................................................................................................................................119 14.4 Lactate Kinetics ............................................................................................................................................................... 120 14.5 Delayed Onset Muscle Soreness (DOMS) ....................................................................................................................... 121 14.6 Flexibility and Stretching Techniques ............................................................................................................................. 121 14.7 Exercise and the Cardiovascular System ......................................................................................................................... 122 14.8 Exercise and the Respiratory System .............................................................................................................................. 122 14.9 Physiologic Changes with Aging ..................................................................................................................................... 122 References ................................................................................................................................................................................. 123
TABLE 14.1 Key Clinical Considerations
myosin detaches from the actin. The ATP is again converted to ADP, and the energy from this reaction allows the myosin head to return to its original position in preparation for another contraction. Thus, two ATPs are required for each contraction cycle.7,12
A motor unit is made up of a single alpha motor neuron and all the muscle cells it innervates. The number of muscle cells per motor unit varies from tens to thousands. Smaller motor units are involved in ne motor movements, while large motor units are involved in gross motor activities. When a motor unit is activated, all of the muscle cells it innervates contract. This is called the “all or none law.”7,8,12
Muscles have special sensory organs called muscle spindles, which are innervated by gamma motor neurons. Muscle spindles are intrafusal muscle bers that sense and respond to muscle position and movement. Rapid stretching of the muscle is sensed by the muscle spindles, and they signal the muscle to contract and resist the rapid stretch (e.g., patellar tendon reex). Golgi tendon organs are located inside of the tendons. When excessive muscle force is sensed by the golgi tendon organ, it causes the muscle to relax, which protects the muscle from tearing.12
