Coupon Accepted Successfully!


The Mechanism and Regulation of Muscle Contraction

As noted previously, the sarcomere is the functional unit of muscle contraction. The thin filaments are composed of globular actin protein subunits polymerized to form a long chain, two of which are wrapped around each other to form the filament. Thick filaments, on the other hand, are composed of two myosin chains wrapped around each other; each chain is a long, fibrous protein, with an exposed globular “head” at its end and many globular “cross-bridges” protruding along its length. Given the arrangement of thick and thin filaments in the sarcomere, how can we explain and understand the contraction of a muscle fiber?
One of the remarkable aspects of muscle fiber contraction is that although the entire muscle fiber becomes shorter during contraction, nothing actually gets shorter at the molecular level. Instead, the myosin and actin filaments slide past each other, so that they take up less space, in a process explained by the “sliding filament” model. According to this model, when the muscle fiber is stimulated, the head and cross-bridges on the myosin molecule approach and interact with binding sites on the actin molecule. The head hydrolyzes ATP, which provides the energy by which the myosin cross-bridges and head propel the thin filament closer to the center of the sarcomere. Repeated cycles of attachment, movement, and breakage cause the actin filaments to slide past the myosin molecules, so that, at full contraction, the Z lines, which are directly attached to the thin filaments, are almost touching the myosin heads (see Figure 17.4). Thus as the sarcomere gets “smaller”, due to changes in the relative positions of the thick and thin filaments, the myofibril as a whole is seen to contract, pulling the bone to which it is attached.
Since skeletal muscles contract in response to voluntary signals from the nervous system, we must explain the mechanism by which the course of events detailed above is set into action and controlled. As we observed in Chapter 14, motor neurons are nervous tissue cells in pathways leading from the brain to an effector organ, usually a muscle. The axon of a motor neuron interacts with the sarcolemma of a muscle cell at the neuromuscular junction, which resembles a neuronal synapse in many ways. Often a motor neuron is connected to several muscle fibers, which it stimulates to contract simultaneously. The motor neuron and all its associated muscle fibers are referred to as a motor unit. The axon releases the neurotransmitter acetylcholine in response to an incoming action potential, which binds to postsynaptic receptors on the muscle cell sarcolemma, triggering a depolarization of the sarcolemma which quickly spreads throughout the muscle fiber by way of the transverse tubules. The acetylcholine is quickly degraded by the enzyme acetylcholinesterase in order to “reset” the system for the next signal.
How does the depolarization of the membrane lead to contraction?
Two other groups of proteins are involved in the regulatory process. Tropomyosin molecules are long and thin, and are associated with globular troponin molecules. When the muscle fiber is relaxed, the tropomyosin interacts with the actin filaments so that the binding sites for myosin cross-bridge formation are covered. When the membrane is depolarized, electrical changes occur that cause gated channels for the positively charged ion calcium to open in the membrane of the sarcoplasmic reticulum (SR). Since large amounts of calcium are stored inside the SR, it quickly rushes out into the cytoplasm (often called the sarcoplasm) of the muscle cell. Calcium subsequently binds to troponin, causing a conformational change which shifts tropomyosin off the myosin binding sites of the thin filaments, allowing cross-bridge formation, ATP hydrolysis, and the sliding of the filaments. When the sarcolemma is repolarized, calcium is pumped back into the sarcoplasmic reticulum by active transport, the muscle fiber relaxes, and the cycle is ready to begin again.

Test Your Skills Now!
Take a Quiz now
Reviewer Name