It’s your first day of residency in the emergency room and a woman comes in who cannot move her arm after lifting heavy weights. You can’t find the consulting physician anywhere. You quickly review your knowledge of muscle structure and function...
In general, skeletal muscles attach to bone pulling on it to move the body. The effect is motion under voluntary control like walking and lifting weights. The biceps brachii skeletal muscle, biceps for short, flexes the forearm. Small changes at the protein level drive motion through the muscle myofilament, myofibril, fiber and fascicle hierarchy to culminate in whole muscle contraction.
The biceps pull on the scapula and the radius to flex the forearm. During contraction the biceps becomes 50% shorter. The large decrease in length at the whole muscle level is the result of many small changes.
Contraction of the biceps brachii flexes the forearm. How is such a large change in length produced?
If a weightlifter strained and tore a muscle from the bone, would the muscle's ability to move the body be affected? How about its ability to contract?
Skeletal muscle like the biceps is made up of smaller bundles called fascicles. The number of fascicles in a muscle and their arrangement determines the muscle’s power and range of motion.
Muscles with parallel fascicles decrease the most in length while fascicles that fan out at one end and converge at the other have better fine control over the angle of contraction. Based on their arrangement, for what range of motion are the biceps fascicles suited?
The muscle fiber is the cellular unit of a muscle. Muscle fibers stretch the entire length of the fascicles they are bundled into. In the biceps fascicles, and therefore fibers, run parallel to the long axis of the muscle.
The cellular components or organelles of muscle fibers have different names, but many of the same counterparts as other cells of the body, with two notable exceptions – which are?
A myofibril is an organelle unique to muscle fibers and is responsible for muscle contraction. The more myofibrils there are, the stronger the force of contraction. Myofibrils contain myofilaments: proteins assembled in long strands. The highly-ordered and close-knit organization of myofilaments gives the myofibril its distinctive banded appearance.
A sarcomere is the smallest repeating unit of a myofibril. At rest, a sarcomere is 2.2 µm long, but can decrease to 1.2 µm during contraction. On average, a myofibril is 10,000 sarcomeres long – how long is it when at rest? How about when it’s contracted?
In myofibrils, thin and thick myofilaments are organized so they interdigitate. A thick filament is composed of hundreds of myosin proteins, the myosin 'tails’ coiling to form the body of the thick filament. Extending from the thick filament body are the myosin ‘heads’, which stretch to interact with actin proteins in the thin filament during muscle contraction.
Thick filaments pull the thin filaments past them during contraction, increasing the zone of overlap and shrinking the region known as the H band in the sarcomere. This action is repeated over the entire length of each myofibril. As the myofibril shortens, so does the muscle fiber, the fascicle, and the whole muscle.
Is an initial zone of overlap important when the muscle is at rest? What if a weightlifter overly extended a muscle, pulling the thin and thick filaments so they no longer overlapped?
Myosin is the muscle’s molecular motor. It alternates shape between a ‘flexed’ and ‘rigor’ shape in order to push actin past it in a rowing motion. This action slides the myofilaments past each other, collapsing the sarcomere, and contracting the muscle.
What would happen to a weightlifter who uses up all available ATP during a workout? Where would the myosin cross-bridge cycle get stuck and what effect does this have on the muscle?