What is the functional unit of a skeletal muscle called?

Choice A reason: This is incorrect. Tropomyosin does not block myosin; it blocks the active sites on actin filaments, preventing myosin binding until calcium binds to troponin.

Choice B reason: Tropomyosin is not a chemical activator. Activation of myosin heads occurs through ATP hydrolysis and calcium-mediated exposure of actin binding sites.

Choice C reason: This is correct. Tropomyosin is a regulatory protein that wraps around actin filaments and blocks myosin-binding sites. When calcium binds to troponin, tropomyosin shifts, exposing these sites and allowing contraction to proceed.

Choice D reason: Tropomyosin does not function in the synaptic cleft nor inhibit calcium there. Calcium regulation in the synaptic cleft involves neurotransmitter release mechanisms, not tropomyosin.

Choice E reason: Tropomyosin is not a receptor. The receptor for motor neuron neurotransmitters (e.g., acetylcholine) is the nicotinic acetylcholine receptor located on the sarcolemma of the muscle cell.

Choice A reason: The central canal, also known as the Haversian canal, is located at the center of each osteon in compact bone. It contains blood vessels and nerve fibers that supply nutrients and signals to the bone cells.

Choice B reason: Adipose tissue is found in yellow marrow, primarily in the medullary cavity of long bones, not in the central canal of osteons.

Choice C reason: Yellow marrow and spicules are found in the medullary cavity and spongy bone, respectively. They are not components of the central canal.

Choice D reason: Cartilage is not present in the central canal. Interstitial lamellae are layers of bone matrix found between osteons, not within the canal.

Choice E reason: Red marrow is found in spongy bone and certain flat bones, not in the central canal. While nerves are present, the absence of blood vessels in this choice makes it incomplete.