The structural and functional unit of a skeletal muscle is a:

Choice A reason: While coordination is important in highly mobile joints, this is not the primary reason for the vulnerability of the glenohumeral joint.

Choice B reason: Ligaments around the shoulder joint are indeed more flexible to allow movement, but this alone does not fully explain the joint’s instability.

Choice C reason: Power generation is not significantly reduced due to mobility. Muscles around the shoulder can generate substantial force.

Choice D reason: Blood vessels are generally well protected and not easily compromised by joint movement. This is not a common consequence of shoulder mobility.

Choice E reason: This is the correct answer. The glenohumeral joint is a ball-and-socket joint with a shallow socket, allowing extensive range of motion. However, this anatomical configuration makes it relatively unstable and prone to dislocation, especially during forceful or awkward movements.

Choice A reason: Smooth muscle does contract in response to electrical signals, but the mechanism is more complex and often involves calcium influx through voltage-gated channels rather than a classic action potential as seen in skeletal muscle.

Choice B reason: Cardiac muscle cells typically have one or two centrally located nuclei, not many. This distinguishes them from skeletal muscle cells, which are multinucleated.

Choice C reason: Cardiac muscle cells are found exclusively in the heart, not in large blood vessels. Large vessels contain smooth muscle in their walls, not cardiac muscle.

Choice D reason: Smooth muscle cells lack T tubules. T tubules are found in skeletal and cardiac muscle cells and are involved in rapid transmission of action potentials.

Choice E reason: Skeletal muscle cells are long, cylindrical, and multinucleated. This is a defining feature of skeletal muscle tissue and makes this statement true.