What is the functional unit of a skeletal muscle called?

Choice A reason: Hinge joints allow movement in one plane, such as flexion and extension. Examples include the elbow and knee, but the joint between the radius and ulna does not function this way.

Choice B reason: This is correct. The proximal radioulnar joint is a pivot joint, allowing rotational movement such as pronation and supination of the forearm. The radius rotates around the ulna, enabling the palm to turn up or down.

Choice C reason: Plane or gliding joints allow sliding movements between flat surfaces, such as those between carpal bones. This does not apply to the radius and ulna.

Choice D reason: Saddle joints allow movement in two planes and are found in the thumb (carpometacarpal joint). The radius and ulna do not form a saddle joint.

Choice E reason: Ball-and-socket joints allow movement in multiple directions and are found in the shoulder and hip. The radius and ulna do not form this type of joint.

Choice A reason: Smooth and cardiac muscle are capable of contracting without direct nervous stimulation. Cardiac muscle has intrinsic pacemaker cells that generate rhythmic contractions, and smooth muscle can respond to chemical signals, stretch, and local factors independently of neural input.

Choice B reason: Skeletal muscle requires nervous stimulation to contract. It is under voluntary control and cannot initiate contraction on its own, making this choice incorrect.

Choice C reason: While cardiac muscle can contract independently, this choice omits smooth muscle, which also has autonomous contractile ability. Therefore, it is incomplete.

Choice D reason: Skeletal muscle is entirely dependent on neural input for contraction. It cannot contract without stimulation from motor neurons.

Choice E reason: Smooth muscle can contract without nervous stimulation, but this choice excludes cardiac muscle, which also shares this property. Thus, it is not the best answer.