Question

select the correct answer.

a skeletal muscle is a composition of several components bundled one into the other. at which structural level in the muscle does contraction occur to bring about contraction in the whole muscle?

a. myofibril

b. muscle fiber

c. myoneural junction

d. tendon

e. fascicle

Explanation:

To determine where muscle contraction starts to cause whole - muscle contraction, we analyze each option:

• Option A: Myofibrils are made of actin and myosin filaments. The sliding of these filaments (the sliding filament theory) occurs at the myofibril level, and this is the basic level where the contraction process (the interaction of actin and myosin) takes place. This contraction at the myofibril level within a muscle fiber ultimately leads to the contraction of the whole muscle.
• Option B: A muscle fiber is a single muscle cell that contains many myofibrils. While muscle fibers do contract, the actual molecular - level contraction (the sliding of filaments) happens within the myofibrils inside the muscle fiber. So the initial contraction event (the sliding of actin and myosin) is at the myofibril level, not the muscle fiber level as a whole in terms of the contraction mechanism's origin.
• Option C: The myoneural junction is the site where a motor neuron connects with a muscle fiber. It is involved in transmitting the signal to start contraction (through the release of neurotransmitters), but it is not the site where the contraction (the sliding of filaments) actually occurs.
• Option D: Tendons are connective tissues that attach muscles to bones. They do not contract; their role is to transmit the force generated by the contracting muscle to the bone.
• Option E: A fascicle is a bundle of muscle fibers. It is a structural unit of the muscle but is not the site where the contraction (the sliding of actin and myosin filaments) takes place. The contraction within the fascicle is due to the contraction of the muscle fibers (and their myofibrils) within it.

Answer:

A. myofibril