Final answer:
Muscle fibers may be replaced by fat or fibrous connective tissue when their motor neurons are severed. This process is known as atrophy, and it involves the breakdown and loss of sarcomeres and myofibrils within the muscle fibers.
Step-by-step explanation:
When the motor neurons of muscle fibers are severed, a series of events can happen that affect the muscular tissue. One possibility is that these fibers may atrophy. This atrophy results in the reduction of size and strength in the muscle because the sarcomeres and myofibrils within the muscle fibers are broken down and lost.
In some cases, if atrophy progresses and becomes prolonged, the muscle cells can die and be replaced by connective tissue and adipose tissue, tissues without the contractile capability of muscle tissue. Therefore, the correct answer to the student's question is B. – the muscle fibers may be replaced by fat or fibrous connective tissue over time.
Skeletal muscle tissue, in particular, does not change the number of fibers but can undergo hypertrophy or atrophy in response to various stimuli. After severe muscle cell damage or loss, satellite cells can aid in repair, although they cannot replace the lost muscle cells entirely.
Meanwhile, smooth muscle can regenerate more effectively due to stem cells called pericytes. Cardiac muscle tissue, when damaged, is mostly replaced by scar tissue which lacks the ability to contract efficiently.