Final answer:
Skeletal muscles can atrophy when not used due to reduced neural stimulation. Smooth muscles, found in organs, do not atrophy in the same way because they are regulated by the autonomic nervous system and continue to receive neural stimulation even with decreased activity.
Step-by-step explanation:
When a person leads a sedentary life, their skeletal muscles can atrophy due to lack of use. However, smooth muscles, which are found in organs like the stomach and blood vessels, do not atrophy in the same way. Skeletal muscle atrophy occurs because the muscles do not receive sufficient neural stimulation to maintain normal mass, tone, and strength. Neural inputs from the brain and spinal cord play a crucial role in maintaining the size and strength of skeletal muscles. When a limb is immobilized, as in the case of being in a cast, neural stimulation to the muscles is reduced or absent, leading to atrophy. In contrast, smooth muscles are not under voluntary control and are regulated by the autonomic nervous system. Even with decreased activity, smooth muscles still receive neural stimulation, allowing them to maintain their structure and function.