Final answer:
The autonomic nervous system (ANS) is responsible for involuntary and subconscious muscle functions, including vital functions like heart rate and digestion.
Step-by-step explanation:
The division of the nervous system primarily responsible for regulating all involuntary and subconscious muscle functions is the autonomic nervous system (ANS). This system controls functions that affect cardiac or smooth muscle tissue, and glands' secretions. The autonomic nervous system operates below the level of consciousness and regulates vital functions such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. It is split into two main divisions: the sympathetic nervous system, which orchestrates the body's rapid involuntary response to dangerous or stressful situations, and the parasympathetic nervous system, which conserves energy as it slows the heart rate and increases intestinal and gland activity.