Select the description below that illustrates a difference between a sphincter and a circular muscle.

• A. Circular muscles are only found in the heart, while sphincters are found in the digestive tract.
• B. Both sphincter and circular muscles are always found in the limbs.
• C. A sphincter muscle controls the opening and closing of a passageway, while a circular muscle does not necessarily regulate passageways.
• D. Sphincter muscles are voluntary, whereas circular muscles are always involuntary.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the definitions of sphincter muscles and circular muscles. Sphincter muscles are specialized circular muscles that control the opening and closing of passageways, such as the esophagus or anus. Circular muscles, on the other hand, are general muscles arranged in a circular pattern but do not necessarily regulate passageways.
2. Step 2: Analyze the options provided in the problem. Each option describes a potential difference between sphincter and circular muscles. Evaluate the accuracy of each statement based on anatomical and physiological knowledge.
3. Step 3: Eliminate incorrect options. For example, the statement 'Circular muscles are only found in the heart, while sphincters are found in the digestive tract' is incorrect because circular muscles are found in various locations, not just the heart. Similarly, 'Both sphincter and circular muscles are always found in the limbs' is incorrect because these muscles are not limited to the limbs.
4. Step 4: Focus on the correct statement. The correct answer is: 'A sphincter muscle controls the opening and closing of a passageway, while a circular muscle does not necessarily regulate passageways.' This accurately describes the functional difference between the two types of muscles.
5. Step 5: Clarify the voluntary vs. involuntary aspect. The statement 'Sphincter muscles are voluntary, whereas circular muscles are always involuntary' is not universally true. Some sphincter muscles, like the external anal sphincter, are voluntary, while others, like the lower esophageal sphincter, are involuntary. Circular muscles can also be involuntary depending on their location and function.