Which of the following sphincters is under voluntary control?

NHM UP Staff Nurse - 2016

Applied Physiology

Easy

Appeared in: NHM UP Staff Nurse - 2016

Explanation

The external anal sphincter is composed of striated (skeletal) muscle, which is distinct from the smooth muscle of the other listed sphincters.
Skeletal muscle is innervated by the somatic nervous system, specifically the pudendal nerve (S2-S4), which allows for conscious, voluntary control.
This voluntary control is essential for the social aspect of continence, allowing an individual to decide when to defecate.

Option A: The pyloric sphincter is a ring of smooth muscle at the junction of the stomach and the duodenum. Its function is regulated by the autonomic nervous system and hormones, making it involuntary.
Option B: The cardiac sphincter, also known as the lower esophageal sphincter (LES), is made of smooth muscle. It is under involuntary control and its primary role is to prevent gastroesophageal reflux.
Option C: The internal anal sphincter is composed of smooth muscle and is a continuation of the circular muscle of the rectum. It is under involuntary control and is responsible for most of the resting pressure of the anal canal, preventing leakage of stool.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Voluntary vs. Involuntary Control of Gastrointestinal Sphincters as background academic context rather than a clinical decision trigger.
Fecal continence depends on the coordinated function of both the involuntary internal sphincter (maintaining resting tone) and the voluntary external sphincter (providing conscious squeeze pressure).
Nurses assess anal sphincter tone during digital rectal exams. Damage to the pudendal nerve, often from childbirth, pelvic surgery, or trauma, can weaken the external sphincter and lead to fecal incontinence.
Patient education for bowel training programs often focuses on strengthening the pelvic floor and external anal sphincter muscles through exercises like Kegels.

How to Approach the Question

First, identify the key term in the question: 'voluntary control'.
Recall the fundamental physiological principle: voluntary actions are controlled by skeletal (striated) muscle, while involuntary actions are managed by smooth muscle.
Systematically evaluate each option based on its muscle type.
Recognize that the pyloric, cardiac, and internal anal sphincters are all part of the internal digestive tract and function automatically (involuntary smooth muscle).
Identify the external anal sphincter as the exception, being composed of skeletal muscle to allow for conscious control over the final stage of defecation.
Select the option that aligns with the principle of skeletal muscle being under voluntary control.

Concept Tested & Keywords

Concept Tested: Voluntary vs. Involuntary Control of Gastrointestinal Sphincters
Stem keywords: sphincters, voluntary control
Lead-in keywords: Which
Negative lead-in flag: false

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E6 Anatomy Applied Ashalatha 4e p. 134-136

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