science

Internal anal sphincter

Internal anal sphincter is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Internal anal sphincter rather than just read about it. In short: The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.

Key takeaways

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Reference excerpt

The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum. The internal anal sphincter aids the sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Its action is entirely involuntary. It is normally in a state of continuous maximal contraction to prevent leakage of faeces or gases. Sympathetic stimulation stimulates and maintains the sphincter's contraction, and parasympathetic stimulation inhibits it. It becomes relaxed in response to distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter to maintain continence, and also contracts during the bulbospongiosus reflex.

Structure The internal anal sphincter is the specialised thickened terminal portion of the inner circular layer of smooth muscle of the large intestine. It extends from the pectinate line (anorectal junction) proximally to just proximal to the anal orifice distally (the distal termination is palpable). Its muscle fibres are arranged in a spiral (rather than a circular) manner. At its distal extremity, it is in contact with but separate from the external anal sphincter.

Innervation The sphincter receives extrinsic autonomic innervation via the inferior hypogastric plexus, with sympathetic innervation derived from spinal levels L1-L2, and parasympathetic innervation derived from S2-S4. The internal anal sphincter is not innervated by the pudendal nerve (which provides motor and sensory innervation to the external anal sphincter).

Function The sphincter is contracted in its resting state, but reflexively relaxes in certain contexts (most notably during defecation). Transient relaxation of its proximal portion occurs with rectal distension and post-prandial rectal contraction (the recto-anal inhibitory reflex and sampling reflex, respectively) while the distal portion of the sphincter remains contracted and the external anal sphincter becomes contracted to maintain continence; this transient relaxation allows passage of stool into the proximal anal canal - this filling is sensed.

Continence The IAS contributes 55% of the resting pressure of the anal canal. It is very important for bowel continence, especially for liquid and gas. When the rectum fills beyond a certain capacity, the rectal walls are distended, triggering the defecation cycle. This begins with the rectoanal inhibitory reflex (RAIR), where the IAS relaxes. This is thought to allow a small amount of rectal contents to descend into the anal canal where specialized mucosa samples whether it is gas, liquid or solid. Problems with the IAS often present as degrees of fecal incontinence (especially partial incontinence to liquid) or mucous rectal discharge.

Physiology

Neurophysiology Sympathetic stimulation is mediated by alpha-2 adrenergic receptors and results in contraction of the sphincter. Parasympathetic stimulation is mediated by muscarinic acetylcholine receptors and results in relaxation of the sphincter. Nitrergic stimulation also produces relaxation which has pharmacological significance.

Clinical significance

Clinical pharmacology Nitrergic pharmaceutical agents produce relaxation of the muscular tone of the sphincter and are applicable in pathological contexts where this tone is abnormally increased.

Regenerative medicine In 2011, it was announced by the Wake Forest School of Medicine that the first bioengineered, functional anal sphincters had been constructed in a laboratory made from muscle and nerve cells, providing a proposed solution for anal incontinence.

Additional images

See also External anal sphincter Levator ani

References This article incorporates text in the public domain from page 426 of the 20th edition of Gray's Anatomy (1918)

External links Pelvis at The Anatomy Lesson by Wesley Norman (Georgetown University) (Rectum)

Illustrations

Worked examples

Example 1 — a first encounter with Internal anal sphincter

Start with the simplest possible case. Write down what Internal anal sphincter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Internal anal sphincter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Internal anal sphincter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Internal anal sphincter

In research: Internal anal sphincter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Internal anal sphincter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Internal anal sphincter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anus, Perineum, so understanding it makes those chapters shorter.
In everyday life: Look for Internal anal sphincter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Internal anal sphincter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Internal anal sphincter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Internal anal sphincter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Internal anal sphincter in simple terms?

Why does Internal anal sphincter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Internal anal sphincter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Internal anal sphincter.