What is the nerve supply of the internal anal sphincter?

Internal Anal Sphincter Nerve Supply

Autonomic Innervation Pattern

The internal anal sphincter receives dual autonomic innervation primarily from the inferior rectal branches of the pelvic plexus (sympathetic-dominant) and secondarily from the myenteric plexus of the distal rectum (parasympathetic), with sympathetic input being excitatory rather than inhibitory as in most gastrointestinal smooth muscle. 1

Anatomical Origin and Course

The nerve supply to the internal anal sphincter originates from distinct anatomical locations:

• Inferior rectal branches of the pelvic plexus arise from the posterolateral corner of the prostate in males or the lower paracolpium in females, at approximately the 2-3 o'clock position 2
• These nerves descend along the internal aspect of the levator ani muscle and enter the anal canal on the anterolateral side from the superior aspect of the levator ani 3, 2
• The nerves run into the conjoint longitudinal muscle coat before reaching the internal anal sphincter 3
• At a level slightly above the squamous-columnar epithelial junction, these nerves join with branches from Auerbach's myenteric plexus of the distal rectum 3, 2

Fiber Composition and Neurotransmitters

The autonomic nerve supply contains multiple fiber types with distinct functional roles:

Sympathetic Innervation (Dominant)

• Tyrosine hydroxylase-positive (adrenergic) fibers constitute the majority of nerve supply to the internal anal sphincter 3, 4, 2
• Sympathetic input is excitatory to the internal anal sphincter, maintaining tonic contraction—this is an exception to the typical inhibitory sympathetic effect seen elsewhere in the gastrointestinal tract 1
• The sympathetic content in nerves supplying the internal anal sphincter is substantially higher than in nerves supplying the distal rectum 3

Parasympathetic Innervation

• Neuronal nitric oxide synthase-positive (nitrergic) fibers provide inhibitory parasympathetic innervation 3, 4, 2
• Vasoactive intestinal polypeptide-positive fibers are present but few in number 2
• Vesicular acetylcholine transporter-positive (cholinergic) fibers include both nitrergic and non-nitrergic components 4
• Parasympathetic innervation is both excitatory (cholinergic) and inhibitory (non-adrenergic, non-cholinergic/nitrergic) 1

Structural Distribution Within the Sphincter

The nerve architecture differs markedly from the distal rectum:

• The internal anal sphincter contains short, longitudinally running sympathetic and parasympathetic nerves, with sympathetic nerves being dominant 3
• In contrast, the distal rectum has parasympathetic nerves forming a lattice-like plexus in the circular smooth muscles 3
• All intramural ganglion cells in the distal rectum are neuronal nitric oxide synthase-positive and tyrosine hydroxylase-negative, and are restricted to above the squamous-columnar epithelial junction 3
• The major autonomic nerve input to the internal anal sphincter originates from the inferior rectal branches of the pelvic plexus rather than from the distal rectum 3

Clinical Implications

Surgical Vulnerability

• The inferior rectal branches are vulnerable during radical prostatectomy or tension-free vaginal tape insertion due to their origin at the posterolateral corner of the prostate or lower paracolpium 2
• Low anterior resection of rectal cancer will likely damage these nerves because of their intersphincteric course 2
• Injury to the inferior rectal branches during surgery results in loss of innervation to the major part of the internal anal sphincter 3

Functional Consequences

• The internal anal sphincter is responsible for >70% of resting anal pressure and generates tone essential for fecal continence 5
• Denervation leads to reduced and unstable internal anal sphincter tone, contributing to fecal incontinence, particularly nocturnal incontinence 1
• The complex control mechanism independent of simple nerve supply makes evaluation of sphincteric function after surgery difficult 2