Innervation
Parasympathetic
The anterior division of third and fourth sacral nerves in both sides form the nervi erigenti which give branches to the rectum; there they relay in the ganglia of the Auerbach plexus. They act as motor nerves to the bowel wall, inhibitors to the sphincter, and sensor fibers to rectal distention.
Sympathetic
These nerves originate in the second, third and fourth lumbar ganglia and the preaortic plexus. They form the hypogastric plexus at the level of the fifth lumbar vertebra; from there, they descend through the posterolateral pelvic walls as the presacral nerves, which join the pelvic ganglion on either side of the pelvis. They act as inhibitors of the bowel wall and motor nerves to the internal sphincter.
Somatic Innervation to the Voluntary Muscles
Most of the levator muscle, particularly its upper portion (ischiococcygeus) and the most anterior part (including the vertical fibers that we call muscle complex), usually known as pubococcygeus portions, receives innervation from the anterior roots of the third and fourth sacral nerves.
These roots unite to form a trunk that runs on the upper surface of the levator muscle. The pudendal nerve, which arises from the second, third and fourth sacral nerves, also innervates these muscles.
The lower portion of the levator muscle, known as puborectalis, as well as the external sphincter, form an inseparable structure that receives its innervation by the perineal branch of the fourth sacral nerve and also from the inferior hemorrhoidal and perineal branches of the pudendal nerves.
Sensory Innervation
The anal canal, including a zone about one centimeter above the pectinate line and extending down to the skin, is exquisitely sensitive, as demonstrated by Duthie and Gairns; they described nerve endings as capable of detecting:
- Pain (free intraepithelial)
- Touch (Meissner's corpuscles)
- Cold (Krause's end-bulbs)
- Pressure or tension (corpuscles of Pacini an Golgi-Mazzoni)
- Friction (genital corpuscles)
The rectum (above anal canal) is not sensitive to the stimuli mentioned above; there is, however, some sensation to rectal distension which is thought to be provided by the parasympathetic innervation of the smooth muscle and by proprioceptive receptors located in the voluntary muscle mechanism surrounding the rectum.
For more information or to request an appointment, please contact the Colorectal Center at Cincinnati Children's.
