Electrophysiological characterization of human rectal afferents

Abstract

It is presumed that extrinsic afferent nerves link the rectum to the central nervous system. However, the anatomical/functional existence of such nerves has never previously been demonstrated in humans. Therefore, we aimed to identify and make electrophysiological recordings in vitro from extrinsic afferents, comparing human rectum to colon. Sections of normal rectum and colon were procured from anterior resection and right hemicolectomy specimens, respectively. Sections were pinned and extrinsic nerves dissected. Extracellular visceral afferent nerve activity was recorded. Neuronal responses to chemical [capsaicin and "inflammatory soup" (IS)] and mechanical (Von Frey probing) stimuli were recorded and quantified as peak firing rate (range) in 1-s intervals. Twenty-eight separate nerve trunks from eight rectums were studied. Of these, spontaneous multiunit afferent activity was recorded in 24 nerves. Peak firing rates increased significantly following capsaicin [median 6 (range 3-25) spikes/s vs. 2 (1-4), P < 0.001] and IS [median 5 (range 2-18) spikes/s vs. 2 (1-4), P < 0.001]. Mechanosensitive "hot spots" were identified in 16 nerves [median threshold 2.0 g (range 1.4-6.0 g)]. In eight of these, the threshold decreased after IS [1.0 g (0.4-1.4 g)]. By comparison, spontaneous activity was recorded in only 3/30 nerves studied from 10 colons, and only one hot spot (threshold 60 g) was identified. This study confirms the anatomical/functional existence of extrinsic rectal afferent nerves and characterizes their chemo- and mechanosensitivity for the first time in humans. They have different electrophysiological properties to colonic afferents and warrant further investigation in disease states.

Keywords: electrophysiology; human; rectal afferents.

Fig. 1.

Diagram showing tissues procured for the study. A: rectal tissue, acquired from the distal (aboral) end of an anterior resection specimen. B: colonic tissue, acquired from the proximal (oral) end of an anterior resection specimen (i) or the distal (aboral) end of a right hemicolectomy specimen (ii).

Fig. 2.

A: tissue pinned serosa side up (×1 magnification). Arrows indicate mesenteric nerves running in neurovascular bundles. B: view under a dissecting microscope (×2 magnification). Arrows indicate mesenteric nerves. C: nerve trunks dissected free from surrounding connective tissue.

Fig. 3.

Specialized tissue chamber containing a separate paraffin-filled nerve recording well that could be isolated from the main chamber.

Fig. 4.

A: typical trace of spontaneous nerve activity (horizontal axis: time, s; vertical axis: voltage, mV), recorded from a rectal nerve (patient: 56-yr-old male). The specimen was of rectal tissue (8 cm from anal verge) acquired from anterior resection performed for rectosigmoid adenocarcinoma. Arrows indicate spontaneous nerve action potentials. The dashed lines show 1 of these action potentials at a faster time base, with a typical biphasic waveform. B: typical nerve response to repeated mechanical probing with a 4.0 g von Frey hair applied to a single marked hot spot on the preparation. Application is indicated by the interval bars. C: hot spot responses to a decreasing series of mechanical stimuli applied by von Frey hairs of 3 different stiffnesses. Firing rates decreased as probing force was reduced, confirmed on corresponding plot of instantaneous frequency (Hz), which could be used to identify the threshold according to the decreasing method of limits.

Fig. 5.

Line graph showing changes in peak (A) and mean (B) firing rates in rectal nerve activity before/after application of inflammatory soup (IS). The individual lines (black) represent changes in firing rates for individual rectal nerves. The solid gray line in each graph indicates median values of firing rates before and after chemical stimulation, with median values indicated in gray text. The changes in firing rates before and after chemical stimulation were all statistically significant (P < 0.001).

Fig. 6.

Line graph showing changes in peak (A) and mean (B) firing rates in rectal nerve activity before/after capsaicin. The individual lines (black) represent changes in firing rates for individual rectal nerves. The solid gray line in each graph indicates median values of firing rates before and after chemical stimulation, with median values stated in gray. The changes in firing rates before and after chemical stimulation were all statistically significant (P < 0.001).