Two independent networks of interstitial cells of cajal work cooperatively with the enteric nervous system to create colonic motor patterns

Abstract

Normal motility of the colon is critical for quality of life and efforts to normalize abnormal colon function have had limited success. A better understanding of control systems of colonic motility is therefore essential. We report here a hypothesis with supporting experimental data to explain the origin of rhythmic propulsive colonic motor activity induced by general distention. The theory holds that both networks of interstitial cells of Cajal (ICC), those associated with the submuscular plexus (ICC-SMP) and those associated with the myenteric plexus (ICC-MP), orchestrate propagating contractions as pacemaker cells in concert with the enteric nervous system (ENS). ICC-SMP generate an omnipresent slow wave activity that causes propagating but non-propulsive contractions ("rhythmic propagating ripples") enhancing absorption. The ICC-MP generate stimulus-dependent cyclic depolarizations propagating anally and directing propulsive activity ("rhythmic propulsive motor complexes"). The ENS is not essential for both rhythmic motor patterns since distention and pharmacological means can produce the motor patterns after blocking neural activity, but it supplies the primary stimulus in vivo. Supporting data come from studies on segments of the rat colon, simultaneously measuring motility through spatiotemporal mapping of video recordings, intraluminal pressure, and outflow measurements.

Keywords: ENS; ICC; colon; colonic motility; peristalsis.

Figure 1

Two motor patterns, intraluminal pressure changes, and outflow. (A) A spatiotemporal map of rat colonic motility (circular muscle activity) over a 20-min period. Black is narrowing of the lumen, hence circular muscle contraction and white is relaxation or widening of the lumen. For each map, the Y-axis is distance along the colon (proximal colon = bottom third, mid colon = middle third, distal colon = top third); the X-axis is time. Thirteen rhythmic propulsive motor complexes are seen to propagate anally in the proximal colon [arrows underneath (A)] and some are continuing to propagate into the mid colon at decreased velocity. Within these motor complexes, rhythmic propagating ripples occur at a much higher frequency and propagate in a different direction [arrows in (D,E)]. The dotted line in (A) indicates the position of the intraluminal pressure transducer. The rhythmic propulsive motor complexes in the mid colon are accompanied by strong increases in intraluminal pressure (B) and outflow of content (C), measured simultaneously. The outflow (C) was collected in a cylinder and measured as increase in pressure measured at the bottom of the cylinder. (D) Enlargement of box in the mid colon. (E) Enlargement of box in the proximal colon.

Figure 2

Interaction between the two motor patterns. (A) The rhythmic propagating ripples are part of the rhythmic propulsive motor complexes. One can see a marked increase in amplitude (blackness) of the ripple contractions within the rhythmic propulsive motor complexes as compared to the time period in between these complexes. However, the propagation of the two types of activity appears independent. (B) When the rhythmic propulsive motor complexes are very strong, the rhythmic propagating ripples appear to summate and relatively long-lasting constrictions (broad black bands) occur. (C) Intraluminal pressure changes accompanying the rhythmic propulsive motor complexes in A at the dotted line. Strong intraluminal pressure changes occur during the rhythmic propulsive motor complexes but they consist of contractions at the rhythm of the rhythmic propagating ripples. (D) The rhythmic propulsive motor complexes are accompanied by strong intraluminal pressure increases. Here, the high frequency intraluminal pressure transients are seen to summate which leaves a single pressure change [occurring at the dotted line in (B)]. (E) Presented here is a single line from the data file underlying (A) (at dotted line), showing in a different way the fact that the rhythmic propagating ripples have a much higher amplitude during the time frame of the rhythmic propulsive motor complexes than outside of that time frame. (F) Single line from data file underlying (B) at dotted line.

Figure 3

Are rhythmic propulsive motor complexes neurogenic? Spatiotemporal maps (A,C,E,G) and corresponding intraluminal pressure recordings [(B,D,F,H) respectively] of the same preparations under control conditions (A,B), 100 μM lidocaine (C,D), 100 μM lidocaine + 1 μM carbachol (E,F) and 100 μM lidocaine + 4 μM carbachol (G,H). (I) Stretch evoked contractions of rat colon circular muscle strips in vitro (muscle bath), occurring at the same frequency as the rhythmic propulsive motor complexes, were not influenced by the addition of 100 μM lidocaine (added at arrow).

Figure 4

Distribution of rat and mouse colonic ICC and cholinergic neurons. c-Kit immunostaining of ICC–MP (A) and ICC–SMP (C) in whole-mounts of the rat colon and a cross-section (B). LM, longitudinal muscle; MP, myenteric plexus; CM, circular muscle; SubM, submucosa. Both ICC–SMP and ICC–MP form dense networks at the level of the submuscular plexus and the myenteric plexus respectively. ICC–SMP run parallel to the circular muscle layer. (D–G) Double immunostaining of VAChT (red) and c-Kit (green) to show the cholinergic nerves and ICC–MP (D–F) or ICC–IM (G) in whole-mounts of the rat (D,E) and mouse (F,G) colons. ICC–IM closely align the boundaries of myenteric ganglia (E) or nerve strands (D,F). All arrows indicate intimate contacts between ICC processes and cholinergic nerves. In (F), two processes (arrows #1 and #2) from two mouse ICC–MP cell bodies (labeled as 1 and 2), respectively, are closely associated with two cholinergic nerves at the level of myenteric plexus.

Figure 5

Schematic drawing of the proposed electrophysiological basis of the rhythmic propulsive motor complexes. (A) Smooth muscle cells of the colon may only receive slow wave activity propagated from ICC–SMP (top line). This may be associated with small amplitude contractions if the slow waves barely pass threshold (dotted line) for generation of action potentials dependent on activation of L-type calcium channels. When the rhythmic transient depolarizations hypothesized to come from ICC–MP under stimulated conditions (middle line) are also received by the smooth muscle cells, de cells will obviously rhythmically depolarize and the slow waves will be pushed well above the threshold and hence action potentials and hence contractions may become much more prominent. (B) Actual electrical recordings from the rat colon circular muscle, modified from previously published figures (Pluja et al., 1).Top line shows slow waves originating from ICC–SMP, bottom line shows in addition a transient depolarization coming from ICC–MP. (C) Synchronized rhythmic calcium transients from two ICC–MP of the mouse colon (Lowie and Huizinga, unpublished). Y-axis are in arbitrary units, the X-axis bar = 5 s. (D) Activity recorded from circular muscle associated with ICC–SMP of the mouse colon (top line) and activity recorded from a longitudinal smooth muscle cell probably influenced by ICC–MP; modified figure from (Yoneda et al., 4).