PMC

A, oral balloon distension applied to the intact segment evoked a transient IJP (descending inhibition) in the CM at both the oral and the anal electrodes. B, anal balloon distension evoked a transient EJP (ascending excitation) in the CM at both the oral and the anal electrodes.

Maintained circumferential stretch generated oral EJPs and anal IJPs in CM. Anal elongation (140%) increased the amplitude, frequency and degree of synchronization between the oral EJP and anal IJP. Oral elongation (140%) applied in conjunction with anal elongation depressed the junction potentials. The addition of l-NA (100 μm) reversed the inhibitory effects of oral elongation.

Strips of LM were stripped away to reveal underlying CM. Ca²⁺ transients were measured in adjacent regions of interest aligned with either the LM or the CM. In the absence of nifedipine, oral elongation suppressed Ca²⁺ transients in both the LM and CM at the same time. B, in contrast, anal elongation increased the Ca²⁺ transients in the LM and CM at the same time.

A and C, maintained circumferential stretch generated oral EJPs and anal IJPs in CM. Maintained oral elongation (140% increase in slack length) immediately suppressed this activity. Anal elongation (140%) in the presence of oral elongation had no effect. B and D, maintained oral elongation (140%) immediately suppressed this activity. Following the addition of l-NA (100 μm) between the partitions, which restored coordinated EJPs and anal IJPs, the further addition of anal elongation increased the amplitude, frequency and degree of synchronization between oral EJPs and anal IJPs.

A, the polarized peristaltic reflex is activated by circumferential stretch, giving a synchronized oral EJP and anal IJP in the CM (see expanded traces). When elongation was applied to the oral end of the colon under circumferential stretch, the oral EJP and anal IJP were depressed. Open and filled circles indicate oral and anal recording sites in the CM, respectively. B, when the preparation was elongated in the anal direction there was a significant increase in amplitude and frequency of firing of the coordinated junction potentials.

A, circumferential stretch generated coordinated oral EJPs and anal IJPs in CM. Pelvic nerve stimulation (PNS; single pulse at 0.5 ms, 50 V) evoked an oral EJP and anal IJP of large amplitude. B, following the addition of maintained longitudinal stretch to the whole preparation the ongoing oral EJPs–anal IJPs were depressed, as were the responses to PNS. C, l-NA (100 μm) added to the middle chamber (between the partitions) restored both the ongoing activity and the response to PNS, despite the maintained elongation. D, summary of the effects of elongation on the responses to PNS.

A, correlation coefficient (r²; n= 6) between an oral EJP and its corresponding anal IJP evoked by circumferential stretch. a, under circumferential stretch only. b, following the addition of anal elongation (to 140% of slack length). c, following the further addition of oral elongation (140% of slack length). d, plus the addition l-NA (100 μm) to the middle chamber. Significance level: *P < 0.05 compared to control; #P < 0.001 compared to anal elongation. B, amplitude of the oral EJP plotted against the corresponding anal IJP. C, following the addition of anal elongation (140%; n= 6). D, following the further addition of oral stretch (140%). E–H, the temporal coordination between an oral EJP and its corresponding IJP (sum of n= 6 preparations). E, control, during only circumferential stretch. F, following the addition of anal elongation; note the narrowing of the ‘funnel’ indicating higher temporal correlation. G, following the further addition of oral elongation. H, following the further addition of l-NA (100 μm).

Circumferential stretch activates mechanosensitive ascending interneurons (AI) and descending interneurons (DI) underlying ongoing peristaltic reflex activity. AIs and DIs synapse with one another, activating excitatory motor neurons orally and inhibitory motor neurons anally to both the LM and CM layers (see ). Following colonic elongation descending (NOS positive) and ascending (ACh) mechanosensitive interneurons are activated by longitudinal stretch. Descending NOS-positive interneurons release NO that generate IPSPs in AI and DI, thus depressing their activity. In contrast, the ascending interneurons activated by elongation release ACh, evoking FEPSPs in AI and DI, thus enhancing their activity. Pelvic nerve stimulation also activates both AI and DI, probably by also generating FEPSPs in these neurons.

A, circumferential stretch activates intrinsic polarized reflexes, consisting of an EJP at the oral electrode and an IJP at the anal electrode. Application of hexamethonium (100 μm) applied to the whole preparation abolished ongoing peristaltic reflex activity. Neither oral nor elongation had any detectable effect in the presence of hexamethonium. B, in a colon with no circumferential stretch, there was no polarized reflex present and colonic elongation (to 140% of slack length) of the anal or oral ends gave no detectable response in the CM. When elongation is applied to the oral end of the colon under circumferential stretch, the oral EJP and anal IJP are depressed. Open and filled circles indicate oral and anal recording sites in the CM, respectively

A, spatio-temporal map (ST Map) of spontaneous emptying of fecal pellets from a full colon; dark areas show the dilated regions of colon produced by fecal pellets. Lower panels show images of pellet expulsion at different times, which correspond to those on the ST Map (dashed lines). B, there is a logarithmic relationship between the number of pellets in a full colon and its overall length (y= 29.5ln(x) + 98.5). The correlation between the raw data and the logarithmic curve fit was r²= 0.98 (P < 0.001; n= 5). C, as pellets were expelled sequentially from the full colon, colonic shortening was associated with an exponential (y= 0.9e^−0.02x) increase in pellet velocity; the correlation between the raw data and the exponential curve fit was r²= 0.80 (P < 0.001; n= 5).