If opioids increase segmenting activity, how is that constipating?

[ibarne242]

Opioids inhibit presynaptic cholinergics, which increases colonic phasic segmenting activity (that I don't get, with the presynaptics inhibited then why is the musculature spasming), and I don't see how that's constipating?
I've looked into this and tried to find an answer but no sources seem to like directly address this...

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[Atom612]

We haven't covered the GI system or pharm yet in my program, but from what I understand opioids increase contraction of circular smooth muscle tissue (non-propulsive) but decrease longitudinal smooth muscle (propulsive) contraction. This leads to longer fecal transit time. Opioids also increase anal sphincter contraction and decrease its ability to relax, which makes evacuation even harder.

 

[chasm-e-baddoor]

The enteric nervous system has both excitatory and inhibitory aspects. As you understand, opioids inhibit the pathways. But you have to think of it as inhibiting the FUNCTION of the pathway to understand the mechanism. When opioids inhibit the excitatory pathways, peristalsis is decreased. When opioids inhibit the inhibitory pathways , the wall pressure and contraction needed to produce the mixing segmentation patterns occur. Both of these compound to result in increase in transit time.

The opioids also act on the submucosal plexus which is responsible for controlling water and electrolyte excretions. Opioids cause less fluid.

Then I guess there is the anal sphincter tone too.