Microbial Enteric Neurophysiology

The myenteric plexus contains a type of neuron that innervates the mucosa, and projects to all other classes of myenteric neurons. This neuron has a specific multipolar morphotype and possesses a characteristic action potential whose firing frequency is moderated a potassium current dependent post-spike slow after-hyperpolarisation. The neuron, termed AH cell, is functionally an intrinsic intestinal primary afferent neuron (IPAN) with chemosensory and mechano-sensory responses to adequate stimuli. With about 80% of sensory neuropeptide containing nerve fibres in the mucosa belonging to IPANs they are ideally positioned to respond to signals from commensal or probiotic gut bacteria. Application of Lactobacillus rhamnosus JB-1 or Bacteroides fragilis to the mucosal epithelium evokes sensory action potentials in the IPANs within a few seconds of administering the bacteria. In the case of B. fragilis, a capsular polysaccharide A is necessary and sufficient to carry the signal from microbe to neuron. This sensory activation leads to a longer-term increase in the intrinsic excitability of the IPAN sensory neuron network caused at least in part by a decrease in a calcium dependent potassium conductance. We have identified a molecular and neuronal target of commensal signalling. The neuron’s rapid sensory responses may activate the neuronal circuits that mediate some of the motility and central nervous system effects of ingesting beneficial microorganisms.