Fig. 1.

Potential neural pathways from gut bacteria to the CNS. Sensory neurons include intrinsic primary afferent neurons (IPANs) in the myenteric plexus (MP) of the enteric nervous system, and vagal and spinal extrinsic primary afferent neurons. IPANs are multipolar with their somata and all neurites confined to the intestine. Vagal and spinal primary afferent neurons are pseudounipolar with somata extrinsic to the intestine; they have collaterals that enter enteric ganglia and form synapses with enteric neurons. Sympathetic and myenteric ganglia reciprocally innervate each other. Some 90 % of sensory neuropeptide containing axons that innervate the mucosal layers derive from intrinsic rather than extrinsic primary afferent neurons. Chemicals including hormones released from epithelial cells act on adjacent primary afferent neuron axons (curved arrows). Cell wall components or secreted products, including neurotransmitters, of microorganisms in the lumen or attached to epithelial cells may induce epithelial cells to release transmitter molecules that in turn modulate neural signaling, or act directly on primary afferent axons. MP myenteric plexus, CM circular muscle, Muc mucosa