The hatchling Xenopus tadpole nervous system circuit
This simplified and incomplete diagram shows the main groups of neurons (circles colour coded by the transmitter released) that present evidence suggests may control the tadpole's behaviour.
p = pineal photoreceptor, pg = pineal ganglion cell, dmd= diencephalic/mesencephalic descending, Tn = trigeminal noxious, Tp = trigeminal pressure, Tt = trigeminal touch, R = Raphe/spinal, ri = reticulospinal inhibitory, d = descending, c = commissural, mn = motoneuron, dl = dorsolateral, dlc = dorsolateral commissural, RB = Rohon- Beard, KA = Kolmer- Agdhur
On the left of the diagram are the stimuli that start swimming. (dim light, touch, noxious substance). On the right are the stimuli that stop swimming (pressure).
Circles are groups of neurons colour coded by the transmitter they release (ascending interneurons are not included). Blobs and triangles are inhibitory and excitatory synapses.
Swimming can be initiated by:
- Touching the skin which excites Rohon- Beard (RB) neurons in the trunk or trigeminal touch (Tt) neurons in the head. In the spinal cord Rohon- Beard neurons excite sensory interneurons (dorsolateral (dl) and dorsolateral commissural (dlc)). Excitation then travels to the neurons of the central pattern generator for swimming (white boxes on either side).
- dimming the illumination excites pineal photoreceptors (p). These then excite pineal ganglion cells (pg) which excite diencephalic/mesencephalic descending neurons (dmd) whose axons project to the hindbrain to excite the swimming central pattern generator.
The central pattern generator for swimming consistes of descending (d) and commissural (c) interneurons, and motoneurons (mn). During swimming these neurons all fire a single action potential on each cycle of swimming. The alternation of activity is organised by glycinergic inhibition from commissural interneurons. After initiation, swimming activity is sustained by positive feedback excitation from glutamatergic descending interneurons and cholinergic motoneurons.
Swimming normally stops when the tadpole bumps into solid objects and adheres with mucus secreted by a cement gland on the front of the head. Bumping the head skin or cement gland excites trigeminal pressure (Tp) receptors which excite GABAergic midhindbrain reticulospinal (ri) neurons. These project into the spinal cord to inhibit the central pattern generator neurons and terminate swimming.