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Figure 1
Neuron types in hatchling Xenopus spinal cord shown diagrammatically in transverse section (top) and opened like a book (below). The defining features of each neuron type are listed below and citations can be found in the text. (1) Rohon‐Beard (RB) skin sensory neurons, dorsal somata, peripheral axon innervating trunk skin to detect touch (few spikes), and stronger potentially damaging stimuli (many spikes), central ascending and descending axons in dorsal tract distribute excitation along the CNS, glutamatergic. (2) Dorsolateral commissural (dlc) sensory pathway interneurons, multipolar dorsolateral superficial somata with dorsal dendrites, ventral axon crosses to the other side to ascend (some branch to descend), strongly excited by RBs (2‐amino‐3‐(5‐methyl‐3‐oxo‐1,2‐oxazol‐4‐yl)propanoic acid receptors), amplify and distribute input (across and rostral to brain) for reflex bend away and initiation of swimming, inhibited during struggling, glutamatergic. (3) Dorsolateral (dla) sensory pathway interneurons, as dlcs but axon ascends ipsilaterally. (4) Excitatory commissural (ecINs) sensory pathway interneurons, anatomy as dlcs but more ventral dendrites, weakly excited by RBs (N‐methyl‐D‐aspartate receptors) recruited during RB firing to pressure, contribute to initiation of struggling, glutamatergic. (5) Commissural (cINs) reciprocal inhibitory interneurons, unipolar somata in mid to dorsal positions, dendrites mid to ventral from initial ventral axon which crosses to ascend or branch, active during swim and struggle, inhibit neurons on the opposite side to organize left right alternation of activity during swimming and struggling, contribute to rhythm generation in both activities, glycinergic. (6) Ascending (aINs) recurrent inhibitory interneurons, unipolar somata, and dendrites from soma or initial segment of axon in mid position, axon ascends but always forms descending branch, active during swim and struggle to limit/modulate firing in sensory pathway and rhythmically active neurons, glycinergic, express engrailed‐1. (7) Descending (dINs) excitatory interneurons, multipolar somata with dendrites in mid positions, axon descends but especially in hindbrain can form ascending branch, fire long‐duration action potentials during swimming to drive head to tail firing of all other neurons during swimming, have conditional pacemaker properties contributing to rhythm generation, electrical coupling leads to tightly synchronous firing, co‐release glutamate and acetylcholine. (8) Descending repetitive (dINrs) excitatory interneurons, anatomy as dINs but no ascending axons, recruited during press stimuli to drive mns during struggling, glutamatergic. (9) Motoneurons (mns) multipolar somata in ventral to mid positions with dendrites in similar positions, axon exits near soma to excite muscles or forms descending branch exiting more caudally, local electrical coupling synchronizes firing, cholinergic. (10) Kolmer‐Agdhur (KA) neurons, somata at ventro‐lateral edge of neural canal with cilia and microvilli in canal, ascending very ventral axon, express gamma‐amino‐butyricacid (GABA), of unknown but probably sensory function. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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Figure 4
Dorso‐ventral distribution of spinal neurons, axons, and dendrites. A: Examples of neurobiotin filled neurons in lateral views to show the dorso‐ventral positions of somata, dendrites, and axons. Dendrites emerge from the black somata (arrowheads). Axons are on the same side as the soma except for dlcs where they cross ventrally then branch. B: Histograms summarize distributions where 0% is ventral and 100% is the dorsal edge of the spinal cord. Distributions are expressed as the probability that a soma, dendrite, or 50 μm segment of axon will lie in a particular dorso‐ventral position.
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