Neural control and proprioceptive load matching in reflex respiratory movements of fishes

A brief anatomical description of topographical arrangements of respiratory neurons in the fish respiratory medulla introduces a discussion centered on neural controls of reflex breathing movements during rhythmic and ram gill ventilation, especially the role of proprioception in "load matching." Paralysis experiments show that proprioceptive signals are processed in medullary respiratory areas of the brain. Some neurons depend completely on proprioceptive input for their activity, while others "mix" proprioceptive and rhythmic information. The existence of direct proprioceptive control loops is revealed by fast compensating changes in electrical activity of respiratory muscles following mechanical interference with respiratory movements. Responses of respiratory neurons to short artificial twitches of respiratory muscles show that some neurons process proprioceptive information at higher levels of integration, often with a long latency and after effect. Other neurons react immediately to the stimulus and often show properties of elements in proprioceptive control systems in which length receptors facilitate and tension receptors inhibit motor neuron activity. Stimulus-response averaging applied to neuron activity and respiratory electromyograms, and to electrical stimuli and neurograms, leads to the conclusion that motor neurons and neurons mediating sensory signals from the respiratory muscles are situated close together in the medullary respiratory areas. Neuroanatomical studies are discussed which indicate that the sensory projection fields of some trigeminal fibers terminate adjacent to the cranial motor nuclei. These presumably project onto motor neurons forming fast disynaptic reflex arcs relaying sensory trigeminal information.