effects of cold block of the spinal cord on chest wall reflexes

ABSTRACT

It was previously established that passive movements of the chest wall induced by artificial respiration elicits reflex activity in both alpha and gamma motomeurons which persists in the spinal preparation. In this work re-examination of these results at segmental level was performed, with the intention to show the behaviour of the chest wall inflation and deflation reflexes at levels of CO[2] just at threshold for rhythm generation, on the one hand, and their response to cold blocking of the spinal cord on the other. In the first instance, the reflexes fluctuate in activity with the central respiratory drive [CROP] indicating a measure of coexistence as proposed by Sears. Hence, these results support the view that the normal functional operation of the inhibitory phase of the CROP is to prevent reflex excitation of the antagonistic muscle contraction. In the second instance, with application of the cold thermode to the dorsolateral surface of the spinal cord, the alpha spikes gradually decreased in number before completely abolished. Eventually the cold block had interrupted the descending supraspinal drives and subsequently brought the mean level of the membrane potential of the expiratory alpha motoneurones to its resting level. Thus the loss of these inputs at segmental level specifically reduces the efficacy of synaptic transmission from expiratory muscle spindle afferents. Although not pump modulated, the tonic discharging gammas reduced in number with the progress of cooling, This seems to be one of the sources facilitating the inflation reflex as their reduced firing reduced the amplitude of the reflex. Results were further discussed