CHANGES IN ACTION POTENTIALS OF SINGLE PHRENIC MOTOR NEURONS DURING ACTIVITY

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Abstract
Repetitive stimulation of the medullary inspiratory area in the cat produces a sequence of changes in the action potentials of single phrenic motor neurons as recorded with micro-electrodes (less than 6[mu] in tip diam.) in the cervical cord. The 1st changes seen were an increase both in the duration of the negative phase and in the amplitude of the positive phase of the diphasic soma discharge. During a period of hyperactivity after stimulation ceased discharges as high as 400/sec. were achieved. The total duration of the refractory period could be measured as 11 msec. during this hyperactivity. A further change during "fatigue" produced a deflection in the downward limb of the negative phases of the single unit potential attributed to slowing of conduction of the impulse from the soma into the initial unmyelinated region of the axon. The last detectable change after prolonged activity before the unit ceased firing was the appearance of labile monophasic discharges suggesting synaptically induced non-propagating depolarization of the soma.