Key result
Cardiac receptors with vagal nonmyelinated afferents had a low resting discharge (1.0 +/- 0.15 impulses/sec), with atrial receptor discharge increasing linearly with transmural pressure.
Cardiac receptors with nonmyelinated vagal afferents exhibit low resting discharge but respond dynamically to changes in transmural pressure during respiration and hemodynamic loading.
Some cardiac receptors with nonmyelinated vagal afferents are silent at rest in cats; leaves open their reflex role in conscious humans.
Activity from left atrial and left ventricular receptors with nonmyelinated vagal afferents (mean conduction velocity, 1.2 m/sec) was recorded in 13 closed-chest spontaneously breathing cats anesthetized with alpha-chloralose. The anatomic position of each receptor was determined by probing the opened heart at the conclusion of the experiment. Three of eight left atrial receptors and four of five left ventricular receptors were silent under resting conditions. The mean discharge frequency under resting conditons for the six receptors displaying spontaneous activity was 1.0 +/- 0.15 impulse/sec. Thus cardiac receptors with vagal nonmyelinated afferent have a low resting discharge in spontaneously breathing cats. The frequency and pattern of discharge of atrial but not of ventricular receptors was altered during spontaneous respiration. The atrial receptors discharged with cardiac rhythmicity during end inspiration and early expiration when transmural pressure was greatest and were silent for the remainder of the respiratory cycle. Whe respiration was augmented by CO2 breathing or blood volume was increased, the rate of discharge was a linear function of atrial transmural pressure. Eleven ventricular receptors with nonmyelinated afferents (mean conduction velocity, 1.0 m/sec) were exposed to graded volume expansion and phenylephrine infusion in eight open-chest and three spontaneously breathing cats. Raising left ventricular end-diastolic pressure alone increased the frequency of discharge, and a concomitant increase in systolic pressure caused a further increase in firing.
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Thames et al. (1977) studied this question. Spontaneous respiration and physiological interventions (CO2 breathing, volume expansion, phenylephrine) vs. Resting conditions was evaluated on Discharge frequency of atrial and ventricular receptors. Cardiac receptors with vagal nonmyelinated afferents had a low resting discharge (1.0 +/- 0.15 impulses/sec), with atrial receptor discharge increasing linearly with transmural pressure.
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