1. Single unit recordings from type B atrial receptors were obtained from slips of the left cervical vagus nerve during acute increases and decreases in atrial pressure produced by infusion of isotonic saline and withdrawal of blood. 2. Atrial receptor discharge reached a peak frequency at the peak of the ‘v’ wave of the atrial pulse, a time when the rate of change of atrial pressure was zero. 3. The receptor discharge exhibited hysteresis when increases in atrial pressure were compared to decreases in atrial pressure during circulatory volume expansion and haemorrhage, respectively. This hysteresis was not due to a change in atrial compliance. 4. The hysteresis was still observed when the pulse pressure of the ‘v’ wave was plotted against receptor discharge. 5. Type B atrial receptors did not show adaptation after 15 min of a sustained increase in left atrial pressure. 6. Acute depression of atrial contractility with propranolol increased resting fibre discharge as a result of the associated increase in atrial pressure.
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