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1. Afferent discharges from single atrial mechanoreceptors were studied in vitro in isolated, perfused cat hearts, by balloon inflation and by localized endocardial stretching with step and ramp wave forms. 2. Receptor excitability could be maintained for several hours with perfusion, but failed reversibly within a half hour without it. 3. Responses to pure step stimuli were purely phasic, adapting within a few seconds as negative fractional powers of time. Ramp responses showed corresponding behaviour; 4. Median discharge sensitivities were 12 Hz ml.-1 for step balloon inflation and 110 Hz strain-1 for step local stretching, so that 1 ml. balloon expansion was equivalent to 10% local stretch. 5. Ramp stimulation indicated two distinct receptor groups, a 'high-pass latency' (HPL) group for which threshold decreased with increasing ramp velocity, and a larger 'low-pass latency' (LPL) group with the opposite behaviour. 6. The endocardial deformation due to local stretching comprised about half simple extension and half shear, so that half the effective lineal stretch would occur in end filaments oriented within 20 degrees of the stretch axis. 7. Of five receptors tested for directionality by local stretching in two perpendicular directions, four showed significant directionality in step sensitivity and/or ramp response latency, indicating the capability of receptive fields to encode changes in endocardial shape.

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