Action potentials in a 'non-spiking' neurone: graded responses and spikes in the afferent P fibre of the crab thoracic-coxal muscle receptor organ.

The small diameter P neurone (7 microns) is here characterized as an afferent of the crab thoracic-coxal muscle receptor organ (TCMRO), in addition to the S and T neurones (50-60 microns). The P neurone employs both passively conducted, graded amplitude receptor potentials and, superimposed upon these, either graded active membrane responses or constant amplitude action potentials. Intracellularly recorded receptor potential waveforms reflect the dynamic component of stretch more strongly than receptor length, while action potentials at the onset of stretch accentuate the phasic response characteristics of this neurone.