1. Intracellular and extracellular recordings from the two large-diameter S and T sensory fibres of the posterior thoracico-coxal muscle receptor in shore crabs confirm the graded, dynamic-static nature of the receptor potentials evoked by stretching the receptor muscle, and the lack of afferent impulses.
2. Slow ramp-function stretches evoke receptor potentials with characteristic shapes, which differ between the two fibres in several respects:
(i) The dynamic component in the S fibre resembles an algebraic sum of length and velocity responses and a variable initial ‘acceleration’ (?) transient, while in the T fibre it commonly declines (‘adapts’) during stretching, especially at greater velocities and starting lengths.
(ii) On release of stretch the S fibre usually exhibits a ‘negative velocity response’, but the T fibre repolarizes rapidly often with a slight hyperpolarization.
(iii) The dynamic response of the T fibre is generally greater than that of the S fibre, and increases more steeply and approximately logarithmically with stretch velocity over a 10- to 50-fold range.
(iv) The ‘static response’ or degree of depolarization increases fairly linearly with receptor length in the S fibre but very non-linearly in the T fibre.
(v) The T fibre displays pronounced hysteresis in its dynamic and static responses at increasing and decreasing lengths, but the S fibre shows little hysteresis.
(vi) The T fibre but not the S fibre commonly shows small rapid oscillations or ‘noise’ superimposed upon strongly depolarized ‘static’ potentials.
(vii) The S and T responses may be affected reciprocally by some forms of receptor muscle contraction.
3. Graded receptor potentials evoked in the ‘D’ fibre by stretching the non-muscular depressor-receptor strand of the coxo-basal joint show little hysteresis.
4. Receptor muscle fibres respond to motor nerve stimulation or spontaneous motor impulses from the thoracic ganglion with slow, facilitating and summating excitatory junctional potentials.
5. The mechanisms underlying the differences between S and T responses, and their functional significance to the animal, are discussed, and comparisons are drawn with other muscle receptors.