Figure 3. Responses of a single unit to stretch by imposed length and imposed load.

A, the preparation was stretched by ‘imposed length’ by 3 mm at 5 mm s⁻¹, held for 30 s, evoking an active contraction just after the onset of stretch that declined over the next 5–10 s. Rectal mechanoreceptor firing more closely followed force rather than length. B, the same preparation was stretched by ‘imposed load’ in which the counterweight on the isotonic transducer was increased by 30 mN, for approximately 30 s. Note that increasing the load evoked firing. In this particular unit, an active contraction reduced firing in the unit. C, the same preparation was stretched by graded increase in ‘imposed load’ (100 μm s⁻¹), evoking initially bursts of firing that were associated with contraction, then constant firing. Note that at some points active contractions of the preparation appeared to be associated with a decrease in firing (asterisks), possibly reflecting ‘offloading’ of the receptor (see Discussion).