The role of the cuticular stress detector, CSD1 in locomotion and limb autotomy in the crabCardnus maenas

Abstract

1. CSD1, situated between the insertion of the anterior BI levator muscle (AL) and the breakage plane in the BI, is sensitive to strains imposed upon the BI by isometric contractions of the AL muscle (Figs. 2b; 7c; 10b). 2. The posterior levator muscle (PL) is supplied by two axons (Fig. 2c), one, slow unit elicits small ejp's in PL muscle fibres (Figs. 4b, d; 5b) and has no influence upon PL tendon orientation (Fig. 3 a). The other, fast PL unit elicits large fast ejp's (Figs. 4a, b, c; 5a) and results in PL tendon rotation (Fig. 3b; Table 1). 3. Reflex connections from CSD1 excite the PL fast unit and inhibit activity in AL units and the PL slow unit (Figs. 2c; 7a b; 9). 4. During normal locomotion AL and PL fire synergisticly (Fig. 8b, c), with alternate firing from the BI depressor muscle (Fig. 8a, b). PL activity results from central excitation of the slow unit (Fig. 8c), the PL muscle provides little, if any support to the limb. 5. Accidental autotomy is prevented by CSD1 inhibiting AL activity should the breakage plane be threatened (Fig. 9), but to achieve autotomy when the limb is injured, CSD1 reflex inhibition of AL units is over-ridden or bypassed (Fig. 11) while excitation of the PL fast unit by CSD1 is retained to switch AL force onto the breakage plane.