The control of load compensation in the hermit crab, Pagurus pollicarus: Characterization of a reflex discharge from postural motoneurons and its action in modulating force production

Date of Completion

January 2004


Biology, Neuroscience|Biology, Animal Physiology




Oceanic hermit crabs occupy large gastropod shells. Shells are carried above, rather than dragged upon, the ocean floor during normal behavior. This is accomplished in part by the superficial muscles of the abdomen, which are activated in a tri-phasic pattern by local mechanoreceptors. Reflex activation of motoneurons consists of an initial burst, a period of inhibition, and a late discharge. ^ Results reported here indicate that this motor system acts in a feedforward fashion: (1) the reflex persists in the absence of peripheral feedback; (2) gain from change in afferent rate to change in reflex discharge is very small; (3) there is no relationship between the site of cuticular touch and the intensity or pattern of motoneuron activation; and (4) the reflex is generated by centrally located premotor neurons and does not require more than a single afferent potential (a trigger). ^ Although the tri-phasic reflex generates force in the postural muscle rapidly, this force rarely exceeds about 25% of maximum. Rate of force production is high early in the reflex, but the following decline in rate is comparable. We address the mechanisms underlying the transition from the first to second phase of motoneuron spiking and their contribution this change in force production. Two mechanisms are described: (1) the centrally generated ipsps of phase 2, and (2) a profound spike frequency adaptation (SFA) that is activated by phase 1 spiking that is voltage sensitive, and which is attenuated by transient repolarization of the membrane. ^ To address the question of whether tri-phasic motoneuron activation offers advantages over monophasic activation, variability in five parameters of reflex firing is assessed. Each of these parameters is correlated with neighboring reflex parameters and force production. Phase 3 frequency shows the greatest correlation with reflex force production. Phase 3 spike rate increases as a function of phase 2 duration and this is by means of SFA attenuation. Paradoxically, the relationship between phase 2 duration and force is weak. This cannot be explained by changes at the neuromuscular junction. We suggest that a non-linearity of force production, originating from intra-muscular dynamics, contributes to reflex force production. ^