Eyecup Withdrawal in the Crab, Carcinus, and Its Interaction With the Optokinetic Response

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Abstract
Protective withdrawal of the eyecup is caused by a burst of impulses in two axons of the optic tract, one to muscles 19 a, 19 b and 20 a, the other to muscles 18, 20b, 21 and 22. At a reflex eyecup withdrawal other concurrent activity is mechanically overridden; the tonic activity in only one muscle is inhibited centrally. At a ‘spontaneous’ withdrawal, however, all motor activity to that eyecup is inhibited. The largest muscle, 19 a, inactive in other eyecup movements, is the prime mover in withdrawal, and some tonic fibres of this muscle hold the eyecup withdrawn. Two muscles which move the eyecup toward the mid line on optokinetic responses are excited during a withdrawal. It is therefore possible for one muscle to contribute to movements in opposite directions. Repeated reflex withdrawal of an eyecup moving towards the mid line inhibits the optokinetic response of the other eye. Weak stimulation of an eyecup region by a variety of means, including withdrawal, improves the optokinetic response of that eyecup and sometimes of the other eyecup