Motoneurone Discharges to the Eyecup Muscles of the Crab Carcinus

Abstract

During imposed tilt the eyecup of the crab tends to preserve an absolute position which depends upon the tonic activity of eight of the nine eyecup muscles. The detailed activity of all these muscles during imposed tilt in different planes has been recorded intracellularly. The slow- and fast-motoneurone discharges to the eyecup muscles differ in that the former have intervals which are more variable at lower frequencies but the latter are more variable at higher frequencies. The standard deviation of the interval between impulses is 20–30% of the mean interval for a wide range of frequencies of the tonic motoneurones. This large scatter is tolerable because the slow muscle fibres are sluggish and because the eyecup is also visually stabilized by a system of long time-constant. In roll the two eyecups move in opposite directions relative to the midline of the animal. In pitch the two eyecups move in the same direction relative to the body of the animal, but in neither case is there a correlation between individual impulses to the muscles of the right and left eyecups which are active at the same time. Possible mechanisms of linkage between the two eyecups are discussed.