In vivo recording of the ocular circadian rhythm in Aplysia.

A circadian rhythm in optic nerve impulse frequency was recorded in vivo from unrestrained Aplysia. The mean free-running period (23.9 h) and phase angle for entrainment to light cycles are comparable to values estimated from isolated eyes. The circadian waveform of eyes recorded in vivo is irregular and thus similar to eyes recorded in vitro with the cerebral ganglion attached. However, optic nerve activity recorded in vivo differs from in vitro records in that efferent modulation of afferent impulse patterning is intermittent and is usually associated with the initiation of head or body movements.