Factors regulating the discharge frequency in optomotor fibres of Carcinus maenas.

An extensive study has been made of the optomotor system in Carcinus by Horridge with various collaborators (Barnes & Horridge, 1969a, b; Burrows & Horridge, 1968a-c; Horridge, 1966a-e, 1968; Horridge & Burrows, I968a-c; Horridge & Sandeman, 1964). In these investigations many features were discovered by the use of various and ingenious techniques. The reason for the present paper and its sequel is that during a study of the responses from the optic nerve of this species (Wiersma, I 970 ), advantage was taken of the frequent occasions in which action potentials were obtained from optomotor fibres or muscles, and some additional facts were thereby discovered. In the present pape1 we will deal especially "With those influences which affect the discharge rates in various optomotor fibres. In the crayfish it has been noted that most motor fibres to the eye muscles are strongly influenced by the state of excitement shown by the animal (Wiersma & Oberjat, 1968). Though such an influence has been mentioned in passing by Burrows & Horridge (Burrows & Horridge, 1968c; Horridge, I966e), it was not studied in any detail. It was found that an increase in the state of excitement in Carcinus leads also to increased discharges in most optomotor fibres, but that at least one specific motor neurone is inhibited by this state. In addition it was observed that there was a pronounced change in habituation to constant stimuli when 'spring' and 'summer' animals were compared.