Responses of the Carp Pit Organ to Chemical and Thermal Stimuli

Abstract

1. Electrical responses to chemicals and thermal changes were recorded from the buccal nerve branch innervating pit organs on the snout region of the carp, Cyprinus carpio.
2. Spontaneous discharges recorde from the buccal nerve branch were quite active, High spontaneous activity was also observed in most single fibers emamined and the mean rate of the spontaneous discharges in 52 fibers was 68.8±38.0 impulses/5 sec.
3. Responses to electrolytes were clear, but those to non-electrolytes were obtained neither from the whole nerve prparation nor from the single fibers. Pit organs are considered to be ion detectors.
4. The threshold of HCL was below 10-5M and that of NaCl was a little higher than this value. High sensitivity to acids and salts may be useful for the detection of the water quality.
5. Response patterns to chemicals wre quite uniform and no specific fiber was found in the buccal nerve branch. Since high correlation coeffcients were obtanied among the sensitivities to various chemicals, it may be assumed that the pit organ can not discriminate the variety of taste qualities and that it functions as a primitive type of chemoreceptor.
6. In the whole nerve response, thermal changes produced two response patterns: excitatory to the cooling and inhibitory to the warming. Temperature sensitive fibers, which were also sensitive chemicals, were classified into 4 types according to their response patterns. Pit organs, may be considered to act as thermal receptors as well as well as chemoreceptors.