Mechanoreceptor activity in the gills of the carp. I. Gill filament and gill raker mechanoreceptors.

Physiological properties of gill filament and gill raker mechanoreceptors in the gills of spontaneously breathing carp, Cyprinus carpio L., were analysed. Stroking stimuli applied to gill filaments elicited a phasic mechanoreceptive response, which was recorded from neurons in the epibranchial ganglia. Sustained deflection resulted in a short on-off response. The same neurons were also activated by slight movements of lamellae on a gill filament. The receptive field extended over all the lamellae of one filament at most, but generally covered a small part of it, including both dorsal and ventral lamellae. Deflection of gill rakers also elicited a brief response in epibranchial ganglion neurons. The threshold of both filament-related and gill raker mechanoreceptors was relatively high. They did not respond during normal respiration. It was therefore argued that these receptors do not function in normal respiratory control, but rather serve against mechanical damage from excessive pressure or particles in the water.

Mechanoreceptor activity in the gills of the carp. II. Gill arch proprioceptors.

The presence of gill arch proprioceptors in the gills of a teleost, Cyprinus carpio L., is demonstrated and their firing characteristics are analysed. Spontaneous activity of gill arch proprioceptors was recorded from epibranchial ganglia. In paralysed fish the mean discharge rate for 16 receptors ranged from 11.3 impulses X sec-1 (SD 0.3) to 25.7 impulses X sec-1 (SD 0.4). The discharge rate could be influenced by displacement of the main elements of the respiratory pumping system. The receptors showed a tonic response. Their firing frequency was approximately linearly related to gill arch position and, hence, showed a respiratory modulation during ventilation. Gill arch adduction caused a decrease and abduction an increase in discharge rate. In actively breathing fish the mean firing frequency of 30 neurons ranged from 9.9 impulses X sec-1 (SD 0.7) to 40.1 impulses X sec-1 (SD 6.7). These gill arch proprioceptors are located in the cartilaginous strip between the epibranchial and the ceratobranchial of each gill arch and are innervated by the pretrematic branches of the vagal branchial nerves. The role these proprioceptors play in the regulation of gill movements during both feeding and ventilation is discussed.