SHORT COMMUNICATION Development of Spontaneous Activity and Response Properties of Primary Lagenar Neurons in the Chick

Abstract Here, we report for the first time developmental changes in spontaneous activity and in response properties of single nerve fibers from the macular chick lagena. Such aspects are important in order to get insight into the functional role of the lagena which remains undetermined. For this purpose, we used intracellular and extracellular single-unit recording techniques in an isolated inner ear preparation from the chicken at ages E15 and P1. At E15, afferent fibers displayed a low irregular spontaneous dis-charge rate (41 ± 14 spikes/s, CV = 1.17 ± 0.1), which was replaced by regular high frequency spontaneous activity at P1 (CV = 0.48 ± 0.8, 89 ± 27 spikes/s). Dur-ing the developmental period including E15, the percent-age of silent neurons was 60 % while that of P1 was 40%. The synaptic activity was higher at E15 than at P1. The action potential waveform generated at E15 had small amplitude and derivative depolarization, and consequently, a large duration in correlation with respect to action potential waveform at P1 (respectively: 53 ± 2 vs. 65 ± 3 mV, 60 ± 11 vs. 109 ± 20 mV/ms, 3.6 ± 0.4 vs. 1.1 ± 0.12 ms). In addition, we recognized two response dynamics to the injection of current steps: phasic, or rap-idly adapting neurons and tonic, or slowly adapting neurons. Our results indicate similar developmental pro-cesses for the lagena as described for the vestibular system in other species, in agreement with the known morpho-logical characteristics of this otholitic end organ. The presence of more than one subtype of afferent neuron also correlates with previous reports on vestibular afferents with analogous electrophysiological properties, strongly sug-gesting the vestibular nature of the lagena