Development of vagal innervation to the muscle of the avian gizzard

Abstract

The development of the vagal innervation to the gizzard has been investigated in chick embryos and young chicks. The membrane potentials, first measurable on the 15th day ofincubation, was −54 ±0.5 mV and increased with development to −67 ± 0.4 mV. The latter value was attained 5 days after hatching and persisted thereafter. Stimulation of intramural nerves elicited a cholinergic, excitatory junction potential (EJP) for the first time, only in a small fraction of cells, on the 20th day of incubation. Within 3 days after hatching, cholinergic transmission showed the same features as in older chicks. Stimulation of the vagus nerve elicited membrane potential responses before hatching but as early as 4 days after hatching, non-adrenergic, inhibitory junction potentials (IJPs) were evoked. In the next 10 days or so, the IJP was replaced with a cholinergic EJP as seen in mature tissues. After atropine (0.1–1μM) treatment, both vagal and intramural nerve stimulations evoked a non-adrenergic IJP in a small fraction of cells immediately after hatching. The fraction of cells exhibiting the IJP increased with growth and reached 100% 5 days after hatching. Hexamethonium (50 or 100μM) abolished the vagally-evoked EJPs. The vagally-evoked IJPs remained unchanged after application of hexamethonium in the early days after hatching, but later they were abolished in about half of the cells. The present results suggest that the cholinergic, excitatory transmission and the non-adrenergic, inhibitory transmission to smooth muscle cells of the chicken gizzard begin to function at about the time of hatching and attain maturation within 5 days after hatching, and that the ganglionic transmission in the vagal excitatory and inhibitory pathways to this organ takes more time than the establishment of both types of neuromuscular transmission.