Evidence for exclusive adrenergic innervation of feather muscles (mm. pennati) in the chicken. Histochemical studies and experiments with 5-hydroxydopamine.

Feather follicles in the avian skin are interconnected by well-defined bundles of smooth muscle cells, which are responsible for the erection and depression of feathers and thus play an important role in thermoregulation. The depressing and erecting muscle bundles were found to receive a very dense supply of unmyelinated nerve fibres that displayed ultrastructural and histochemical characteristics of noradrenergic axons (formaldehyde- and glyoxylic acid-induced catecholamine fluorescence; uptake to 5-hydroxydopamine). No nerve fibres were encountered showing histochemical acetylcholinesterase activity. There was no indication of the presence of peptidergic or purinergic nerve endings. The neuromuscular space usually ranged from 40-60 nm in width and contained a basal lamina. Occasionally, this space was reduced to approximately 20 nm. At such close neuromuscular contacts a basal lamina was lacking, and focal densities beneath the pre- and postsynaptic plasma membrane were observed. Since no gap junctions between muscle cells were detected, the dense supply with noradrenergic nerve fibres indicates a high amount of directly innervated smooth muscle cells. An additional finding of the present study was the observation that high local concentrations of 5-hydroxydopamine led to degeneration of noradrenergic nerve endings.