Responsiveness and ultrastructure of slowly adapting type I cutaneous mechanoreceptors in vitamin A deficient rats

Single-unit recordings were made from afferent nerve fibres supplying slowly adapting type I (s.a. I) cutaneous mechanoreceptors in anaesthetized vitamin A deficient and control rats. Trains of thirty repetitive mechanical stimuli with 0.1 s rise time, 1.9 s plateau phase, and 0.7 s interstimulus interval were applied. A feed-back mechanism maintained the force of stimulation at 20 mN during the plateau phases and the contact force between stimuli at 0.5 mN. All displacement values in the group of vitamin A deficient rats were significantly larger than the corresponding control values. Residual indentations were increased by 70-100% while maximal indentations were only about 40% higher. These results indicate a non-linear increase in compliance of the skin and underlying tissues. S.a. I receptors were found to be significantly less responsive in vitamin A deficient animals. Mean numbers of impulses were about 25% lower in the vitamin A deficient group than in controls throughout the entire train of thirty stimuli. In vitamin A deficient rats, Merkel cells and adjoining nerve terminals showed signs of degeneration of a variety of cell organelles, particularly the mitochondria. Degenerative changes induced by vitamin A deficiency especially in the Merkel cells appeared to be a major cause of the reduction of responsiveness in s.a. I receptors.