Mechanical properties of skin and responsiveness of slowly adapting type I mechanoreceptors in rats at different ages.

Slowly adapting type I (s.a. I) cutaneous mechanoreceptors were studied in young (3-4 months old) and adult (9-11 months old) 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 of stimuli and the contact force between stimuli at 0.5 mN. During the first few stimuli in a train residual indentation at contact force increased rapidly. Maximal indentation required to maintain the force of stimulation of 20 mN increased as well but to a smaller extent. Thus, the stroke amplitudes of individual stimuli decreased with increasing stimulus number. All displacement values in the group of adult rats were consistently reduced to 62 +/- 3% of the respective values in the group of young rats, indicating a linear decrease in skin compliance in the force range of 0.5-20 mN. Nervous responses to individual stimuli decreased from about 200 impulses for stimulus number 1 to about 60 impulses for stimulus number 30. Significant differences in the number of impulses between young and adult rats were observed from stimulus number 9 to number 16 only. It is concluded that the design of the s.a. I receptor allows maintained high tactile sensitivity in response to force-related stimuli irrespective of age-induced changes in mechanical properties of the skin and underlying tissues.