Multiunit neural responses to strong finger pulp vibration. I. Relationship to age.

The effect of age on mechanoreceptive function in the distal part of the fingers was studied in 12 healthy subjects aged 18-64 years. A recording microelectrode was inserted into a sensory fascicle of the median nerve at the wrist. Multiunit mechanoreceptor activity was recorded from the fascicular field, which is typically restricted to the ulnar or radial half of one finger, corresponding to the innervation zone of one digital nerve. Strong standardized vibration (40 Hz) was applied to the finger pulp, with the amplitude of the vibration pulses high enough to induce maximal neural impulse volleys. With the same microelectrode recording position, maximal neural impulse volleys were also induced by electrical pulses applied to the ventral digital nerve supplying the finger pulp. In each subject, the areas of the mechanically and the electrically induced responses were measured (after integration and averaging) and the ratio of mechanically to electrically induced responses (MR/ER) was determined. The MR/ER ratio decreased with increasing age of the subject. This finding was considered to indicate that it is the peripheral parts of the sensory units, involved in the mechanoelectrical transfer functions, that exhibit the most pronounced degenerative changes during the ageing process. This is in line with previous histological findings of pronounced age-related degeneration of Meissner corpuscles and other mechanoreceptive end organs in the finger pulps.