Functional capacities of tactile afferent fibres in neonatal kittens

Abstract

Responses were recorded from individual tactile afferent fibers isolated by microdissection from the median nerve of pentobarbitone-anesthetized neonatal kittens (1-5 days post-natal age). Adult cats were compared. Neonatal fibers with receptive fields on the glabrous skin of the foot pads were classified into 2 broad groups, a slowly adapting class (40%) responding throughout a 1 s period of steady indentation and a rapidly adapting or dynamically sensitive class comprising 60% of units. Fibers in these 2 groups had overlapping conduction velocities in the range 4.3-7.5 m/s and were presumably developing Group II afferents of the adult. Neonatal slowly adapting fibers qualitatively resembled their adult counterparts. They displayed graded stimulus-response relations over the steepest segment of the curves with mean slopes of 15.7 impulses/100 .mu.m of indentation. Plateau levels of response were often reached at amplitudes of skin indentation of < 0.5-0.7 mm. Dynamically sensitive fibers with receptive fields on the glabrous skin were studied using sinusoidal cutaneous vibration. In the adult they were divided into 2 distinct classes. In the neonate they formed a continuum with criteria of sensitivity or responsiveness. In response to vibration neonatal fibers differed from adult fibers in the following quantitative indices: sensitivity, as measured by absolute thresholds and thresholds for a 1:1 pattern of response, was higher in the neonate than in the adult at all frequencies > 50 Hz and differed by an order of magnitude at frequencies .mchgt. 200 Hz; responsiveness based on the mean impulse rate evoked at a fixed amplitude of cutaneous vibration; band width of vibratory sensitivity was confined to approximately 5-300 Hz in the neonate but covered the range 5-800 Hz in the 2 adult classes; capacity for coding information about vibration frequency. Impulse activity of neonatal fibers was less tightly phase-locked to the vibratory stimulus and showed a poorer reflection of the periodic nature of the vibratory stimulus than impulse patterns of adult units. Tactile receptors and afferent fibers in the neonate are functionally immature. Their restricted coding capacities imply peripheral tactile sensory mechanisms impose limits on the ability of the new-born animal to derive information about its tactile environment.