Tactile sensory coding during development: signaling capacities of neurons in kitten dorsal column nuclei

Abstract

The functional maturation of cuneate neurons was studied using reproducible tactile stimulation procedures and quantitative assessment of coding capacities in anesthetized (N2O/O2 plus barbiturate infusion) or decerebrate cats from six different age groups; neonatal (1-5 days), 10-15, 25-30, 55-63, and 83-90 days, and an adult group. Cuneate neurons were distinguished from input fibers to the nucleus on criteria of spike configuration and time course and on response profiles. Extracellular spike durations underwent a progressive shortening with age, reaching the adult range at approximately 3 mo when background activity levels also reached maturity. Despite an increase in conduction-path length, response latencies decreased to adult values by 25-30 days of age, presumably reflecting the increased conduction velocity in the input fibers. In each age group three functional classes of neurons responsive to tactile stimulation of the footpads were identifiable. One consisted of slowly adapting neurons whose stimulus-response relations resembled those of the adult in responsiveness and dynamic range by 25-30 days postnatal age. The remaining neurons were purely dynamically sensitive, but among them two classes could be distinguished by their differential sensitivity to cutaneous vibration, one receiving rapidly adapting (RA) fiber input the other pacinian corpuscle (PC) input. The combined bandwidth of vibration sensitivity in dynamically sensitive cuneate neurons expands from approximately 5-300 Hz in the neonate to the mature range of 5-1,000 Hz by 1 mo of age. The PC class of cuneate neurons showed an upward shift in peak sensitivity from 30-200 Hz in the neonate to 100-300 Hz at 10-15 days, reaching adult values of 200-600 Hz by 25-30 days. Over this period absolute thresholds dropped by an order of magnitude at 200-500 Hz reflecting the threshold trends in primary PC fibers. During the first 1-2 postnatal months, the capacity of cuneate PC neurons to signal information in a pattern code about vibration frequencies around 300 Hz is restricted because of poor phase locking and low responsiveness, which preclude an impulse periodicity reflecting the vibratory frequency.(ABSTRACT TRUNCATED AT 400 WORDS)