REORGANIZATION OF THE RACCOON CUNEATE NUCLEUS AFTER PERIPHERAL DENERVATION.
Douglas D. Rasmusson and Stacey A. Northgrave.
Department of Physiology and Biophysics, Dalhousie University, Halifax,
Nova Scotia, Canada B3H 4H7.
APStracts 4:184N, 1997.
ABSTRACT
The effects of peripheral nerve transection on the cuneate nucleus were
studied in anesthetized raccoons using extracellular, single-unit recordings.
The somatotopic organization of the cuneate nucleus was first examined in
intact, control animals. The cuneate nucleus in the raccoon is organized with
the digits represented in separate cell clusters. The dorsal cap region of the
cuneate nucleus contains a representation of the claws and hairy skin of the
digits. Within the representation of the glabrous skin, neurons with rapidly
adapting properties tended to be segregated from those with slowly adapting
properties. The representations of the distal and proximal pads on a digit
were also segregated. Electrical stimulation of two adjacent digits provided a
detailed description of the responses originating from the digit that contains
the tactile receptive field (the on-focus digit) and from the adjacent (off-
focus) digit. Stimulation of the on-focus digit produced a short latency
excitation in all 99 neurons tested, with a mean of 10.5 ms. These responses
had a low threshold (450 æA). Stimulation of an off-focus digit activated 65%
of these neurons. These responses had a significantly longer latency (15.3 ms)
than on-focus responses and the threshold was more than twice as large.
Two to five months after amputation of digit 4, 97 cells were tested with
stimulation of digits 3 and 5. A total of 44 were in the intact regions of the
cuneate nucleus. They had small RFs on intact digits and their responses to
electrical stimulation did not differ from the control neurons. The remaining
53 neurons were judged to be deafferented and in the 4th digit region on the
basis of their location with respect to intact neurons. All but two of these
cells had receptive fields that were much larger than normal, often including
more than one digit and part of the palm. When compared to the off-focus
control neurons, their responses to electrical stimulation had lower
thresholds and an increased response probability and magnitude. The latencies
of these cells did not decrease, however, and were the same as the off-focus
control values. The enhanced responses of the deafferented neurons to adjacent
digit stimulation indicates that there is a strengthening of synapses that
were previously ineffective. The increased proportion of neurons that could be
activated after amputation suggests that there is also a growth of new
connections.
Received 4 April 1996; accepted in final form 5 August 1997.
APS Manuscript Number J460-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997