Changes in the response properties of neurons in the ventroposterior
lateral thalamic nucleus of the raccoon after peripheral deafferentation.
Rasmusson , Douglas D.
Department of Physiology & Biophysics, Dalhousie University Halifax, N.S.,
Canada, B3H 4H7.
APStracts 2:0374N, 1995.
SUMMARY AND CONCLUSIONS
1. Single neurons in the ventroposterior lateral thalamic nucleus were studied
in 10 anesthetized raccoons, 4 of which had undergone amputation of the fourth
digit 4-5 months before recording. Neurons with receptive fields on the
glabrous skin of a forepaw digit were examined in response to electrical
stimulation of the "on-focus" digit that contained the neuron's receptive
field and stimulation of an adjacent, "off-focus" digit. 2. In normal raccoons
all neurons responded to on-focus stimulation with an excitation at a short
latency (mean 13 ms), while only 63% of the neurons responded to off-focus
digit stimulation. The off-focus responses had a longer latency (mean 27.2 ms)
and a higher threshold than the on-focus responses (800 and 452 [mu]A,
respectively). Only 3 of 32 neurons tested with off-focus stimulation had both
a latency and a threshold within the range of on-focus values. Inhibition
following the excitation was seen in the majority of neurons with both types
of stimulation. 3. In the raccoons with digit removal the region of the
thalamus that had lost its major peripheral input (the "deafferented" region)
was distinguished from the normal third and fifth digit regions on the basis
of the sequence of neuronal receptive fields within a penetration and
receptive field size as described previously (Rasmusson 1995). 4. Almost all
of the neurons in the deafferented region (91%) were excited by stimulation of
one or both adjacent digits. The average latency for these responses was
shorter (15.3 ms) and the threshold was lower than was the case with off-focus
stimulation in control animals. These values were not significantly different
from the responses to on-focus stimulation in the animals with digit
amputation. 5. These results confirm that reorganization of sensory pathways
can be observed at the thalamic level. In addition to the changes in the
somatotopic map that have been shown previously using mechanical stimuli, the
present paper demonstrates an improvement in several quantitative measures of
single unit responses. Many of these changes suggest that this reorganization
could be explained by an increased effectiveness of pre-existing, weak
connections from the off-focus digits; however, the increase in the proportion
of neurons responding to stimulation of adjacent digits may indicate that
sprouting of new connections also occurs.
Received 22 September 1995; accepted in final form 11 December 1995.
APS Manuscript Number J635-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95