Journal of Neurophysiology, Vol 39, Issue 3 582-600, Copyright © 1976 by APS
Dynamic response and transfer characteristics of joint neurons in somatosensory thalamus of the cat
T. C. Yin and W. J. Williams
1. The dynamic response of neurons sensitive to knee joint rotation in the
cat somatosensory thalamus was studied during sinusoidal variation of joint
angle. The input sine waves were applied with a precise voltage-controlled,
electromechanical actuator. The average rate of discharge of a single cell
was considered as the output parameter. Describing functions of the sensory
system were extracted by correlation and spectral analysis techniques. The
effects of anesthetic, bias angle, and excursion angle were investigated.
Discrete and swept sinusoidal waveforms between 0.1 and 7.0 Hz were used as
inputs.2. The majority of joint cells in the thalamus were rapidly adapting
and had frequency-response curves that were characterized as highpass
filters. Although the major features of the response curves for individual
cells were very similar, they could not all be fit with a single transfer
function. The describing function of all the rapidly adapting cells
averaged together was well fit by a transfer function that could be termed
velocity sensitive in the bandwidth between 0.1 and 6.5 Hz. Most of these
phasic cells showed a phase-locking tendency, particularly at high
frequencies.3. The dynamics of the response for the rapidly adapting cells
was relatively independent of anesthetic, bias angle, and excursion angle.
Threshold and saturation effects were exhibited by some cells for very
small (less than 1 degree) and large (greater than 10 degrees) input
amplitudes, respectively. In addition a few (17%) showed a bidirectional
response, i.e., responded at both flexion and extension of the limb. The
anesthetic had a strong effect in depressing the spontaneous discharge of
the cells and seemed to change the character of the tonic response by
introducing a bursting component.4. The transfer characteristic of the
thalamic cells was found to be a single-pole low-pass filter plus a time
delay. The optimized value for the filter was found to have a corner
frequency of 6.0 Hz with a time delay of 6 ms.5. Of the knee joint
sensitive cells, 17% were slowly adapting or tonic, and more tonic cells
were found in the unanesthetized animals. Only one tonic cell was studied
in detail, and its dynamic characteristics were similar to that of the
slowly adapting joint receptors at low frequencies. In this respect the
rapidly adapting and slowly adapting joint cells in the thalamus have
strikingly different frequency-response curves, the former curves have a
much steeper slope in the magnitude.6. The functional implications of these
results and of other recent findings in relation to the probable role of
joint receptors in mediating proprioception are discussed.
