Burst responses in thalamic relay cells of the awake, behaving cat.
Guido, William & Theodore Weyand.
Department of Anatomy and Neuroscience Center for Excellence, LSU Medical
Center, New Orleans, LA 70112.
APStracts 2:0218N, 1995.
SUMMARY AND CONCLUSIONS
1. Relay cells of the dorsal lateral geniculate nucleus (LGN) respond in one
of two modes: tonic or burst . The purpose of this study was to determine if,
and under what conditions, burst responses occur in LGN cells of an awake,
behaving animal. 2. We recorded the extracellular responses of cells located
in the A-layers of LGN in two cats trained to perform a simple fixation task.
In our paradigm, head position was fixed and gaze monitored using the scleral
search coil technique. They were trained to fixate on a small target light
while we probed the receptive field of an isolated LGN cell with drifting
gratings. 3. Many LGN cells (37/48; 77%) recorded in the awake cat showed some
form of burst firing. However, the degree of burstiness a given cell displayed
was relatively low. The probability of recording a burst response during any
sec within the fixation task was 0.09. This reflects the fact that during
wakefulness, LGN cells are generally in a state of relative depolarization and
the low threshold Ca 2+ conductance underlying burst firing is inactivated. 4.
The majority of bursts occurred (262/377; 71%) during visual stimulation.
These comprised the initial response to the grating and were confined to the
early phase of fixation. As the cat continued to hold fixation and as
subsequent cycles of the grating passed through the cell's receptive field,
the response shifted from a burst to a tonic firing pattern. Some bursts
(67/377, 18%) were related to eye movements. All of these were postsaccadic,
and most occurred >150 msec from the onset of the eye movement. Finally, some
bursts were neither visually driven nor related to eye movements. However,
these spontaneous bursts were infrequent (41/377; 11%) and never rhythmic. 5.
Burst firing in LGN cells has a dual purpose. During quiescent states such as
slow wave sleep, they support the decoupling of retinal signals from LGN.
During the waking state, bursts can facilitate signal transmission during
target acquisition and early phases of fixation.
Received 9 June 1995; accepted in final form 20 July 1995.
APS Manuscript Number J370-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.