Responses of neurons in the gracile nucleus of cats to innocuous and noxious stimuli: basic characterization and antidromic activation from the thalamus.

1. Responses to innocuous and noxious mechanical and thermal stimuli were recorded from 90 neurons in the gracile nucleus of anesthetized cats. Cells were tested by antidromic activation for projections to the contralateral ventrobasal thalamus. 2. Cells were characterized broadly by their responses to mechanical stimuli as 1) responding only to tapping (16%), 2) fast-adapting to low-intensity mechanical stimuli (33%), or 3) slowly adapting (51%; most with a fast-adapting component to their responses). All fast-adapting cells and those slowly adapting cells that were tested with noxious heat were further categorized on the basis of their patterns of firing and responses to stimuli. These plus the tap-responsive cells comprised a more restricted sample of 76 categorized cells. 3. Many (22) slowly adapting cells responded to noxious heat (69% of tested slowly adapting cells; 29% of all categorized cells), either on the first application (9 cells) or after sensitization (13 cells), indicating input originating in nociceptors. Nearly all of these (21) responded more to intense pressure than to innocuous pressure. The majority of slowly adapting cells not responsive to noxious heat (5 of 8) or not tested with it (8 of 12) also responded more to intense than to innocuous pressure, suggesting possible input originating in nociceptors. Most cells that responded to noxious heat also had both rapidly and slowly adapting responses with low thresholds. Many were recorded in the range of the cluster region of the gracile nucleus. 4. Cells antidromically activated from the thalamus projected to the rostral part of the ventral posterior lateral nucleus, regardless of their physiological category, and included many with nociceptive input. Latencies of antidromic activation were shorter at more caudal locations in the gracile nucleus, indicating higher conduction velocities to the thalamus. Responses of antidromically activated cells to low-intensity phasic stimuli tended to be greater than those of cells not antidromically activated. 5. Background activity of the neurons was low, most firing at less than one spike/s. Antidromically activated cells had higher background activity than cells not antidromically activated. 6. The results indicate a greater proportion and more widespread distribution of cells with nociceptive input in the cat gracile nucleus than has been previously recognized. Many of these projected to the ventrobasal thalamus, showing that information originating in nociceptors can reach the thalamus through a dorsal column-medial lemniscal pathway in cats.