SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .2. RESPONSES TO INNOCUOUS AND NOXIOUS MECHANICAL ANDTHERMAL STIMULI

1. The goal of this study was to gather data that would increase our u nderstanding of nociceptive processing by spinothalamic tract (STT) ne urons that receive inputs from the hand and arm. Fifty neurons in the cervical enlargement of urethan-anesthetized rats were antidromically activated from the contralateral posterior thalamus. A stimulating ele ctrode was moved systematically within an anterior-posterior plane in the thalamus until a point was located where the smallest amount of cu rrent antidromically activated the neuron. The antidromic thresholds a t each of these lowest threshold points was less than or equal to 30 m u A; the mean antidromic threshold was 15.4 +/- 1.0 (SE) mu A. Lowest threshold points were found primarily in the posterior thalamic group (Po), zona incerta, and in or near the supraoptic decussation. 2. The recording sites of 47 neurons were marked and recovered. Recording sit es were located in the superficial dorsal horn (SDH, n = 15), deep dor sal horn (DDH, n = 31), and ventral horn (n = 1). Recording sites were located across the mediolateral extent of the SDH. Within the DDH, re cording sites were concentrated laterally in nucleus proprius and dors ally in the lateral reticulated area. The locations of the recording p oints confirm previous anatomic descriptions of STT neurons in the cer vical enlargement. 3. Cutaneous excitatory receptive fields were restr icted to the ipsilateral forepaw or forelimb in 67% (10/15) of the neu rons recorded in the SDH and 42% ( 13/31) of the neurons recorded in t he DDH. Neurons having larger, more complex receptive fields were also commonly encountered. Thirty-three percent (5/15) of the neurons reco rded in the SDH and 58% (18/31) recorded in the DDH had receptive fiel ds that were often discontinuous and included areas of the ipsilateral shoulder, thorax, and head, including the face. 4. Innocuous and noxi ous mechanical stimuli were applied to the receptive field of each neu ron. Fifty percent (25/50) responded to innocuous mechanical stimuli b ut responded at higher frequencies to noxious stimuli (wide dynamic ra nge, WDR). Forty-four percent (22/50) responded only to noxious stimul i (high threshold, HT). Six percent (3/50) responded preferentially to innocuous stimuli (low threshold, LT). WDR and HT neurons were record ed in both the SDH and DDH, including nucleus proprius, an area not ty pically associated with nociceptive transmission at other levels of th e cord. Sixty percent (9/15) of the units recorded in the SDH were cla ssified as WDR neurons; the other 40% (6/15) were classified HT. Forty -eight percent( 15/31) of the units recorded in the DDH were classifie d as WDR neurons and 42% ( 13/31) as HT. All neurons classified as LT were recorded in the DDH (3/31). 5. Eighty-six percent (30/35) of the neurons that were tested responded to noxious heat. The calculated mea n threshold was 43.2 degrees C. The stimulus-response relationship was described by a power function with an exponent of 3.5, indicating tha t the responses were highly accelerating functions in which small incr ements in noxious heat produced robust increases in firing rates. The mean responses of neurons recorded in the SDH or DDH to heat were not significantly different. Similarly, there was no difference in the mea n responses of neurons classified as WDR or HT.