Spinothalamic and spinohypothalamic tract neurons in the cervical enlargement of rats. II. Responses to innocuous and noxious mechanical and thermal stimuli

1. The goal of this study was to gather data that would increase our understanding of nociceptive processing by spinothalamic tract (STT) neurons 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 electrode was moved systematically within an anterior-posterior plane in the thalamus until a point was located where the smallest amount of current antidromically activated the neuron. The antidromic thresholds at each of these lowest threshold points was ≤30 μA; the mean antidromic threshold was 15.4 ± 1.0 (SE) μA. 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 sites were located in the superficial dorsal horn (SDH, n = 15), deep dorsal horn (DDH, n = 31), and ventral horn (n = 1). 3. Cutaneous excitatory receptive fields were restricted to the ipsilateral forepaw or forelimb in 67% (10/15) of the neurons recorded in the SDH and 42% (13/31) of the neurons recorded in the DDH. 4. Innocuous and noxious mechanical stimuli were applied to the receptive field of each neuron. Fifty percent (25/50) responded to innocuous mechanical stimuli but responded at higher frequencies to noxious stimuli (wide dynamic range, WDR). Forty-four percent (22/50) responded only to noxious stimuli (high threshold, HT). Six percent (3/50) responded preferentially to innocuous stimuli (low threshold, LT). 5. Eighty-six percent (30/35) of the neurons that were tested responded to noxious heat. The calculated mean threshold was 43.2°C. The stimulus-response relationship was described by a power function with an exponent of 3.5, indicating that the responses were highly accelerating functions in which small increments in noxious heat produced robust increases in firing rates. 6. Twenty-nine percent (10/35) of the tested neurons responded to cooling of their receptive fields. The calculated threshold for a response to cooling was 29.1°C, a temperature near normal skin temperature. The majority of neurons that responded to cooling responded throughout a wide range of both innocuous and noxious cooling stimuli. 7. The estimated mean conduction velocity of the axons between the recording site and the contralateral posterior diencephalon was 20.8 m/s (range 8.8-38.6 m/s). Conduction velocities of neurons recorded in the SDH and DDH were not significantly different. Similarly, there was no difference in the conduction velocities of WDR and HT neurons. 8. Twenty-six of the neurons described here were tested for additional projections within the diencephalon. Eighty-five percent (22/26) could also be antidromically activated from the hypothalamus (STT/SHT). Fifteen percent (4/26) could be backfired only within the thalamus (STT). 9. The cervical enlargement of rats contains many neurons located in the SDH and DDH that project to (or through) the contralateral posterior diencephalon. These neurons are highly responsive to noxious stimuli applied to the forelimb and may, therefore, contribute to the processing and sensation of nociceptive input from the upper extremity. Therefore, in many ways, their responses to noxious stimulation are similar to those of primate STT neurons. Thus, the present physiological results, along with previous anatomic studies, suggest that STT neurons in the cervical enlargement of rats may prove to be a useful model of the STT in primates.