SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .1. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THE THALAMUS AND HYPOTHALAMUS

1. Seventy-seven neurons in the cervical enlargement of rats anestheti zed with urethan were initially antidromically activated using current s less than or equal to 30 mu A from the contralateral posterior thala mus. A goal of these experiments was to determine the course of physio logically characterized spinal axons within the diencephalon. Therefor e, in 38 cases, additional antidromic mapping was done throughout the mediolateral extent of the diencephalon at multiple anterior-posterior planes. 2. Electrolytic lesions marking the recording sites were reco vered for 71 neurons. Thirty-one were located in the superficial dorsa l horn (SDH); 39 were in nucleus proprius or the lateral reticulated a rea of the deep dorsal horn (DDH), and one was in the ventral horn. 3. Eight of 38 (21%) neurons that were tested for more anterior projecti ons could only be antidromically activated with currents less than or equal to 30 mu A from sites in the contralateral posterior thalamus. S uch neurons are referred to as spinothalamic tract (STT) neurons. Lesi ons marking the lowest threshold points for antidromic activation were located in or near the posterior thalamic group (Po). At more anterio r levels, considerably higher currents were required for antidromic ac tivation or it was not possible to activate the neurons with currents up to 500 mu A. Four of these neurons were physiologically characteriz ed and each responded preferentially to noxious mechanical stimuli (wi de dynamic range, WDR). Each of the three neurons that were tested res ponded to noxious heat stimuli. These findings confirm anatomic studie s that have shown that a number of STT axons terminate in Po and sugge st that such axons that originate in the cervical enlargement carry no ciceptive input from the upper extremity. 4. In 15 cases, electrode pe netrations were made systematically throughout much of the contralater al ventrobasal complex (VbC). In 17 cases, penetrations were made thro ughout the intralaminar nuclei contralaterally, including the central lateral nucleus (CL). Surprisingly, only one of the examined axons was antidromically activated with low currents from CL and one from VbC, although both of these nuclei are known to receive sizeable inputs fro m the STT. 5. Many of the axons (27 of the 38 tested, 71%) that were i nitially antidromically activated from the contralateral posterior tha lamus could also be antidromically activated with low currents (less t han or equal to 30 mu A) and at increased latencies from sites located anteriorly in the contralateral hypothalamus. Such neurons are referr ed to as spinothalamic tract/spinohypothalamic tract (STT/SHT) neurons . Lesions marking the lowest threshold points for antidromic activatio n in the hypothalamus were concentrated in or near the supraoptic decu ssation, optic tract, and caudal part of the optic chiasm. Responses o f 20 of these neurons to innocuous and noxious mechanical stimuli were determined. Eleven were classified as WDR neurons. The other nine wer e classified as high threshold (HT) neurons. Fourteen of 17 (82%) resp onded to noxious heat. These results confirm our earlier studies and d emonstrate that many nociceptive neurons in the cervical enlargement p roject to both the thalamus and hypothalamus. distances from the lowes t threshold points were generated to determine the effective spread of current within the hypothalamus. The maximum effective spread of curr ents of 30 mu A was similar to 400 mu m (13 mu m/mu A). The mean thres hold for antidromic activation at the lowest threshold points was simi lar to 15 mu A, and thus it appears that the effective current spread away from the lowest threshold points was probably <200 mu m.