SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE SACRAL SPINAL-CORD OF RATS .1. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THECERVICAL CORD AND DIENCEPHALON

1. A goal of this study was to determine the sites in the diencephalon to which neurons in sacral spinal segments of rats project. Therefore , 95 neurons were recorded extracellularly in spinal segments L(6)-S-2 Of rats that were anesthetized with urethan. These neurons were activ ated initially antidromically with currents less than or equal to 30 m u A from a monopolar stimulating electrode placed into the contralater al posterior diencephalon. The mean +/- SE current for antidromic acti vation from these sites was 16 +/- 0.8 mu A. These neurons were record ed in the superficial dorsal horn (4%), deep dorsal horn (89%), and in termediate zone and ventral horn (4%). 2. Systematic antidromic mappin g techniques were used to map the axonal projections of 41 of these ne urons within the diencephalon. Thirty-three neurons (80%) could be act ivated antidromically with currents less than or equal to 30 mu A only from points in the contralateral thalamus and are referred to as spin othalamic tract (STT) neurons. Eight neurons (20%) were activated anti dromically with low currents from points in both the contralateral tha lamus and hypothalamus, and these neurons are referred to as spinothal amic tract/spinohypothalamic tract (STT/SHT) neurons. Three additional neurons were activated antidromically with currents less than or equa l to 30 mu A only from points within the contralateral hypothalamus an d are referred to as spinohypothalamic tract (SHT) neurons. The dience phalic projections of another 51 neurons were mapped incompletely. The se neurons are referred to as spinothalamic/unknown (STT/U) neurons to indicate that it was not known whether their axons ascended beyond th e site in the thalamus from which they initially were activated antidr omically. 3. For 31 STT neurons, the most anterior point at which anti dromic activation was achieved with currents less than or equal to 30 mu A was determined. Fourteen (45%) were activated antidromically only from sites posterior to the ventrobasal complex (VbC) of the thalamus . Sixteen STT neurons (52%) were activated antidromically with low cur rents from sites at the level of the VbC, but not from more anterior l evels. One STT neuron (3%) was activated antidromically from the anter oventral nucleus of the thalamus. 4. STT/SHT neurons were antidromical ly activated with currents less than or equal to 30 mu A from the medi al lemniscus (ML), anterior pretectal nucleus (Apt), posterior nuclear group and medial geniculate nucleus (Po/MG), and zona incerta in the thalamus and from the optic tract (OT), supraoptic decussation, or lat eral area of the hypothalamus. No differences in the sites in the thal amus from which STT and STT/SHT neurons were activated antidromically were apparent. Five STT/SHT neurons (62%) were activated antidromicall y from points in the thalamus in the posterior diencephalon and from p oints in the hypothalamus at more anterior levels. Three STT/SHT neuro ns (38%) were activated antidromically with currents less than or equa l to 30 mu A from sites in both the thalamus and hypothalamus at the s ame anterior-posterior level of the diencephalon. All three of these S TT/SHT neurons projected to the intralaminar nuclei (parafascicular or central lateral nuclei) of the thalamus. 5. Seven STT/SHT neurons wer e tested for additional projections to the ipsilateral brain. Two (29% ) were activated antidromically with currents less than or equal to 30 mu A and at longer latencies from sites in the ipsilateral diencephal on. One could only be activated antidromically from the hypothalamus i psilaterally. The other was activated antidromically at progressively increasing latencies from points in the ipsilateral brain that extende d as far posteriorly as the posterior pole of the MG. 6. Fifty-eight S TT, STT/SHT, and STT/U neurons were classified as low-threshold (LT), wide dynamic range (WDR), or high-threshold (HT) neurons based on thei r responsiveness to innocuous and noxious mechanical stimuli applied t o their cutaneous receptive fields. There were no statistically signif icant differences among the numbers of LT, WDR, and HT neurons categor ized as STT or STT/SHT neurons. For STT neurons, the most anterior poi nts from which LT neurons were activated antidromically with low curre nts were located in and adjacent to the VbC. In contrast, the most ant erior points from which nociceptive (WDR and HT) neurons were activate d antidromically tended to be located posterior to the VbC in the ML, Apt, OT, and Po/MG. This difference in the apparent termination patter ns of LT and nociceptive (WDR and HT) STT neurons was statistically si gnificant. 7. STT and STT/SHT neurons also were tested for their respo nsiveness to either colorectal (CrD) or vaginal (VaD) distention, or t o both. Nine of 23 STT neurons (39%) and three of four STT/SHT neurons (75%) were excited by CrD. Five of 14 STT neurons (36%) and two of fo ur STT/SHT neurons (50%) were excited by VaD. Four STT and two STT/SHT neurons were excited by both CrD and VaD and two STT neurons were exc ited by CrD but not VaD. There were no statistically significant diffe rences among the numbers of STT and STT/SHT neurons that were excited by either CrD or VaD. 8. Using a second stimulating electrode, 80 STT, STT/SHT, and STT/U neurons also were activated antidromically with cu rrents less than or equal to 30 mu A from the contralateral white matt er of upper cervical segments. More than 90% of these neurons were act ivated from the lateral half of the contralateral lateral funiculus. T wenty-two of these neurons were activated antidromically from both C-4 and C-2. At all cervical levels, the majority was activated antidromi cally from the ventrolateral funiculus (VLF). No segregation of axons in the dorsolateral funiculus or VLF according to diencephalic project ions, recording site, or physiological classification was apparent. 9. Conduction velocities from the recording point to sites in C-4, C-2, and the brain from which STT, STT/SHT, and STT/U neurons were activate d antidromically were calculated. The mean +/- SE conduction velocity to C-4 was 22.5 +/- 1.0 m/s, to C-2 was 21.5 +/- 1.9 m/s, and to the i nitial low-threshold point in the brain was 19.8 +/- 0.9 m/s. The mean conduction velocity to the brain was significantly slower than those to C-4 and C-2. The mean conduction velocities to C-4 and C-2 were nut significantly different. IO. Our findings demonstrate that neurons in spinal segments L(6)-S-2 Of rats issue axons that ascend co