Spinothalamic and spinohypothalamic tract neurons in the cervical enlargement of rats. III. Locations of antidromically identified axons in the cervical cord white matter

1. Fifty-five neurons in the cervical enlargement (C₆-C₈) of urethan- anesthetized rats were antidromically activated from the contralateral posterior diencephalon. In all cases, antidromic thresholds were ≤30 μA. The locations of the axons of these neurons within the white matter of segments C₂-C₆ were determined by tracking systematically using a second antidromic stimulating electrode. 2. The recording locations of 51 neurons were marked and recovered. Twenty neurons were recorded in the superficial dorsal horn (SDH) and 31 were in the deep dorsal horn (DDH). Eighty-three lowest threshold points for antidromic activation within the white matter of segments C₂-C₆ were determined for these 51 neurons. The mean antidromic threshold at these points was 9.5 ± 0.5 (SE) μA. 3. In segments C₅-C₆, 88% (7/8) of the lowest threshold points of the examined axons were located in the contralateral ventral funiculus, indicating that the majority of examined axons crossed the midline within one or two segments. 4. In segments C₃-C₄, 32% (14/44) of all examined axons were found in the dorsal lateral funiculus (DLF) and 66% (29/44) were within the ventral quadrant [ventral lateral funiculus (VLF) and ventral funiculus (VF)]. Sixty-nine percent (11/16) of the axons of neurons recorded in the SDH were located in the contralateral DLF and 31% (5/16) were located in the ventral quadrant (VQ). 5. In segment C₂, 74% (23/31) of all examined axons were found in the DLF, 23% (7/31) were in the VQ, and 3% (1/31) were in the dorsal horn. Thus, the percentage of all examined axons in the DLF in C₂ was ~2.5 times greater than it was in C₃-C₄. 6. Thirty-one neurons (56% of the sample) were classified according to their responses to innocuous and noxious mechanical stimulation of their receptive fields. Fifteen units were classified as wide dynamic range (WDR) neurons, 15 as high-threshold (HT) neurons, and 1 as a low-threshold (LT) neuron. Therefore, 97% of the examined neurons responded differentially or specifically to noxious stimuli. 7. Twenty-nine of the neurons described here were tested for additional projections within the diencephalon. Seventy-nine percent (23/29) could also be antidromically activated from the hypothalamus [spinothalamic tract/spinohypothalamic tract (STT/SHT)]. Twenty-one percent (6/29) could be backfired only from within the thalamus (STT). 8. The mean estimated conduction velocity (CV) of axons from the recording sites in C₆-C₈ to the lowest threshold points for antidromic activation in segment C₂ was 23.4 ± 1.2 (SE) m/s. The CVs of axons ascending in the DLF did not significantly differ from those of axons in the VLF. 9. Plots of the currents required for antidromic activation versus distances from the lowest threshold points indicated that the maximum effective spread of currents of 30 μA were typically <600 μm. The calculated distance that current spread from the average lowest threshold point was 120 μm. 10. Our findings extend previous studies in at least three ways. First, they demonstrated that nearly all of the examined axons, including those in the DLF, carried nociceptive information. Second, they indicated that the majority of examined neurons recorded in either SDH or DDH had axons that ascended in the DLF in C₂. Third, they indicated that axons that ascended to the thalamus and hypothalamus frequently shifted their position dorsally in the white matter as they passed through upper cervical segments.