SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .3. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THE CERVICAL CORD WHITE-MATTER

1. Fifty-five neurons in the cervical enlargement (C-6-C-8) of urethan-anesthetized rats were antidromically activated from the contralateral posterior diencephalon. In all cases, antidromic thresholds were less than or equal to 30 mu A. The locations of the axons of these neurons within the white matter of segments C-2-C-6 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 C-2-C-6 were determined for these 51 neurons. The mean antidromic threshold at these points was 9.5 +/- 0.5 (SE) mu A. For 26 neurons, the lowest threshold point for antidromic activation was determined at one segmental level. We also attempted to determine whether individual axons maintained their position as they ascended through the cervical cord white matter. In 25 cases, lowest threshold points were determined at two or more segmental levels. 3. In segments C-5-C-6, 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 C-3-C-4, 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). In contrast, only 11% (3/28) of the axons of neurons recorded in the DDH were located in the contralateral DLF and 86% (24/28) were located in the VQ. Therefore, in segments C-3-C-4, the locations of axons differed significantly. Those from neurons recorded in the SDH were located primarily in the DLF and those from neurons recorded in the DDH were located principally in the VQ. 5. In segment C-2 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 C-2 was similar to 2.5 times greater than it was in C-3-C-4. Seventy-nine percent (11/14) of the axons of neurons recorded in the SDH were located in the DLF and 21% (3/14) were located in the VQ. In contrast to their locations within C-3-C-4, 65% (11/17) of the axons of neurons in the DDH were located in the DLF in C-2 and only 35% (6/17) were found in the VQ. These findings indicate that a large number of the axons of neurons in the DDH shift their positions from the VQ in C-3 and C-4 to the DLF in C-2 This conclusion was supported by the studies in which individual axons were examined at two or more segmental levels. The location of axons of 13 neurons recorded in the DDH were examined both in C-3 or C-4 and in C-2. In C-3 or C-4, all 13 axons were located in the VQ. However, in 54% (7/13) of these cases, the axons were located in the DLF in C-2 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. In segments C-3-C-4, 26% (7/27) of the lowest threshold points for antidromic activation of axons of neurons classified as WDR or HT were located in the DLF and 74% (20/ 27) were in the VQ. In contrast, 68% (13/19) of both types were located in the contralateral DLF in C-2. These findings indicate that both the DLF and VQ contain ascending nociceptive axons and that in C-2, the majority of such axons are located in the DLF.