SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .3. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THE CERVICAL CORD WHITE-MATTER
Rj. Dado et al., SPINOTHALAMIC AND SPINOHYPOTHALAMIC TRACT NEURONS IN THE CERVICAL ENLARGEMENT OF RATS .3. LOCATIONS OF ANTIDROMICALLY IDENTIFIED AXONS IN THE CERVICAL CORD WHITE-MATTER, Journal of neurophysiology, 71(3), 1994, pp. 1003-1021
1. Fifty-five neurons in the cervical enlargement (C-6-C-8) of urethan
-anesthetized rats were antidromically activated from the contralatera
l posterior diencephalon. In all cases, antidromic thresholds were les
s than or equal to 30 mu A. The locations of the axons of these neuron
s within the white matter of segments C-2-C-6 were determined by track
ing systematically using a second antidromic stimulating electrode. 2.
The recording locations of 51 neurons were marked and recovered. Twen
ty neurons were recorded in the superficial dorsal horn (SDH) and 31 w
ere in the deep dorsal horn (DDH). Eighty-three lowest threshold point
s 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 segmen
tal level. We also attempted to determine whether individual axons mai
ntained their position as they ascended through the cervical cord whit
e 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 low
est threshold points of the examined axons were located in the contral
ateral ventral funiculus, indicating that the majority of examined axo
ns 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 latera
l funiculus (DLF) and 66% (29/44) were within the ventral quadrant [ve
ntral lateral funiculus (VLF) and ventral funiculus (VF)]. Sixty-nine
percent (11/16) of the axons of neurons recorded in the SDH were locat
ed 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 r
ecorded in the DDH were located in the contralateral DLF and 86% (24/2
8) were located in the VQ. Therefore, in segments C-3-C-4, the locatio
ns 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 i
n 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 great
er than it was in C-3-C-4. Seventy-nine percent (11/14) of the axons o
f neurons recorded in the SDH were located in the DLF and 21% (3/14) w
ere 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 indic
ate 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 conclusio
n 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 respon
ses to innocuous and noxious mechanical stimulation of their receptive
fields. Fifteen units were classified as wide dynamic range (WDR) neu
rons, 15 as high-threshold (HT) neurons, and 1 as a low-threshold (LT)
neuron. Therefore, 97% of the examined neurons responded differential
ly 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/ 2
7) 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 th
e DLF and VQ contain ascending nociceptive axons and that in C-2, the
majority of such axons are located in the DLF.