Reorganization of somatosensory cortical areas 3b and 1 after unilateral section of dorsal columns of the spinal cord in squirrel monkeys

Abstract

An incomplete lesion of the ascending afferents from the hand in the dorsal columns of the spinal cord in monkeys is followed after weeks of recovery by a reactivation of much of the territory of the hand representations in primary somatosensory cortex (area 3b). However, the relationship between the extent of the dorsal column lesion and the amount of cortical reactivation has not been clear. Largely, this is due to the uncertainties about axon sparing after spinal cord lesions. Here, we unilaterally sectioned dorsal column afferents in the cervical spinal cord (C4-C6) in adult squirrel monkeys. After weeks of recovery, cholera toxin subunit B (CTB) was injected into the distal phalanges to label normal and surviving afferents to the cuneate nuclei representing the hands. Days later, the responsiveness of neurons in cortical areas 3b and 1 to tactile stimulation on the hand was evaluated in a microelectrode mapping session. The sizes and densities of CTB-labeled patches in the cuneate nuclei of both sides were quantified and compared. The results indicate that extensive reactivations of the hand representations in cortical areas 3b and 1 occur contralateral to the spinal cord lesion, even when <1% of labeled dorsal column terminations in the cuneate nucleus remained. These results raise the possibilities that secondary afferents from innervated neurons in the spinal cord contribute to the reactivation, and that the reactivation of area 1 is not completely dependent on inputs from area 3b.

Figure 1.

Terminations of peripheral afferents in the dorsal horn of the spinal cord and cuneate nucleus of the brainstem labeled by injections of CTB into the distal phalange of digits 1, 3, and 5 of a squirrel monkey (SM-O). A, A horizontally cut CTB-immunoreacted section of the spinal cord showing the location of the lesion on the upper right and labeled terminal fields after tracer injections. Cervical segments 4–7 are indicated, and foci of labels from injections in digits 1, 3, and 5 are marked with arrows. Injection sites in the digits are indicated by black dots on the schematic drawings of hands on the bottom of A. B, Transverse view of spinal cord through cervical segment C4–C5 with the reconstructed extent of the lesion in black. The lesion was reconstructed from a series of horizontally cut sections. C, A series of coronally cut CTB-immunoreacted sections through dorsal column nuclei of brainstem. The cuneate nucleus is outlined, and numbers 1, 3, and 5 mark foci of afferents labeled by injections in digits 1, 3, and 5. Note that there are only a few detectable foci of axon fibers on the lesioned (right) side (arrows). The numbers in the section centers indicate the position of each section in the series. Note that not all sections are included. CuN, Cuneate nucleus of brainstem.

Figure 2.

Terminations of peripheral afferents in the dorsal horn of the spinal cord and cuneate nucleus of SM-C. A, A portion of the cervical spinal cord through C2–C8. The arrow points to the lesion on the right side of the spinal cord. Scale bars, 5 mm. B, Foci of peripheral afferents labeled in the dorsal horn of the spinal cord by injections of tracer CTB into the distal phalange of digits 1, 3, and 5 of both hands. Traces of the first and second lesion are on the upper right. C, A transverse view of spinal cord through cervical segment C4–C5 indicating the extent of lesion (black). The lesion was reconstructed from a series of horizontally cut sections. The arrows mark the preserved zones in the dorsal columns. D, A series of sections through the cuneate nucleus of the brainstem. The numbers in the section centers indicate the position of each section in the series. The cuneate nucleus is outlined, and numbers 1, 3, and 5 mark foci of afferents labeled by injections in digits 1, 3, and 5. Only a few weakly labeled foci survived on the side of lesion (marked by arrows, or numbers). For other conventions, see Figure 1.

Figure 3.

Terminations of peripheral afferents in the dorsal horn of the spinal cord and cuneate nucleus of SM-D. A, A portion of cervical spinal cord through C2–C8. The arrow points to the lesion on the right side of the spinal cord. Scale bar, 5 mm. B, Foci of the peripheral afferent terminations in the dorsal horn of the spinal cord labeled by injections of tracer CTB into the distal phalange of digits 1, 3, and 5. The dorsal column section was at cervical cord C5–C6 on the right side of the spinal cord, which is at the level of peripheral afferents from digit 2. The right half of section 17 and both halves of section 23 are shown. C, Transverse view of spinal cord through cervical segment C4–C5 indicating the extent of lesion (black). The lesion was reconstructed from a series of horizontally cut sections. D, A series of coronally cut CTB-immunoreacted sections through the dorsal column nuclei of the brainstem. The cuneate nucleus is outlined, and numbers 1, 3, and 5 mark labeled foci of afferents from digits 1, 3, and 5. More foci of axons are labeled on the lesioned right side in SM-D than in the other two monkeys, especially for D1 foci (see Figs. 1, 2). However, the density of the labeled terminations in the D1 territory was reduced on the lesion side compared with those of the intact side. The numbers in the section centers indicate the position of each section in the series. For other conventions, see Figure 1.

Figure 4.

A–C, Bar graphs showing the areal size of CTB-labeled axon arbor foci in the cuneate nuclei of the brainstem from three lesioned monkeys. The numbers on the x-axis are the distance (in micrometers) measured from the beginning of the obex. The negative values indicate the measured distances were caudal to the obex. The values on the y-axis are the areal size (in square millimeters) of the combined foci of CTB label for each section through the cuneate nucleus of the brainstem. Values from the intact and lesioned sides are compared. a–c, Topographical maps of hand territories in areas 3b and 1 of the same three monkeys in A–C depict regions where neurons had weak to strong responses to tactile stimulation. The x marks penetrations with unresponsive neurons. Neuronal response sites for different parts of the hand are grouped and outlined, and territories for D1 to D5 and palm pads are indicated. Shadings from dark to white indicate the proportion of good responses for each responding zone of the hand from high to low. 3a, Area 3a; 3b, area 3b; 1, area 1; CS, central sulcus; FA, forearm; GR, good response; H, hand; M, medial; P1, pad 1; PTH, palm pad thenar; R, rostral; WR, weak response; Wr, wrist.

Figure 5.

A–D, Bar graphs showing the relative extents and densities of CTB-labeled axon arbors in the digit 1 territory of rotunda subdivision of the cuneate nuclei (A, B), and the dorsal horn of the spinal cord (C, D) of SM-D. The value of label density was defined by inversed lightness of averaged values of reflected light from red, green, and blue channels (see Materials and Methods). The Wilcoxon matched-pairs signed-rank test was used to compare the difference between lesioned side with the intact side. p < 0.05 is considered significant. A, A comparison of the areal extent of the foci of label from D1 in the cuneate nucleus of the lesioned and intact sides. Measurements were taken from the middle-rostral sector of the cuneate nucleus where the rotunda subdivision is located as this is where CTB-labeled foci from digits 1, 3, and 5 could be distinguished on both sides. B, A comparison of the densities of the foci of label for digit 1 in the rotunda subdivision of the cuneate nucleus. C, The areal extent of foci of labels from D1, D3, and D5 in the dorsal horn of the spinal cord of the two sides. There are no significant differences in the extents of labeling in the spinal cord on the two sides, indicating that the dorsal column lesion did not disrupt most of the terminations in the dorsal horn. D, However, a comparison of the densities of the labeled foci in the dorsal horn of the spinal cord after injections in digits 1, 3, and 5 reveal that only the foci for digit 1 were significantly less dense on the lesioned side compared with the intact side, suggesting the lesion at the C6 level (Fig. 3) may have disrupted some of the D1 afferent terminations in the dorsal horn. *p < 0.05. No significant differences were observed for foci for digits 3 and 5. Values in C and D are reported as the mean ± SD.

Figure 6.

The organization of digit representations of area 3b in three squirrel monkeys on the side ipsilateral to lesions of the dorsal columns of the spinal cord. The fourth squirrel monkey (SM-M) was a surgical control monkey in which a laminectomy was performed at the C4–C5 level of cervical vertebrae; however, the dorsal columns were not touched. Maps of each case were reconstructed based on the locations of receptive fields for neurons at each electrode penetration (see Materials and Methods). The schematic drawings of hands for each case depict the examples of the neuronal receptive fields as they progressively shifted from distal to proximal portions of the digits as electrode penetrations moved from rostral to caudal in cortical area 3b. The solid dots mark good responses; the open circles mark weak responses; the open diamonds mark very weak responses to hard taps; the dashed regions mark responses to hairy or dorsal skin; x marks microelectrode penetrations with no responses; stars mark electrolytic lesions. In all cases, the vast majority of responses in areas 3b and 1 were good, and the somatotopic organizations of areas 3b and 1 appeared to be normal. A, Anterior; M, medial; L, lateral; P, posterior. For other conventions, see Figures 1 and 4.

Figure 7.

The somatotopic map of areas 3b and 1 of a squirrel monkey (SM-O) obtained 7 weeks after section of the contralateral dorsal columns. A, The somatotopic map shows the locations of microelectrode penetrations. Solid dots mark penetrations with good responses; black open circles mark those with weak responses; black diamonds mark those with very weak responses to hard taps; x marks microelectrode penetrations with no responses. Heavy black dashed lines mark area boundaries based on myelin architecture. Note that the vast majority of microelectrode penetrations over unaffected face cortex are marked with good responses. B, Receptive fields obtained with microelectrode recordings are outlined on the drawing of the glabrous and dorsal sides of the hand of SM-O. The dashed lines on the drawing of hand indicate that the exact boundaries of the RFs were hard to define due to very weak responses to hard taps. Numbers or letters on the drawings of the hands indicate the receptive fields recorded from matching microelectrode penetrations in the map. Areas 3a, 3b, and 1 are indicated. CS, Central sulcus; dD1 and dD2, distal phalange of D1 and D2, respectively; GR, good response; M, medial; NR, no response; pD4, proximal phalange of D4; R, rostral; P2, pad 2; PTH, pad thenar; WR, weak response; vWR, very weak response to hard taps. For other conventions, see Figures 4 and 6.

Figure 8.

The somatotopic map obtained 9 weeks after a second, effective lesion of the contralateral dorsal columns in a squirrel monkey (SM-C). A, The cortical map of somatosensory areas 3b and 1 shows the locations of microelectrode penetrations, responsiveness, and nonresponsive zones. Solid dots mark penetrations with good responses; black open circles mark those with weak responses; black diamonds mark those with very weak responses to hard taps; x marks microelectrode penetrations with no responses; heavy black dashed lines mark area boundaries based on myelin architecture. B, Receptive fields obtained with microelectrode recordings are outlined on drawings of the glabrous and dorsal sides of the hand of a squirrel monkey. The dashed lines on the drawing of the hand indicate that the exact boundaries of the RFs were hard to define due to very weak responses to hard taps. Numbers or letters on the drawings of the hands indicate the receptive fields recorded from matching microelectrode penetrations in the map. For other conventions, see Figures 4 and 6.

Figure 9.

A somatotopic map obtained 8 weeks after a lesion of the contralateral dorsal columns in SM-D. On the left, solid dots mark penetrations with good responses; black open circles mark those with weak responses; black diamonds mark those with very weak responses to hard taps; x marks microelectrode penetrations with no responses. On the right, receptive fields obtained with microelectrode recordings are outlined on drawings of the glabrous and dorsal side of the hand of a squirrel monkey. Numbers or letters on the drawings of the hands indicate the receptive fields matching microelectrode penetrations in the map. The detailed topographical organizations of the most spared digits 1 and 2 are shown on the left top two panels. By dividing the regions representing digits 1 and 2 into three sectors (rostral, middle, and caudal), we show that the representations of the distal part of digits are not limited to the rostral portions of digit representations of area 3b, as reported in normal monkeys (Sur et al., 1982), but they also appear in the middle and caudal sectors of area 3b. In the bottom half of the illustration, digits 3–5 were represented in partly deprived cortex in an abnormal fashion, reflected by the appearance of silent zones and abnormal somatotopy. The receptive fields did not progressively shift as the electrode penetrations moved systematically from rostral to caudal (e.g., D3 representation). In addition, the representations of D3–D5 were fragmented. For conventions, see Figures 4 and 6.

Figure 10.

The somatosensory maps of areas 3a and 1 and neuronal receptive fields on the forelimb in SM-D obtained 8 weeks after an incomplete lesion on the contralateral dorsal column. A, The somatotopic map on the top shows the locations of microelectrode penetrations and electrolytic lesions (stars). Solid dots mark penetrations with good responses; black open circles mark those with weak responses; black diamonds mark those with very weak responses to hard taps; x marks microelectrode penetrations with no responses; heavy black dashed lines mark area boundaries based on myelin architecture. B, Receptive fields obtained with microelectrode recordings are outlined on drawings of the glabrous and dorsal side of the hand of a squirrel monkey. The dashed lines on the drawing of hand indicate that the exact boundaries of the RFs were hard to define due to very weak responses to hard taps. Numbers or letters on the drawings of the hands indicate the receptive fields recorded from the respective microelectrode penetrations in the map. For conventions, see Figures 4 and 6.