Patterned activity via spinal dorsal quadrant inputs is necessary for the formation of organized somatosensory maps

Abstract

The normal development of the somatosensory system requires intact sensory inputs from the periphery during a critical window of time early in development. Here we determined how the removal of only part of the ascending spinal inputs early in development affects the anatomical and neurophysiological development of the somatosensory system. We performed spinal overhemisections in rat pups at C3/C4 levels on the third day after birth. This procedure hemisects the spinal cord on one side and transects the dorsal funiculus on the other side. When the rats were 6-8 months old, the responsiveness and somatotopy of the primary somatosensory cortex (S1) contralateral to the hemisection were determined using standard multiunit mapping techniques. Sections of the flattened cortex were processed for cytochrome oxidase activity, Nissl substance, or myelin. We found that histologically apparent modules that are normally present in the regions of the forepaw and the hindpaw representations were absent, whereas the lateral barrel field representing the face was completely normal. The neurons in the forepaw regions of S1 either did not respond to the stimulation of the skin of any region of the body or responded to the stimulation of the upper arm afferents that enter the spinal cord rostral to the site of the lesion. The results show that a lack of normal sensory inputs via ascending pathways in the dorsal spinal cord during early development results in massive anatomical and neurophysiological abnormalities in the cortex. Intact crossed spinothalamic pathways are unable to support the normal development of the forepaw barrels.

Figure 1.

The organization of S1 of rats as revealed in Nissl-, CO-, and myelin-stained sections of the flattened cortex. A, An outline diagram showing the locations of the mystacial vibrissae, lips, forepaw, hindpaw, and trunk representations in S1. Primary visual and auditory cortices are marked for reference. B, Photomontage of Nissl-stained sections from the flattened cortex of a rat showing barrels and modules in the S1. The barrels appear with a cell-sparse “hollow” surrounded by cell-dense “walls.” C, In CO-stained sections, the barrels are visible as CO-dark modules surrounded by CO-light septa. The distinct modular pattern is visible in the vibrissae, lips, forepaw, and hindpaw regions. In the trunk region, the staining is uniformly dark. The CO-dark patches lateral to S1 correspond to second somatosensory (S2) and parietal ventral (PV) areas. D, We also stained sections of the flattened cortex for myelin. In myelin-stained sections through the middle layers of the cortex, the barrels appear as myelin-light patches, whereas the septa are darkly stained. The regions of the trunk representation are uniformly stained as for CO and Nissl. In the deeper layers, the staining pattern is reversed with myelin-dark barrels and myelin-light septa (see Fig. 6).

Figure 6.

Changes in the modular pattern and somatotopy in S1 as a result of neonatal spinal overhemisection in rat 99-43. The modular pattern in the deeper layers of S1 cortex revealed by staining sections of the flattened cortex for myelin. Note that the modules show a pattern that is reverse of that seen in the middle layers (compare with Figs. 1 D, 5A). In deeper layers, the barrels are myelin dark surrounded by myelin-light septa. The modules are absent in the forepaw and hindpaw regions, although they are normal in the face region. The modules are not clearly visible in the rostral face representation because this image is from the photomicrograph of a single section, and part of the modules were in other sections attributable to uneven flattening. (The image shown in Fig. 5A is a photomontage.)

Figure 2.

Changes in the modular pattern and somatotopy in S1 as a result of neonatal spinal overhemisection in rat 99-29. A, The modular pattern in the S1 cortex revealed by staining sections of the flattened cortex for CO activity. Note that the modules are absent in the forepaw and hindpaw regions, although they are normal in the face region. In the forepaw region, only a few faint modules in the lateralmost and medialmost regions are discernable. B, Electrode penetration sites and responsiveness in S1 cortex. Neuronal responses in the region of the face and lower lip representations remain normal. The neurons respond vigorously to light touch (large filled circles) on the whiskers and hairs (compare with Fig. 1). In the regions of the forepaw and hindpaw at most of the sites, the neurons were unresponsive (× symbols) or responded weakly to cutaneous (small filled circles) or deep (small open circles) stimulation. At these responsive sites, the receptive fields of neurons are located on the skin of the arm, shoulder, or neck, the inputs that enter the spinal cord rostral to the lesion (see E). There are no responses in the forepaw region of the S1 cortex to the stimulation of the forepaw. Electrolytic microlesions made to help overlay the histochemically visible map in CO-stained sections with the electrophysiological map are marked with arrows in A and stars in B. The expected normal outline of the body representation (white outline in B) is approximated based on the face representation for this rat and the body representation in a normal rat (see Fig. 1). R, Rostral; M, medial. C, Reconstruction of the spinal cord lesion site in a coronal plane showing the extent of the lesion. The damaged portion is shown in black. Note that the overhemisection is complete. D, Photographs of the spinal cord showing the lesion (arrows) in a dorsal (left) and a ventral (right) view. E, Receptive fields at selected numbered locations in the forepaw region of S1.

Figure 3.

Changes in the modular pattern and somatotopy in S1 as a result of neonatal spinal overhemisection in rat 99-33. A, The modular pattern in the S1 cortex revealed by staining sections of the flattened cortex for CO activity. Note that the modules are absent in the forepaw and hindpaw regions, although they are normal in the face region. In the forepaw region only, a few faint modules in the lateralmost and medialmost regions are discernable. B, Electrode penetration sites and responsiveness in S1 cortex. Neuronal responses in the region of the face and lower lip representations remain normal. The neurons respond vigorously to light touch (large filled circles) on the whiskers and hairs (compare with Fig. 1). In the region of the forepaw at many of the sites, the neurons were unresponsive (× symbols) or responded weakly to cutaneous (small filled circles) stimulation. At these responsive sites, the receptive fields of neurons are located on the skin of the arm or shoulder, the inputs that enter the spinal cord rostral to the lesion (see E). There are no responses in the forepaw region of the S1 cortex to the stimulation of the forepaw. Electrolytic microlesions made to help overlay the histochemically visible map in CO-stained sections with the electrophysiological map are marked with arrows in A and stars in B. The expected normal outline of the body representation (white outline in B) is approximated based on the face representation for this rat and the body representation in a normal rat (see Fig. 1). R, Rostral; M, medial. C, Reconstruction of the spinal cord lesion site in a coronal plane showing the extent of the lesion. Note that the overhemisection is complete except for small remaining fibers in the ventromedial region. D, Photographs of the spinal cord showing the lesion (arrows) in a dorsal (left) and a ventral (right) view. E, Receptive fields at selected numbered locations in the forepaw region of S1.

Figure 4.

Changes in the modular pattern and somatotopy in S1 as a result of neonatal spinal overhemisection in rat 99-35. A, The modular pattern in the S1 cortex revealed by staining sections of the flattened cortex for Nissl substance. Note that the modules are absent in the forepaw and hindpaw regions, although they are normal in the face region. The forepaw and the hindpaw regions have small uniform cell-dense regions devoid of any modules (compare with Fig. 1 B). B, Electrode penetration sites and responsiveness in S1 cortex. Neuronal responses in the region of the face and lower lip representations remain normal. The neurons respond vigorously to light touch (large filled circles) on the whiskers and hairs (compare with Fig. 1). In the regions of the forepaw and hindpaw at most of the sites, the neurons were unresponsive (× symbols) or responded to cutaneous stimulation (filled circles). At these responsive sites, the receptive fields of neurons are located on the skin of the arm, shoulder, neck, or lower lip, the inputs that enter the spinal cord rostral to the lesion (see D). There are no responses in the forepaw region of the S1 cortex to the stimulation of the forepaw. Electrolytic microlesions made to help overlay the histochemically visible map in the Nissl-stained sections with the electrophysiological map are marked with arrows in A and stars in B. The expected normal outline of the body representation (dark outline in B) is approximated based on the face representation for this rat and the body representation in a normal rat (see Fig. 1). R, Rostral; M, medial. C, Reconstruction of the spinal cord lesion site in a coronal plane showing the extent of the lesion. D, Receptive fields at selected numbered locations in the forepaw region of S1 cortex.

Figure 5.

Changes in the modular pattern and somatotopy in S1 as a result of neonatal spinal overhemisection in rat 99-43. A, The modular pattern in the S1 cortex revealed by staining sections of the flattened cortex for myelin. Note that the modules are absent in the forepaw and hindpaw regions, although they are normal in the face region. The forepaw and the hindpaw regions show dark uniform staining as for the trunk region (compare with Fig. 1 D; also see Fig. 6). B, Electrode penetration sites and responsiveness in S1 cortex. Neuronal responses in the region of the face and lower lip representations remain normal. The neurons respond vigorously to light touch (large filled circles) on the whiskers and hairs (compare with Fig. 1). In the regions of the forepaw and hindpaw at nearly all of the sites, the neurons were unresponsive (× symbols). At a few sites, the neurons responded to cutaneous (filled circles) or deep (small open circles) stimulation. At these responsive sites, the receptive fields of neurons are located on the skin of the arm, shoulder, or neck, the inputs that enter the spinal cord rostral to the lesion (see D). There are no responses in the forepaw region of the S1 cortex to the stimulation of the forepaw. Electrolytic microlesions made to help overlay the histochemically visible map in the myelin-stained sections with the electrophysiological map are marked with arrows in A and stars in B. The expected normal outline of the body representation (dark outline in B) is approximated based on the face representation for this rat and the body representation in a normal rat (see Fig. 1). R, Rostral; M, medial. C, Reconstruction of the spinal cord lesion site in a coronal plane showing the extent of the lesion. D, Receptive fields at selected numbered locations in the forepaw region of S1 cortex.

Figure 7.

Plot of the transganglionic neuronal tracer B-HRP in a section from the lower medulla of rat 99-43. The tracer was injected in the skin of the upper arm (between the shoulder and the elbow) at 13 different locations. Presence of the tracer in the brainstem shows that dorsal column inputs from this part of the arm were not interrupted by the spinal lesion because they entered the spinal cord rostral to the lesion. DMV, Dorsal motor nucleus of the vagus nerve; Hy, hypoglossal nucleus; IO, inferior olivary nucleus; Py Tr, pyramidal tract.

Figure 8.

Photomicrographs of sections through the lower medulla stained for cytochrome oxidase activity. A, A complete section through the medulla of a normal rat. The region shown at a higher magnification in the other panels is boxed. B, C, High-magnification photomicrographs from lower medulla in the region of the cuneate nucleus (Cu) from two normal rats. Note the CO-dark clusters in the pars rotunda of the cuneate nucleus (outlined with dashed line) in which dorsal column inputs terminate. D-F, Photomicrographs of the brainstem in the region of the cuneate nucleus at approximately the same rostrocaudal plane as for B and C from three rats, 99-27, 99-33, and 99-41, that had undergone neonatal spinal overhemisections. Note that the pars rotunda of the cuneate nucleus (dashed outline) has a uniform appearance lacking CO-dark clusters. Gr, Nucleus gracilis. Scale bar shown in F also applies to B-E.