The burst-like firing of spinal neurons in rats with peripheral inflammation is reduced by an antagonist of N-methyl-D-aspartate.

Ankle inflammation was induced in rats by subcutaneous injection of complete Freund's adjuvant and the firing properties of spinal neurons receiving afferent input from the inflamed areas were studied four to six days later. Comparable neurons in normal rats were also studied. In normal animals the response of neurons to ankle compression consisted of a brief burst of action potentials followed by sustained firing during stimulus application. On cessation of the stimulus there was no prolonged afterdischarge. In rats with an inflamed ankle, compression of the ankle produced firing while the stimulus was applied, but with 17 of 22 neurons there was a prolonged (219 +/- 55 s) post-stimulus afterdischarge. All neurons studied in rats with peripheral inflammation fired with intermittent bursts of action potentials, particularly during the afterdischarge and spontaneous firing. The N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonopentanoate was ejected microiontophoretically near the cells studied. The major effect was a near abolition of bursts present in spontaneous firing and post-stimulus afterdischarges with a lesser reduction in firing during stimulus application. Effects on afterdischarge duration were variable. Since firing in bursts is known to increase transmitter release at some sites in the brain, it is proposed that when the relevant spinal neurons fire in bursts, additional intraspinal pathways are recruited and this contributes to the expanded receptive fields of neurons and possibly to the enhanced pain experienced by manipulation of inflamed peripheral tissues.

Crossed receptive field components and crossed dendrites in cat sacrocaudal dorsal horn.

The hypothesis that sacrocaudal dorsal horn neurons with crossed receptive field components on the tail have dendrites which cross to the contralateral dorsal horn was tested in a combined electrophysiological and morphological study. Dorsal horn cells in the sacrocaudal spinal cord of anesthetized cats were penetrated with horseradish peroxidase-filled microelectrodes. After mapping their low threshold mechanoreceptive fields, cells were iontophoretically injected with horseradish peroxidase. A sample of 16 well-stained cells was obtained in laminae III and IV. Cells with receptive fields crossing the dorsal midline of the tail (n = 8) had somata in the lateral ipsilateral dorsal horn, and some of these cells (5/8) had dendrites which crossed to the lateral contralateral dorsal horn. Cells with receptive fields spanning the ventral midline (n = 2) were located near the center of the fused dorsal horn, and one of these had bilateral dendrites in this region. Cells with receptive fields on the lateral tail, crossing neither the dorsal nor the ventral midline (n = 6), had cell bodies in the middle of the ipsilateral dorsal horn; half had only ipsilateral dendrites, and half had crossed dendritic branches. Although the relationship between cell receptive field (RF) location (RF center, expressed as distance from tips of toes) and mediolateral location of the cell body was statistically significant, the correlation between crossed RF components and crossed dendritic branches was not significant.

Changes in substance P and 5-HT binding in the spinal cord dorsal horn and lamina 10 after dorsolateral funiculus lesions.

The present study was undertaken to determine whether changes in receptor binding of substance P (SP) and 5-hydroxytryptamine (5-HT) occur in lumbar spinal cord laminae 2, 3, 4 and 10 following interruption of descending SP and 5-HT input. These transmitters and spinal cord regions have been implicated in nociceptive and visceral functions. Quantitative receptor binding autoradiography was used to assess the binding of 2 nM [3H]SP and 2 nM [3H]5-HT to lumbar spinal cord sections taken from normal rats and rats with unilateral thoracic dorsolateral funiculus (DLF) lesions. Postoperative survival times ranged from 1 to 28 days. Substance P binding was above normal in laminae 2 and 3 ipsilateral to the lesion and in contralateral lamina 2 at 1 day postoperatively (DPO), and declined thereafter, reaching below normal levels by 28 DPO. Substance P binding in lamina 10 was significantly above normal at 7 and 14 DPO, but not at 1 or 28 DPO. Binding of 5-HT was above normal at 7 DPO in lamina 2 ipsilateral to the lesion, lamina 3 contralaterally, and lamina 10 bilaterally. These increases were not sustained, however, and at 28 DPO 5-HT binding was significantly below normal in laminae 2-4 bilaterally. The bilateral effects seen in the present study are consistent with the bilaterality of descending thoracic DLF projections demonstrated by the Fink-Heimer method.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of 5-HT1 binding sites in cat spinal cord.

Quantitative analysis of high affinity [3H]5-HT binding to 5-HT1 receptors in the cervical, thoracic, lumbar, and sacral spinal cord of the cat revealed specific binding throughout the grey matter, with the highest levels of binding in laminae II and III, and the lowest levels in laminae I and VII. Relatively high levels were also observed in the thoracic intermediolateral cell column. There were no significant differences in the degree of binding between various segmental levels. Comparison of these data with published maps of 5-HT immunoreactivity reveals--with the exception of lamina I--a close correspondence between the degree of immunoreactivity and the degree of 5-HT binding. These results suggest that 5-HT plays an important role in a variety of spinal cord sensory, motor and autonomic functions.

Influence of map scale on primary afferent terminal field geometry in cat dorsal horn.

1. Thirty-one physiologically identified primary afferent fibers were labeled intracellularly with horseradish peroxidase (HRP). 2. A computer analysis was used to determine whether the distribution of cutaneous mechanoreceptive afferent terminals varies as a function of location within the dorsal horn somatotopic map. 3. An analysis of the geometry of the projections of these afferents has shown that 1) terminal arbors have a greater mediolateral width within the region of the foot representation than lateral to it, 2) terminal arbors have larger length-to-width ratios outside the foot representation than within it, and 3) the orientation of terminal arbors near the boundary of the foot representation reflects the angle of the boundary. Previous attribution of mediolateral width variations to primary afferent type are probably in error, although there appear to be genuine variations of longitudinal extent as a function of primary afferent type. 4. Nonuniform terminal distributions represent the first of a three-component process underlying assembly of the monosynaptic portions of cell receptive fields (RFs) and the somatotopic map. The other two components consist of the elaboration of cell dendritic trees and the establishment of selective connections. 5. The variation of primary afferent terminal distributions with map location is not an absolute requirement for development of the map; for example, the RFs of postsynaptic cells could be assembled with the use of a uniform terminal distribution for all afferents, everywhere in the map, as long as cell dendrites penetrate the appropriate portions of the presynaptic neuropil and receive connections only from afferent axons contributing to their RFs.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthetic blockade of the dorsolateral funiculus enhances evoked activity of spinal cord dorsal horn neurons.

1. A previous study of cat lumbar dorsal horn neurons found reduced responsiveness to A-fiber stimulation 1.5-12 h after thoracic dorsolateral funiculus (DLF) lesions. The present study was undertaken to determine whether this was due to the loss of descending activity or to factors specifically associated with injury by examining the response properties of dorsal horn cells before and during lidocaine blockade of the ipsilateral DLF. Electric shocks applied to the dorsal columns were used to search for dorsal horn cells. Noxious and nonnoxious cutaneous mechanical stimuli and graded electrical stimuli applied to the tibial nerve were used to activate peripheral afferent fibers. Cells were classed as low threshold (LT), high threshold (HT), or multireceptive (MR), according to their responses to natural stimuli. Baseline data were collected from a total of 58 cells. Twelve of these were further studied after lidocaine injection of the DLF. All cells examined with lidocaine were in dorsal horn laminae III-V. 2. All cells responded to activation of tibial nerve A fibers. However, the median threshold for the HT and MR cells (200 microA) was significantly higher than that of the LT cells (75 microA). Some cells in each class were also activated by C fibers (10, 70, and 64% of the LT, HT, and MR cells, respectively). 3. For the cells that were further characterized by lidocaine blockade of the DLF, all LT cells (n = 3) responded only to A-fiber stimulation, and all HT (n = 3) and MR cells (n = 6) responded to both A- and C-fiber stimulation. 4. For LT cells, responses evoked by mechanical and electrical stimuli were unaltered by lidocaine blockade. 5. HT and MR cells showed enhanced responses to electrical stimulation of C fibers during DLF blockade. There was no consistent effect of the blockade on A-fiber-evoked responses. 6. Two of three HT and four of six MR cells studied with lidocaine had spontaneous activity, which exhibited a small but significant increase during DLF blockade. 7. Receptive fields for noxious stimulation expanded in two of six MR cells during DLF blockade. Two of three HT cells developed responses to tactile stimuli during the blockade. 8. In two additional cells (1 HT and 1 MR), spontaneous activity and responses to C-fiber input increased after the DLF was cut.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatotopic organization of single primary afferent axon projections to cat spinal cord dorsal horn.

Horseradish peroxidase injection of identified low threshold cutaneous mechanoreceptor (LTCM) primary afferent axons was used to assess the somatotopic organization of hindlimb projections to laminae III and IV of cat dorsal horn. Multiple injections in the same animals were used to assess bilateral symmetry and precision. Thirty-one axons were injected, with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic relations between their receptive field (RF) centers and the centers of their dorsal horn projections were similar to the somatotopic relations between dorsal horn cell RF centers and cell locations. Very few reversals of mediolateral somatotopic gradients (proximodistal RF location as a function of mediolateral projection center) were observed. Two afferents with nearly identical RFs in 1 animal had nearly identical projections. These observations held for many different combinations of receptor types. A simple mathematical model was used to demonstrate that assembly of dorsal horn cell RFs via passive sampling of the presynaptic neuropil by dorsal horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take into account the functional selectivity of connections required to produce RFs smaller than those predicted by the passive assembly model.

Characteristics of dorsal horn neurons expressing subliminal responses to sural nerve stimulation.

The present study was designed (1) to characterize the subliminal responses of dorsal horn neurons to stimulation of the sural nerve, and (2) to correlate the type of response to this stimulus with the responses to natural mechanical stimulation of the skin. To accomplish this, intracellular and extracellular recordings were carried out in L6 and L7 dorsal horn neurons in the cat. The excitatory responses of each cell to electrical stimulation of the sural nerve and to mechanical stimulation of the skin were noted. Of 35 dorsal horn cells recorded intracellularly, 11 responded with impulses to sural nerve stimulation, 9 responded with excitatory postsynaptic potentials (EPSPs) but not impulses, and 15 had no excitatory responses to this stimulus. The type of response to sural nerve stimulation was strongly correlated with receptive field modality. Most cells receiving an input from high-threshold cutaneous mechanoreceptors responded with impulses or gave no excitatory response to sural nerve stimulation, whereas most cells that had only low-threshold mechanoreceptor input responded with EPSPs only or gave no response. In cells with only low-threshold (LT) mechanoreceptive input, response to sural nerve stimulation was highly correlated with receptive field locus. Those LT cells with no excitatory responses to sural nerve stimulation had receptive fields confined to the foot and/or toes, whereas those that gave EPSPs had more proximal receptive fields. The possible significance of these data with reference to changes observed after lesions, such as increased response to sural nerve stimulation, increased receptive field size, and somatotopic reorganization, is discussed.

Temporally dependent changes in response properties of dorsal horn neurons after dorsolateral funiculus lesions.

Previous studies in this laboratory have shown 1) that 19% of L6 and L7 dorsal horn cells in normal cats respond only with excitatory postsynaptic potentials (EPSPs) to sural nerve stimulation, and 2) that the distribution of dorsal horn neurons responding with impulses to sural nerve stimulation is increased in cats with chronic lateral funiculus lesions. The present study was undertaken to determine whether strengthening of subliminal sural nerve projections could account for the changes seen after lateral funiculus lesions and to explore the nature of these changes in greater detail. Ipsilateral L6 and L7 dorsal horn cells of cats with T12 dorsolateral funiculus (DLF) lesions were studied electrophysiologically at less than 1-30 days postoperatively (DPO) and were compared with similar cells recorded in normal cats. The major results were as follows. 1. Responsiveness to peripheral stimulation was depressed for up to 3 DPO following the lesions. 2. The percentage of L6 and L7 dorsal horn cells showing spontaneous activity was elevated at 3 DPO and declined to normal levels by 28 DPO. 3. The percentage of cells that responded to sural nerve stimulation increased over 3-30 DPO and was significantly greater than normal at 28-30 DPO. The increase in the percentage of cells giving impulses to sural nerve stimulation at 28-30 DPO was similar to the percentage of cells with subliminal responses to sural nerve stimulation in normal cats. A subpopulation of identified postsynaptic dorsal column neurons also showed a significant increase in the percentage that responded to sural nerve stimulation after DLF lesions. 4. The percentage of cells that had a cutaneous receptive field (RF) overlapping the region of skin innervated by the sural nerve also increased over time and was significantly greater than normal at 28-30 DPO. 5. Median low-threshold or high-threshold RF areas were not significantly greater than normal ipsilateral to DLF lesions at any survival time. 6. The proportions of low-threshold (LT), high-threshold (HT), and multireceptive (MR) cells were not significantly different from normal at any survival time from less than 1 to 30 DPO.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute and chronic effects of the neurolytic agent ricin on dorsal root ganglia, spinal cord, and nerves.

The short- and long-term effects of ricin injections into nerves have been evaluated with light microscopy in the dorsal root ganglia, spinal cord, and peripheral nerves in rats and cats. Dorsal root ganglion cells initially exhibited chromatolysis, followed by gliosis and cell death. These changes were associated with Fink-Heimer degeneration in the somatotopically appropriate region of the dorsal horn. There were no signs of chromatolysis in dorsal horn neurons in ricin-injected animals, but chromatolytic motoneurons were observed. Ricin produced acute necrosis of injected nerves and dissolution of axoplasm. At long survival times (greater than 4 weeks) some apparently regenerating axons were seen in the injection sites of rats. Cell counts indicated that a substantial percentage of dorsal root ganglion neurons associated with the injected nerves were killed, but the presence of regenerating axons suggested that some cells survived the ricin treatment. Although the lesion may not always be complete, even with maximum sublethal doses, this method appears to be useful for specifically destroying afferent fibers associated with a particular nerve without transynaptic destruction of dorsal horn neurons.

Lack of central sprouting of primary afferent fibers after ricin deafferentation.

A new deafferentation technique, the application of ricin to peripheral nerves, was used to test for collateral sprouting of undamaged primary afferent fibers within the adult mammalian spinal cord dorsal horn. The right sciatic nerves in rats were injected with ricin 14 to 57 days prior to bilateral labelling of dorsal rootlets with horseradish peroxidase. To equate the number of surviving dorsal root fibers on the two sides, the left sciatic nerves were injected 5 days prior to labelling. In each animal, horseradish peroxidase was applied to a bilateral pair of lumbar or low thoracic dorsal rootlets 18 hours prior to sacrifice to test for sprouting by labelling primary afferent fibers and terminals in the right (experimental) and left (control) dorsal horns. Although there is overlap of degenerated and intact primary afferent fields in this preparation, a postulated precondition for sprouting (Murray and Goldberger: J. Neurosci. 6:3205-3217, '86), we found no evidence for sprouting of undamaged, myelinated afferent fibers in the experimental dorsal horns. The pattern of labelling was symmetrical in all animals, and the density of labelling was not consistently greater on the experimental side. These results support the conclusions of Rodin et al. (J. Comp. Neurol. 215:187-198, '83) and Rodin and Kruger (Somatosens. Res. 2:171-192, '84), who also found no sprouting in the rat's dorsal horn after surgical deafferentation, and do not support the assertion that the difference between the results of those studies and earlier studies in cats was due to a lack of overlap of degenerated and intact dorsal roots in the rat.

Electrical stimulation reveals relatively ineffective sural nerve projections to dorsal horn neurons in the cat.

Electrical stimulation of the sural nerve (SN) revealed input from sural nerve afferents to L6 and L7 dorsal horn neurons that were not apparent using natural mechanical stimuli, especially in cells with variable latency responses to SN stimulation. Nearly all (31/32) cells that had reliable, fixed latency responses to SN stimulation also had an excitatory receptive field (RF) in the region of skin innervated by the sural nerve (SN region). About one-third (20/57) of the cells with variable latency responses to SN stimulation, however, had an RF outside the SN region. Most (130/146) cells with no response to SN stimulation had RFs outside the SN region. There were no obvious differences between variable latency cells with RFs in the SN region vs those with RFs outside it in latency of response to SN stimulation, recording depth, RF sizes or modality properties. In a subsample of 31 postsynaptic dorsal column neurons all cells responding to SN stimulation also had an RF in the SN region. Strengthening of relatively ineffective projections from the sural nerve by lesions might be expected to lead to an increase in the proportion of cells responding with impulses to natural stimulation of the skin innervated by the sural nerve, and, hence, to an increase in average RF size.

The boundary of proximal hindlimb representation in the dorsal horn following peripheral nerve lesions in cats: a reevaluation of plasticity in the somatotopic map.

Some investigators have reported that deafferentation markedly alters the somatotopic organization of the dorsal horn in adult mammals, whereas others, including the present investigator, have not. Failure to observe changes in somatotopy might be attributable to insufficient deafferentation, inadequate sampling, limitation of the observations to a population of neurons that does not exhibit the phenomenon, or a variety of technical factors. The most striking changes in somatotopy have been reported following total deafferentation of the medial dorsal horn by sciatic and saphenous nerve lesions (Devor and Wall, 1981a; Lisney, 1983). The present study was an attempt to replicate this result to determine which explanations might account for the failure of some earlier investigations to observe changes in somatotopic organization. Other variables to be considered include type of anesthesia, type of electrode, and mapping and reconstruction procedures. The somatotopic organization of the lumbar dorsal horn in the adult cat was examined ipsilateral to sciatic and saphenous nerve lesions at postoperative survival times of 1-98 days. Half of these experiments were performed using chloralose anesthesia and half using pentobarbital. Microelectrodes were used to record single and multiunit activity in transverse rows of penetrations across the dorsal horn of L6 and adjacent segments. The results of these experiments were compared with data taken from dorsal horns ipsilateral to intact nerves. Sciatic and saphenous nerve lesions completely abolished dorsal horn neuronal responses to foot and toe stimulation at all survival times, yet there was no significant shift of proximal hindlimb representation into the medial dorsal horn following these lesions at any survival time. It is suggested that the method of anatomical reconstruction of recording sites used by investigators reporting such changes might best explain their findings.

Tactile receptor discharge and mechanical properties of glabrous skin.

Current knowledge of the functional properties of mammalian cutaneous mechanoreceptors is reviewed with special reference to receptors associated with the glabrous skin of the raccoon and squirrel monkey hand. Four physiologically defined mechanoreceptor types are recognized: Pacinian afferents, rapidly adapting (RA), and slowly adapting type I (SAI), and slowly adapting type II (SAII). The SAI category is divided into moderately slowly adapting and very slowly adapting (VSA) types in terms of the duration of their response to a prolonged mechanical displacement of skin. Although both RA and SA units are capable of signaling displacement ramp velocity, the pattern of discharge during ramp stimulation may vary widely among units. SAI units also code the depth of skin displacement, but there is no best-fitting function describing the relationship. Static discharge is also markedly influenced by prior ramp velocity. Both raccoon and squirrel monkey VSA units show wide variation in the regularity of their discharge during static displacement. The rate of adaptation of SAI units is less when constant force stimuli are applied to the skin than when constant displacement stimuli are applied. This is partly attributable to mechanical properties of the skin. When either constant force or constant displacement stimuli are spaced too closely in time, there is a progressive (trial-to-trial) decrement in response rate, accounted for in part by failure of the skin to recover to its initial resting level.

Receptive field organization and response properties of spinal neurones with axons ascending the dorsal columns in the cat.

Micro-electrode recordings were made from single post-synaptic axons in the dorsal columns of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The recordings were made from the L5 segment and the axons were shown to project to the upper cervical level. Forty-eight units were recorded and the axons had conduction velocities of 22-61 ms-1, averaging 38.3 ms-1. Excitatory receptive fields were complex in many units, being made up of clearly defined, separate, low and high threshold areas. The receptive fields were often discontinuous. Only a few units behaved as if they received excitatory input from a single class of mechanoreceptors. A minority (13%) of units had labile, excitatory receptive fields that expanded in size during the recording period. About 40% of the units had inhibitory receptive fields. These were of two main types: either small and within or adjacent to the excitatory field, or large and separated from or adjacent to the excitatory field. The great majority of units had resting discharges upon isolation and these consisted of single impulses or bursts of impulses at short intervals separated by longer, irregular periods. The time course of inhibition produced by electrical stimulation of cutaneous nerves suggested presynaptic inhibitory components to the inhibition. Some inhibitory curves were very prolonged with maxima at about 100 ms and total durations of up to 400 ms. The complexity of the receptive field organization in these dorsal horn neurones is discussed, as is their possible significance as input neurones to the dorsal column nuclei.

Effects of dorsal root section on spinocervical tract neurones in the cat.

1. In nine cats, dorsal roots L(6) and L(7) were sectioned under pentobarbitone anaesthesia using strict aseptic precautions. In two cats all lumbosacral roots except L(6) were sectioned under similar conditions. 27-75 days after the dorsal rhizotomies, under chloralose anaesthesia, neurones of the spinocervical tract (s.c.t.) were recorded electrophysiologically both extracellularly and intracellularly, their response properties and receptive fields were analysed, and they were injected with horseradish peroxidase (HRP). Twenty-five cells were recorded extracellularly and twenty were recorded intracellularly and stained with the enzyme.2. Of the twenty neurones stained with HRP, one was located in caudal L(5), eleven in L(6) and eight in L(7). Five cells had no receptive field, either before or after current injection, two cells had no field before injection but after current injection responded to either electrical stimulation of peripheral nerve or to brusque tapping on the limb; six neurones responded only to tapping before current injection and two of these showed a more clearly defined receptive field after current injection; six neurones had a receptive field and responded with impulses to hair movement and/or pressure within a well-defined area but this area was surrounded by skin from which excitatory post-synaptic potentials could be elicited; only one neurone had an apparently normal receptive field.3. Of the twenty-five neurones recorded extracellularly, eleven had no receptive field, four responded only to tapping on the limb, three had receptive fields consisting of an area of low threshold plus an area responding to brusque or noxious mechanical stimulation, and seven neurones had apparently normal receptive fields.4. Neurones with no observable receptive fields were either silent with no resting activity, or had very low rates of background activity consisting of isolated single impulses or short bursts of impulses. No neurones in de-afferented segments had high rates of background activity.5. There were no alterations in the normal somatotopy of the dorsal horn. All fifteen s.c.t. neurones injected with HRP and which had a localizable receptive field had fields appropriately located in the somatotopic map. There were no electro-physiological signs of inappropriate connexions.6. The dendritic trees of fifteen neurones injected with HRP were reconstructed. With the exception of one cell which appeared grossly disorganized (but which was the one cell with an apparently normal receptive field), there were no obvious differences between the dendritic trees of de-afferented cells and normal cells from the same and other cats. Their dendritic trees had similar rostro-caudal, medio-lateral and dorso-ventral extents to normal cells, and appeared to exhibit similar degrees of branching. There was a suggestion that de-afferented neurones had rather more of their dorsally-directed distal dendrites invading lamina II than normal cells, but the sample size was too small to make this more than a possibility.7. It is concluded that after dorsal rhizotomy, s.c.t. neurones lose part or all of their afferent input and that within 75 days do not show any signs of new or inappropriate connexions.

Increased receptive field size of dorsal horn neurons following chronic spinal cord hemisections in cats.

The somatotopic organization of the 17 dorsal horn was studied using extra-cellular recordings in normal cats, and in cats with acute or chronic spinal cord hemisection at T13, sparing the dorsal columns. Based on data concerning recovery of function and collateral sprouting of afferents following hemisections, we predicted that the lesion would result in increases in receptive field size and decreases in the specificity of the somatotopic map. In normal animals, the usual mediolateral, rostrocaudal and dorsoventral somatotopic sequences were found. Following acute hemisections (6 h-5 days), there were changes in spontaneous and evoked activity, but receptive field sizes and somatotopic organization remained unchanged. Following chronic hemisections (88-174 days), proximal hindlimb receptive fields in the lateral dorsal horn ipsilateral to the lesion increased dramatically in size and were significantly larger than similar receptive fields on the contralateral side. The largest of these fields extended from the dorsal midline to the middle of the foot. Receptive field sizes elsewhere in the dorsal horn remained unchanged, as did somatotopic organization in general. These findings indicate that hemisections result in a complex series of changes consisting of an early stage of anatomically generalized changes in excitability and a later stage of highly localized changes in receptive field size. Possible mechanisms for these changes, as well as their relationship to recovery of function, are discussed.

Increased response to sural nerve input in the dorsal horn following chronic spinal cord hemisection.

Monosynaptic input from sural nerve afferents to dorsal horn neurons was mapped bilaterally using electrical stimulation in normal cats and cats with spinal cord hemisections. Animals hemisected 6 h-5 days previously did not differ significantly from normals and the sides of the cord did not differ in either group. In animals hemisected 88-182 days previously there were significantly more sites responsive to sural nerve input ipsilateral to the hemisection, than contralateral to it.