JOURNALOF NEUROPHYSIOLOGY Vol. 66, No. 3, September 1991. Printed in U.S.A.

Synaptic Potentials Evoked by Convergent Somatosensory and Corticocortical Inputs in Raccoon Somatosensory Cortex: Substrates for Plasticity E. SMITS, D. C. GORDON, S. WITTE, D. D. RASMUSSON, AND P. ZARZECKI Medical Research Council Group in Sensory-Motor Physiology, Department of Physiology, Queen S University, Kingston, Ontario K7L 3N6; and Department of Physiology and Biophysics, Dalhousie University, [email protected], Nova Scotia B3H 4H7, Canada only is each digit represented on its own gyrus, but each digit representation can be subdivided functionally. Two of gested as a mechanism for the early changes in cortical topo- the subregions process information from skin of the digits. graphic maps that follow alterations of sensory activity. For such a The “glabrous zone” for each digit is dominated by inputs mechanism to operate, convergent sensory inputs must already from the glabrous skin of that single digit. Because receptive exist in the normal cortex. fields are well localized and sensory stimuli evoking neuro2. We tested for topographic and cross-modality convergence nal discharge have such specific submodality properties, the in primary somatosensory cortex of raccoon. The representation glabrous zones are suited for locating and signalling the of glabrous skin of forepaw digits was chosen because, even though physical properties of punctate stimuli on single digits. The it is dominated by inputs from the glabrous skin of a single digit, it “heterogeneous zone” of SI is very different. Adequate stimnevertheless comes to respond to stimulation of other digits when, uli are not always submodality- and place-specific. The hete.g., a digit is removed. erogeneous zone, therefore, is considered to function in the 3. Intracellular recordings were made from 109 neurons in the representation of glabrous skin of digit 4. Neurons were tested for representation of complex stimuli. These include simultasomatosensory inputs with electrical and natural stimulation of neous stimulation of more than one digit or of both hairy and glabrous skin surfaces (Doetsch et al. 1989), as occurs digits. 4. Excitatory postsynaptic potentials (EPSPs)were evoked in during haptic exploration (Lederman et al. 1988), a more 100% of the neurons ( 109/ 109) by electrical stimulation of gla- common use of the forepaw in raccoon than in most quadbrous skin of digit 4, and in 79% (3 1 of 39) by vibrotactile stimula- rupeds (Cole 19 12). tion. These marked differences in receptive field properties for 5. Glabrous skin of digit 4 was not the sole source of somato- neurons of the glabrous and heterogeneous zones occur desensory inputs. A minority of neurons generated EPSPsafter electrical stimulation of hairy skin of digit 4 (10 of 98 neurons, 10%). spite direct corticocortical connections between these two Electrical stimulation of digits 3 or 5 evoked EPSPsin 22 of 103 subregions of SI (Doetsch et al. 1988b). In particular, there neurons (2 1%). Natural stimulation (vibrotactile or hair bending) is no indication in the place- and modality-specific recepwas also effective in most of these latter cases (digit 3, 6/7; tive fields of neurons in the glabrous zone that they have a digit 5, 9/10). corticocortical input from the convergent heterogeneous 6. Intracortical microstimulation of the “heterogeneous zone” zone. Nevertheless, when cortical reorganization is evoked was used to test for corticocortical connections to neurons in the (e.g., by removal of a digit), the glabrous zone takes on some glabrous zone. We found a higher proportion of corticocortical properties that are normally found only in the heterogeEPSPsamong neurons that had convergent somatosensory inputs neous zone. Neurons become responsive to stimulation of when compared to neurons having inputs only from digit 4. This other digits and other receptor classes, i.e., apparently new suggeststhat a corticocortical pathway could be relaying some of inputs are expressed from previously ineffective sources the convergent somatosensory inputs. 7. We conclude that convergent somatosensory and corticocor- (Kelahan and Doetsch 1984; Rasmusson 1982). A variety of mechanisms has been proposed to account tical EPSPs,present in normal brain and available for unmasking, could be substrates for reorganizations of cortical topographic for these cortical changes (Wall 1988). One mechanism involves an “unmasking” of preexisting but ineffective inmaps. puts. According to this scheme, neurons in the glabrous zone with receptive fields restricted to one digit would nevINTRODUCTION ertheless generate excitatory postsynaptic potentials The representation of the raccoon forepaw in primary (EPSPs) after stimulation of adjacent digits. The conversomatosensory cortex (3) has been studied repeatedly (e.g., gent somatosensory EPSPs from these “off-focus” digits Feldman and Johnson 1988; Johnson et al. 1982; Kelahan might be relayed through the corticocortical pathway beand Doetsch 1984; Rasmusson 1982; Welker and Seiden- tween the heterogeneous and glabrous zones. In normal stein 1959). SI of the raccoon is organized somatotopically, cortex these EPSPs would be too weak to evoke action poas in other mammals, but there is a singular precision in the tentials, or their excitatory effects would be suppressed by way the map is related to the cortical sulcal pattern. Not intracortical inhibitory processes. Inputs from adjacent dig-

SUMMARY

AND

CONCLUSIONS

1. “Unmasking” of weak synaptic connections has been sug-

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its, necessary for the mechanism of unmasking, have never been demonstrated in receptive fields of neurons in the glabrous zone of the normal animal. Accordingly, the present study was undertaken to test the hypothesis that there are inputs from multiple digits and from different skin surfaces on the same digit, even for neurons in the glabrous zone. Intracellular recording techniques were used because such convergent inputs are not suspected from descriptions of receptive fields. If convergent somatosensory and corticocortical inputs are present in normal cortex, then enhancement of these could contribute to cortical map reorganizations. METHODS

Animal preparation Thirteen raccoons, Procyon Z&or, were captured from the wild as weanlings (< 1 kg body weight) by a licensed trapper under a Scientific Collector’s Permit from the Ontario Ministry of Natural Resources. They were vaccinated, communally housed, and fed a mixed diet of dry chow and raw vegetables. Experiments were performed 6 to 10 mo after the animals were captured (mean body weight, 6.9 kg). Anesthesia was induced by ketamine hydrochloride (200 mg im) and continued with pentobarbital sodium (30.0 mg/kg iv). Supplemental doses of pentobarbital sodium (1 .O-6.0 mg/kg) were given as required throughout the experiment to prevent spontaneous movements and to suppress withdrawal reflexes. Two doses of dexamethasone (0.5 mg/kg each iv) were given -2 h apart to counteract cerebral edema and inflammation of the pia. No neuromuscular paralysis was used. The animal breathed through a tracheostomy. Core body temperature was monitored continuously via a rectal probe and maintained at 3638°C by a feedback controlled heating pad. The somatosensory cortex was exposed by craniotomy. A double-cylinder Teflon chamber (Zarzecki and Asanuma 1979) was fixed to the skull over the craniotomy with screws and dental impression wax. The dura was opened and retracted. The chamber was sealed after filling with warm paraffin oil. This facilitated stable intracellular recording by eliminating brain movements of cardiovascular and respiratory origin. Polaroid photographs of the cortical surface were made to record locations of electrode penetrations and surface recordings.

Somatosensory

d3 record

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Somatic stimulation The forepaw contralateral to the exposed somatosensory cortex was prepared for electrical and natural somatic stimulation of its hairy and glabrous surfaces. Intradermal electrodes were used to stimulate glabrous and hairy skin of digits 3,4, and 5 (d3, d4, and d5). The test electrical stimulus had an intensity of 1 mA (measured as a voltage drop acrossa seriesresistor)and a pulse duration of 0.1 ms and was repeated at 1 Hz. Mechanical stimuli were delivered to glabrous skin of the digits by an electromagnetic driver controlled by a custom-built waveform generator (Department of Biomedical Engineering, Queen’s University) and mounted on an x-y-z manipulator. Movements of the l.O-mmdiam vibrating stylus were measured with an optically coupled circuit calibrated with a piezoelectric accelerometer and a doubleintegrating amplifier. The sinusoidal vibrotactile stimulus had a maximum frequency of 700 Hz and a maximum amplitude of 40 pm. For hair deflection, 40- to 60-ms pulses of nitrogen gas were delivered from 1.O-mm-diam nozzles 0.5-l .O cm from the skin. The gas pulses were controlled by solenoid valves and monitored with a pressure-sensitivetransducer calibrated by a water column. The maximum pressure exerted by the gas pulses was 0.5 kPa at the skin surface.

Placement of cortical recording electrodes Microelectrodes for intracellular recording were placed in the cortical representation of d4. Two methods were used to locate the cortical representation of d4 (Fig. 1). First, the cortical representation of the glabrous skin of the forepaw digits was identified by anatomic landmarks. This is more reliable in raccoon than in other mammalian speciesbecause the representations of individual digits are consistently located with respect to the sulci of the triradiate complex (Fig. 1). Second, electrophysiological mapping was done to verify the location of these digit representations. The distal pads of d3, d4, and d5 were stimulated electrically through intradermal electrodes. Cortical potentials were recorded with a surface electrode from the gyral crowns of the cortex around the triradiate complex. The mapping procedure confirmed in all cases the conclusions made from examining surface landmarks. The position at which the cortical response had the maximum amplitude after stimulation of d4 was located approximately midway between those of d3 and d5.

epsps

B Corticocortical

CORTICAL

FIG. 1. Convergent somatosensory and corticocortical EPSPs recorded from 1 neuron in SI. The intracellular recording microelectrode was positioned in the representation of glabrous skin of d4 (record, cortex inset). ICMS electrodes were positioned rostrally in the heterogeneous zone (ICMS, inset). Four digitized sweeps are superimposed in each panel. A: EPSP evoked by electrical stimulation of d4: B: EPSP from stimulation of d3. C: short-latency corticocortical EPSP evoked by ICMS (50 PA). Voltage calibration, 10 mV; time calibration, 50 ms (A and B) or 25 ms (C).

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Intracellular

recording

Cortical neurons were recorded intracellularly with microelectrodes filled with 2 M potassium acetate and advanced with a piezoelectric stepping microdrive mounted on the lid of the caudal chamber. Intracellular recordings were analyzed if they met two criteria: 1) the neurons were impaled in the cortical representation of d4 and 2) these neurons had membrane potentials of at least -40 mV. The membrane potential criterion was chosen arbitrarily, but some indication of its validity was the observation that neurons in this group also had spontaneous synaptic activity and action potentials between 30 and 70 mV in amplitude. Synaptic potentials were evoked in all of these impaled neurons after electrical or natural stimulation of the digits. Neurons with membrane potentials of lessthan -40 mV did not share these characteristics. The postsynaptic potentials recorded from cortical neurons were stored on analog tape for off-line analysis. For each neuron, sources of somatosensory and corticocortical EPSPswere determined. Latencies of EPSPs were measured in 4- 16 (usually 16) individual data sweeps,not in waveform averaged records, and are given as mean and standard deviation of these individual values.

Intracortical

microstimulation

(ICA4S)

Corticocortical pathways were activated by ICMS. Three glassinsulated tungsten microelectrodes were held with dental impression wax in an array of stainless steel guide tubes mounted in the lid of the rostra1 cylinder of the cortical chamber. Each electrode was advanced into the cortex to a depth of -0.5 mm by briefly heating the dental impression wax. The electrodes for ICMS were positioned within the cortical representation of d4, but rostra1 to the region where the largest cortical surface potential was evoked by electrical stimulation of the glabrous skin of that digit. The stimulated cortical region was judged to be the heterogeneous zone (Feldman and Johnson 1988; Johnson 1985). Effects of ICMS restricted to the grey matter could be attributed to corticocortical pathways originating from the stimulated region of cortex in the heterogeneous zone, identified electrophysiologically and by surface topography. Current spread to white matter, on the other hand, could activate corticocortical pathways from distant cortical areas or thalamocortical fibers in their subcortical path. Accordingly, electrodes for ICMS were placed in the upper layers of cortex to decreasethe possibility of current spread to subcortical white matter. Low intensities and short pulse durations were used to reduce current spread and prevent noxious effects (Asanuma and Arnold 1975; Asanuma et al. 1976). The microelectrodes for ICMS were connected to a switch box so that stimulus currents could be passed through any combination of one or more electrodes. The indifferent electrode was inserted under the skin of the scalp. Negative pulses of 0.2 ms duration and a search intensity of 50 PA were used to stimulate the cortex. The stimulus pulse was followed immediately by a positive pulse of 0.1 ms duration to reduce stimulus artifact. Stimulating currents were triggered from two constant-voltage isolated stimulators, one for each polarity of stimulus. Stimulus current was measured as the voltage drop acrossa seriesresistor. The electrodes used for ICMS were also used to record field potentials and responsesof single neurons to electrical stimulation of the digits. For electrodes correctly placed in the heterogeneous zone, it was expected that neuronal responses would be evoked after stimulation of the glabrous skin of d4 (“on-focus”), as well as the glabrous and hairy skin of one or more off-focus digits (d3 and d5). These recordings provided further confirmation that the ICMS electrodes were located in the heterogeneous zone for d4.

Histological

analysis

The purpose of the histological analysis was to evaluate the extent of stimulus current spread from the IC MS electrodes. Electro-

ET

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lytic lesions were made with each productive stimulating microelectrode (-5 PA, 10 s) and the lesions were located in histological sections. Estimation of current spread was carried out to identify casesin which currents for ICMS could have spread beyond the grey matter of cortex. These caseswere eliminated from the final data pool. As we are not aware of a previous study of current spread in raccoon somatosensory cortex, our estimates of current spread were based on a study of pyramidal tract neurons in cat motor cortex in which identical stimulating microelectrodes were used (Asanuma et al. 1976). This seemsjustified because the corticocortical neurons that were the target of our ICMS are pyramidally shaped, as are the pyramidal tract cells of motor cortex. RESULTS

The final data pool consists of 109 neurons recorded intracellularly in the representation of d4 glabrous skin in SI. All evoked synaptic responses were initially depolarizing, presumably EPSPs. In some cases, EPSPs were followed by hyperpolarizing components, presumably inhibitory postsynaptic potentials (IPSPs). Somatosensory and corticocortical inputs were detected by the presence of EPSPs. Neurons responded to electrical stimulation

of the digits

RESPONSESFROMDIGIT4. All of the impaled neurons generated EPSPs in response to electrical stimulation of glabrous skin on the distal pad of d4. A characteristic EPSP evoked by electrical stimulation of d4 is shown in Fig. 1. The EPSPs evoked in this neuron by 13 consecutive stimuli delivered to glabrous skin of d4 had a mean latency of 14.6 t 0.6 (SD) ms. Some neurons responded to electrical stimulation of the hairy surfaces of d4 as well as to electrical stimulation of its glabrous skin. Responses from both hairy and glabrous surfaces of d4 were found for 35 of 98 neurons (36%). There was concern that this apparent convergence of inputs from afferent fibers in glabrous and hairy skin could have resulted from inadvertent spread of stimulus currents from electrodes in hairy skin to afferent fibers in glabrous skin of the same digit. For 10 of these neurons, however, there was clear evidence that the responses originated from separate groups of afferent fibers in the different regions of skin. For these cases (Fig. 2, +), latencies of EPSPs evoked by stimulation through electrodes in glabrous skin of d4 (range, 13.620.4 ms; 16.1 t 2.0 ms, mean t SD) were shorter than latencies of EPSPs evoked in the same neuron from electrodes in hairy skin of the same digit (range, 17.8-26.8 ms; 2 1.0 t 2.4 ms, mean t SD, Mann-Whitney U test, P < 0.01). Therefore, at least in these cases, the EPSPs after stimulation through electrodes in the two skin surfaces could not have been evoked by activation of a common set of afferent fibers. Because of the possibility of current spread, neurons in which responses from electrodes in glabrous and hairy skin had similar latencies (Fig. 2, l ) were not classified as having inputs from two skin surfaces of the same digit. RESPONSESFROMDIGITS~AND s. Effectivesourcesofinputs to individual neurons in the SI representation of glabrous skin of d4 were not always restricted to the on-focus digit. In

addition

to the ubiquitous

responses to stimulation

of the

glabrous skin of d4,22 of 102 neurons had inputs from an off-focus digit (d3, n = 17 or d5, n = 10). Five of these

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eight of these neurons. Five neurons responded to vibration of all three digits (Fig. 3, A-C). Latencies of responses to mechanical stimulation covered a wide range (Table 2). Latency differences of EPSPs evoked from separate digits were most apparent when they were compared for individual neurons (Fig. 4B). The latencies of EPSPs evoked from the glabrous skin of d4 were shorter than EPSPs evoked from d3 or d5. This difference was significant for all but one neuron (Kruskal-Wallis test, P < 0.02). The differences among latencies of EPSPs evoked in the same neurons by vibrotactile stimulation of d4 and of d3 or d5 ranged from 2.9 to 43.6 ms (24.4 t 14.7, mean t SD).

+ 26

CORTICAL

glabrous

(ms)

2. Latencies of EPSPs evoked by electrical stimulation of glabrous and hairy skin of d4. Each data point represents the mean latency of EPSPs evoked from glabrous skin in 1 neuron plotted against the mean latency of EPSPs evoked from hairy skin in the same neuron. For some neurons (+), the latencies of EPSPs from glabrous skin were shorter than EPSPs in the same neurons after stimulation of hairy skin (Mann-Whitney U test, P -c 0.0 1). Current spread between dorsal and ventral surfaces of the digit was suspect in a 2nd group of neurons because latencies of responses from glabrous and hairy skin were not different (0). Data from 26 neurons with EPSPs from both skin surfaces of d4. FIG.

neurons responded to stimulation of the glabrous skin of all three of the tested digits. The latencies of responses from glabrous skin of d4 relative to responses from glabrous skin of d3 or d5, recorded from the same neurons, are illustrated in Fig. 4A. Responses from glabrous skin of d4 had shorter latencies than EPSPs evoked in the same neuron from d3 in all cases and from d5 in all but one neuron (Kruskal-Wallis test, P < 0.02). When EPSPs were evoked from more than one digit in the same neurons, the differences in latencies of responses evoked by electrical stimulation of d4 and of off-focus digits ranged from 0.4 to 12.0 ms (5.3 t 2.9, mean t SD). Neurons responded to natural stimulation

of the digits

RESPONSES FROM VIBROTACTILE STIMULATION. Forty-one neurons were tested with vibrotactile stimulation of glabrous skin on the same digits from which EPSPs could be evoked by electrical intradermal stimulation. Thirty-three of these neurons (80%) responded to this form of mechanical stimulation. For example, the effective mechanical stimulus applied to glabrous skin in the case illustrated in Fig. 3 was a forward and back movement of the stylus, 9.0 pm in amplitude and 2.8 ms in duration. Fifteen neurons were tested for a response to vibrotactile stimulation of more than one digit. EPSPs were evoked from multiple digits in

Somatosensory EPSPs evoked by hair bending were far less common than responses from vibrotactile stimulation of glabrous skin. Only three neurons responded to hair bending among 25 tested (Fig. 30). The effective hairs were located on d4 for two neurons and on both d3 and d4 for one neuron. In one case, a receptive field was isolated to three hairs, deflected by a pressure of 0.5 kPa. When these hairs were cut off the EPSPs evoked by the air jet stimulus disappeared without change in the response evoked by electrical skin stimulation. All three of the neurons with EPSPs evoked by hair bending also had EPSPs evoked by vibrotactile stimulation of glabrous skin. In other words, all of the neurons with input from hair follicle afferent fibers displayed convergence of inputs from another cutaneous submodality. Neurons responded to corticocortical inputs Corticocortical EPSPs after ICMS of the heterogeneous zone were recorded in 4 1 of 69 neurons (59%). A characteristic corticocortical EPSP is illustrated in Fig. 1C. The mean latency of 16 consecutive responses in this neuron was 3.5 -+ 0.4 (SD) ms. Intensities of ICMS evoking corticocortical EPSPs ranged from 40 to 70 PA. Thus it was necessary to examine the relationship between latency and stimulus intensity before comparing latencies across the neuronal sample. Latencies were measured at a total of 32 different intensities of ICMS in 11 neurons. Statistical analysis of the relationship between latency and stimulus intensity for each neuron revealed that for 8 of 11 neurons there was a significant difference between latency values across the lowest part of the tested intensity range (between - 10 and 30 ,uA, Wilcoxon signed-rank test, P < 0.02). In contrast, there was no difference between latency values obtained from individual neurons when stimulus intensities were varied between 40 and 70 PA (P > 0.05). Accordingly, latency data were pooled for all corticocortical EPSPs evoked by 240 PA. Latencies of corticocortical EPSPs evoked by ICMS of 240 PA ranged from 0.9 to 9.4 ms (Fig. 5). Twenty-one of the 40 neurons with corticocortical responses had latencies of 53.9 ms. There were no corticocortical EPSPs with latencies between 3.9 and 4.9 ms. Nineteen neurons had latencies of 24.9 ms. Convergence of somatosensory and corticocortical EPSPs The corticocortical EPSPs were not distributed randomly across the neuronal sample. There was a relationship be-

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D

d3

d3

ii B

d4

C

d5

FIG. 3. Characteristic EPSPs evoked by mechanical stimulation. These responses from glabrous and hairy skin were recorded in a single neuron. The latencies of the EPSPs evoked by mechanical stimulation of glabrous skin of d3 (A), d4 (B) and d5 (C) were 21.2, 16.9, and 52.5 ms, respectively. i, waveform of the mechanical stimulus evoking responses A-C, a skin displacement of -9.0 pm. D: EPSPs from bending of hairs on d3. ii, waveform of the stimulus for hair bending, peak pressure of 0.5 kPa reached 40 ms after its onset. Voltage calibration, 10 mV; time calibration, 50 ms.

*

I 0.5 kPa

tween the presence of corticocortical EPSPs in individual neurons and the pattern of somatosensory inputs from the digits to these same neurons (Fig. 6). In the sample of neurons with somatosensory EPSPs only from the glabrous

A.

digital

El8ctriCal

skin of d4, 25 of 52 (48%) had corticocortical EPSPs. The corticocortical EPSPs were of short latency (53.9 ms) in 12 of these neurons (23%). On the other hand, 11 of 13 neurons (85%) with convergent somatosensory inputs had cor-

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4. Latencies of somatosensory EPSPs evoked from separate digits. A: mean latencies of EPSPs from electrical stimulation of glabrous skin of d4 in individual neurons are plotted against mean latencies of EPSPs from glabrous skin of d3 or d5 in the same neurons. Data from 19 neurons with responses from d4 combined with d3 or d5. B: mean latencies of EPSPs from mechanical stimulation of glabrous skin of d4 are plotted against mean latencies of EPSPs in the same neurons from vibrotactile stimulation of glabrous skin of d3 or d5. Data from 8 neurons. FIG.

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ticocortical EPSPs. Eight of these 13 neurons (62%) had corticocortical EPSPs in the shortest latency range. Neurons that had inputs from more than one digit were more likely to have corticocortical EPSPs (x2, P < 0.025), and these corticocortical EPSPs were more likely to be in the short-latency range of 53.9 ms (x2, P < 0.01). Thus the corticocortical input from the heterogeneous zone to the glabrous zone of SI seems to be targeted more specifically to neurons with convergent somatosensory inputs. A corticocortical relay for the off-focus inputs is suggested also by the relative latencies of EPSPs from on-focus and off-focus digits. Recall that responses evoked from glabrous skin of d4 had shorter latencies than those evoked in the same neurons from glabrous skin of d3 or d5 (Fig. 4a). These differences between latencies of EPSPs evoked by electrical stimulation of on-focus and off-focus digits were compared with the corticocortical latency for eight of these same neurons. There was only one case in which the difference in latencies of on-focus and off-focus responses was less than the corticocortical latency. Obviously, for this one neuron, the off-focus somatosensory input could not have been relayed by the activated corticocortical neurons. On the other hand, for seven of the eight neurons, the latency of input from d3 or d5 was longer than the response from d4 by a time only slightly greater (1.4- 1.6 ms) than the latency of the corticocortical EPSP. Thus the corticocortical connection could have relayed the off-focus sensory input found for these same neurons.

Latency Distribution of Corticocortical Epsps

693

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corticocortical s 3.9

latency ms

100 90

10 0

single

digit

multidigit

input

input

In=521

(n=13)

FIG. 6. Corticocortical EPSPs were more common in neurons with convergent somatosensory inputs. Neurons are divided into those that had somatosensory inputs evoked by electrical stimulation of only d4 or from more than 1 digit. The filled portions of the bars indicate the proportions of neurons with corticocortical latencies of 53.9 ms. n, number of neurons with somatosensory inputs from a single or from multiple digits.

DISCUSSION

8

Topographic and submodality convergence in the glabrous zone of SI

1

2

3

4

5

Latency

6

7

8

9

lo

(ms)

5. Latency distribution of corticocortical EPSPs evoked by ICMS of the heterogeneous zone. Latency bins are of 0.5 ms duration. Latencies are plotted from 40 neurons. FIG.

CORTICAL

The regions of raccoon primary somatosensory cortex in which neurons typically discharge only to stimulation of glabrous skin of a single digit have always been interpreted as important for localizing punctate stimuli on single digits and signaling their physical properties (Johnson 1985; Johnson et al. 1982). Even using the more sensitive technique of intracellular recording to detect somatosensory inputs, we found that a majority of neurons in the representation of the glabrous skin of d4 had somatosensory inputs only from the on-focus digit. However, for some neurons (-20% of the tested sample), EPSPs could be evoked from more than this one somatic source. Properties of electrical and natural stimuli have to be taken into account before interpreting these results as topographic and submodality convergence. Conclusions about topographic convergence of somatosensory effects from more than one digit can be drawn from results obtained with electrical stimulation of the glabrous skin of the digits. In particular, because spread of stimulus current between digits at the 1-mA test intensity is not probable, the 22 cases (out of 103 tested neurons) in which EPSPs were evoked by electrical stimulation of more than one digit (Fig. 1) are interpreted as unambiguous evidence of convergent inputs from more than one location. Spread of effective mechanical stimuli between digits or from the digits to afferent fibers in the palm is a concern

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because of the low threshold of many of the afferent fibers TABLE 2. Latencies of EPSPs evoked by mechanical activated by vibrotactile stimuli (Pubols and Pubols 1976). For the high-stimulus velocities that we used, however, Digit 3 there is a sharp decline in the amplitude of transmitted skin Digit 4 Digit 5 movement within a few millimeters from the stimulus Range, ms 22.8-63.7 16.7-32.6 29.1-55.5 probe (Pubols 1987). Therefore, inadvertent spread of stimMean t SD, ms 45.3 t 17.4 21.4 AI 3.9 47.8 + 9.5 uli between digits is not a probable explanation for our find- Effective/tested 617 31/39 9/10 ings of topographic convergence with the use of low-ampliEPSP, excitatory postsynaptic potential. tude vibrotactile stimulation of distal digit pads (e.g., 9 pm in Fig. 3). Spread of mechanical stimuli is even less plausible in the cases in which EPSPs evoked by vibrotactile stim1986). These thalamocortical paths bring inputs to the repulation of one digit occurred in the same neuron with resentations of glabrous skin from their on-focus digits. EPSPs evoked by stimuli deflecting a few hairs on another However, there is no direct thalamocortical path that could digit (Fig. 3). Furthermore, EPSPs evoked by mechanical carry the inputs that we found from off-focus digits, bestimulation of separate digits almost always had different cause the representation of glabrous skin of d4 receives thalatencies (Fig. 4B). This would not be expected if mechanical stimuli delivered to separate digits had escaped to a lamocortical connections only from the somatotopically common group of afferent fibers, e.g., in the palm. There- appropriate thalamic subnucleus. The off-focus inputs may fore our conclusion of topographic convergence is sup- involve the corticocortical projections known to pass directly from the heterogeneous zone to the glabrous zone ported by findings using both electrical and natural stimula(Doetsch et al. 1988b), or multisynaptic intracortical pathtion of the digits. Some of the results from electrical stimulaways through the cortical gray matter (Doetsch et al. 1988a; tion can also be interpreted as evidence for submodality Rasmusson and Nance 1986). The heterogeneous zone is convergence. Cases in which single neurons were activated by electrical stimulation of hairy and glabrous skin on the suspected as the source of off-focus inputs because of its same digit and in which current spread between the skin mix of on-focus and off-focus inputs. These result from surfaces seemed unlikely (Fig. 2) are at least suggestive of projections to the heterogeneous zone for each digit not submodality convergence because of the very dissimilar re- only from the somatotopically appropriate subnucleus of the ventral posterolateral nucleus (the same ones that proceptor complements in hairy and glabrous skin. However, our strongest evidence for submodality convergence is the ject to the cortical glabrous zones), but also from adjacent three cases, of 25 neurons tested, in which neurons were thalamic subnuclei representing off-focus digits (Doetsch et activated by two forms of natural stimulation (Fig. 3). We al. 1988a). A corticocortical relay from the heterogeneous zone is further implicated by the relationship that we found conclude from the combined observations obtained with between somatosensory and corticocortical inputs to the electrical and natural stimulation that there is convergence upon some neurons in the glabrous zone of SI from more same neurons. than one location on the paw and from more than one kind Corticocortical EPSPs more probable in neurons with of cutaneous receptor. multidigit inputs Pathways from the digits to SI If a corticocortical path is involved in the transfer of conFor each digit zone in the forepaw representation of SI vergent somatosensory inputs to neurons in the representation of glabrous skin, then neurons with multidigit inputs there is an on-focus digit that provides the predominant somatosensory input. Off-focus inputs are from sources should have a larger proportion of corticocortical EPSPs other than the predominant one. The shorter latencies that than neurons with EPSP evoked only from a single digit. we found for inputs from the on-focus digit (Tables 1 and We found this to be the case; neurons with convergent somatosensory inputs had more corticocortical EPSPs than 2), also detected by Doetsch et al. (1989) with extracellular recordings, are consistent with anatomic descriptions of dif- neurons with inputs from the glabrous skin of only a single digit. They were also more likely to have the shortest-laferent routes from on-focus and off-focus digits to the cortical representations of glabrous skin. The representation of tency corticocortical EPSPs. Thus the neuronal couplings exist for at least some convergent somatosensory inputs to the glabrous skin of each digit receives its own thalamocororiginate in the heterogeneous zone, a region known to retical projection from a distinct part of the ventral posterolaceive afferent information from more than one digit and teral nucleus (Doetsch et al. 1988a; Rasmusson and Nance more than one receptor class. TABLE

1.

Latencies of EPSPs evoked by electrical stimulation

of glabrous skin

Range, ms Mean _+ SD, ms Effective/tested

Digit 3

Digit 4

Digit 5

14.4-25.7 19.9 + 3.2 171102

11.7-25.9 15.3 -t 1.9 109/109

14.0-3 1.3 21.1 + 6.3 10191

EPSP, excitatory postsynaptic potential.

Implications

for cortical plasticity

In spite of the corticocortical connection to the placeand modality-specific glabrous zone from the convergent heterogeneous zone (Doetsch et al. 1988b), neurons in the glabrous zone respond almost exclusively to stimulation of glabrous skin of a single digit (Johnson 1985; Johnson et al. 1982). Corticocortical effects from the heterogeneous zone do not result in complex receptive field properties for neu-

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SUBSTRATES

FOR CEREBRAL

rons in the glabrous zone in normal cortex. However, neuronal properties in the glabrous zone change when a digit is amputated (Carson et al. 198 1; Kelahan et al. 198 1; Kelahan and Doetsch 1984; Rasmusson 1982). In this situation, neurons in t.he glabrous zone become responsi ve to stim .ulation. of the Palm, adjacent digits, and both glabrous and hai ry Sk in. In other words, neuronal properties in the cortical area depri .ved of its predominant sensory in put become more similar to those normally found for neurons in the heterogeneous zone. T ‘his change suggests a function for the corticocortical pathwa ,y between these two cortical areas. It could operate during cortical reorganizations to transfer new somatosensory properties to a malleable representation of the digits. It is evident that in normal cortex the inputs from off-focus digits that we have detected by intracellular recording of EPSPs are not expressed in receptive field properties of neurons in the glabrous zone. We propose that during cortical reorganization inputs I from m ultiple digits becom .e expressed as receptive fields because ofa progressive increase in the transfer of somatosensory information by preexisting corticocortical connections. The corticocortical pathway from the heterogeneous zone to the glabrous zone has recently been investigated by intracellular recording in raccoons after digit removal (Smits et al. 1989; Witte et al. 1990). Reorganization is also evoked by events less drastic than peripheral nerve sections, such as temporary nerve blocks (Calford and Tweedale 199 l), differential behavioral use of skin surfaces (Jenkins et al. 1990), or joining the skin on two adjacent digits (Clark et al. 1988). In fact, the capacity for reorganization may be a part of normal cortical function of every individual (Edelman 1978; Merzenich 1987; Merzenich and Kaas 1982; Merzenich et al. 1984). Corticocortical pathways may also be involved in the cortical plasticity evident in these situations. N. Carr and M. Hurt provided expert technical assistance. This research was supported by awards from the Medical Research Council of Canada to P. Zarzecki (MT 7373) and D. D. Rasmusson (MT 6673). E. Smits, D. C. Gordon, and S. Witte received Graduate Awards from Queen’s University. Address for reprint requests: P. Zarzecki, Medical Research Council Group in Sensory-Motor Physiology, Dept. of Physiology, Queen’s University, Kingston, Ontario K7L 3N6, Canada. Received 2 1 March 199 1; accepted in final form 1 May 199 1. REFERENCES H. AND ARNOLD, A. P. Noxious effects of excessive currents used for intracortical microstimulation. Brain Res. 96: 103- 107, 1975. ASANUMA, H., ARNOLD, A., AND ZARZECKI, P. Further study on the excitation of pyramidal tract cells by intracortical microstimulation. Exp. Brain Res. 26: 443-46 1, 1976. CALFORD, M. B. AND TWEEDALE, R. Acute changes in cutaneous receptive fields in primary somatosensory cortex after digit denervation in adult flying fox. J. Neurophysiol. 65: 178- 187, 199 1. CARSON, L., KELAHAN, A. M., RAY, R. H., MASSEY, C. E., AND D~ETSCH, G. S. Effects of early peripheral lesions on the somatotopic organization of the cerebral cortex. CZin. Neurosurg. 28: 532-546, 198 1. CLARK, S. A., ALLARD, T., JENKINS, W. M., AND MERZENICH, M. M. Receptive fields in the body-surface map in adult cortex defined by temporally correlated inputs. Nature Lond. 332: 444-445, 1988. COLE, L. W. Observations of the senses and instincts of the raccoon. J. Anim. Behav. 2: 299-309, 19 12. D~ETSCH, G. S., STANDAGE, G. P., JOHNSTON, K. W., AND LIN, C.-S. ASANIJMA,

CORTICAL

PLASTICITY

695

Thalamic connections of two functional subdivisions of the somatosensory forepaw cerebral cortex of the raccoon. J. Neurosci. 8: 1873- 1886, 1988a. DOETSCH, G. S., STANDAGE, G. P., JOHNSTON, K. W., AND LIN, C.-S. Intracortical connections of two functional subdivisions of the somatosensory forepaw cerebral cortex of the raccoon. J. Neurosci. 8: 18871900, 1988b. DOETSCH, G. S., STONEY, S. D., JR., AND HAUGE, D. H. Convergent inputs to single neurons in SmI forepaw digit cortex of raccoons. Sot. Neurosci. Abstr. 15: 1053, 1989. EDELMAN, G. M. Group selection and phasic reentrant signaling: a theory of higher brain function. In: The Mindful Brain, edited by G. M. Edelman and V. B. Mountcastle. Cambridge, MA: MIT Press, 1978, pp. 5 l-95. FELDMAN, S. H. AND JOHNSON, J. I., JR. Kinesthetic cortical area anterior to primary somatic sensory cortex in the raccoon (Procyon Zotor). J. Comp. Neural. 277: 80-95, 1988. JENKINS, W. M., MERZENICH, M. M., OCHS, M. T., ALLARD, T., AND GUIC-ROBLES, E. Functional reorganization of primary somatosensory cortex in adult owl monkeys after behaviorally controlled tactile stimulation. J. Neurophysiol. 63: 82- 104, 1990. JOHNSON, J. I., JR. Thalamocortical organization in the raccoon: comparison with the primate. Exp. Brain Res. Suppl. 10: 294-3 12, 1985. JOHNSON, J. I., JR. OSTAPOFF, E.-M., AND WARACH, S. The anterior border zones of primary somatic sensory (Sl) neocortex and their relations to cerebral convolutions shown by micromapping of peripheral projections to the region of the fourth forepaw digit representation in raccoons. Neuroscience 7: 9 15-936, 1982. KELAHAN, A. M., DOETSCH, G. S. Time-dependent changes in the functional organization of somatosensory cerebral cortex following digit amputation in adult raccoons. Somatosens. Res. 2: 49-8 1, 1984. KELAHAN, A. M., RAY, R. H., CARSON, L. V., MASSEY, C. E., AND D~ETSCH, G. S. Functional reorganization of adult raccoon somatosensory cerebral cortex following neonatal digit amputation. Brain Res. 223: 151-159, 1981. LEDERMAN, S. J., BROWSE, R. A., AND KLATZKY, R. L. Haptic processing of spatially distributed information. Percep. Psychophys. 44: 222-232, 1988. MERZENICH, M. M. Dynamic neocortical processes and the origins of higher brain functions. In: The Neural and Molecular Bases of Learning, edited by J.-P. Changeux and M. Konishi. New York: Wiley, 1987, p. 337-358. MERZENICH, M. M. AND KAAS, J. H. Reorganization of mammalian somatosensory cortex following peripheral nerve injury. Trends Neurosci. 5: 434-436, 1982. MERZENICH, M. M., NELSON, R. J., STRYKER, M. P., CYNADER, M. S., SCHOPPMANN, A., AND ZOOK, J. M. Somatosensory cortical map changes following digit amputation in adult monkeys. J. Comp. Neural. 224: 591-605, 1984. PIJBOLS, B. H., JR. Effect of mechanical stimulus spread across glabrous skin of raccoon and squirrel monkey hand on tactile primary afferent fiber discharge. Somatosens. Res. 4: 273-308, 1987. PUBOLS, B. H., JR. AND PUBOLS, L. M. Coding of mechanical stimulus velocity and indentation depth by squirrel monkey and raccoon glabrous skin mechanoreceptors. J. Neurophysiol. 39: 773-787, 1976. RASMUSSON, D. D. Reorganization of raccoon somatosensory cortex following removal of the fifth digit. J. Comp. Neural. 205: 3 13-326, 1982. RASMUSSON, D. D. AND NANCE, D. M. Non-overlapping thalamocortical projections for separate forepaw digits before and after cortical reorganization in the raccoon. Brain Res. Bull. 16: 399-406, 1986. SMITS, E., GORDON, D. C., ZARZECKI, P., AND RASMUSSON, D. D. Plasticity of raccoon somatosensory cortex: roles of convergent sensory inputs and corticocortical EPSPs. Sot. Neurosci. Abstr. 15: 1052, 1989. WALL, J. T. Variable organization in cortical maps of the skin as an indication of the lifelong adaptive capacities of circuits in the mammalian brain. Trends Neurosci. 11: 549-557, 1988. WELKER, W. I. AND SEIDENSTEIN, S. Somatic sensory representation in the cerebral cortex of the raccoon (Procyon lotor). J. Comp. Neural. 111: 469-502, 1959. WITTE, S., SMITS, E., ZARZECKI, P., AND RASMUSSON, D. D. New somatosensory EPSPs in reorganized raccoon cortex without a change in the incidence of corticocortical EPSPs. Sot. Neurosci. Abstr. 16: 44, 1990. ZARZECKI, P. AND ASANUMA, H. Proprioceptive influences on somatosensory and motor cortex. Prog. Brain Res. 50: 113-l 19, 1979.

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