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Primary motor cortex receives input from area 3a in macaques
Brain Research, 537 (1990) 367-371 Elsevier
367
BRES 24463
Primary motor cortex receives input from area 3a in macaques M.F. H u e r t a I a n d T.P. P o n s 2 1Department of BioStructure and Function, University of Connecticut Health. Center, Farmington, CT 06030 (U.S.A.) and Laboratory of Neuropsychology, NIMH, Bethesda, MD 20892 (U.S.A.) (Accepted 25 September 1990)
Key words: Primary motor cortex; Cortical area 3a; Sensorimotor; Primate; Connectional topography; Intracortical microstimulation
Intracortical microstimulation was used to define topographic sectors and the rostral border of primary motor cortex in adult macaques (Macaca mulatta). In the same animals, injections of fluorescent tracers were made within defined regions of primary motor cortex. Retrogradely labeled neurons were topographicallydistributed in area 3a, with most neurons located in layer III, and fewer neurons situated in layers V and IV. These findings suggest that muscle afferent information, thought to be important in a closed-loop mode of function, may reach primary motor cortex directly from cortical area 3a.
Non-cutaneous somatosensory information, including that originating in muscle receptors, is known to reach primary motor cortex in primates, and such information is considered to be important in mechanisms of motor control 1'2'5'16'24. Since cortical area 3a is dominated by inputs relaying muscle receptor information 6'7'14A7'26, this area might he expected to supply such input to the adjacent primary motor cortex. Previous anatomical experiments, however, have yielded contradictory results, some appearing to support 3'12, and others to not support this hypothesis 11'13. The issue of whether area 3a projects directly to primary motor cortex has been complicated by disagreements regarding the definition of area 3a n'18'24, and the level of certainty as to whether retrograde tracer injection sites were limited to primary motor cortex, the rostral border of which is difficult to assess on the basis of cytoarchitecture25. On the other hand, a consensus on the cytoarchitectonic definition of area 3a appears to be emerging from the literature, with most investigators generally agreeing with the definition of Vogt and Vogt3'11"18"21. Moreover, while there is still disagreement about whether the rostral border of primary motor cortex is clear on the basis of cytoarchitecture, this border has been shown to be distinguishable on the basis of intracortical microstimulation 15'18'22. In the present study, therefore, intracortical microstimulation and anatomical tract tracing techniques were used in combination to examine the question of whether cells in area 3a project to primary motor cortex in macaques, and
if SO, whether this projection is topographically organized. Each of 3 adult monkeys (Macaca mulatta) was initially anesthetized with ketamine hydrochloride (20 mg/kg b.wt.) administered intramuscularly, and was given supplemental doses of ketamine (typically 20-30 mg/kg) as needed to maintain an anesthetic state 23. In latter parts of the experiment, sedation was aided by intramuscular injections of acepromazine (20-30 mg/kg). Throughout the experiment, which lasted 5-7 h, the animal was kept on a heating pad, and its temperature, respiration, and heart rate were closely monitored. Under aseptic conditions, a large craniotomy was made over the frontal lobe, and the underlying dura mater was cut to expose the cortical surface. The locations of the rostral border of primary motor cortex and of the various topographic sectors of primary motor cortex were defined via intracortical microstimulation delivered at multiple sites 4'8. Different anatomical tracers were then injected into physiologically defined sectors of primary motor cortex. Although each animal received injections of up to 5 different tracers, only data resulting from injections of the retrogradely transported tracers Diamidino yellow and Fast blue are presented here. Intracortical stimuli consisted of 40 ms trains of symmetric biphasic square wave pulses of 0.4 ms of constant current delivered at 300 Hz via a tungsten microelectrode with an impedance of 0.2-0.4 MI2 at 1000 Hertz. As in other investigations, M-I was defined as the precentral cortex from which visible or palpable muscle contractions could be evoked using current levels of 30
Correspondence: M.F. Huerta, Department of BioStructure and Function, University of Connecticut Health Center Farmington, CT 06030, U.S.A. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
368 /~A or less. In agreement with others 18, cortex rostral to primary motor cortex required current levels greater than 30/~A to elicit muscle contractions. Volumes of 0.4-0.7 /A of a 3% solution of Diamidino yellow or Fast blue were injected with pressure via a microsyringe into physiologically defined topographic sectors (orofacial, forelimb, or hindlimb) of primary motor cortex. Following survival periods of 6-11 days, the animals were euthanized and transcardially perfused with 4% paraformaldehyde in 0.1 M cacodylate buffer or in 0.1 M phosphate buffer. After serially cutting the brain into 40 /~m sections on a freezing microtome, a one in 10 or a one in 5 series of sections was mounted, and the label was plotted using fluorescence microscopy. While injection sites extended through all cortical layers, they did not encroach upon the underlying white matter. As described in more detail elsewhere 8, the cortical region containing the injection site (shown in Figs. 1 and 2) was graphically reconstructed from drawings of frontal sections in which the injection site was plotted. The drawings of frontal sections were aligned and related to landmarks on surface photographs of the region. Such landmarks included blood vessels, locations of marking lesions, and/or sulci. From dorsal or dorsolateral views, the injection sites appeared round, with irregular edges (Figs. 1 and 2). The 3-dimensional shapes of the injection sites resembled cylinders or parabolas; in the latter case, the widest part of the parabola occupied the most superficial layers. Cytoarchitectonic features were added to plots of label from adjacent sections which were stained for Nissl substance. Area 3a was defined according to the criteria of Vogt and Vogt 2x. In case 88-55 (Fig. 1), movements of the jaw and tongue were produced by intracortical microstimulation of lateral primary motor cortex, while movements of forelimb structures were elicited by stimulation of a medially adjacent region of primary motor cortex. In this case, Fast blue was injected into the orofacial sector and Diamidino yellow into the forelimb sector of primary motor cortex. By comparing the extent of the injection site with intracortical microstimulation data, both injection sites were found to be confined to primary motor cortex. Within area 3a, neurons containing Fast blue were situated rostral and lateral to those containing Diamidino yellow, and the blue- and yellow-labeled cells were segregated from each other. Cells in area 3a projecting to the forelimb and orofacial sectors of primary motor cortex were concentrated in layer Ill, with additional cells present in layers V and IV. In case 89-44 (Fig. 2A), intracortical microstimulation was used to define a region of the hindlimb sector of medial primary motor cortex, and Fast blue was injected into cortex from which movements of the first toe were
evoked. By comparing the intracortical microstimulation data with the reconstructed injection site, it was determined that the rostral border of this injection site was at least 1.3 mm caudal to the rostral border of primary motor cortex. Retrogradely labeled neurons were distributed in medial area 3a, but not in other parts of area 3a. These cells were concentrated in layer III. In case 89-43 (Fig. 2B) Diamidino yellow was injected into cortex from which intracortical microstimulation evoked forelimb movements, as in case 88-55 (Fig. 1). Both the medial-lateral location and laminar pattern of labeled neurons in area 3a were similar in the two cases. Most labeled neurons occupied layer III, with fewer cells present in layers V and IV. In each of these cases, the laminar distribution of labeled cells differed markedly in areas 3a and parts of area 4 outside the injection site. Lamina II of area 4 contained more labeled cells than lamina II in the adjacent area 3a (Figs. 2A and 2B). This increased density of labeled cells in lamina II of area 4 corresponded well with the cytoarchitectonically defined border between areas 3a and 4. In most earlier reports it has been suggested that area 3a does not project to primary motor cortex in macaques 5"9'1°'11'13. In contrast to this prevalent view, the present results clearly demonstrate such a projection, and confirm and extend the results of another recent report 3. Combined microstimulation and anatomieal methods used in this study enabled us to determine that the injection sites were confined to primary motor cortex. Since in the present study the location of the rostral border of the primary motor cortex was determined before tracer injections were made, it was possible to make tracer injections closer to the rostral border than would be prudent without such knowledge. Placement of injections within the rostral part of primary motor cortex is important because this particular zone contains the preponderance of primary motor cortex neurons that respond to stimulation of receptors located in deep tissues, including receptors innervating muscles 19'21. Given the fact that area 3a is dominated by information arising in deep receptors, and assuming that the projection of area 3a to primary motor cortex is conveying such information, it is likely that the rostrally located primary motor cortical neurons which respond to stimulation of deep receptors are the targets of this projection. Thus, by placing the injection sites in the rostral part of primary motor cortex the probability of labeling cells in area 3a was increased. Besides permitting identification of the rostral border of primary motor cortex, the use of intracortical microstimulation in the present study allowed identification of topographic sectors of primary motor cortex; injections were made within the orofacial, forelimb and hindlimb
369
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S.h F2 F1 E 1
V • V
10mm
Yellow injection site Fast Blue injection site
Arcuate Sulcus x No movement evoked ~<30~A • Movement evoked ~<30~A of: E: Elbow Sh: Shoulder F: Finger Tg: Tongue J: Jaw V: Vibrissae
T~
T,g
x Sulcus
5mm
Cg
S
A
Fig. 1. Case in which Fast blue was injected into the orofacial sector, and Diamidino yellow into the forelimb sector, of rostral primary motor cortex. Top panel: a lateral view of the left hemisphere, with rostral to the left and dorsal to the top. The injection sites are drawn in black and the bold outline indicates region enlarged on the right. Enlarged region shows the relationship between the intracortical microstimulation data and the extent of each injection site. Small dots indicate electrode penetrations from which movements were evoked at current levels at, or below, 30 g A and X's indicate penetrations from which movements could not be evoked at these current levels. Bottom panel: frontal sections showing the locations of neurons retrogradely labeled with Fast blue (dots) or Diamidino yellow (open circles). Levels from which these sections were taken are indicated by corresponding letters in top panel.
370
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./ /%
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Fig. 2. A: case in which Fast blue was injected into the hindlimb sector of primary motor cortex. In the left panel, the relationship between intracortical microstimulation sites and the extent of the injection site (black) is shown, with rostral to the left and dorsal to the top. Numerals indicate lowest level of current (in/~A) required to elicit a consistent movement of a given body part (T, toe; Lg, leg; Tr, trunk; FA, forearm). Other conventions and abbreviations as in Fig. 1. To the right is a frontal section (labeled cells not illustrated) with an enlargement of the enclosed region illustrating the laminar pattern of labeled cells (dots) in area 3a and adjacent cortical areas 3b and 4, with cortical layers of area 3a indicated; arrowheads mark the borders of area 3a. B: case in which Diamidino yellow was injected into the forelimb sector of primary motor cortex. Mierostimulation data obtained in this case are not flhistrated. In the top left panel is a dorsolateral view of the left hemisphere, with the injection site drawn in black and the levels corresponding to the illustrated sections indicated. Other conventions as in Figs. 1 and 2A.
371 sectors of primary m o t o r cortex. A r e a 3a n e u r o n s were
The general question of which pathways supply the
retrogradely labeled following injections in each of these topographic sectors. Moreover, the retrogradely labeled
m a j o r somatosensory activation of primary m o t o r cortex in macaques remains unresolved5'24. Nevertheless, the
n e u r o n s were distributed topographically within area 3a. N e u r o n s projecting to the hincUimb sector of primary m o t o r cortex were situated in medial area 3a, those
present work demonstrates the existence of a topographic and lamina specific projection from area 3a to primary
projecting to the forelimb sector of primary motor cortex were situated in a more lateral part of area 3a, and those projecting to the orofacial sector of primary m o t o r cortex were situated in a still more lateral part of area 3a. This topography was apparent from comparisons made across
m o t o r cortex, and suggests this pathway as a prime candidate for supplying the primary m o t o r cortex with information arising in muscle receptors.
cases as well as from comparison of multiple retrograde tracers within single cases.
We thank Adrienne Gagnon, James Diehi, Chris Halsell, and Dr. Lynn Huerta for their technical help, and Dr. Mortimer Mishkin for his support and comments on the manuscript. Supported in part by UCorm HCRAC grant and NS25874 (to M.F.H.).
1 Evarts, E.V., Transcortical reflexes: their properties and functional significance. In A.W. Goodwin and I. Darian-Smith (Eds.), Hand Function and the Neocortex, Springer-Verlag, New York, 1985, pp. 130-154. 2 Evarts, E.V., Motor cortex output in primates. In E.G. Jones and A. Peters (Eds.), Cerebral Cortex, Vol. 5, Plenum, New York, 1986, pp. 217-241. 3 Ghosh, S., Brinkman, C. and Porter, R., A quantitative study of the distribution of neurons projecting to the precentral motor cortex in the monkey (M. fascicularis), J. Comp. Neurol., 259 (1987) 424-444. 4 Gould, H.J., Cusick, C.G., Pons, T.P. and Kaas, J.H., The relationship of corpus callosum connections to electrical stimulation of motor, supplementary motor, and the frontal eye fields in owl monkeys, J. Comp. NeuroL, 247 (1986) 297-325. 5 Hepp-Reymond, M.-C., Functional organization of motor cortex and its participation in voluntary movements. In H.D. Steklis and J. Erwin (Eds.), Comparative Primate Biology, Vol. 4, Alan R. Liss, New York, 1988, pp. 501-624. 6 Hore, J., Preston, J.B., Durkovic, R.G. and Cheney, P.D., Responses of cortical neurons (areas 3a and 4) to ramp stretch of hindlimb muscles in the baboon, J. Neurophysiol., 39 (1976) 484-500. 7 Huang, C.-S., Sirisko, M.A., Hiraba, H., Murray, G.M. and Sessle, B.J., Organization of the primate face motor cortex as revealed by intracortical microstimulation and electrophysiological identification of afferent inputs and corticobulbar projections, J. NeurophysioL, 59 (1988) 796-818. 8 Huerta, M.F. and Kaas, J.H., Supplementary eye field as defined by intracortical microstimulation: connections in macaques, J. Comp. Neurol., 293 (1990) 299-330. 9 Jones, E.G., Connectivity of the primate sensory-motor cortex. In E.G. Jones and A. Peters (Eds.), Cerebral Cortex, Vol. 5, Plenum, New York, 1986, pp. 113-183. 10 Jones, E.G., Ascending inputs to, and internal organization of, cortical motor areas. In G. Book, M. O'Connor and J. Marsh (Eds.), Motor Areas of the Cerebral Cortex, Wiley, Sussex, 1987, pp. 21-39. 11 Jones, E.G. and Porter, R., What is area 3a?, Brain Res. Rev., 2 (1980) 1-43. 12 Jones, E.G. and PoweU, T.ES., Connexions of the somatic sensory cortex of the rhesus monkeys. I. Ipsilateral cortical connexions, Brain Research, 93 (1969) 477-502.
13 Jones, E.G., Coulter, J.D. and Hendry, S.H.C., Intracortical connectivity of architectonic fields in the somatic sensory motor and parietal cortex of monkeys, J. Comp. Neurol., 181 (1978) 291-348. 14 Lucier, G.E., Ruegg, D.C. and Wiesendanger, M., Responses of neurones in motor cortex and in area 3a to controlled stretches of forelimb muscles in cebus monkeys, J. Physiol., 251 (1975) 833-853. 15 McGuinness, E., Sivertsen, D. and Allman, J.M., Organization of the face representation in macaque motor cortex, J. Comp. Neurol., 193 (1980) 591-608. 16 Phillips, C.G., Motor apparatus of the baboon's hand. The Ferrier Lecture, Proc. Roy. Soc. Ser. B, 173 (1969) 141-174. 17 Sakai, T. and Preston, J.B., Evidence for a transcortical reflex: primate corticospinal tract neuron responses to ramp stretch of muscle, Brain Research, 159 (1978) 463-467. 18 Sessle, B.J. and Wiesendanger, M., Structural and functional definition of the motor cortex in the monkey (Macaca fascicularis), J. Physiol., 323 (1982) 245-265. 19 Strick, P.L. and Preston, J.B., Sorting of somatosensory afferent information in primate motor cortex, Brain Research, 156 (1978) 364-368. 20 Tanji, J. and Wise, S.P., Submodality distribution in sensorimotor cortex of the unanesthetized monkey, J. Neurophysiol., 45 (1981) 467-481. 21 Vogt, C. and Vogt, O., Allgemeinere Ergebnisse uuserer Hirnforschnng, J. Psychol. Neurol., 25 (1919) 279-439. 22 Weinrich, M. and Wise, S.P., The premotor cortex of the monkey, J. Neurosci., 2 (1982) 1329-1345. 23 White, P.E, Way, W. L. and Woolsey, Ketamine - - its pharmacology and therapeutic uses, Anesthesiology, 56 (1982) 119-136. 24 Wiesendanger, M. and Miles, T., Ascending pathway of lowthreshold muscle afferents to the cerebral cortex and its possible role in motor control, Physiol. Rev., 62 (1982) 1234--1270. 25 Wise, S.P., The nonprimary motor cortex and its role in cerebral control of movement. In G.M. Edelman, W.E. Gall and W.M. Cowan (Eds.), Dynamic Aspects of Neocortical Function, Wiley, New York, 1984, pp. 525-556. 26 Wise, S.P. and Tanji, J., Neuronal responses in sensorimotor cortex to ramp displacements and maintained positions imposed on hindlimb of the unanesthetized monkey, J. Neurophysiol., 45 (1981) 482-500.
367
BRES 24463
Primary motor cortex receives input from area 3a in macaques M.F. H u e r t a I a n d T.P. P o n s 2 1Department of BioStructure and Function, University of Connecticut Health. Center, Farmington, CT 06030 (U.S.A.) and Laboratory of Neuropsychology, NIMH, Bethesda, MD 20892 (U.S.A.) (Accepted 25 September 1990)
Key words: Primary motor cortex; Cortical area 3a; Sensorimotor; Primate; Connectional topography; Intracortical microstimulation
Intracortical microstimulation was used to define topographic sectors and the rostral border of primary motor cortex in adult macaques (Macaca mulatta). In the same animals, injections of fluorescent tracers were made within defined regions of primary motor cortex. Retrogradely labeled neurons were topographicallydistributed in area 3a, with most neurons located in layer III, and fewer neurons situated in layers V and IV. These findings suggest that muscle afferent information, thought to be important in a closed-loop mode of function, may reach primary motor cortex directly from cortical area 3a.
Non-cutaneous somatosensory information, including that originating in muscle receptors, is known to reach primary motor cortex in primates, and such information is considered to be important in mechanisms of motor control 1'2'5'16'24. Since cortical area 3a is dominated by inputs relaying muscle receptor information 6'7'14A7'26, this area might he expected to supply such input to the adjacent primary motor cortex. Previous anatomical experiments, however, have yielded contradictory results, some appearing to support 3'12, and others to not support this hypothesis 11'13. The issue of whether area 3a projects directly to primary motor cortex has been complicated by disagreements regarding the definition of area 3a n'18'24, and the level of certainty as to whether retrograde tracer injection sites were limited to primary motor cortex, the rostral border of which is difficult to assess on the basis of cytoarchitecture25. On the other hand, a consensus on the cytoarchitectonic definition of area 3a appears to be emerging from the literature, with most investigators generally agreeing with the definition of Vogt and Vogt3'11"18"21. Moreover, while there is still disagreement about whether the rostral border of primary motor cortex is clear on the basis of cytoarchitecture, this border has been shown to be distinguishable on the basis of intracortical microstimulation 15'18'22. In the present study, therefore, intracortical microstimulation and anatomical tract tracing techniques were used in combination to examine the question of whether cells in area 3a project to primary motor cortex in macaques, and
if SO, whether this projection is topographically organized. Each of 3 adult monkeys (Macaca mulatta) was initially anesthetized with ketamine hydrochloride (20 mg/kg b.wt.) administered intramuscularly, and was given supplemental doses of ketamine (typically 20-30 mg/kg) as needed to maintain an anesthetic state 23. In latter parts of the experiment, sedation was aided by intramuscular injections of acepromazine (20-30 mg/kg). Throughout the experiment, which lasted 5-7 h, the animal was kept on a heating pad, and its temperature, respiration, and heart rate were closely monitored. Under aseptic conditions, a large craniotomy was made over the frontal lobe, and the underlying dura mater was cut to expose the cortical surface. The locations of the rostral border of primary motor cortex and of the various topographic sectors of primary motor cortex were defined via intracortical microstimulation delivered at multiple sites 4'8. Different anatomical tracers were then injected into physiologically defined sectors of primary motor cortex. Although each animal received injections of up to 5 different tracers, only data resulting from injections of the retrogradely transported tracers Diamidino yellow and Fast blue are presented here. Intracortical stimuli consisted of 40 ms trains of symmetric biphasic square wave pulses of 0.4 ms of constant current delivered at 300 Hz via a tungsten microelectrode with an impedance of 0.2-0.4 MI2 at 1000 Hertz. As in other investigations, M-I was defined as the precentral cortex from which visible or palpable muscle contractions could be evoked using current levels of 30
Correspondence: M.F. Huerta, Department of BioStructure and Function, University of Connecticut Health Center Farmington, CT 06030, U.S.A. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
368 /~A or less. In agreement with others 18, cortex rostral to primary motor cortex required current levels greater than 30/~A to elicit muscle contractions. Volumes of 0.4-0.7 /A of a 3% solution of Diamidino yellow or Fast blue were injected with pressure via a microsyringe into physiologically defined topographic sectors (orofacial, forelimb, or hindlimb) of primary motor cortex. Following survival periods of 6-11 days, the animals were euthanized and transcardially perfused with 4% paraformaldehyde in 0.1 M cacodylate buffer or in 0.1 M phosphate buffer. After serially cutting the brain into 40 /~m sections on a freezing microtome, a one in 10 or a one in 5 series of sections was mounted, and the label was plotted using fluorescence microscopy. While injection sites extended through all cortical layers, they did not encroach upon the underlying white matter. As described in more detail elsewhere 8, the cortical region containing the injection site (shown in Figs. 1 and 2) was graphically reconstructed from drawings of frontal sections in which the injection site was plotted. The drawings of frontal sections were aligned and related to landmarks on surface photographs of the region. Such landmarks included blood vessels, locations of marking lesions, and/or sulci. From dorsal or dorsolateral views, the injection sites appeared round, with irregular edges (Figs. 1 and 2). The 3-dimensional shapes of the injection sites resembled cylinders or parabolas; in the latter case, the widest part of the parabola occupied the most superficial layers. Cytoarchitectonic features were added to plots of label from adjacent sections which were stained for Nissl substance. Area 3a was defined according to the criteria of Vogt and Vogt 2x. In case 88-55 (Fig. 1), movements of the jaw and tongue were produced by intracortical microstimulation of lateral primary motor cortex, while movements of forelimb structures were elicited by stimulation of a medially adjacent region of primary motor cortex. In this case, Fast blue was injected into the orofacial sector and Diamidino yellow into the forelimb sector of primary motor cortex. By comparing the extent of the injection site with intracortical microstimulation data, both injection sites were found to be confined to primary motor cortex. Within area 3a, neurons containing Fast blue were situated rostral and lateral to those containing Diamidino yellow, and the blue- and yellow-labeled cells were segregated from each other. Cells in area 3a projecting to the forelimb and orofacial sectors of primary motor cortex were concentrated in layer Ill, with additional cells present in layers V and IV. In case 89-44 (Fig. 2A), intracortical microstimulation was used to define a region of the hindlimb sector of medial primary motor cortex, and Fast blue was injected into cortex from which movements of the first toe were
evoked. By comparing the intracortical microstimulation data with the reconstructed injection site, it was determined that the rostral border of this injection site was at least 1.3 mm caudal to the rostral border of primary motor cortex. Retrogradely labeled neurons were distributed in medial area 3a, but not in other parts of area 3a. These cells were concentrated in layer III. In case 89-43 (Fig. 2B) Diamidino yellow was injected into cortex from which intracortical microstimulation evoked forelimb movements, as in case 88-55 (Fig. 1). Both the medial-lateral location and laminar pattern of labeled neurons in area 3a were similar in the two cases. Most labeled neurons occupied layer III, with fewer cells present in layers V and IV. In each of these cases, the laminar distribution of labeled cells differed markedly in areas 3a and parts of area 4 outside the injection site. Lamina II of area 4 contained more labeled cells than lamina II in the adjacent area 3a (Figs. 2A and 2B). This increased density of labeled cells in lamina II of area 4 corresponded well with the cytoarchitectonically defined border between areas 3a and 4. In most earlier reports it has been suggested that area 3a does not project to primary motor cortex in macaques 5"9'1°'11'13. In contrast to this prevalent view, the present results clearly demonstrate such a projection, and confirm and extend the results of another recent report 3. Combined microstimulation and anatomieal methods used in this study enabled us to determine that the injection sites were confined to primary motor cortex. Since in the present study the location of the rostral border of the primary motor cortex was determined before tracer injections were made, it was possible to make tracer injections closer to the rostral border than would be prudent without such knowledge. Placement of injections within the rostral part of primary motor cortex is important because this particular zone contains the preponderance of primary motor cortex neurons that respond to stimulation of receptors located in deep tissues, including receptors innervating muscles 19'21. Given the fact that area 3a is dominated by information arising in deep receptors, and assuming that the projection of area 3a to primary motor cortex is conveying such information, it is likely that the rostrally located primary motor cortical neurons which respond to stimulation of deep receptors are the targets of this projection. Thus, by placing the injection sites in the rostral part of primary motor cortex the probability of labeling cells in area 3a was increased. Besides permitting identification of the rostral border of primary motor cortex, the use of intracortical microstimulation in the present study allowed identification of topographic sectors of primary motor cortex; injections were made within the orofacial, forelimb and hindlimb
369
Sh
S.h F2 F1 E 1
V • V
10mm
Yellow injection site Fast Blue injection site
Arcuate Sulcus x No movement evoked ~<30~A • Movement evoked ~<30~A of: E: Elbow Sh: Shoulder F: Finger Tg: Tongue J: Jaw V: Vibrissae
T~
T,g
x Sulcus
5mm
Cg
S
A
Fig. 1. Case in which Fast blue was injected into the orofacial sector, and Diamidino yellow into the forelimb sector, of rostral primary motor cortex. Top panel: a lateral view of the left hemisphere, with rostral to the left and dorsal to the top. The injection sites are drawn in black and the bold outline indicates region enlarged on the right. Enlarged region shows the relationship between the intracortical microstimulation data and the extent of each injection site. Small dots indicate electrode penetrations from which movements were evoked at current levels at, or below, 30 g A and X's indicate penetrations from which movements could not be evoked at these current levels. Bottom panel: frontal sections showing the locations of neurons retrogradely labeled with Fast blue (dots) or Diamidino yellow (open circles). Levels from which these sections were taken are indicated by corresponding letters in top panel.
370
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89-44 I~k52oIo ,:
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,
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•
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./ /%
F3., d., F~,
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Suleus 5mm
B
_
ST
Fig. 2. A: case in which Fast blue was injected into the hindlimb sector of primary motor cortex. In the left panel, the relationship between intracortical microstimulation sites and the extent of the injection site (black) is shown, with rostral to the left and dorsal to the top. Numerals indicate lowest level of current (in/~A) required to elicit a consistent movement of a given body part (T, toe; Lg, leg; Tr, trunk; FA, forearm). Other conventions and abbreviations as in Fig. 1. To the right is a frontal section (labeled cells not illustrated) with an enlargement of the enclosed region illustrating the laminar pattern of labeled cells (dots) in area 3a and adjacent cortical areas 3b and 4, with cortical layers of area 3a indicated; arrowheads mark the borders of area 3a. B: case in which Diamidino yellow was injected into the forelimb sector of primary motor cortex. Mierostimulation data obtained in this case are not flhistrated. In the top left panel is a dorsolateral view of the left hemisphere, with the injection site drawn in black and the levels corresponding to the illustrated sections indicated. Other conventions as in Figs. 1 and 2A.
371 sectors of primary m o t o r cortex. A r e a 3a n e u r o n s were
The general question of which pathways supply the
retrogradely labeled following injections in each of these topographic sectors. Moreover, the retrogradely labeled
m a j o r somatosensory activation of primary m o t o r cortex in macaques remains unresolved5'24. Nevertheless, the
n e u r o n s were distributed topographically within area 3a. N e u r o n s projecting to the hincUimb sector of primary m o t o r cortex were situated in medial area 3a, those
present work demonstrates the existence of a topographic and lamina specific projection from area 3a to primary
projecting to the forelimb sector of primary motor cortex were situated in a more lateral part of area 3a, and those projecting to the orofacial sector of primary m o t o r cortex were situated in a still more lateral part of area 3a. This topography was apparent from comparisons made across
m o t o r cortex, and suggests this pathway as a prime candidate for supplying the primary m o t o r cortex with information arising in muscle receptors.
cases as well as from comparison of multiple retrograde tracers within single cases.
We thank Adrienne Gagnon, James Diehi, Chris Halsell, and Dr. Lynn Huerta for their technical help, and Dr. Mortimer Mishkin for his support and comments on the manuscript. Supported in part by UCorm HCRAC grant and NS25874 (to M.F.H.).
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