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Types of synapses contacting the soma of corticotectal cells in the visual cortex of the cat
Neuroscience Vol. 7, No. 9, pp. 2159 to 2163. 1982 Printed in Great Britain
0306-4522/82/092159-05$03.00/O
Pergamon Press Ltd 0 1982IBRO
TYPES OF SYNAPSES CONTACTING THE SOMA OF CORTICOTECTAL CELLS IN THE VISUAL CORTEX OF THE CAT H. KENNEDY INSERM, Unit& 94, Laboratoire de Neuropsychologie Exptrimentale, 69500 Bran, France
16 avenue du Doyen L&pine,
Abstract-Retrograde transport of horseradish peroxidase has been used to single out a distinct functional cortical cell type for ultrastructural study. Following injection of horseradish peroxidase into the superior colliculus, labelled pyramidal cells were found in layer V of the visual cortex. Examination of the labelled corticotectal cells from the visual cortex showed that their cell bodies received two types of synaptic contacts. one from boutons containing spherical vesicles and one from boutons containing flattened vesicles. The possible functional significance of this dual type of input is pointed out.
Studies exploring the receptive field (RF) properties of single neurons in the visual cortex have shown that there are well defined physiological cell types.13 Correlation between physiological properties and a given morphological category, however, has proved more difficult. Direct correlations of this type require that RF of a cell be characterized before the same cell is injected with a histochemical marker. These studies have shown that there is no strict correlation between simple/complex RF classification and stellate/pyramida1 categories.’ Cells projecting from the visual cortex to superior colliculus (SC) constitute a group where correlation between cell structure and RF properties has been firmly established. Antidromic activation of corticotectal cells by electrical stimulation of SC has made it possible to identify these neurons and determine their RF characteristics.” Corticotectal neurons are pyramidal cells found in layer V. 8*12*‘8 The majority of synaptic contacts on pyramidal cells are on their dendritic spines.4 Most of these are of the asymmetrical type with spherical vesicles 4*6,‘4.16 and are putative excitatory synapses.24 The spine-free initial portion of the apical and parent stem dendrites, as well as the soma, receive fewer contacts which are always of the symmetrical type with flattened vesicles4~6~‘4*16 and are putative inhibitory synapses. 24 Two of the principal studies to date on cat visual cortex have looked at only a limited number of pyramidal cells and have concentrated on the superficial layers.4.“’ Garey examined pyramidal cells in all layers of area 17 in the cat and found only symmetrical synapses with flattened vesicles on the somas of pyramidal cells.6 In view of the fact that the
Abbreviations: F- or S-type of synapse, synapses with flattened or sperical vesicles, respectively; HRP, horseradish peroxidase; PMLS, posterior-medial lateral suprasylvian area; RF, receptive field; SC, superior colliculus.
RFs of superficial pyramidal cells are different from those of deeper pyramidal cells, 2*7*8,‘7 it is possible that there are underlying morphological differences. Colonnier has suggested that layer V pyramidal cells have a different synaptology from more superficial pyramidal cells.3 The fact that layer V pyramidal cells have been rarely encountered in electron-microscopy has made the issue of differences between superficial and deep pyramidal cell synaptology difficult to determine.3 We report here on the types of synapses found on the somas of corticotectal cells. In electron-microscopic sections, synapses are characterised by a widening of the extracellular space (synaptic cleft), the presence of synaptic vesicles on the presynaptic side of the contact which are often clustered against the presynaptic membrane and cytoplasmic opacities adjacent to the pre- and post-synaptic membranes. The presence or absence of a post-synaptic differentiation has led to synapses being classified as asymmetrical or symmetricaL However, the width of this thickening is variable and tends to be thinner on synapses contacting the soma and this can lead to a variable number of intermediate classes of synapses.26 With respect to the relevance of the postsynaptic opacity for classifying a synapse, it is worth mentioning a recent study which has shown that retinal afferents to the suprachiasmatic nucleus can make both symmetrical and asymmetrical contacts, but that in both cases the vesicles are spherical.” It has been suggested that differences in synaptic vesicle morphology are related to the presence of different chemical transmitter substances.24 Hence, we have used only the shape of vesicles for classification purposes. Synapses were classified as being S-type or F-type according to the size and shape of the synaptic vesicles.4v24 S-type synapses contain spherical vesicles which are relatively homogeneous in shape and size. F-type synapses have smaller vesicles which are flattened. The variability of the shape and size of the
2159
H. KI:NNI.IIY
2160
vesicles of F-type synapses has led to their being described as pleomorphic.4 It has been shown that the shape of synaptic vesicles can be modified by the fixation process,l~‘S.‘l.‘” This made it imperative to control carefully the osmolarity fixation process,
and to standardize
the
501 30 20 10 01
01
2 3L567
8910
01
23&567,8,9
0 10
EXPERIMENTAL
$1.,,~~~~,
PROCEDURES
Five normal adult cats were used. After trephining the skull. one cerebral hemisphere was retracted from the midline so that the SC could be seen between the splenium of the corpus callosum and the bony tentorium. Horseradish peroxidase (HRP) (0.1 ~1 of 30”/,; HRP Serva) was injected into the SC under visual control using a 1~11 Hamilton syringe. After a survival period of 24 h, the animals were anaesthetized and killed by intracardiac perfusion with l”,, paraformaledhyde and I?;, glutaraldehyde in Na cacodylate buffer (pH 7.6). The osmolarity of the fixative was 800 milliosmoles and this value was maintained in all histological procedures. The brain was then removed and the medial bank of the splenial gyrus (area 17) as well as the suprasylvian gyrus (posterior-medial lateral suprasylvianPMLS area of Palmer et u/.~“) were carefully dissected out and placed in cold (4‘ C) buffered fixative for 8 h. Area 17 samples corresponded to cortex subserving the central visual field (Tusu et ~1.‘~).The brain specimens were then for I2 h. washed in cold 5”/, buffered sucrose solution 80 ilrn sections were cut with a tissue chopper. The sections were then incubated according to the methods of Graham & Karnovsky, to in order to demonstrate retrogradely transported HRP. The reacted sections were put in cold buffered sucrose solution and placed under a dissecting microscope. Small 2 mm squares of tissue containing labelled cells were cut out. These blocks were osmicated and embedded in Epon. Thick (0.5 pm) sections were cut on a microtome. mounted on glass and stained with toluidine blue and viewed under dark-field illumination to locate HRP-positive cells. The block was cut down until a section passing through the nucleus of the labelled cell was obtained. The block was trimmed and thin sections of this neuron were cut. mounted on Formvar-covered grids and lightly stained with lead citrate and uranyl acetate. The grids were examined in a Philips 300 electron-microscope. Photographs were taken of all synapses contacting the somas of cells containing HRP reaction product and which were sectioned through the nucleus,
RESULTS Under the light-microscope, HRP reaction product was found in layer V pyramidal cells in area 17 and previous PMLS confirming the results of authors.8~‘2~‘E In the electron-microscope, the HRP reaction product appeared as electron-dense bodies largely restricted to the soma. S-type and F-type synapses were found contacting the soma and the base of the dendrites of corticotectal cells in area I7 and PMLS. No differences were found between either the total number of synapses or the proportion of S-type and F-type synapses contacting the corticotectal cells in area I7 or PMLS. and therefore cells from both areas have been pooled. A total of 84 corticotectal cells were examined and 176 synapses were found
20 30 I
l-2
C
Fig. I. Distribution of synapses on corticotcctal cells. Ordinate: numbers of cells. abscissa: number of synapses. A: total number of synapses. 9: S-type synapses. C: F-type synapses. Shading shows those cells with no synapses.
contacting the soma. One hundred and twenty-six synapses were F-type and 50 were S-type. The distribution of the total number of synapses found contacting corticotectal cells is shown in Fig. IA. The number of S-type synapses contacting labelled cells is shown in Fig. 1B and the number of F-type synapses in Fig. IC. S-type synapses were found on 43”/(, of corticotectal cells and F-type on 66’::,. The mean number of S-type synapses per cell was 0.59 (f standard error of 0.1 I) and the mean number of F-type synapses was I.5 per cell (+ standard error of 0.19). The distribution of S-type and F-type synapses, as revealed by a chi square test. were not significantly different. In 28%, of cells examined both types of synapse were found on a single soma. This is important with respect to the above-mentioned influence of fixation procedure on synaptic vesicle morphology. Examples of a S-type and F-type synapse contacting the soma of area I7 corticotectal cells are shown in Fig 2 and 3.
DISCUSSION
The present result, showing that a functionally distinct group of pyramidal cells in layer V have F-type and S-type synapses, appears to be in contradiction with the results of others who have found evidence of only F-type synapses on the somas of pyramidal cells.4.h.14,“’ The discrepancy could be due to the fact that the previous studies tended to examine pyramidal cells from more superficial layers and the presence of S-type synapses on the soma could be a feature distinctive of corticotectal cells or of all pyramidal cells in deep layers of visual cortex. In either case. because there are few S-type synapses contacting the somas of corticotectal cells. it is imperative to examine a number of cells to reveal their existence. We felt it was necessary to verify that HRP-treated material would not affect the vesicle morphology of synapses contacting more superficial pyramidal cells. HRP was injected into areas 18 and I9 and examination of 20 labelled cells in area I7 showed that the soma of these cells were uniquely contacted by F-type synapses. Retaining the view that spherical vesicles are found in excitatory synapses and in view of the fact that there
Fig. 2. Examples
of synapses contacting area 17 corticotectal cells. A: Two S-type synapses cal vesicles. 8: Two F-type synapses with flattened vesicles. x 50,000
2161
with spheri-
Fig. 3. Horseradish peroxidase-labelled corticotectal cell. Open arrow indicates synapse with spherical vesicles. solid arrow synapses with flattened vesicles. Arrow head indicates one of several HRP grains. Asterix indicates a synapse with spherical vesicles not contacting the labelled cell.
2161
Corticotectal
cells in the visual cortex
are no known thalamic inputs to layer V, there are at least four possible origins of this input to corticotectal cells: corticocortical interconnections, collateral systerns of corticotectal neurons, excitatory interneurons, and projection from pyramidal cells in layers II and III.’ Excitatory interneurons have been postulated in order to explain known columnar arrangements of cells having similar RF properties.22 The presence of
of the cat
2163
S-type synapses on cell soma could be important in view of the suggestion that synapses occupying such a position are priviledged in determining the firing of the cell5 Acknowledyewenfs-C. Baleydier contributed much useful advice and encouragement. N. Boyer gave invaluable technical assistance.
REFERENCES 1. Bodian D. (1970) An electron-microscope characterization of classes of synaptic vesicles by means of controlled aldehyde fixation. J. Cell. Biol. 44, 115-124. 2. Camarda R. M. & Rizzolatti G. (1976) Receptive fields of cells in the superficial layers of the cat’s area 17. Expl Bruin Res. 24, 423-427. 3. Colonnier M. (1967) The fine structural arrangement of the cortex. Archs Neural. Psychiar., Chicago 16, 651-657. 4. Colonnier M. (1968) Synaptic patterns on different cell types in the different laminae of the cat visual cortex. An electron-microscope study. Brain Res. 9, 268-287. 5. Colonnier M. & Rossignol S. (1969) Heterogeneity of the cerebral cortex. In Basic Mechanism of the Epikpsies (eds Jasper, Ward & Hope). J. & A. Churchill. London. 6. Garey L. J. (1971) A light- and electron-microscope study of the visual cortex of the cat and monkey. Proc. R. Sm. B 179. 2140. 7. Gilbert C. D. (I 977) Laminar differences in receptive field properties of cells in cat primary visual cortex. .I. Physiol.. Lond. 268, 391421. 8. Gilbert C. D. & Kelly J. P. (1975) The projections of cells in different layers of the cat’s visual cortex. J. camp. Neural. 163, 81~106. 9. Gilbert C. D. & Wiesel T. N. (1979) Morphology and intracortical projections of functionally characterized neurons in the cat visual cortex. Nature, Land. 280, 12&125. 10. Graham R. C. & Karnovsky M. J. (1966) Glomerular permeability ultrastructural cytochemical studies using peroxidase as protein tracers. J. exp. Med. 124, 1123-I 133. Il. Giildner F. H. & Wolff J. R. (1978) Retinal afferents form Gray Type I and Type II synapses in the suprachiasmatic nucleus (Rat). Expl. Bruin Res. 32. 83-89. 12. Hiillander H. (1974) On the origin of corticotectal projections in the cat. Expl Brain Res. 21, 433-439. 13. Hiibel D. H. & Wiesel. T. N. (1959) Receptive fields of single neurons in the cat’s striate cortex. J. Physiol. Land. 148, 574-591. 14. Jones E. G. & Powell T. P. S. (1970) Electron-microscopy of the somatic sensory cortex of the cat-l. Cell types and synaptic organization. Phi/. Trans. R. Sot. Ser. B 257. 1-l I. 15. Korneliussen H. (1972) Elongated profiles of synaptic vesicles in motor end-plates. Morphological effects of fixative variations. J. Neurocytol. I. 279-296. 16. Le Vay S. (1977) Synaptic patterns in the visual cortex of the cat and monkey. Electron-microscopy of Golgi preparations. J. c’omp. Neural. 150. 53-86. 17. Leventhal A. G. & Hirsch H. V. B. (1978) Receptive field properties of neurons in different laminae of visual cortex of the cat. J. Neurophysiol. 41. 948-962. 18. Magalhaes-Castro H. H., Sariva P. E. S. & Magalhaes-Castro B. (1975) Identification of corticotectal cells of the visual cortex of cats by means of horseradish peroxidase. Bruin Rrs. 83. 474479. 19. Palmer L. A. & Rosenquist A. C. (1974) Visual receptive fields of single striate cortical units projecting to the superior colliculus in the cat. Brain Res. 67, 27-42. 20. Palmer C. A.. Rosenquist A. C. & Tusa R. J. (1978) The retinotopic organization of lateral suprasylvian visual areas in the cat. J. cornp, Neural. 177, 237-256. 21. Paula-Barbosa M. (1975) The duration of aldehyde fixation as a flattening factor of synaptic vesicles. Cell. Tissue Rrs. 164, 63-72. 22. Szentagothai J. (1978) The neuron network of the cerebral cortex: a functional interpretation. Proc. R. Sot. B. 201. 219-248. 23. Tusa R. J., Palmer L. A. & Rosenquist A. C. (1978) The retinotopic organization of area I7 (striate cortex) in the cat. J. camp. Neural. 177, 213-236. 24. Uchizono K. (1965) Characteristics of excitatory and inhibitory synapses in the central nervous sysem of the cat. Nature. Lond. 207, 642-643. 25. Valdivia 0. (1971) Methods of fixation and the morphology of synaptic vesicles. J. camp. Nrurol. 142, 257-274. 26. White E. L. & Rock M. P. (1980) Three dimensional aspects and synaptic relationships of a Golgi-impregnated spiny stellate cell reconstructed from serial thin sections. J. NeurocytoL 9, 615-636. (Accepted 29 March 1982)
0306-4522/82/092159-05$03.00/O
Pergamon Press Ltd 0 1982IBRO
TYPES OF SYNAPSES CONTACTING THE SOMA OF CORTICOTECTAL CELLS IN THE VISUAL CORTEX OF THE CAT H. KENNEDY INSERM, Unit& 94, Laboratoire de Neuropsychologie Exptrimentale, 69500 Bran, France
16 avenue du Doyen L&pine,
Abstract-Retrograde transport of horseradish peroxidase has been used to single out a distinct functional cortical cell type for ultrastructural study. Following injection of horseradish peroxidase into the superior colliculus, labelled pyramidal cells were found in layer V of the visual cortex. Examination of the labelled corticotectal cells from the visual cortex showed that their cell bodies received two types of synaptic contacts. one from boutons containing spherical vesicles and one from boutons containing flattened vesicles. The possible functional significance of this dual type of input is pointed out.
Studies exploring the receptive field (RF) properties of single neurons in the visual cortex have shown that there are well defined physiological cell types.13 Correlation between physiological properties and a given morphological category, however, has proved more difficult. Direct correlations of this type require that RF of a cell be characterized before the same cell is injected with a histochemical marker. These studies have shown that there is no strict correlation between simple/complex RF classification and stellate/pyramida1 categories.’ Cells projecting from the visual cortex to superior colliculus (SC) constitute a group where correlation between cell structure and RF properties has been firmly established. Antidromic activation of corticotectal cells by electrical stimulation of SC has made it possible to identify these neurons and determine their RF characteristics.” Corticotectal neurons are pyramidal cells found in layer V. 8*12*‘8 The majority of synaptic contacts on pyramidal cells are on their dendritic spines.4 Most of these are of the asymmetrical type with spherical vesicles 4*6,‘4.16 and are putative excitatory synapses.24 The spine-free initial portion of the apical and parent stem dendrites, as well as the soma, receive fewer contacts which are always of the symmetrical type with flattened vesicles4~6~‘4*16 and are putative inhibitory synapses. 24 Two of the principal studies to date on cat visual cortex have looked at only a limited number of pyramidal cells and have concentrated on the superficial layers.4.“’ Garey examined pyramidal cells in all layers of area 17 in the cat and found only symmetrical synapses with flattened vesicles on the somas of pyramidal cells.6 In view of the fact that the
Abbreviations: F- or S-type of synapse, synapses with flattened or sperical vesicles, respectively; HRP, horseradish peroxidase; PMLS, posterior-medial lateral suprasylvian area; RF, receptive field; SC, superior colliculus.
RFs of superficial pyramidal cells are different from those of deeper pyramidal cells, 2*7*8,‘7 it is possible that there are underlying morphological differences. Colonnier has suggested that layer V pyramidal cells have a different synaptology from more superficial pyramidal cells.3 The fact that layer V pyramidal cells have been rarely encountered in electron-microscopy has made the issue of differences between superficial and deep pyramidal cell synaptology difficult to determine.3 We report here on the types of synapses found on the somas of corticotectal cells. In electron-microscopic sections, synapses are characterised by a widening of the extracellular space (synaptic cleft), the presence of synaptic vesicles on the presynaptic side of the contact which are often clustered against the presynaptic membrane and cytoplasmic opacities adjacent to the pre- and post-synaptic membranes. The presence or absence of a post-synaptic differentiation has led to synapses being classified as asymmetrical or symmetricaL However, the width of this thickening is variable and tends to be thinner on synapses contacting the soma and this can lead to a variable number of intermediate classes of synapses.26 With respect to the relevance of the postsynaptic opacity for classifying a synapse, it is worth mentioning a recent study which has shown that retinal afferents to the suprachiasmatic nucleus can make both symmetrical and asymmetrical contacts, but that in both cases the vesicles are spherical.” It has been suggested that differences in synaptic vesicle morphology are related to the presence of different chemical transmitter substances.24 Hence, we have used only the shape of vesicles for classification purposes. Synapses were classified as being S-type or F-type according to the size and shape of the synaptic vesicles.4v24 S-type synapses contain spherical vesicles which are relatively homogeneous in shape and size. F-type synapses have smaller vesicles which are flattened. The variability of the shape and size of the
2159
H. KI:NNI.IIY
2160
vesicles of F-type synapses has led to their being described as pleomorphic.4 It has been shown that the shape of synaptic vesicles can be modified by the fixation process,l~‘S.‘l.‘” This made it imperative to control carefully the osmolarity fixation process,
and to standardize
the
501 30 20 10 01
01
2 3L567
8910
01
23&567,8,9
0 10
EXPERIMENTAL
$1.,,~~~~,
PROCEDURES
Five normal adult cats were used. After trephining the skull. one cerebral hemisphere was retracted from the midline so that the SC could be seen between the splenium of the corpus callosum and the bony tentorium. Horseradish peroxidase (HRP) (0.1 ~1 of 30”/,; HRP Serva) was injected into the SC under visual control using a 1~11 Hamilton syringe. After a survival period of 24 h, the animals were anaesthetized and killed by intracardiac perfusion with l”,, paraformaledhyde and I?;, glutaraldehyde in Na cacodylate buffer (pH 7.6). The osmolarity of the fixative was 800 milliosmoles and this value was maintained in all histological procedures. The brain was then removed and the medial bank of the splenial gyrus (area 17) as well as the suprasylvian gyrus (posterior-medial lateral suprasylvianPMLS area of Palmer et u/.~“) were carefully dissected out and placed in cold (4‘ C) buffered fixative for 8 h. Area 17 samples corresponded to cortex subserving the central visual field (Tusu et ~1.‘~).The brain specimens were then for I2 h. washed in cold 5”/, buffered sucrose solution 80 ilrn sections were cut with a tissue chopper. The sections were then incubated according to the methods of Graham & Karnovsky, to in order to demonstrate retrogradely transported HRP. The reacted sections were put in cold buffered sucrose solution and placed under a dissecting microscope. Small 2 mm squares of tissue containing labelled cells were cut out. These blocks were osmicated and embedded in Epon. Thick (0.5 pm) sections were cut on a microtome. mounted on glass and stained with toluidine blue and viewed under dark-field illumination to locate HRP-positive cells. The block was cut down until a section passing through the nucleus of the labelled cell was obtained. The block was trimmed and thin sections of this neuron were cut. mounted on Formvar-covered grids and lightly stained with lead citrate and uranyl acetate. The grids were examined in a Philips 300 electron-microscope. Photographs were taken of all synapses contacting the somas of cells containing HRP reaction product and which were sectioned through the nucleus,
RESULTS Under the light-microscope, HRP reaction product was found in layer V pyramidal cells in area 17 and previous PMLS confirming the results of authors.8~‘2~‘E In the electron-microscope, the HRP reaction product appeared as electron-dense bodies largely restricted to the soma. S-type and F-type synapses were found contacting the soma and the base of the dendrites of corticotectal cells in area I7 and PMLS. No differences were found between either the total number of synapses or the proportion of S-type and F-type synapses contacting the corticotectal cells in area I7 or PMLS. and therefore cells from both areas have been pooled. A total of 84 corticotectal cells were examined and 176 synapses were found
20 30 I
l-2
C
Fig. I. Distribution of synapses on corticotcctal cells. Ordinate: numbers of cells. abscissa: number of synapses. A: total number of synapses. 9: S-type synapses. C: F-type synapses. Shading shows those cells with no synapses.
contacting the soma. One hundred and twenty-six synapses were F-type and 50 were S-type. The distribution of the total number of synapses found contacting corticotectal cells is shown in Fig. IA. The number of S-type synapses contacting labelled cells is shown in Fig. 1B and the number of F-type synapses in Fig. IC. S-type synapses were found on 43”/(, of corticotectal cells and F-type on 66’::,. The mean number of S-type synapses per cell was 0.59 (f standard error of 0.1 I) and the mean number of F-type synapses was I.5 per cell (+ standard error of 0.19). The distribution of S-type and F-type synapses, as revealed by a chi square test. were not significantly different. In 28%, of cells examined both types of synapse were found on a single soma. This is important with respect to the above-mentioned influence of fixation procedure on synaptic vesicle morphology. Examples of a S-type and F-type synapse contacting the soma of area I7 corticotectal cells are shown in Fig 2 and 3.
DISCUSSION
The present result, showing that a functionally distinct group of pyramidal cells in layer V have F-type and S-type synapses, appears to be in contradiction with the results of others who have found evidence of only F-type synapses on the somas of pyramidal cells.4.h.14,“’ The discrepancy could be due to the fact that the previous studies tended to examine pyramidal cells from more superficial layers and the presence of S-type synapses on the soma could be a feature distinctive of corticotectal cells or of all pyramidal cells in deep layers of visual cortex. In either case. because there are few S-type synapses contacting the somas of corticotectal cells. it is imperative to examine a number of cells to reveal their existence. We felt it was necessary to verify that HRP-treated material would not affect the vesicle morphology of synapses contacting more superficial pyramidal cells. HRP was injected into areas 18 and I9 and examination of 20 labelled cells in area I7 showed that the soma of these cells were uniquely contacted by F-type synapses. Retaining the view that spherical vesicles are found in excitatory synapses and in view of the fact that there
Fig. 2. Examples
of synapses contacting area 17 corticotectal cells. A: Two S-type synapses cal vesicles. 8: Two F-type synapses with flattened vesicles. x 50,000
2161
with spheri-
Fig. 3. Horseradish peroxidase-labelled corticotectal cell. Open arrow indicates synapse with spherical vesicles. solid arrow synapses with flattened vesicles. Arrow head indicates one of several HRP grains. Asterix indicates a synapse with spherical vesicles not contacting the labelled cell.
2161
Corticotectal
cells in the visual cortex
are no known thalamic inputs to layer V, there are at least four possible origins of this input to corticotectal cells: corticocortical interconnections, collateral systerns of corticotectal neurons, excitatory interneurons, and projection from pyramidal cells in layers II and III.’ Excitatory interneurons have been postulated in order to explain known columnar arrangements of cells having similar RF properties.22 The presence of
of the cat
2163
S-type synapses on cell soma could be important in view of the suggestion that synapses occupying such a position are priviledged in determining the firing of the cell5 Acknowledyewenfs-C. Baleydier contributed much useful advice and encouragement. N. Boyer gave invaluable technical assistance.
REFERENCES 1. Bodian D. (1970) An electron-microscope characterization of classes of synaptic vesicles by means of controlled aldehyde fixation. J. Cell. Biol. 44, 115-124. 2. Camarda R. M. & Rizzolatti G. (1976) Receptive fields of cells in the superficial layers of the cat’s area 17. Expl Bruin Res. 24, 423-427. 3. Colonnier M. (1967) The fine structural arrangement of the cortex. Archs Neural. Psychiar., Chicago 16, 651-657. 4. Colonnier M. (1968) Synaptic patterns on different cell types in the different laminae of the cat visual cortex. An electron-microscope study. Brain Res. 9, 268-287. 5. Colonnier M. & Rossignol S. (1969) Heterogeneity of the cerebral cortex. In Basic Mechanism of the Epikpsies (eds Jasper, Ward & Hope). J. & A. Churchill. London. 6. Garey L. J. (1971) A light- and electron-microscope study of the visual cortex of the cat and monkey. Proc. R. Sm. B 179. 2140. 7. Gilbert C. D. (I 977) Laminar differences in receptive field properties of cells in cat primary visual cortex. .I. Physiol.. Lond. 268, 391421. 8. Gilbert C. D. & Kelly J. P. (1975) The projections of cells in different layers of the cat’s visual cortex. J. camp. Neural. 163, 81~106. 9. Gilbert C. D. & Wiesel T. N. (1979) Morphology and intracortical projections of functionally characterized neurons in the cat visual cortex. Nature, Land. 280, 12&125. 10. Graham R. C. & Karnovsky M. J. (1966) Glomerular permeability ultrastructural cytochemical studies using peroxidase as protein tracers. J. exp. Med. 124, 1123-I 133. Il. Giildner F. H. & Wolff J. R. (1978) Retinal afferents form Gray Type I and Type II synapses in the suprachiasmatic nucleus (Rat). Expl. Bruin Res. 32. 83-89. 12. Hiillander H. (1974) On the origin of corticotectal projections in the cat. Expl Brain Res. 21, 433-439. 13. Hiibel D. H. & Wiesel. T. N. (1959) Receptive fields of single neurons in the cat’s striate cortex. J. Physiol. Land. 148, 574-591. 14. Jones E. G. & Powell T. P. S. (1970) Electron-microscopy of the somatic sensory cortex of the cat-l. Cell types and synaptic organization. Phi/. Trans. R. Sot. Ser. B 257. 1-l I. 15. Korneliussen H. (1972) Elongated profiles of synaptic vesicles in motor end-plates. Morphological effects of fixative variations. J. Neurocytol. I. 279-296. 16. Le Vay S. (1977) Synaptic patterns in the visual cortex of the cat and monkey. Electron-microscopy of Golgi preparations. J. c’omp. Neural. 150. 53-86. 17. Leventhal A. G. & Hirsch H. V. B. (1978) Receptive field properties of neurons in different laminae of visual cortex of the cat. J. Neurophysiol. 41. 948-962. 18. Magalhaes-Castro H. H., Sariva P. E. S. & Magalhaes-Castro B. (1975) Identification of corticotectal cells of the visual cortex of cats by means of horseradish peroxidase. Bruin Rrs. 83. 474479. 19. Palmer L. A. & Rosenquist A. C. (1974) Visual receptive fields of single striate cortical units projecting to the superior colliculus in the cat. Brain Res. 67, 27-42. 20. Palmer C. A.. Rosenquist A. C. & Tusa R. J. (1978) The retinotopic organization of lateral suprasylvian visual areas in the cat. J. cornp, Neural. 177, 237-256. 21. Paula-Barbosa M. (1975) The duration of aldehyde fixation as a flattening factor of synaptic vesicles. Cell. Tissue Rrs. 164, 63-72. 22. Szentagothai J. (1978) The neuron network of the cerebral cortex: a functional interpretation. Proc. R. Sot. B. 201. 219-248. 23. Tusa R. J., Palmer L. A. & Rosenquist A. C. (1978) The retinotopic organization of area I7 (striate cortex) in the cat. J. camp. Neural. 177, 213-236. 24. Uchizono K. (1965) Characteristics of excitatory and inhibitory synapses in the central nervous sysem of the cat. Nature. Lond. 207, 642-643. 25. Valdivia 0. (1971) Methods of fixation and the morphology of synaptic vesicles. J. camp. Nrurol. 142, 257-274. 26. White E. L. & Rock M. P. (1980) Three dimensional aspects and synaptic relationships of a Golgi-impregnated spiny stellate cell reconstructed from serial thin sections. J. NeurocytoL 9, 615-636. (Accepted 29 March 1982)