EXPERIMENTAL
NEUROLOGY
Electrophysiological in the Ventral
46,506-520
and Lateral
LAWRENCE
Departments
of Anatomy
Received
September
H.
(1975)
Morphological Geniculate
MATHERS
AND
Properties of Neurons Nucleus of the Rabbit GIAN
G.MASCETTI~
and Neurology, Stanford University Stanford, California 94305
School
of Medicine,
6, 1974; revision received September 21,1974
The ventral lateral geniculate nucleus of the rabbit has been studied with light and electron microscopy, as well as single unit extracellular recordings. There are two basic types of neurons in the nucleus, and three synaptic types, designated RS, RL and F. There are also synaptic glomeruli, in which RL axons are always involved. The arrangement of these synaptic types and their percentage of occurrence closely resembles that in dorsal thalamic nuclei. Retinal projections are found in both external and internal sectors of the nucleus, shown by both degeneration and autoradiagraphic techniques. Retinal projections to the internal sector are sparser and degenerate more rapidly than those to the external sector. Electrophysiological studies revealed some neurons in both external and internal sectors with receptive fields resembling those in the dorsal lateral geniculate nucleus. There were, however, a significant number of neurons with an indefinite visual response or no visual response at all. The possible role of the ventral lateral geniculate nucleus in visual function is discussed.
INTRODUCTION While several studies have been devoted to examination of the morphology and electrophysiology of the dorsal lateral geniculate nucleus, relatively few have appeared dealing with the ventral lateral geniculate nucleus. This nucleus is present throughout the mammalian Class (19), and appears to be proportionately smaller in primates and carnivores than it is in rodents. It has often been reported that the ventral lateral geniculate nucleus (or its primate homologue, the pregeniculate nucleus) is composed of an 1 Present address : Lab de Neurofisiologica, Depto. Neurobiologia, Instituto de Ciemcias Biologicas, Universidad Catolica de Chile, Casilla 114-D, Santiago, Chile. The authors wish to thank Harris Yeates and Mary Smith for valuable technical aid, and Grace Lee and Elinor Yeates for preparation of the manuscript. Supported by NIH Grant Nos. EYO0691 and NS 11669. 506 Copyright All rights
1975 by Academic Press, Inc. o? reproduction in any form reserved.
GENICULATE
NUCLEUS
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external sector which receives retinal input, and an internal one which does not (7, 14, 19). More recent reports have suggested the presence of retinal input in the internal sector, although it appears more sparse than that in the external sector (11, 28) and seems to degenerate more rapidly than that in the external sector. We have confirmed these newer observations in our experiments. Both sectors evidently receive input from the visual cortex (7, 10, 24, 25). There also appears to be a projection from the superior colliculus to the ventral lateral geniculate nucleus (10, 19, 20). Two recent studies have increased our knowledge of the efferent connections of the nucleus (5, 28), demonstrating prominent projections to the superior colliculus and pretectum. The cellular and synaptic morphology of the ventral lateral geniculate nucleus, thus far examined only in the rat, seems approximately equivalent to that in dorsal thalamic nuclei, with two main varieties of neurons, two varieties of round-vesicled axon terminals and two varieties of flat-vesicled axon terminals (25). Many data have been accumulated on the morphology and physiology of the rabbit visual system (4, 17, 22). We still have little notion of the importance of the ventral lateral geniculate nucleus. It is known that the retinal input is visuotopically organized (18). A recent study of the monkey pregeniculate nucleus suggests that many of its neurons experience alterations in firing rate in correspondence with saccades (3) while most dorsal lateral geniculate neurons do not. In the present study we are interested in the ultrastructure of the ventral lateral geniculate nucleus, and the types of receptive fields associated with its neurons. Careful attention has also been paid to any differences between the internal and external sectors and the similarity of their structure to that of dorsal thalamic nuclei. We have also compared the receptive field properties of ventral lateral geniculate nucleus neurons with those already described in other regions of the rabbit visual system (4). MATERIALS
AND
METHODS
Twenty-six ad& Dutch-belted rabbits were used in this study. Of this total 17 were used for electrophysiological studies. Animals were prepared for electrophysiological recording by being anesthetized with sodium pentobarbital (30 mg/kg). The animal was then placed in a stereotaxic apparatus and a screw affixed to the skull with dental cement. Openings were also made in the skull over the coordinates for the ventral lateral geniculate nucleus, but the piece of bone was replaced and held by bone wax. When these procedures were completed, the animal was given an injection of penicillin and returned to his cage for several days. On the day of recording, the animal was anesthetized with halothane
508
MA’l!HERS
AND
MASCETl’t
(Fluothane, Ayerst) and a tracheotomy performed. Long-duration topical anesthetic (Anucaine, Calvin) was applied to this and all other wound margins. After a trachea tube was sutured in place, an intravenous dose of gallamine triethiodide (Flaxedil, Abbot) was administered and the animal placed on artificial respiration. This drug was continued at a rate of 20 mg/hr throughout the experiment. The animal was then placed in a recording chamber and suspended in place with the bolt previously attached to the skull. In this manner the visual field was clear and no ear-, eye-, or pressure bars were necessary. The ECG was monitored continuously as an index of the animal’s health and freedom from discomfort. Body temperature was maintained at 37-39 C and 75-100 ml of 5% dextrose in saline was given subcutaneously. The pupil was dilated with atropine sulfate (Isoptoatropine, Alcon), and the eye fitted with a +l diopter contact lens. The optic disc was plotted on a tangent screen placed 57 cm from the animal. Visual stimuli used were either whole eye illumination with light flashes (50 msec, 325-1400 cd/m2), or luminous spots and bars (l-l.5 log units above a -1.5 log cd/m2 background), or cardboard shadows (1 log unit below a -0.5 to -1.5 log cd/m2 background) projected onto the screen. Unit activity was recorded with tungsten microelectrodes (3-5 megohm impedance, tested at 1000 Hz), and was amplified and displayed conventionally. The signals were monitored with a loudspeaker, and some were stored with an Ampex FR 1300 tape recorder for further study. Evoked potentials were recorded through the same electrodes and could be averaged on-line with a CAT Mnemetron computer. For each recording experiment the location of the electrode tip was marked at two or three locations by passing a 40 *clamp direct current for 15-30 set under deep anesthesia. The brain was then sectioned and stained for cell bodies. Only those units whose location could be verified as being in the ventral lateral geniculate nucleus were included in the results. Nine rabbits were used for morphological study of the ventral lateral geniculate nucleus. In five of these, one eye was removed 2, 4 or 7 days prior to killing. These animals were then deeply anesthetized with sodium pentobarbital and perfused through the left ventricle with chilled buffered 4% paraformaldehyde-0.5% glutaraldehyde. The brains were removed and small pieces of tissue removed from slabs containing the ventral lateral geniculate nucleus. These samples were osmicated, dehydrated, embedded in Epon-Araldite, and prepared for electron microscopy. The remaining portions of the brain were then sectioned at 30 pm and prepared according to a modified Nauta-Gygax technique. Two additional animals were prepared for electron microscopy only. In these, buffered 3% glutaraldehyde was used as the perfusate. All electron microscopic work was done with a Siemens IA microscope.
IXW~JLATE
NUCL~S
509
The last two animals each underwent injections of 30-60 pliter of tritiated proline into the left eye. Following a 24-hr survival, the animals were anesthetized and perfused with 270 paraformaldehyde-2% glutaraldehyde. The brains were embedded in paraffin, sections cut at 15 pm, coated with Kodak NTB-2 emulsion and exposed for 1 wk. Sections were then developed and lightly counterstained with cresyl violet. Golgi-stained rabbit brains were available from a collection previously assembled. This set includes Golgi-Kopsch and Rapid Golgi material, and observations on the neuronal morphology of ventral lateral geniculate nucleus were made from this set. RESULTS Morjho!ogy of the Ventral Lateral Geniculate Nuclezrs. Neurons in the ventral lateral geniculate nucleus fall into two broad categories, type I neurons 15-25 pm in diameter, and type II neurons S-20 pm in diameter. The type I neurons have very smooth long dendrites, which in some cases are oriented in the plane of the optic tract (Fig. lb), and in some cases are oriented radially as is the case in the dorsal lateral geniculate nucleus neurons. Similarly elongated dendrites are reported for large neurons in the rat ventral lateral geniculate nucleus (25). Type II neurons (Fig. lc) have much less extensive dendritic arbors. Their axon is locally ramifying, and the dendrites exhibit numerous protrusions. The so-called intraganglionic medullary lamina separates the dorsal lateral geniculate nucleus from the ventral lateral geniculate nucleus (Fig. la). It appears that there are fewer large cells present in the dorsal half of the ventral lateral geniculate nucleus. Also, cells in the dorsal half have more distinctly elongated dendrites, appearing to fit into the interstices between axons. In the ventral portions of the ventral lateral geniculate nucleus, the axonal fascicles are less numerous, and neurons have a more nearly radial arrangement of dendrites. There is a conspicuous cell-poor strip dividing the external from the internal segments. This strip is occupied by some of the larger bundles of axons found in the ventral lateral geniculate nucleus. With the electron microscope, the external sector of the ventral lateral geniculate nucleus appears to be heavily populated with myelinated axons. The largest of these are gathered into large fascicles, and between these fascicles are found the neurons of ventral lateral geniculate nucleus. In the internal sector of ventral lateral geniculate nucleus the neuropil is thickly populated with myelinated axons, but these are of a smaller diameter than those in the external sector and are not gathered into conspicuous fascicles. The largest neurons (15-22 pm in diameter) have a large amount of rough endoplasmic reticulum, accounting for their basophilia in Nissl preparations. The nucleus is usually but not invariably indented at one or more
510
MATHERS
AND MASCETTI
FIG. 1. (a) The external and internal sectors of the ventral lateral geniculate nucleus (vLGN) are bordered ventrally and dorsally by the intraganglionic medullary lamina (IML) and the optic tract (OT). The cell-poor region between the external and internal sectors may be seen. The ventral portion of dorsal lateral geniculate nucleus (dLGN) is also shown. (b) A rapid Golgi preparation of a Type I neuron in the external sector of vLGN. (c) A Golgi-Kopsch preparation of a Type II neuron, with small arrows indicating two dendritic protrusions. (d) Low-power electron micrograph of a Type I neuron in the external sector. The large arrows indicate portions of two axon fascicles. (e) Low-power electron micrograph of a Type II neuron, with smooth-surfaced nucleus and larger nucleus/cytoplasm ratio than in the Type I neuron.
places around its circumference. There are usually one to three synaptic junctions visible on the perikaryal membrane in one plane of section, and much of the rest of the perikaryal surface is abutted by dendritic and glial processesof the neuropil.
GENICULATE
NUCLEUS
511
The type II neurons are typically 8-20 pm in diameter and have a relatively small amount of endoplasmic reticulum in their cytoplasm. The nucleus usually has a smooth surface and is nearly spherical in shape. There ‘are usually two to five synaptic junctions present on the perikaryal membrane in one plane of section (Fig. le) . The synaptic population is basically similar to that found in dorsal thalamic relay nuclei. Presynaptic profiles containing round synaptic vesicles and forming asymmetric synaptic junctions comprise about 75% of all synaptic profiles seen. Of this group, about 60% are small in size (less than 4 pm in diameter) and contact a postsynaptic dendrite of about the same size. These terminals are classed as RS in (Fig. 2a). The remaining 15% are large, 4-10 pm in diameter, and make asymmetric contacts, usually on dendrites. By analogy with other studies these terminals are called RL (Figs. 2b, 2d). Occasionally a flat-vesicle-containing profile is postsynaptic to these large terminals. These RL terminals are similar to optic afferent axons described in dorsal lateral geniculate nucleus (9, 29). The RS terminals typically synapsed upon small dendrites and were distributed singly throughout the neuropil. They do not enter into axoaxonal synapses. The RL terminals were usually associated with one or two postsynaptic dendrites, and occasionally a postsynaptic F axon. They are not randomly distributed throughout the neuropil, but instead occur in distinctive arrangement with postsynaptic structures. These “glomeruli” are less complex than those seen in dorsal thalamic nuclei (8, 13, 16), in that there are seldom more than two postsynaptic structures visible in any single section plane. Axons, designated F in Fig. 2c, synapse upon both large and small dendrites in equal proportion. The role of F axons in RL axon synaptic glomeruli is controversial, with the suggestion being made increasingly that these F axon profiles may in fact be presynaptic dendrites (6, 21). The F axon profiles observed here did not exhibit ribosomes or other evidence of dendritic origin, and thus until other criteria for identification are available we will continue to describe them as axons. About 25% of the synaptic terminals seen contained flattened vesicles and made symmetric synaptic junctions on dendrites of various sizes (Fig. 2~). As mentioned above, flat-vesicled profiles (F axons) were occasionally postsynaptic to RL axons. Retinal Projections. In five rabbits, one eye was removed either 2, 4 or 7 days prior to killing. Nauta preparations showed that in the 2-day survival there was evidence of terminal degeneration in both the internal and external sectors of the contralateral ventral lateral geniculate nucleus. The degeneration fragments in the external sector were coarser and denser than in the internal sector. In the 4-day survival, most of the degeneration debris has been eliminated from the internal sector, while it persists
512
MATHERS
AND
MASCETTI
FIG. 2. Synaptic types in ventral lateral geniculate nucleus.(a) A synapse(RS) upon a dendrite. (b) An RL synapseupon two dendrites.Note lengthsof synaptic
zones in each. (c) An RS and F synapse upon a dendrite. The difference in synaptic thickenings is clearly seen, and the clear arrows indicate examples of flattened vesicles. (d) An RL synapse upon two dendrites. In this case the accumulation of mitochondria inside the terminal and glial lamellae are well shown.
prominently in the external sector. This is in accord with a previous report by Schneider (23). In the 7-day survival, the internal sector is virtually free of debris, although there is some evidence of axonal degeneration in the external sector. Electron microscopic examination of axonal degeneration showed that degeneration was confined to RL terminals. As has been shown in the dorsal lateral geniculate, the RL terminals in the external sector of the ventral lateral geniculate nucleus passthrough an early stage of filamentous degenerative change (Fig, 3a, b, c), then become dense and shrunken, and
GENICULATE
NUCLEUS
513
FIG. 3. Degeneration of retinal axon terminals in ventral lateral geniculate nucleus. (a) Early phase of filamentous degeneration (f) in a degenerating RL terminal (dRL). (b) More advanced filamentous changes, with vesicles and mitochondria accumulating in the center. (c) Longitudinal section of a degenerating RL terminal, with severe filamentous hyperplasia. (d) Later stage of RL degeneration with marked darkening of the terminal. Mitochondria and vesicles can still be discerned within. The white arrow indicates the degenerating terminal and the dark arrow a persisting synaptic thickening.
finally are engulfed by glial processes (Fig. 3d). It was noteworthy that while in the case of the Z-day enucleation, nearly all degenerating RI, terminals were in a filamentous phase, at 4 days and 7 days there was a mixture of filamentous and dense RL terminals. So while it appears that RL terminals pass through an initial filamentous phase and later become dense,there is not in any sensea synchronous process of axonal change.
514
MATHERS
AND
MASCETTI
To the contrary, the internal sector of ventral lateral geniculate nucleus showed very little sign of filamentous change in RL terminals, even at 2 days survival. Nearly all RL terminals were undergoing a dense degenerative process at this early stage. Even at 7 days, however, when the light microscopic evidence of degeneration was gone from the internal sector, there was still some evidence of dense RL terminal degeneration. The ipsilateral ventral lateral geniculate nucleus received a small amount of retinal input, confined to the lateralmost areas. No attempt was made to study this with the electron microscope. The experiments involving two rabbits with intraocular injections of SHproline demonstrated the earlier-reported projection of retinal axons to the external sector of the ventral lateral geniculate nucleus. In addition, however, there was evidence of a projection to the internal sector of the ventral lateral geniculate nucleus. Grain counts in the internal sector revealed a density of approximately one-half that in the external sector. There appeared to be no differences in grain density between those parts of the ventral geniculate nucleus containing neurons with elongated dendrites and those with more radially oriented dendrites. Electrophysiology. The following observations were made during 17 experiments involving 109 neurons. The criteria for neuronal action potentials were those of Bishop, Burke and Davis (2), and were particularly crucial in recording from an area such as ventral lateral geniculate nucleus, where so many optic tract axons are intermingled with neurons. All visual responses were elicited by presenting light to the eye contralateral to the recording site. We made no attempt to measure ipsilateral visual response. The topographic arrangement of visual response in the ventral lateral geniculate nucleus is in accord with that reported by Hughes (12) in the rabbit and Montero, Brugge and Beitel (18) in the rat. Briefly, the ventral lateral geniculate nucleus has a representation of the visual field in which the nasal periphery of the field is represented most dorsally and the temporal periphery most ventrally. The more lateral regions of the nucleus represent the dorsal region of the visual field. It was often possible to note passage of the electrode from the dorsal lateral geniculate nucleus to the ventral lateral geniculate nucleus by the large shift of receptive fields from the temperoventral to the nasodorsal areas of the visual field. Neurons were classified according to a system defined in Stewart, Chow and Masland (26). Briefly, the classifications are as follows : (a) concentric, central region with an antagonistic surround ; (b) uniform, a central excitatory or inhibitory region with no antagonistic surround ; (c) motion, sensitive to movement of dark or light stimulus, irrespective of selective, configuration or direction of movement ; (d) directionally sensitive to movement in one best direction, and also exhibiting a null direction, in which movement elicits no response or may inhibit spontaneous
GENICULATE
TABLE RECEPTIVE
CONC
515
NUCLEUS
1
FIELD PROPERTIES OF VENTRAL GENICULATE NUCLEUS NEURONS” UNIF
MOT
DIR
IND
LATERAL
NR
TOTAL
External sector
27
9
1
1
13
1
52
Internal sector
16
6
2
1
18
14
57
Total vLGN
43
1.5
3
2
31
1.5
109
concentric; UNIF, no response;
uniform; vLGN,
MOT, ventral
0 Abbreviations: IND, indefinite;
CONC, NR,
motion; DIR, directional; lateral geniculate nucleus.
activity ; and (e) indefinite. Cells in this category respond to light, but have no discernible borders to the receptive field area, or in some instances respond only to whole-eye illumination. No cells were found in the ventral lateral geniculate nucleus which had receptive fields resembling the simple, complex, or oriented-directionally selective types found in the rabbit striate cortex (4). Table 1 gives the numbers of cells found with each receptive field type in the internal and external sectors of the ventral lateral geniculate nucleus. Of the total of 109 neurons, 52 were located in the external sector of ventraI lateral geniculate nucleus and 57 in the internal sector. One apparent difference between the internal and external sectors is the significant proportion of nonresponsive neurons found in the internal sector (24%), versus a very small percentage in the external sector (2%). The nonresponsive cells found in the internal sector were sampled in six different animals and were found in association with all other receptive field types. There is no indication that they were confined to one region of the internal sector. Nonresponsive cells may be genuinely silent in the presence of visual stimuli, or may have complicated stimulus requirements which we did not discover. Both the internal and external sectors have a significant percentage of neurons with indefinite receptive fields. These were found in 11 of the 17 animals studied, and occurred in the same electrode passes in which cells with clearly defined receptive fields were recorded. In most cases great effort was made to delineate a receptive field, even when the local area of response was in an extreme peripheral region of the visual field. The vast majority of dorsal lateral geniculate neurons with concentric lnd uniform fields were reported by Stewart, Chow and Masland (26) to
516
MATHERS
AND
MASCETTI
have receptive fields less than 20” in diameter. In ventral lateral geniculate nucleus concentric and uniform cells (58 cells), the same approximate range of receptive field diameters was found, but there is a greater percentage of those with larger diameters. This is illustrated graphically in Fig. 4, where concentric receptive field diameters from both are compared. As might be predicted, field size of the ventral lateral geniculate nucleus concentric receptive fields increased with distance from the optic nerve head. A very small number of motion-sensitive and directional cells (4) was found in this study. They were located in central parts of the nucleus, however, and were certainly a part of the ventral lateral geniculate nucleus. It is worth noting that concentric, directional, and a type of motionsensitive cell are all found in the rabbit’s retina (l), so that it is not surprising that such receptive fields should also be reported for neurons in ventral lateral geniculate nucleus. The superior colliculus of the rabbit also has some cells with directional and motion sensitivity (15). DISCUSSION Morphological Properties. There has been little previous work on the ventral lateral geniculate nucleus, owing to its small size, and the fact that its location near the optic tract made lesioning techniques difficult to apply. It is well established that throughout much of the mammalian Class, the ventral lateral geniculate nucleus exists with an internal and external sector
ilF
. . . . . . . . . vLGN
CELLS
FIELD DIAMETER-
DEGREES
FIG. 4. This graph illustrates a comparison of receptive field diameters for concentric cells in the ventral lateral geniculate nucleus and the dorsal lateral geniculate nucleus [dorsal lateral geniculate nucleus data from Stewart, Chow and Masland (26) 1.
GENICULATE
NUCLEUS
517
(19). We are as yet quite ignorant of the significance of this visuotopically organized nucleus in the ventral thalamus, either from the standpoint of comparative neurology or the function of the visual system. This morphological examination of the rabbit ventral lateral geniculate nucleus suggests that its structure is quite reminiscent of that of the relay nuclei in the dorsal thalamus (13, 22). There are two prominent neuron types, one evidently the relay neuron and one the interneuron. Like dorsal thalamic nuclei, the synaptic population is predominated by small roundvesicle terminals, and has a lesser population of large round-vesicle terminals which constitute the extrinsic afferent input from the retina. There are also flat-vesicle terminals, perhaps dendritic, which presumably are of intrinsic origin. The neuropil also contains synaptic “glomeruli” (29), with RL terminals as a central element. The retinal projection to the ventral lateral geniculate nucleus, formerly believed to be restricted to the external sector, now appears to be distributed to both the internal and external sector. There is a difference, however, in the caliber of degenerating retinal axons in the internal vs. the external sector, and there is a very obvious difference in grain density in the internal vs the external segment following intraocular injection. In addition, the rate of axonal degeneration and phagocytosis of debris in the internal sector appears to be faster than that of the external sector. This has already been observed in the guinea pig (23)) and our findings are in support of this previous report. We plan to pursue further experiments in an effort to learn more about this difference between the retinal projections to the internal and external sectors of the ventral lateral geniculate nucleus. Physiological Properties. The principal impetus for this series of experiments was the observation that retinal input was confined to the external sector. We believed that since the visual cortex projects to both internal and external sector, we would have the opportunity to record from some neurons receiving both retinal and visual cortical input, and others receiving the visual cortical but not the retinal input. Our own findings, and those previously reported by Hendrickson (11) and Swanson, Cowan and Jones (28) made it evident that the ventral lateral geniculate nucleus would not offer this opportunity. The receptive field properties of some of the neurons in the ventral lateral geniculate nucleus are akin to those of neurons in the dorsal lateral geniculate nucleus of the rabbit (2c, while the receptive field properties of other ventral lateral geniculate nucleus neurons are imprecise and indefinite, like those of neurons in parts of the rabbit posterior thalamus and pretectum (27). We have no basis at the present time for speculation on the relative importance of the retinal and the visual cortical input in producing the receptive field properties of ventral lateral geniculate nucleus neurons.
518
MATHERS
AND
MASCETTI
However, there is at least one study in which visual cortical ablation produced little change in neuronal receptive field properties in a rabbit subcortical visual area, the superior colliculus (1.5). This is in marked contrast to the results of similar studies in the cat (30)) where visual cortical ablation altered the directionality and binocular response properties of collicular neurons. The ventral lateral geniculate remains an enigma in the visual system. The visuotopic organization (IS), the similarity of neuronal receptive fields to those in the dorsal lateral geniculate, and the morphologic similarity of the ventral and dorsal lateral geniculate nuclei all suggest that similar functions might be carried via neuronal circuits including these nuclei. The ventral lateral geniculate nucleus has additional and as yet unexplained connections with superior colliculus and pretectum. Nothing in the receptive field properties of neurons studied in these experiments suggested the role in eye movements indicated by these connections. We have as yet no idea of what significance may be attached to morphological differences in the retinal projection to the internal and external sectors. By knowing which areas of the retina project to the ventral lateral geniculate nucleus, and which types of retinal ganglion cells project to each sector, we should have a system through which much may be learned about differences in retinal ganglion cells, and the manner in which they project to thalamic visual nuclei. The recent reports of Swanson, Cowan and Jones (28) and Edwards, Rosenquist and Palmer (5) have reaffirmed earlier beliefs that the ventral lateral geniculate nucleus does not project to the cerebral cortex. Perhaps this gives some hint to the functional neural circuitry in which the ventral lateral geniculate is most important. It appears that the ventral lateral geniculate projects to the zona incerta and lateral terminal nucleus, and it is reasonable to postulate that this is a pathway by which visual inputs can reach some prominent subcortical telencephalic motor centers. The geniculocalcarine system reaches these motor pathways only very indirectly, so perhaps the ventral lateral geniculate nucleus is the origin of more direct input. Until further physiological work can be accomplished, along with behavioral work involving ablation of the ventral lateral geniculate nucleus, we will probably have little more concrete idea about the role of the ventral lateral geniculate nucleus in visual function. REFERENCES 1. BARLOW, H. B., and W. R. LEVICK. 1%5. The mechanism of directionally selective units in rabbit’s retina. J. Physiol. (Lo&don) 178: 477-504. 2, BISHOP, P. O., W. BURKE, and R. DAVIS. 1%2. The identification of single vnits in central visual pathways. J. Physiol. (Lodon) 162: 409-431.
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NUCLEUS
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3. B~~TTNER, U., and A. F. FUCHS. 1973. Influence of saccadic eye movements on unit activity in simian lateral geniculate and pregeniculate nuclei. J. Nenrophysiol. 36: 127-141. 4. CHOW, K. L., R. H. MASLAND, and D. L. STEWART. 1971. Receptive field characteristics of striate cortical neurons in the rabbit. Brain Res. 33: 337-352. 5. EDWARDS, S. B., A. C. ROSENQUIST, and L. A. PALMER. 1974. An autoradiographic study of ventral lateral geniculate projections in the cat. Broi~t Res. 72: 282287. 6. FAMICLIETTI, E. V., and A. PETERS. 1972, The synaptic glomerulus and the intrinsic neuron in the dorsal lateral geniculate nucleus of the cat. f. Con@. Neural. 144 : 285-333. 7. GIOLLI, R. A., and M. D. GUTHRIE. 1%9. The primary optic projections in the rabbit. An experimental degeneration study. J. Camp. Neural. 136: 99-126. 8. GUILLERY, R. W. 1969. The organization of synaptic interconnections in the laminae of the dorsal lateral geniculate nucleus of the cat. 2. Zellforsch. 96: l-38. 9. GUILLERY, R. W., and M. COLONNIER. 1970. Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey. 2. Zellfarsch. 103 : 90-108. 10. HARTING, J. K., and C. R. NOBACK. 1971. Subcortical projections from the visual cortex in the tree shrew (Tupaia glis). Braitt Rrs. 25: 21-33. 11. HENDRICKSON, A. E. 1973. The pregeniculate nucleus of the monkey. Anot. Rec. 175: 341 (abstr.). 12. HUGHES, A. 1971. Topographical relationships between the anatomy and physiology of the rabbit visual system. Dot. Ophthalmol. 30 : 33-159. 13. JONES, E. G., and T. P. S. POWELL. 1969. Electron microscopy of synaptic glomeruli in the thalamic relay nuclei of the cat. Proc. Roy. SOC. London Ser. B 172: 153-171. 14. LATIES, A. M., and J. M. SPRAGUE. 1966. The projection of optic fibers to the visual centers in the cat. J. Camp. Nerrrol. 127: 35-70. 15. MASLAND, R. H., K. L. CHOW, and D. L. STEWART. 1971. Receptive-field characteristics of superior colliculus neurons in the rabbit. J. NeuropJzysioZ. 34: 148-156. 16. MATHERS, L. H. 1972. Ultrastructure of the pulvinar of the squirrel monkey. .I. Comp. Neural. 147: 15-42. 17. MATHERS, L. H., K. L. CHOW, P. D. SPEAR, and P. GROBSTEIN. 1974. Ontogenesis of receptive fields in the rabbit striate cortex. E.@. Bruin Rcs. 19: 20-3.5. 18. MONTERO, V. M., J. F. BRUGGE, and R. E. BEITEL. 1968. Relation of the visual field to the lateral geniculate body in the rat. J. Neurophysiol. 31 : 268-282. 19. NIIMI, K., T. KANASEKI, and T. TAKI~LIOTO. 1963. The comparative anatomy of the ventral nucleus of the lateral geniculate body in mammals. J. Comb. Neztrol. 121: 313-323. 20. RAFOLS, J. A., and H. A. MATZKE. 1970. Efferent projections of the superior colliculus in the opossum. J. Camp. Neural. 138 : 147-160. 21. RALSTON, H. J. 1971. Evidence for presynaptic dendrites and a proposal for their mechanism of action. Nattrre (Loudon) 230 : 585-587. 22. RALSTON, H. J., nd K. L. CHOW. 1973. Synaptic reorganization in the degenerating lateral geniculate nucleus of the rabbit. j. Cop@. Na2rroI. 147: 321350. 23. SCHNEIDER, G. E. 1968. Retinal projections characterized by differential rate of degeneration revealed by silver impregnation. Anat. Rec. 160: 423.
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26. 27.
28. 29. 30.
MASCETTI
W. R., J. TIGGES, and M. TIGGES. 1970. Subcortical projections, cortical associations, and some intrinsic interlaminar connections of the striate cortex in the squirrel monkey (Saimiri). J. Comb. Nezrrol. 140 : 155-173. STELZNER, D. J., D. C. GOODMAN, and P. L. ROSSETTI. 1973. The synaptic organization of the ventral lateral geniculate nucleus pars lateralis of the rat. Anat. Rec. 175: 451 (abstr.). STEWART, D. L., K. L. CHOW, and R. L. MASLAND. 1971. Receptive field characteristics of lateral geniculate neurons in the rabbit. J. Neurophysiol. 34: 139147. STEWART, D. L., L. C. TOWNS, and D. BRIT. 1973. Visual receptive-field characteristics of posterior thalamic and pretectal neurons in the rabbit. Brain Res. 57: 43-57. SWANSON, L. C., W. M. COWAN, and E. G. JONES. 1974. An autoradiographic study of the efferent connections of the ventral lateral geniculate nucleus in the albino rat and the cat. J. Comp. Neural. 156: 143-164. SZENTAC~THAI, J. 1970. Glomerular synapses, complex arrangements, and their operational significance, pp. 427-442. In “The Neurosciences: Second Study Program.” Rockefeller Press, New York. WICKLEGREN, B. G., and P. STERLING. 1969. Influence of visual cortex on receptive fields in the superior colliculus of the cat. J. NeurophysioC. 32: 16-23.
24. SPATZ,
25.
AND
NEUROLOGY
Electrophysiological in the Ventral
46,506-520
and Lateral
LAWRENCE
Departments
of Anatomy
Received
September
H.
(1975)
Morphological Geniculate
MATHERS
AND
Properties of Neurons Nucleus of the Rabbit GIAN
G.MASCETTI~
and Neurology, Stanford University Stanford, California 94305
School
of Medicine,
6, 1974; revision received September 21,1974
The ventral lateral geniculate nucleus of the rabbit has been studied with light and electron microscopy, as well as single unit extracellular recordings. There are two basic types of neurons in the nucleus, and three synaptic types, designated RS, RL and F. There are also synaptic glomeruli, in which RL axons are always involved. The arrangement of these synaptic types and their percentage of occurrence closely resembles that in dorsal thalamic nuclei. Retinal projections are found in both external and internal sectors of the nucleus, shown by both degeneration and autoradiagraphic techniques. Retinal projections to the internal sector are sparser and degenerate more rapidly than those to the external sector. Electrophysiological studies revealed some neurons in both external and internal sectors with receptive fields resembling those in the dorsal lateral geniculate nucleus. There were, however, a significant number of neurons with an indefinite visual response or no visual response at all. The possible role of the ventral lateral geniculate nucleus in visual function is discussed.
INTRODUCTION While several studies have been devoted to examination of the morphology and electrophysiology of the dorsal lateral geniculate nucleus, relatively few have appeared dealing with the ventral lateral geniculate nucleus. This nucleus is present throughout the mammalian Class (19), and appears to be proportionately smaller in primates and carnivores than it is in rodents. It has often been reported that the ventral lateral geniculate nucleus (or its primate homologue, the pregeniculate nucleus) is composed of an 1 Present address : Lab de Neurofisiologica, Depto. Neurobiologia, Instituto de Ciemcias Biologicas, Universidad Catolica de Chile, Casilla 114-D, Santiago, Chile. The authors wish to thank Harris Yeates and Mary Smith for valuable technical aid, and Grace Lee and Elinor Yeates for preparation of the manuscript. Supported by NIH Grant Nos. EYO0691 and NS 11669. 506 Copyright All rights
1975 by Academic Press, Inc. o? reproduction in any form reserved.
GENICULATE
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external sector which receives retinal input, and an internal one which does not (7, 14, 19). More recent reports have suggested the presence of retinal input in the internal sector, although it appears more sparse than that in the external sector (11, 28) and seems to degenerate more rapidly than that in the external sector. We have confirmed these newer observations in our experiments. Both sectors evidently receive input from the visual cortex (7, 10, 24, 25). There also appears to be a projection from the superior colliculus to the ventral lateral geniculate nucleus (10, 19, 20). Two recent studies have increased our knowledge of the efferent connections of the nucleus (5, 28), demonstrating prominent projections to the superior colliculus and pretectum. The cellular and synaptic morphology of the ventral lateral geniculate nucleus, thus far examined only in the rat, seems approximately equivalent to that in dorsal thalamic nuclei, with two main varieties of neurons, two varieties of round-vesicled axon terminals and two varieties of flat-vesicled axon terminals (25). Many data have been accumulated on the morphology and physiology of the rabbit visual system (4, 17, 22). We still have little notion of the importance of the ventral lateral geniculate nucleus. It is known that the retinal input is visuotopically organized (18). A recent study of the monkey pregeniculate nucleus suggests that many of its neurons experience alterations in firing rate in correspondence with saccades (3) while most dorsal lateral geniculate neurons do not. In the present study we are interested in the ultrastructure of the ventral lateral geniculate nucleus, and the types of receptive fields associated with its neurons. Careful attention has also been paid to any differences between the internal and external sectors and the similarity of their structure to that of dorsal thalamic nuclei. We have also compared the receptive field properties of ventral lateral geniculate nucleus neurons with those already described in other regions of the rabbit visual system (4). MATERIALS
AND
METHODS
Twenty-six ad& Dutch-belted rabbits were used in this study. Of this total 17 were used for electrophysiological studies. Animals were prepared for electrophysiological recording by being anesthetized with sodium pentobarbital (30 mg/kg). The animal was then placed in a stereotaxic apparatus and a screw affixed to the skull with dental cement. Openings were also made in the skull over the coordinates for the ventral lateral geniculate nucleus, but the piece of bone was replaced and held by bone wax. When these procedures were completed, the animal was given an injection of penicillin and returned to his cage for several days. On the day of recording, the animal was anesthetized with halothane
508
MA’l!HERS
AND
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(Fluothane, Ayerst) and a tracheotomy performed. Long-duration topical anesthetic (Anucaine, Calvin) was applied to this and all other wound margins. After a trachea tube was sutured in place, an intravenous dose of gallamine triethiodide (Flaxedil, Abbot) was administered and the animal placed on artificial respiration. This drug was continued at a rate of 20 mg/hr throughout the experiment. The animal was then placed in a recording chamber and suspended in place with the bolt previously attached to the skull. In this manner the visual field was clear and no ear-, eye-, or pressure bars were necessary. The ECG was monitored continuously as an index of the animal’s health and freedom from discomfort. Body temperature was maintained at 37-39 C and 75-100 ml of 5% dextrose in saline was given subcutaneously. The pupil was dilated with atropine sulfate (Isoptoatropine, Alcon), and the eye fitted with a +l diopter contact lens. The optic disc was plotted on a tangent screen placed 57 cm from the animal. Visual stimuli used were either whole eye illumination with light flashes (50 msec, 325-1400 cd/m2), or luminous spots and bars (l-l.5 log units above a -1.5 log cd/m2 background), or cardboard shadows (1 log unit below a -0.5 to -1.5 log cd/m2 background) projected onto the screen. Unit activity was recorded with tungsten microelectrodes (3-5 megohm impedance, tested at 1000 Hz), and was amplified and displayed conventionally. The signals were monitored with a loudspeaker, and some were stored with an Ampex FR 1300 tape recorder for further study. Evoked potentials were recorded through the same electrodes and could be averaged on-line with a CAT Mnemetron computer. For each recording experiment the location of the electrode tip was marked at two or three locations by passing a 40 *clamp direct current for 15-30 set under deep anesthesia. The brain was then sectioned and stained for cell bodies. Only those units whose location could be verified as being in the ventral lateral geniculate nucleus were included in the results. Nine rabbits were used for morphological study of the ventral lateral geniculate nucleus. In five of these, one eye was removed 2, 4 or 7 days prior to killing. These animals were then deeply anesthetized with sodium pentobarbital and perfused through the left ventricle with chilled buffered 4% paraformaldehyde-0.5% glutaraldehyde. The brains were removed and small pieces of tissue removed from slabs containing the ventral lateral geniculate nucleus. These samples were osmicated, dehydrated, embedded in Epon-Araldite, and prepared for electron microscopy. The remaining portions of the brain were then sectioned at 30 pm and prepared according to a modified Nauta-Gygax technique. Two additional animals were prepared for electron microscopy only. In these, buffered 3% glutaraldehyde was used as the perfusate. All electron microscopic work was done with a Siemens IA microscope.
IXW~JLATE
NUCL~S
509
The last two animals each underwent injections of 30-60 pliter of tritiated proline into the left eye. Following a 24-hr survival, the animals were anesthetized and perfused with 270 paraformaldehyde-2% glutaraldehyde. The brains were embedded in paraffin, sections cut at 15 pm, coated with Kodak NTB-2 emulsion and exposed for 1 wk. Sections were then developed and lightly counterstained with cresyl violet. Golgi-stained rabbit brains were available from a collection previously assembled. This set includes Golgi-Kopsch and Rapid Golgi material, and observations on the neuronal morphology of ventral lateral geniculate nucleus were made from this set. RESULTS Morjho!ogy of the Ventral Lateral Geniculate Nuclezrs. Neurons in the ventral lateral geniculate nucleus fall into two broad categories, type I neurons 15-25 pm in diameter, and type II neurons S-20 pm in diameter. The type I neurons have very smooth long dendrites, which in some cases are oriented in the plane of the optic tract (Fig. lb), and in some cases are oriented radially as is the case in the dorsal lateral geniculate nucleus neurons. Similarly elongated dendrites are reported for large neurons in the rat ventral lateral geniculate nucleus (25). Type II neurons (Fig. lc) have much less extensive dendritic arbors. Their axon is locally ramifying, and the dendrites exhibit numerous protrusions. The so-called intraganglionic medullary lamina separates the dorsal lateral geniculate nucleus from the ventral lateral geniculate nucleus (Fig. la). It appears that there are fewer large cells present in the dorsal half of the ventral lateral geniculate nucleus. Also, cells in the dorsal half have more distinctly elongated dendrites, appearing to fit into the interstices between axons. In the ventral portions of the ventral lateral geniculate nucleus, the axonal fascicles are less numerous, and neurons have a more nearly radial arrangement of dendrites. There is a conspicuous cell-poor strip dividing the external from the internal segments. This strip is occupied by some of the larger bundles of axons found in the ventral lateral geniculate nucleus. With the electron microscope, the external sector of the ventral lateral geniculate nucleus appears to be heavily populated with myelinated axons. The largest of these are gathered into large fascicles, and between these fascicles are found the neurons of ventral lateral geniculate nucleus. In the internal sector of ventral lateral geniculate nucleus the neuropil is thickly populated with myelinated axons, but these are of a smaller diameter than those in the external sector and are not gathered into conspicuous fascicles. The largest neurons (15-22 pm in diameter) have a large amount of rough endoplasmic reticulum, accounting for their basophilia in Nissl preparations. The nucleus is usually but not invariably indented at one or more
510
MATHERS
AND MASCETTI
FIG. 1. (a) The external and internal sectors of the ventral lateral geniculate nucleus (vLGN) are bordered ventrally and dorsally by the intraganglionic medullary lamina (IML) and the optic tract (OT). The cell-poor region between the external and internal sectors may be seen. The ventral portion of dorsal lateral geniculate nucleus (dLGN) is also shown. (b) A rapid Golgi preparation of a Type I neuron in the external sector of vLGN. (c) A Golgi-Kopsch preparation of a Type II neuron, with small arrows indicating two dendritic protrusions. (d) Low-power electron micrograph of a Type I neuron in the external sector. The large arrows indicate portions of two axon fascicles. (e) Low-power electron micrograph of a Type II neuron, with smooth-surfaced nucleus and larger nucleus/cytoplasm ratio than in the Type I neuron.
places around its circumference. There are usually one to three synaptic junctions visible on the perikaryal membrane in one plane of section, and much of the rest of the perikaryal surface is abutted by dendritic and glial processesof the neuropil.
GENICULATE
NUCLEUS
511
The type II neurons are typically 8-20 pm in diameter and have a relatively small amount of endoplasmic reticulum in their cytoplasm. The nucleus usually has a smooth surface and is nearly spherical in shape. There ‘are usually two to five synaptic junctions present on the perikaryal membrane in one plane of section (Fig. le) . The synaptic population is basically similar to that found in dorsal thalamic relay nuclei. Presynaptic profiles containing round synaptic vesicles and forming asymmetric synaptic junctions comprise about 75% of all synaptic profiles seen. Of this group, about 60% are small in size (less than 4 pm in diameter) and contact a postsynaptic dendrite of about the same size. These terminals are classed as RS in (Fig. 2a). The remaining 15% are large, 4-10 pm in diameter, and make asymmetric contacts, usually on dendrites. By analogy with other studies these terminals are called RL (Figs. 2b, 2d). Occasionally a flat-vesicle-containing profile is postsynaptic to these large terminals. These RL terminals are similar to optic afferent axons described in dorsal lateral geniculate nucleus (9, 29). The RS terminals typically synapsed upon small dendrites and were distributed singly throughout the neuropil. They do not enter into axoaxonal synapses. The RL terminals were usually associated with one or two postsynaptic dendrites, and occasionally a postsynaptic F axon. They are not randomly distributed throughout the neuropil, but instead occur in distinctive arrangement with postsynaptic structures. These “glomeruli” are less complex than those seen in dorsal thalamic nuclei (8, 13, 16), in that there are seldom more than two postsynaptic structures visible in any single section plane. Axons, designated F in Fig. 2c, synapse upon both large and small dendrites in equal proportion. The role of F axons in RL axon synaptic glomeruli is controversial, with the suggestion being made increasingly that these F axon profiles may in fact be presynaptic dendrites (6, 21). The F axon profiles observed here did not exhibit ribosomes or other evidence of dendritic origin, and thus until other criteria for identification are available we will continue to describe them as axons. About 25% of the synaptic terminals seen contained flattened vesicles and made symmetric synaptic junctions on dendrites of various sizes (Fig. 2~). As mentioned above, flat-vesicled profiles (F axons) were occasionally postsynaptic to RL axons. Retinal Projections. In five rabbits, one eye was removed either 2, 4 or 7 days prior to killing. Nauta preparations showed that in the 2-day survival there was evidence of terminal degeneration in both the internal and external sectors of the contralateral ventral lateral geniculate nucleus. The degeneration fragments in the external sector were coarser and denser than in the internal sector. In the 4-day survival, most of the degeneration debris has been eliminated from the internal sector, while it persists
512
MATHERS
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FIG. 2. Synaptic types in ventral lateral geniculate nucleus.(a) A synapse(RS) upon a dendrite. (b) An RL synapseupon two dendrites.Note lengthsof synaptic
zones in each. (c) An RS and F synapse upon a dendrite. The difference in synaptic thickenings is clearly seen, and the clear arrows indicate examples of flattened vesicles. (d) An RL synapse upon two dendrites. In this case the accumulation of mitochondria inside the terminal and glial lamellae are well shown.
prominently in the external sector. This is in accord with a previous report by Schneider (23). In the 7-day survival, the internal sector is virtually free of debris, although there is some evidence of axonal degeneration in the external sector. Electron microscopic examination of axonal degeneration showed that degeneration was confined to RL terminals. As has been shown in the dorsal lateral geniculate, the RL terminals in the external sector of the ventral lateral geniculate nucleus passthrough an early stage of filamentous degenerative change (Fig, 3a, b, c), then become dense and shrunken, and
GENICULATE
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FIG. 3. Degeneration of retinal axon terminals in ventral lateral geniculate nucleus. (a) Early phase of filamentous degeneration (f) in a degenerating RL terminal (dRL). (b) More advanced filamentous changes, with vesicles and mitochondria accumulating in the center. (c) Longitudinal section of a degenerating RL terminal, with severe filamentous hyperplasia. (d) Later stage of RL degeneration with marked darkening of the terminal. Mitochondria and vesicles can still be discerned within. The white arrow indicates the degenerating terminal and the dark arrow a persisting synaptic thickening.
finally are engulfed by glial processes (Fig. 3d). It was noteworthy that while in the case of the Z-day enucleation, nearly all degenerating RI, terminals were in a filamentous phase, at 4 days and 7 days there was a mixture of filamentous and dense RL terminals. So while it appears that RL terminals pass through an initial filamentous phase and later become dense,there is not in any sensea synchronous process of axonal change.
514
MATHERS
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To the contrary, the internal sector of ventral lateral geniculate nucleus showed very little sign of filamentous change in RL terminals, even at 2 days survival. Nearly all RL terminals were undergoing a dense degenerative process at this early stage. Even at 7 days, however, when the light microscopic evidence of degeneration was gone from the internal sector, there was still some evidence of dense RL terminal degeneration. The ipsilateral ventral lateral geniculate nucleus received a small amount of retinal input, confined to the lateralmost areas. No attempt was made to study this with the electron microscope. The experiments involving two rabbits with intraocular injections of SHproline demonstrated the earlier-reported projection of retinal axons to the external sector of the ventral lateral geniculate nucleus. In addition, however, there was evidence of a projection to the internal sector of the ventral lateral geniculate nucleus. Grain counts in the internal sector revealed a density of approximately one-half that in the external sector. There appeared to be no differences in grain density between those parts of the ventral geniculate nucleus containing neurons with elongated dendrites and those with more radially oriented dendrites. Electrophysiology. The following observations were made during 17 experiments involving 109 neurons. The criteria for neuronal action potentials were those of Bishop, Burke and Davis (2), and were particularly crucial in recording from an area such as ventral lateral geniculate nucleus, where so many optic tract axons are intermingled with neurons. All visual responses were elicited by presenting light to the eye contralateral to the recording site. We made no attempt to measure ipsilateral visual response. The topographic arrangement of visual response in the ventral lateral geniculate nucleus is in accord with that reported by Hughes (12) in the rabbit and Montero, Brugge and Beitel (18) in the rat. Briefly, the ventral lateral geniculate nucleus has a representation of the visual field in which the nasal periphery of the field is represented most dorsally and the temporal periphery most ventrally. The more lateral regions of the nucleus represent the dorsal region of the visual field. It was often possible to note passage of the electrode from the dorsal lateral geniculate nucleus to the ventral lateral geniculate nucleus by the large shift of receptive fields from the temperoventral to the nasodorsal areas of the visual field. Neurons were classified according to a system defined in Stewart, Chow and Masland (26). Briefly, the classifications are as follows : (a) concentric, central region with an antagonistic surround ; (b) uniform, a central excitatory or inhibitory region with no antagonistic surround ; (c) motion, sensitive to movement of dark or light stimulus, irrespective of selective, configuration or direction of movement ; (d) directionally sensitive to movement in one best direction, and also exhibiting a null direction, in which movement elicits no response or may inhibit spontaneous
GENICULATE
TABLE RECEPTIVE
CONC
515
NUCLEUS
1
FIELD PROPERTIES OF VENTRAL GENICULATE NUCLEUS NEURONS” UNIF
MOT
DIR
IND
LATERAL
NR
TOTAL
External sector
27
9
1
1
13
1
52
Internal sector
16
6
2
1
18
14
57
Total vLGN
43
1.5
3
2
31
1.5
109
concentric; UNIF, no response;
uniform; vLGN,
MOT, ventral
0 Abbreviations: IND, indefinite;
CONC, NR,
motion; DIR, directional; lateral geniculate nucleus.
activity ; and (e) indefinite. Cells in this category respond to light, but have no discernible borders to the receptive field area, or in some instances respond only to whole-eye illumination. No cells were found in the ventral lateral geniculate nucleus which had receptive fields resembling the simple, complex, or oriented-directionally selective types found in the rabbit striate cortex (4). Table 1 gives the numbers of cells found with each receptive field type in the internal and external sectors of the ventral lateral geniculate nucleus. Of the total of 109 neurons, 52 were located in the external sector of ventraI lateral geniculate nucleus and 57 in the internal sector. One apparent difference between the internal and external sectors is the significant proportion of nonresponsive neurons found in the internal sector (24%), versus a very small percentage in the external sector (2%). The nonresponsive cells found in the internal sector were sampled in six different animals and were found in association with all other receptive field types. There is no indication that they were confined to one region of the internal sector. Nonresponsive cells may be genuinely silent in the presence of visual stimuli, or may have complicated stimulus requirements which we did not discover. Both the internal and external sectors have a significant percentage of neurons with indefinite receptive fields. These were found in 11 of the 17 animals studied, and occurred in the same electrode passes in which cells with clearly defined receptive fields were recorded. In most cases great effort was made to delineate a receptive field, even when the local area of response was in an extreme peripheral region of the visual field. The vast majority of dorsal lateral geniculate neurons with concentric lnd uniform fields were reported by Stewart, Chow and Masland (26) to
516
MATHERS
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have receptive fields less than 20” in diameter. In ventral lateral geniculate nucleus concentric and uniform cells (58 cells), the same approximate range of receptive field diameters was found, but there is a greater percentage of those with larger diameters. This is illustrated graphically in Fig. 4, where concentric receptive field diameters from both are compared. As might be predicted, field size of the ventral lateral geniculate nucleus concentric receptive fields increased with distance from the optic nerve head. A very small number of motion-sensitive and directional cells (4) was found in this study. They were located in central parts of the nucleus, however, and were certainly a part of the ventral lateral geniculate nucleus. It is worth noting that concentric, directional, and a type of motionsensitive cell are all found in the rabbit’s retina (l), so that it is not surprising that such receptive fields should also be reported for neurons in ventral lateral geniculate nucleus. The superior colliculus of the rabbit also has some cells with directional and motion sensitivity (15). DISCUSSION Morphological Properties. There has been little previous work on the ventral lateral geniculate nucleus, owing to its small size, and the fact that its location near the optic tract made lesioning techniques difficult to apply. It is well established that throughout much of the mammalian Class, the ventral lateral geniculate nucleus exists with an internal and external sector
ilF
. . . . . . . . . vLGN
CELLS
FIELD DIAMETER-
DEGREES
FIG. 4. This graph illustrates a comparison of receptive field diameters for concentric cells in the ventral lateral geniculate nucleus and the dorsal lateral geniculate nucleus [dorsal lateral geniculate nucleus data from Stewart, Chow and Masland (26) 1.
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NUCLEUS
517
(19). We are as yet quite ignorant of the significance of this visuotopically organized nucleus in the ventral thalamus, either from the standpoint of comparative neurology or the function of the visual system. This morphological examination of the rabbit ventral lateral geniculate nucleus suggests that its structure is quite reminiscent of that of the relay nuclei in the dorsal thalamus (13, 22). There are two prominent neuron types, one evidently the relay neuron and one the interneuron. Like dorsal thalamic nuclei, the synaptic population is predominated by small roundvesicle terminals, and has a lesser population of large round-vesicle terminals which constitute the extrinsic afferent input from the retina. There are also flat-vesicle terminals, perhaps dendritic, which presumably are of intrinsic origin. The neuropil also contains synaptic “glomeruli” (29), with RL terminals as a central element. The retinal projection to the ventral lateral geniculate nucleus, formerly believed to be restricted to the external sector, now appears to be distributed to both the internal and external sector. There is a difference, however, in the caliber of degenerating retinal axons in the internal vs. the external sector, and there is a very obvious difference in grain density in the internal vs the external segment following intraocular injection. In addition, the rate of axonal degeneration and phagocytosis of debris in the internal sector appears to be faster than that of the external sector. This has already been observed in the guinea pig (23)) and our findings are in support of this previous report. We plan to pursue further experiments in an effort to learn more about this difference between the retinal projections to the internal and external sectors of the ventral lateral geniculate nucleus. Physiological Properties. The principal impetus for this series of experiments was the observation that retinal input was confined to the external sector. We believed that since the visual cortex projects to both internal and external sector, we would have the opportunity to record from some neurons receiving both retinal and visual cortical input, and others receiving the visual cortical but not the retinal input. Our own findings, and those previously reported by Hendrickson (11) and Swanson, Cowan and Jones (28) made it evident that the ventral lateral geniculate nucleus would not offer this opportunity. The receptive field properties of some of the neurons in the ventral lateral geniculate nucleus are akin to those of neurons in the dorsal lateral geniculate nucleus of the rabbit (2c, while the receptive field properties of other ventral lateral geniculate nucleus neurons are imprecise and indefinite, like those of neurons in parts of the rabbit posterior thalamus and pretectum (27). We have no basis at the present time for speculation on the relative importance of the retinal and the visual cortical input in producing the receptive field properties of ventral lateral geniculate nucleus neurons.
518
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However, there is at least one study in which visual cortical ablation produced little change in neuronal receptive field properties in a rabbit subcortical visual area, the superior colliculus (1.5). This is in marked contrast to the results of similar studies in the cat (30)) where visual cortical ablation altered the directionality and binocular response properties of collicular neurons. The ventral lateral geniculate remains an enigma in the visual system. The visuotopic organization (IS), the similarity of neuronal receptive fields to those in the dorsal lateral geniculate, and the morphologic similarity of the ventral and dorsal lateral geniculate nuclei all suggest that similar functions might be carried via neuronal circuits including these nuclei. The ventral lateral geniculate nucleus has additional and as yet unexplained connections with superior colliculus and pretectum. Nothing in the receptive field properties of neurons studied in these experiments suggested the role in eye movements indicated by these connections. We have as yet no idea of what significance may be attached to morphological differences in the retinal projection to the internal and external sectors. By knowing which areas of the retina project to the ventral lateral geniculate nucleus, and which types of retinal ganglion cells project to each sector, we should have a system through which much may be learned about differences in retinal ganglion cells, and the manner in which they project to thalamic visual nuclei. The recent reports of Swanson, Cowan and Jones (28) and Edwards, Rosenquist and Palmer (5) have reaffirmed earlier beliefs that the ventral lateral geniculate nucleus does not project to the cerebral cortex. Perhaps this gives some hint to the functional neural circuitry in which the ventral lateral geniculate is most important. It appears that the ventral lateral geniculate projects to the zona incerta and lateral terminal nucleus, and it is reasonable to postulate that this is a pathway by which visual inputs can reach some prominent subcortical telencephalic motor centers. The geniculocalcarine system reaches these motor pathways only very indirectly, so perhaps the ventral lateral geniculate nucleus is the origin of more direct input. Until further physiological work can be accomplished, along with behavioral work involving ablation of the ventral lateral geniculate nucleus, we will probably have little more concrete idea about the role of the ventral lateral geniculate nucleus in visual function. REFERENCES 1. BARLOW, H. B., and W. R. LEVICK. 1%5. The mechanism of directionally selective units in rabbit’s retina. J. Physiol. (Lo&don) 178: 477-504. 2, BISHOP, P. O., W. BURKE, and R. DAVIS. 1%2. The identification of single vnits in central visual pathways. J. Physiol. (Lodon) 162: 409-431.
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3. B~~TTNER, U., and A. F. FUCHS. 1973. Influence of saccadic eye movements on unit activity in simian lateral geniculate and pregeniculate nuclei. J. Nenrophysiol. 36: 127-141. 4. CHOW, K. L., R. H. MASLAND, and D. L. STEWART. 1971. Receptive field characteristics of striate cortical neurons in the rabbit. Brain Res. 33: 337-352. 5. EDWARDS, S. B., A. C. ROSENQUIST, and L. A. PALMER. 1974. An autoradiographic study of ventral lateral geniculate projections in the cat. Broi~t Res. 72: 282287. 6. FAMICLIETTI, E. V., and A. PETERS. 1972, The synaptic glomerulus and the intrinsic neuron in the dorsal lateral geniculate nucleus of the cat. f. Con@. Neural. 144 : 285-333. 7. GIOLLI, R. A., and M. D. GUTHRIE. 1%9. The primary optic projections in the rabbit. An experimental degeneration study. J. Camp. Neural. 136: 99-126. 8. GUILLERY, R. W. 1969. The organization of synaptic interconnections in the laminae of the dorsal lateral geniculate nucleus of the cat. 2. Zellforsch. 96: l-38. 9. GUILLERY, R. W., and M. COLONNIER. 1970. Synaptic patterns in the dorsal lateral geniculate nucleus of the monkey. 2. Zellfarsch. 103 : 90-108. 10. HARTING, J. K., and C. R. NOBACK. 1971. Subcortical projections from the visual cortex in the tree shrew (Tupaia glis). Braitt Rrs. 25: 21-33. 11. HENDRICKSON, A. E. 1973. The pregeniculate nucleus of the monkey. Anot. Rec. 175: 341 (abstr.). 12. HUGHES, A. 1971. Topographical relationships between the anatomy and physiology of the rabbit visual system. Dot. Ophthalmol. 30 : 33-159. 13. JONES, E. G., and T. P. S. POWELL. 1969. Electron microscopy of synaptic glomeruli in the thalamic relay nuclei of the cat. Proc. Roy. SOC. London Ser. B 172: 153-171. 14. LATIES, A. M., and J. M. SPRAGUE. 1966. The projection of optic fibers to the visual centers in the cat. J. Camp. Nerrrol. 127: 35-70. 15. MASLAND, R. H., K. L. CHOW, and D. L. STEWART. 1971. Receptive-field characteristics of superior colliculus neurons in the rabbit. J. NeuropJzysioZ. 34: 148-156. 16. MATHERS, L. H. 1972. Ultrastructure of the pulvinar of the squirrel monkey. .I. Comp. Neural. 147: 15-42. 17. MATHERS, L. H., K. L. CHOW, P. D. SPEAR, and P. GROBSTEIN. 1974. Ontogenesis of receptive fields in the rabbit striate cortex. E.@. Bruin Rcs. 19: 20-3.5. 18. MONTERO, V. M., J. F. BRUGGE, and R. E. BEITEL. 1968. Relation of the visual field to the lateral geniculate body in the rat. J. Neurophysiol. 31 : 268-282. 19. NIIMI, K., T. KANASEKI, and T. TAKI~LIOTO. 1963. The comparative anatomy of the ventral nucleus of the lateral geniculate body in mammals. J. Comb. Neztrol. 121: 313-323. 20. RAFOLS, J. A., and H. A. MATZKE. 1970. Efferent projections of the superior colliculus in the opossum. J. Camp. Neural. 138 : 147-160. 21. RALSTON, H. J. 1971. Evidence for presynaptic dendrites and a proposal for their mechanism of action. Nattrre (Loudon) 230 : 585-587. 22. RALSTON, H. J., nd K. L. CHOW. 1973. Synaptic reorganization in the degenerating lateral geniculate nucleus of the rabbit. j. Cop@. Na2rroI. 147: 321350. 23. SCHNEIDER, G. E. 1968. Retinal projections characterized by differential rate of degeneration revealed by silver impregnation. Anat. Rec. 160: 423.
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W. R., J. TIGGES, and M. TIGGES. 1970. Subcortical projections, cortical associations, and some intrinsic interlaminar connections of the striate cortex in the squirrel monkey (Saimiri). J. Comb. Nezrrol. 140 : 155-173. STELZNER, D. J., D. C. GOODMAN, and P. L. ROSSETTI. 1973. The synaptic organization of the ventral lateral geniculate nucleus pars lateralis of the rat. Anat. Rec. 175: 451 (abstr.). STEWART, D. L., K. L. CHOW, and R. L. MASLAND. 1971. Receptive field characteristics of lateral geniculate neurons in the rabbit. J. Neurophysiol. 34: 139147. STEWART, D. L., L. C. TOWNS, and D. BRIT. 1973. Visual receptive-field characteristics of posterior thalamic and pretectal neurons in the rabbit. Brain Res. 57: 43-57. SWANSON, L. C., W. M. COWAN, and E. G. JONES. 1974. An autoradiographic study of the efferent connections of the ventral lateral geniculate nucleus in the albino rat and the cat. J. Comp. Neural. 156: 143-164. SZENTAC~THAI, J. 1970. Glomerular synapses, complex arrangements, and their operational significance, pp. 427-442. In “The Neurosciences: Second Study Program.” Rockefeller Press, New York. WICKLEGREN, B. G., and P. STERLING. 1969. Influence of visual cortex on receptive fields in the superior colliculus of the cat. J. NeurophysioC. 32: 16-23.
24. SPATZ,
25.
AND
