Outer plexiform layer

[Techniques] [Bipolar cells] [Horizontal cells] [ON and OFF center pathways] [References]


3. Horizontal cells.

All mammalian retinas have two types of horizontal cell as the laterally interconnecting neurons in the outer plexiform layer (Mariani, 1986). The cat has been studied extensively as a model mammalian retina and the two types, known as A-type and B-type are illustrated below (Kolb, 1974). The A- and B-type HCs are very similar in appearance in the rabbit retina too.

The A-type HC is a large sturdy cell with radiating dendrites covering a dendritic field of 150-250 µm (depending on area of retina) (all neurons of the retina are small in dendritic expanse in the central or foveal retina and increase in dendritic tree size with eccentricity from the central area). The B-type HC in the mammalian retina differs from the A-type in having a smaller bushier (in general) dendritic tree (75-150 µm diameter) and bearing an axon that travels 300 µm or more before ending in a huge expansive axon terminal tree. The dendrites of both A-type and B-type HCs end in cone pedicles while the axon terminals of the B-type HCs end in rod spherules (see above) (Kolb, 1974).

Fig. 9. Horizontal cells in cat retina (59 K jpeg image)

Primates, although obviously mammals, have been thought of as somewhat different in their horizontal cell make-up compared with cats. Originally there was only thought to be one HC type, called an HI, looking like a miniature version of the cat B-type HC (Polyak, 1941). In 1980, we described a second type of HC in the rhesus monkey retina and called it the HII type (Kolb et al., 1980). Most recently we have been able to distinguish a third type, HIII type, of horizontal cell in the human retina (Kolb et al., 1994). Light micrographs and drawings of Golgi stained examples of these three types are illustrated below.


Fig. 10. Light micrograph of human horizontal cells (59 K jpeg image)

Fig. 11. Drawings of monkey horizontal cells (59 K jpeg image)

HI is the classic horizontal cell of primate retina ( Polyak, 1941). It is a small-field cell (15 µm diameter dendritic tree in the fovea, 80-100 µm in the periphery) with stout dendrites giving rise to distinct clusters of round or donut-shaped terminals contacting cones as lateral elements of the ribbon synapses (shown below).

Fig. 12. HI horizontal cell of primate retina (59 K jpeg image)

In peripheral retina the HI cells have much bigger dendritic trees and their radiating dendrites contact as many as 18 cones. The HI cell has a single thick axon that passes laterally in the outer plexiform layer to terminate more than 1mm away in a thickened axon terminal stalk bearing a fan-shaped profusion of lollipop-like terminals. HI axon terminals end in rod spherules as lateral elements of the ribbon synapses (Kolb, 1970) (see below).


Fig. 13. HI horizontal cell terminals (59 K jpeg image)

HIII cells are similar in appearance to HI cells, but are everywhere in the retina 1/3 bigger in dendritic tree size and typically, particularly in peripheral retina, asymmetrical in shape (one or two dendrites are much longer than others). The clusters of terminals contact cones in the same manner as the HI cell terminals and because of their bigger field size contact more cone pedicles (9-12 in foveal retina, 20-25 in peripheral retina). The axon of the HIII has not been conclusively followed to a terminal yet so we know nothing of the nature of the photoreceptor type they contact, although we suspect it is a mixture of rods and cones.

HII cells are more spidery and intricate in dendritic field characteristics than either of the other types (Kolb et al., 1980). Their terminals are not clearly seen as clusters approaching cone pedicles but they are known to end in cone pedicles (Kolb, 1980; Ahnelt and Kolb 1994a,b). HII cells also bear an axon, but this is quite different from that of the other two horizontal cell types. It is short (100-200 µm) curled instead of straight and has contacts to cone pedicles by means of small whispy terminal.



Recent findings from electron microscopic studies of Golgi-stained horizontal cells of the human retina show that there is some colour specific wiring going on for the three cell types (Ahnelt and Kolb, 1994a,b).

Fig. 14. Three cell types of horizontal cells in human retina (59 K jpeg image)




Thus HIs contact medium and long wavelength cones primarily but with a small number of contacts to any short wavelength cones in the dendritic field. HII cells contact short wavelength cones, directing major dendrites to these cones in their dendritic fields where they occur, and contacting with lesser numbers of terminals other types of non-short wavelength cone. The HII cells axon contacts short wavelength cones only. HIII cells have large dendritic terminals in medium and long wavelength cones, seemingly avoiding short wavelength cones in their dendritic tree (Ahnelt and Kolb, 1994b). Thus a wiring diagram can be made (below) that summarizes our present understanding of the spectral connections of the three HC cell types of the primate retina.

Fig. 15. Summary of the spectral connections of the three HC types of the primate retina (59 K jpeg image)


4. ON and OFF center pathways and center surround organization of the retina are initiated at the photoreceptor to bipolar and horizontal cell contacts in the outer plexiform layer.

We know that photoreceptor neurotransmitter (which is glutamate, see Dowling, 1987 and Massey, 1990, for reviews) is released in the dark in the vertebrate retina (Trifonov, 1968). Thus the photoreceptor, whether it be rod or cone is in a depolarized state in the dark. On light stimulation the photoreceptor responds with a hyperpolarization, transmitter release ceases but the postsynaptic bipolar cells respond with either hyperpolarization or depolarization of their membranes.The hyperpolarizing type of bipolar cell is called an OFF-center cell while the depolarizing bipolar cell is called an ON-center cell (Werblin and Dowling, 1969; Werblin, 1991).

The origin of these two important ON-center and OFF-center channels, is the types of synaptic contacts these bipolars make with cone pedicles or rod spherules. Thus, the type of bipolar cell making invaginating contacts (several cone bipolar types, amongst them the IMB type and the rod bipolar type), responds to light with an inverted sign compared with the photoreceptor. They give depolarizing responses to light and are thought to be stimulated via metabotropic glutamate receptors, specifically mGluR6, and signal via a G protein cascade (Jahr, 1999; Dhingra et al. 2001) (see chapters on rod and cone pathways). These depolarizing bipolar cells have an APB sensitive glutamate receptor (Slaughter and Miller, 1981).

Fig. 16. Origin of ON-center and OFF-center channels (59 K jpeg image)

On the other hand, the type of bipolar cell that makes contact with the photoreceptor at a basal junction, responds to light just like the photoreceptor by hyperpolarizing (see above). The hyperpolarizing types are driven via ionotropic (iGluR) AMPA-kainate glutamate channels in their synapses with photoreceptors (Slaughter and Miller, 1983). Hyperpolarizing (basal junction contacting) bipolar cells are the start of OFF-center channels and the depolarizing (invaginating, ribbon-related) bipolar types are the start of ON-center channels through the whole retina and visual system. For, as we shall see in a later chapter, at the level of information transfer between cone bipolar cells and ganglion cells in the inner plexiform layer, only excitatory channels are present. Therefore, the status of the signal transmitted by the ganglion cell to the brain is essentially determined by the nature of the cone bipolar contacting it which in turn is determined by its photoreceptor synapse.


CLICK HERE to see a movie of the intracellular recordings of a cone and bipolar cells (103 K quicktime movie)


In submammalian species intracellular recordings from the cones have indicated that they receive a feed-back inhibitory message from horizontal cells (Baylor et al., 1971). In fish retinas there is morphological evidence that the horizontal cell lateral elements at the triad synapse produce spinules at which feed-back signalling occurs to the cone (Djamgoz and Kolb, 1993, for a review), In mammalian cones there is no unequivocal morphological evidence for this feed-back synapse but some kind of feed-back is expected at this site (see above).


Horizontal cell lateral elements, i.e. horizontal cell dendrites, have vesicles within them that could be directed at the cone membrane. This arrangement of horizontal cell dendrites on either side of the ribbon synapse in the triad, with the bipolar cell dendrites forming the central or basal junction position, is very important. A small local circuit is formed here, that influences the flow of information throughout the whole retina.

Fig. 17. Electron micrograph of synaptic contacts in a cone pedicle (59 K jpeg image)


We know that photoreceptor neurotransmitter is released in the dark in the vertebrate retina. Thus the photoreceptor, whether it be rod or cone is in a depolarized state in the dark. On light stimulation the photoreceptor reacts with a hyperpolarization, transmitter release ceases but the postsynaptic bipolar cells respond with either hyperpolarization or depolarization of their membranes. The horizontal cells responds too, with a hyperpolarization, but a feedback synapse from the horizontal cell to the photoreceptor is also thought to occur. The feedback signal sums information from a network of horizontal cells connected over a wide spatial area of the outer plexiform layer. Thus, this large spatial area influences the photoreceptor and the bipolar cell to include a response coming from a surround region of retina. This local circuit provides the bipolar cell with a center-surround organization.



5. References.

Ahnelt, P. and Kolb, H. (1994a) Horizontal cells and cone photoreceptors in primate retina: A Golgi-light microscope study of spectral connectivity. J. Comp. Neurol. 343, 387-405.

Ahnelt, P. and Kolb, H. (1994b) Horizontal cells and cone photoreceptors in human retina: a Golgi-electron microscopic study of spectral connectivity. J. Comp. Neurol. 343, 406-427.

Baylor, D.A., Fuortes, M.G.F. and O'Bryan, P.M. (1971) Receptive fields of the cones in the retina of the turtle. Journal of Physiology, London, 214, 265-294.

Boycott B.B. and Dowling J.E. 1969) Organization of the primate retina: light microscopy. Phil. Trans. R. Soc., B, 255, 109-184.

Boycott, B. B. and WŠssle, H. (1991) Morphological classification of bipolar cells of the primate retina Eur. J. Neurosci. 3, 1069-1088.

Cajal, S.R. (1892) The Structure of the Retina. (Transl. Thorpe SA, Glickstein M), Thomas, Springfield, Il., 1972.

Djamgoz, M.B.A., and Kolb, H. (1993) Ultrastructural and functional connectivity of intracellularly stained neurones in the vertebrate retina: Correlative analyses. Microscopy Research and Techniques, 24, 43-66.

Dowling, J.E. (1987) The Retina: an approachable part of the brain. The Belknap Press, Harvard University Press, Cambridge, Massachusetts.

Golgi, C. (1885) Sulla fina anatomie degli organi centrali del sistemi nervosa. Calderini.

Hopkins, J.M. and Boycott, B.B. (1995) Synapses between cones and diffuse bipolar cells of a primate retina. J. Neurocytol. 24, 680-694.

Kolb, H. (1970) Organization of the outer plexiform layer of the primate retina: electron microscopy of Golgi-impregnated cells. Philosophical Transactions of the Royal Society, London, B, 258, 261-283.

Kolb H. (1974) The connexions between horizontal cells and photoreceptors in the retina of the cat: electron microscopy of Golgi-preparations. J. Comp. Neurol. 155, 1-14.

Kolb H. (1977) The organization of the outer plexiform layer in the retina of the cat: Electron microscopic observations. J. Neurocytol. 6, 131-153.

Kolb, H., Fernandez, E., Schouten, J., Ahnelt, P., Linberg, K.A. and Fisher, S.K. (1994) Are there three types of horizontal cell in the human retina? J. Comp. Neurol. 343, 370-386

Kolb, H., Goede, P., Roberts, S. McDermott, R. and Gouras, P. (1997) Uniqueness of the S-cone pedicle in the human retina and consequences for color processing. J. Comp. neurol., 386, 443-460.

Kolb, H., Linberg, K. A. and Fisher, S.K. (1992) Neurons of the human retina: A Golgi study. Journal of Comparative Neurology, 31, 147-187.

Kolb, H., Mariani, A. and Gallego, A. (1980) A second type of horizontal cell in the monkey retina. J. Comp. Neurol. 189, 31-44.

Kolb, H. and Nelson, R. (1995) The organization of photoreceptor to bipolar synapses in the outer plexiform layer. In "Neurobiology and Clinical Aspects of The Outer Retina" (Eds. Djamgoz, M.B.A., Archer, S.N. and Vallerga, S.) Chapman and Hall, London. pp. 273-296.

Kolb, H., Nelson, R. and Mariani, A. (1981) Amacrine cells, bipolar cells and ganglion cells of the cat retina: A Golgi study. Vision Research, 21, 1081-1114.

Kouyama, N. and Marshak, D.W. (1992) Bipolar cells specific for blue cones in the macaque retina. Journal of Neuroscience, 12, 1233-1252.

Mariani A.P. (1981) A diffuse, invaginating cone bipolar cell in primate retina. J. Comp. Neurol. 197, 661-671.

Mariani, A. P. (1983) Giant bistratified bipolar cells in monkey retina. The Anatomical Record, 206, 215-220.

Mariani, A.P. (1985) Multiaxonal horizontal cells in the retina of the tree shrew, Tupaia glis. J. Comp. Neurol. 233, 553-563.

Massey, S.C. (1990) Cell types using glutamate as a neurotransmitter in the vertebrate retina. Progress in Retinal Research, 9, 399-425.

Nawy, S. and Jahr, C.E. (1990) Suppression by glutamate of cGMP activated conductance in retinal bipolar cells. Nature, 346, 269-271.

Nelson, R. (1977) Cat cones have rod input: a comparison of the response properties of cones and horizontal cell bodies in the retina of the cat. J. Comp. Neurol. 172, 109-136.

Polyak S.L.(1941) The Retina. The University of Chicago Press, Chicago, Ill.

Slaughter, M.M. and Miller, R.F. (1981) 2-amino-4-phosphonobutyric acid: A new pharmacological tool for retina research. Science, 211: 182-184.

Slaughter, M.M., and Miller, R.F. (1983) An excitatory amino acid antagonist blocks cone input to sign-conserving second-order retinal neurons. Science, 219: 1230-1232.

Trifonov, Y.A. (1968) Study of synaptic transmission between the photoreceptor and the horizontal cell using electrical stimulation of the retina. Biofizika 10 673-680.

Werblin, F. (1991) Synaptic connections, receptive fields, and patterns of activity in the tiger salamander retina. Investigative Ophthalmology and Visual Science, 32: 459-483.

Werblin, F.S., and Dowling, J.E. (1969) Organization of the retina of the mudpuppy, Necturus maculosus. II. Intracellular recording. Journal of Neurophysiology, 32: 339-355.

[Techniques] [Bipolar cells] [Horizontal cells] [ON and OFF center pathways] [References]



Updated: June, 2001