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The Senses

Help Link : Webvision Link : The Retina Link : BIPOLAR CELLS AND HORIZONTAL CELLS
Link : Animation : Receptive Fields in the Retina
Experience : Characterization of an extended glutamate receptor of the on bipolar neuron in the vertebrate retina.

Intuitively, you might expect the photoreceptors to be located in the layer of the retina closest to its inner surface, so that they would receive the incoming light first. But that is not the case. Before reaching the photoreceptors, light must travel through all the other layers of the retina.

The reason for this somewhat paradoxical arrangement is that the light-sensitive pigments in the photoreceptors must be in contact with the layer of epithelial cells at the back of the eye, which provide them with a continuous supply of retinene, a light-sensitive derivative of Vitamin A. Also, after the structural arrangement of the retinene molecules has been changed by the light energy, they are recycled in this epithelium. The dark pigmentation of this epithelium also prevents unabsorbed photons from being reflected back onto the photoreceptors and thus creating light interference that would degrade the image.

Horizontal cells share a special characteristic with amacrine cells: the lack of any extension resembling an axon. These cells in fact have only dendrites, some of which are presynaptic, that is, play the role of axons. The extensions of these cells thus apparently play both roles.


Within the retina, information travels from the photoreceptors to the bipolar cells and then on to the ganglion cells. At each stage along this most direct visual pathway, the responses are modified by the activation of lateral connections involving horizontal and amacrine cells. Thus the analysis of visual stimuli begins even in the retina.


Curiously, in order to reach the photoreceptors, incoming light must first pass through all the other layers of cells in the retina (see sidebar). The first of these is the ganglion cell layer, composed of the bodies of ganglion cells. Next comes the inner plexiform layer, a network of axons and dendrites from ganglion cells, bipolar cells, and amacrine cells. After that comes the inner nuclear layer, composed of the bodies of bipolar, horizontal, and amacrine cells. Next come the outer plexiform layer, composed of the nerve endings of bipolar cells, horizontal cells, and photoreceptor cells, and then the outer nuclear layer, which contains the bodies of the photoreceptor cells. Last comes the outer segment layer, containing the photoreceptors' outer segments, in which the light-sensitive pigments are located. The endings of these outer segments are embedded in the pigment epithelium.

Every type of cell in these layers has its own distinctive distribution pattern and physiological characteristics.

The distribution of the rods and cones in the retina is not uniform. Around the periphery of the retina, rods are far more numerous, whereas in the centre, at the fovea, cones are. The number of photoreceptors connected to a single ganglion cell is also far greater in the peripheral retina. The combined effect of these distributions is to make the peripheral retina more sensitive to light. The tradeoff is that the convergence of many photoreceptors onto a single ganglion cell causes the sharpness of the image to suffer.
For the high visual acuity found in the central retina, a low ratio of photoreceptors to ganglion cells is required. This acuity is also enhanced by the cones in the fovea, which are very small and packed tightly against one another. The farther from the fovea, the larger the cones and the greater the space between them; the rods fill in the remaining space. Despite the high density of cones in the fovea, this region is so small that it contains only 1% of all the cones in the retina.

Regardless of whether nerve impulses from the photoreceptor cells are following the retina's direct pathway or its indirect pathway, in which the horizontal cells are involved, these signals must pass through the bipolar cells to reach the ganglion cells. These signals are transmitted to the bipolar cells in the form of graduated potentials, which may be either depolarizations or hyperpolarizations, depending on whether the bipolar cell is of the ON or OFF type.

Each bipolar cell receives some of its synaptic connections directly from a number of photoreceptors located roughly opposite it. This number ranges from one photoreceptor at the centre of the fovea to thousands in the peripheral retina.

In addition to these direct connections from photoreceptors, each bipolar cell receives some of its afferences from horizontal cells. These cells are in turn connected to a set of photoreceptors that surround the central group which connect to the bipolar cell directly. As a result, the receptive field of a bipolar cell has two components: a central receptive field composed of the information that travels directly from the photoreceptors to the bipolar cell, and a peripheral receptive field composed of information that arrives via the horizontal cells.

Because the horizontal cells are connected laterally to many rods, cones, and bipolar cells, their role is to inhibit the activity of neighbouring cells. This selective suppression of certain nerve signals is called lateral inhibition, and its overall purpose is to increase the acuity of sensory signals. In the case of vision, when light reaches the retina, it may illuminate some photoreceptors brightly and others much less so. By suppressing the signal from these less illuminated photoreceptors, the horizontal cells ensure that only the signal from the well lit photoreceptors reaches the ganglion cells, thus improving the contrast and definition of the visual stimulus.

The retina's amacrine cells have highly diverse morphologies and employ an impressive number of neurotransmitters. Their cell bodies are all located in the inner nuclear layer, while their synaptic endings are located in the inner plexiform layer. By connecting bipolar cells with ganglion cells, they provide an alternative, indirect path between them. Amacrine cells appear to have many functions, most of them as yet unknown.

As regards ganglion cells, various types with distinct functions have been characterized.


Link : Système sensoriel visuel. Link : Visual Responses of Ganglion Cells Link : The Visual Cortex (animations) Link : Animation: Center-surround receptive field

The circuits formed by the amacrine cells in the inner plexiform layer provide additional information to the ganglion cells, possibly by further increasing the centre-surround contrast generated by the horizontal cells.

Whether a given synapse between a photoreceptor cell and a bipolar cell is excitatory or inhibitory may depend either on the type of neurotransmitter released by the photoreceptor or on the type of receptors in the postsynaptic membrane of the bipolar cell. The possibility that one photoreceptor can release two different neurotransmitters is receiving less and less credence, and all indications are that ON and OFF bipolar cells have different molecular receptors.


Bipolar cells have centre-surround receptive fields. The centre of each such field receives direct connections from a small number of photoreceptors, while the surrounding area (called the "surround") receives inputs from a larger set of photoreceptors whose activity is relayed by the horizontal cells.

Light shining on the centre of a bipolar cell's receptive field and light shining on the surround produce opposite changes in the cell's membrane potential. The diagram here uses an ON-centre bipolar cell as an example.

If light is shined on the centre of this cell's receptive field, the first change is a hyperpolarization of the photoreceptor cell, causing depolarization of the bipolar cell, because of the inhibitory nature of the synapse between them. This depolarization in turn excites the following cell, a ganglion cell, causing it to emit action potentials at a higher frequency.

Source: Adapted from J.E. Dowling

Conversely, if light were shined on the surround of the receptive field of this same ON-centre bipolar cell, it would become hyperpolarized. In contrast, another kind of bipolar cell becomes depolarized when an area of darkness strikes the centre of its receptive field, and hyperpolarized when it strikes the surround. Bipolar cells of this kind are called OFF-centre cells.


This centre-surround structure of the receptive fields of bipolar cells is transmitted to the ganglion cells via synapses located in the inner plexiform layer .


Thus, some synapses connect ON-centre bipolar cells to ON-centre ganglion cells, while others connect OFF-centre bipolar cells to OFF-centre ganglion cells. The accentuation of contrasts by the centre-surround receptive fields of the bipolar cells is thereby preserved and passed on to the ganglion cells, and ultimately to the visual cortex.

Human vision depends in large part on our ability to discern contrasts between objects and the backgrounds behind them. The establishment of parallel pathways for the processing of visual information starting in the retina is one of the mechanisms that makes this discrimination possible.

In addition to the simple cells found mainly in layer IV of the visual cortex, there are other cells, outside of layer IV, that respond to a light stimulus only if it has a particular orientation and is moving.

These are called complex cells. They detect movement through two mechanisms. First, when the axons of many simple cells with the same orientation and adjacent but not identical receptive fields converge on a complex cell, it can detect movement from the differences between these fields. Second, complex cells can detect movement through the phenomenon of temporal summation: if a cell that has already been excited once is excited again shortly afterward, its membrane is still depolarized enough that a stimulus that would not normally suffice to trigger another action potential can do so. Thus, when a moving light beam activates several simple cells in succession, the temporal summation of the stimuli applied to them causes the complex cell to respond to the movement.

Complex cells also frequently display selectivity for direction, responding only when the stimulus is moving in one direction and not in the other. And unlike simple cells, complex cells are not fussy about where the band of light is located in their receptive field. Complex cells represent a further level of visual information processing, but certainly not the ultimate one, because researchers have also discovered the existence of hypercomplex cells.

Receptive Fields of Complex Cells



Link : The Visual Cortex (animations)
Research : David H. Hubel Research : David H. Hubel Research : Korbinian Brodmann
Original modules
Experience Module : Brodmann's Cortical Areas Brodmann's Cortical Areas
Experience Module : Cybernetics   Cybernetics

Just as in the other relay points in the visual pathways, there are far more cortical neurons that receive information from the central part of the retina than from its peripheral areas. This "retinotopy" reflects a principle that operates in other parts of the cortex as well: greater sensory or motor precision requires the involvement of a greater cortical surface.



The primary visual cortex, like all the other parts of the neocortex, has a stratified cellular structure. Layers I to VI, originally described by Brodmann, had to be further subdivided as more was learned about the input and output pathways of the visual cortex.

First, layer IV was divided into three sublayers designated IV A, IV B, and IV C. Then layer IV C was itself subdivided into IV Ca and IV Cb when a difference was found between the connectivities of the cells of the upper and lower parts of this sublayer.

The axons of the cells of the lateral geniculate nucleus transmit information from the eye along various pathways that project mainly into layer IV C. In addition, the neighbouring cells in this layer receive receive information from neighbouring areas of the retina, thus preserving a retinotopic structure. We also know that the information flows emerging from the lateral geniculate nucleus use separate channels arising from its internal structure.



In layer IV C, these information streams are received by the stellate cells, whose axons pass them on to the dendrites of the pyramidal cells in layers IV B and III. These pyramidal cells then project their axons to other areas of the cortex. As for the other output pathways from the primary visual cortex, we know that the pyramidal cells in layer V project to the superior colliculus and the pons at the subcortical level, and that the axons from layer VI return massively to the lateral geniculate nucleus, thus exerting a feedback effect on this structure.

This stratification of the visual cortex into horizontal layers can be readily revealed through simple staining of its neurons. But the visual cortex is also organized into vertical columns, which were not detected until electrophysiological recordings were made of these neurons.

David Hubel and Torsten Wiesel were the first scientists to propose this columnar structure superimposed on the horizontal layers. Using microelectrodes to explore the receptive fields of the neurons of the visual cortex, they showed that this cortex can be regarded as a collection of essentially identical columns. The difference from one column to the next comes simply from the portion of the visual field that is assigned to each of them. The succession of functions of the various layers from the top of the column to the bottom remains the same, but each column processes a characteristic (contrast, colour, orientation, movement, etc.) of a different part of the visual field.
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