The receptive cells for vision are in the retina of the eyes and are of two types. One type are the rods, which can detect the intensity of light by the activation of o molecule called photopigment when light strikes it, but not its color since it responds to any wavelength. The second type are cones, and each cone has a photopigment that responds mostly to a given wavelength of light, thus allowing color vision.
The retina of the eye if a thin film covering the inside of the back of the eye and is conformed by three distinct layers. The first layer is made of the receptors, and their distribution is not homogeneous on the retina. Rods predominate the periphery of the retina while cones abound in the center, reaching a maximum at a point called the fovea, that is directly behind the pupil. The fovea is a specialized area for fine visual discrimination and is almost completely made of cones. The second layer is made up of small parallel bipolar neurons, that radiate perpendicular to the retina plane. Bipolar cells are entwined with interconnecting neurons that are called horizontal cells. The third level is made up by ganglion neurons, which are big cells with lots of dendritic ramifications. The axons of ganglionar cells all converge in one point to form a bundle and forming the optic nerve. In the second and third levels there are medium sized neurons called amacrine cells, that spread their processes (dendrites and axons) in a horizontal way to the retina plane, thus interconnecting bipolar and ganglionar cells among them and with each other. Each receptor (rod or cone) makes a synapse with a bipolar cell and it in turn synapses with a ganglion cell. Only one receptor connects to a bipolar cell, but several spatially contiguous bipolar cells synapse with a single ganglion cell. This produces the effect that a ganglion cell will respond to a small restricted receptive field of the whole visual field. This is important since these receptive fields are still found even in the higher neural relays of the visual pathway.
The axons of the ganglion neurons of the retina form the optic nerves that go into the cranium and are joined to each other at a point called the optic chiasm. Here the fibers that come from the nasal part of the retina cross and project to the opposite side, while the fibers from the temporal (lateral of the head) side of the retina go on the same side. This produces the effect that whatever stimuli is presented in the right side of the visual field, will go to the left hemisphere of the brain, and viceversa. The new bundle of fibers made up of axons from the temporal retina of the same side and the fibers from the nasal retina of the other eye is called the optic tract.
The axons of the optic tract divide in three distinct visual pathways. The first pathway ends in the lateral geniculate nucleus of the thalamus, and it processes the visual information necessary for perception. The other two pathways continue into the midbrain. One ends in the pretectum, synapsing on neurons from which fibers go back to the eye to the cilliary ganglions that control pupilary movements. The other midbrain pathway synapses at the superior colliculus, and it controls the visually guided eye movements, which are the adjusting mechanisms that are not voluntary, such as saccadic movement and focus of the eyes. From the lateral geniculate nucleus, neurons that carry the visual information project to the primary visual cortex, which is located in the back of the brain, in the occipital lobe. This cortex is organized in vertical columns, and each column has a receptive field, analogous to the ganglionar receptive fields. The difference is that here not only detection of the presence stimuli is made, but each column may code for such complex attributes as sideways movement, verticality or horizontality, contrast, etc. The visual information then carries on to higer order sensory cortices and association cortices.
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