Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

in both habitats must “choose” whether they will be emmetropic in the air or under water.

Visual Processing: Photoreceptors to Cortex

      Retina

The optical part of the visual process ends with photons striking the outer segments of the photoreceptors. The neuronal part of the visual process begins with the capture of these photons and absorption of their energy by the photopigments in the outer segments of the cones and rods, where a chain of biochemical reactions starts. In addition to these sensory neurons, the retina also contains secondary and higher order neurons and an intricate neural circuitry that performs the initial stages of image processing before trains of electrical impulses are transferred through the axons of the retinal ganglion cells (RGCs) to areas in the brain where secondary processing and eventually visual perception occur. A schematic representation of the retina is shown in Figure 2.13.

      Photoreceptors

The outermost cells in the neural retina are the photoreceptors, which are divided into two classes: rods and cones. Rods and cones differ from each other in their morphology, function, and retinal distribution. Functionally, cone systems are characterized by high resolution of fine details, rapid responses, color perception, and low sensitivity to small fluctuations in light intensity. Rod systems are characterized by poor visual resolution and no color perception, but they are extremely sensitive to minute changes in light levels and detection of motion. Therefore, cones are particularly suitable for daylight photopic vision, whereas rods contribute mostly to dim‐light scotopic vision (Figure 2.14).

      Horizontal and Bipolar Cells

      The somas of both the horizontal and bipolar cells are located in the inner nuclear layer (INL). Both cells serve as second‐order neurons of the retina, connecting, directly or indirectly, the first‐order (photoreceptors) and third‐order neurons (RGCs).

      Other INL Cells

      The INL is populated by three more types of cells, in addition to some displaced RGCs. Little is known about the interplexiform cells, which are neurons with processes in both the outer plexiform layer (OPL) and IPL. In the OPL, they are presynaptic to bipolar cells. In the IPL, they are pre‐ and postsynaptic to amacrine cells, and presynaptic to bipolar cells. Thus, it is believed that they may modulate the synaptic gain between photoreceptors and their second‐order neurons. Müller cells are another class of cells found in the INL, and are the main glial cells of the retina. Müller cells are ependymoglial cells, meaning they have both a structural support and a metabolic role.

      Ganglion Cells

      All information processed by the retina eventually converges on the RGCs, the innermost cell layer in the retina, and its third‐order, final output neuron. Though much signal processing has already occurred in the vertical (photoreceptor to bipolar to RGC) and in the lateral (photoreceptor to horizontal cell to bipolar to amacrine to RGC) pathways, the RGCs are the most complex information processing cells in the retina.

      Optic Nerve

RGC axons constitute the optic nerve fibers. These axons converge at the optic disc, where they are joined in bundles to form the nerve. As the nerve contains RGC axons but no other neuronal cell body, it can be considered a pure white matter tract. However, the nerve does contain several important glial cell populations. These include oligodendrocytes, which contribute to its myelin sheath and formation of nodes of Ranvier, and astrocytes, which have several functions including K⁺ homeostasis and transportation and storage of metabolites (mainly glycogen) used by the axons. RGCs (and some subtypes of amacrine cells) are the only retinal neurons that generate action potentials. Unlike the graded hyperpolarizing or depolarizing responses of other retinal neurons, action potentials are all‐or‐nothing spikes of electrical activity. This means that all of the neuronal processing of the visual signal that has taken place in the retina so far, including information about stimulus size, contrast, color, movement, and location, is coded as alterations in the firing pattern (e.g., short bursts or sustained episodes of firing) and firing rates of the RGCs.

Figure 2.13 Schematic drawing of the mammalian retina with part of the choroid (Ch) on top. Below the retinal pigment epithelium (PE) are the layers of the neuroretina: the outer segments of the photoreceptors (OS), the outer limiting membrane (OLM), the outer nuclear layer with nuclei of cones (C) and rods (R), the neuropil of the OPL, the INL with nuclei of horizontal (H), bipolar (B), and amacrine (A) cells, the inner plexiform layer with synapses in strata, the ganglion cell layer (G), their axons in the nerve fiber layer, and finally the inner limiting membrane (ILM) facing the vitreous. The glial elements of the retina, Müller cells (M) and microglia (Mi), as well as astrocytes (As) embracing retinal blood vessels, are shown.

      Optic Chiasm and Optic Tract

As the optic nerve approaches the optic chiasm, the location of fibers within the nerve gradually shifts in preparation for decussation at the optic chiasm. Generally, fibers from the temporal retina remain in the ipsilateral hemisphere, and fibers from the nasal retina cross over to the contralateral side. The amount of decussation varies between species, perhaps representing a broad evolutionary scale. Birds, as well as many amphibian and reptilian species, have complete crossover of fibers to the contralateral side. A greater proportion of fibers remain on the ipsilateral side in mammals that have developed binocular vision. In the horse, 15% of the fibers stay on the ipsilateral side, as do 25% in the dog and 33% in the cat. In humans, only half of the fibers cross over.

Figure 2.14 (a) The discs of the outer segments (facing the retinal pigment epithelium) of the photoreceptors contain the photopigment required for vision. The photoreceptors' inner segments contain the mitochondria, and together with the outer segments constitute the photoreceptor layer. The rod spherule and cone pedicle are synaptic expansions where their axons synapse with dendrites of bipolar and horizontal cells in the OPL. Portions of Müller's cells (dotted lines) are shown adjoining the rods and cones. (b) Rod and cone bipolar cells show extensive contacts. Horizontal cells also make synapses with both rods and cones. Interconnections are shown between spherules and pedicles.

      The optic tract runs from the optic chiasm to the lateral geniculate nucleus (LGN). Because of decussation at the chiasm, fibers of the optic tract conduct information from the opposite visual field of both eyes. In humans, where roughly 50% of the axons decussate in the chiasm, the left optic tract relays the right visual hemifield of both eyes, and the right optic tract relays both left visual hemifields. In animals, where a greater percentage of fibers cross over, the left optic tract will relay a greater proportion of the right visual field from the right eye and a smaller proportion of the right visual field from the left eye.

      Lateral Geniculate Nucleus

      For most RGC axons, the first synapse occurs in the LGN, which is one of about 10 targets of RGCs in the thalamus. The axons maintain their retinotopic arrangement through the optic nerves, chiasm, and tracts and as they enter the LGN. Here, the RGC axons synapse with dendrites of LGN interneurons (which provide for signal processing) and projecting cells in synaptic glomeruli. In the LGN, RGC axons segregate