Visual Physiology Notes (3) PDF 1-8

THE EYE: VISION Vision is the process by which light is reflected from objects within our environment. Vision is translated by three steps including (1) the need for light to enter the eye and be focused on the retina by the lens, (2) the ability of photoreceptors cells within the retina to transduc...

THE EYE: VISION Vision is the process by which light is reflected from objects within our environment. Vision is translated by three steps including (1) the need for light to enter the eye and be focused on the retina by the lens, (2) the ability of photoreceptors cells within the retina to transduce the signal into an electrical signal, and (3) the processing of the electrical signal by neural pathways. The eye is protected by a bony orbit and is innervated by six eye muscles controlled by the cranial nerves III (oculomotor), IV (trochlear), and VI (abducens). The external eye also consists of both an upper and lower eyelid, along with a lacrimal gland. The lacrimal gland produces tears via innervation by cranial nerve VII (facial). The eye is a hollow structure that is composed of two fluid filled compartments that are separated by the lens structure. The external most compartment is filled with a substance called aqueous humor, which is a plasma-like fluid generated by ciliary epithelium. The posterior chamber (behind the lens) is called the vitreous chamber and holds the vitreous body. This is a clear and gelatinous matrix that maintains the shape of the eyeball. The eyeball is covered by sclera connective tissue, under which the retinal layer lies. The inner vitreous chamber is also infiltrated with blood vessels by entry and exit through the optic disc, and an optic nerve. Other features of the eye include the pupil which can change in diameter in response to changes in light, the cornea which is a transparent disc of tissue that is continuous with the sclera and the ciliary muscles. Nerves (optic nerve) exit through the optic disc. 10 Light first enters the cornea (transparent disc continuous with the sclera) and passes through the aqueous humor. From here light must pass through the pupil and lens and then ultimately focus on the retina, a layer within the internal eye that is light sensitive and contains photoreceptors that are capable are converting light into electrical signals. Light entering the eye is modified in two distinct ways. The first being by the amount of light that is allowed to enter, a result of pupil size. The second being how the light is focused and the changes to lens shape. For the purpose of this question, let us focus only on the pupil. From physics you should recall that light passing through any medium that isn’t uniform tends to bend in different directions. For example, think of light entering a prism and being sent out the opposite side at a different angle than how it entered. Refraction is encountered within the visual system at two points. The cornea is responsible for some degree of refraction, while the lens is the other structure capable of bending light rays. Because the shape of the cornea cannot easily be adjusted, for the purposes of refraction we will address the lens only. This is because the lens has the ability to modify its shape through the existence of accessory elastic elements and muscles. Refraction, the degree at which light will bend, is influenced by two factors. These are the result of the change in medium the light is entering into (e.g., from air to fluid) and also the angle at which the light rays approach the surface they are attempting to pass through. Parallel light rays passing through a uniform rectangular lens will remain parallel and exit similarly on the opposite side. Meanwhile, a concave lens will take parallel light rays and scatter them upon exiting. A convex lens does the opposite and modifies parallel light rays entering the lens so that they converge towards a common point, referred to as a focal point. 11 Parallel light rays that are coming from a far distance (usually greater than 20 feet) pass through a convex lens to converge onto a focal point. However, the focal point exists in the back-center of the retina and to make it possible for the proper focal point placement (and focal length), the lens of the eye has to be adjusted in shape. For far distances, the lens will flatten so that the focal point falls on the retina. For closer objects (less than 20 feet), parallel rays also pass through our convex lens. However, the convergence from this close distance provides a focal point that would extend beyond the retina. Therefore, the lens shape has to be modified so that it is more rounded (shortening the focal length), allowing the focal point to fall directly on the back of the retina. 12 Lens shape is modified through the action of both the ciliary muscle and the zonule (ligaments attaching the lens to the ciliary muscle). The process of changing lens shape is referred to as accommodation. Under normal circumstances the lens appears normal. However, when it is necessary to flattened the lens (as it may be to view distant objects), the ciliary muscles relax while the zonules gain tone. On the other hand, to change the lens to a more rounded shape, the ciliary muscles must contract while allowing the zonules to slacken. Accommodation is an ability that declines with age leading to presbyopia in older individuals, requiring the use of reading glasses to see close objects. Visual deficits: Hyperopia is far-sightedness and results from when the focal point falls behind the retina. Myopia is near-sightedness and results from when the focal point falls in front of the retina. These can be corrected for with convex and concave lenses, respectively. 13 Phototransduction: This occurs within the retina, which is the sensory organ of the eye. The retina, however, only senses as small fraction of total light (within the wavelengths of 400 nm to 750 nm). Photoreceptors on the retina are responsible for the transduction of the light stimulus into a change in membrane potential. Retinal cells are arranged in the opposite order than what one might expect. You may expect the retinal cells to be the first exposed to light rays, however, light must act through a number of other cells in order to reach photoreceptors. For example, the cells that sit between the light stimulus and the photoreceptor cells include ganglion, bipolar, amacrine, and horizontal cells. In the fovea, however, photoreceptors have direct access to light and therefore reflect an area of the retina with the most visual acuity. Convergence is observed within the retina. For example, many photoreceptor cells (15-45) synapse onto one bipolar neuron. Bipolar neurons further converge onto ganglion cells. The axons of these ganglion cells form the optic nerve and exit the eye through the optic disc (the area of the eye where we experience a blind-spot). In the fovea, however, there is no convergence. Instead, there is a 1:1 ratio of photoreceptor cells to bipolar cells. Cell types including horizontal and amacrine cells enhance the signal processing that occurs between bipolar and ganglion cells. 14 Photoreceptors transduce light into electrical signals and come in the form of rods and cones. The ratio of rods to cones is 20:1 except for in the fovea where only cones are present. This high density of cones in the fovea allows it to provide an enhanced visual acuity (sharpness). Rods function in low light and are best suited for nighttime vision, while cones are responsible for color vision and the sharpness (acuity) of our vision. Both of these, though, have the same basic structure. They consist of an outer, inner, and base segment which each have different functions in phototransduction. The outer segment is the tip of the photoreceptor cell and is embedded into the pigment membrane. The pigment membrane absorbs extraneous light that isn’t absorbed by the photoreceptors, preventing an interference in visual processing. The outer segment also consists of a series of stacked discs in which the visual pigments are located. The inner segment of the photoreceptor cell is where the organelles associated with the production of ATP and photopigments are located. These, therefore, have increased numbers of mitochondria. The base segment of the photoreceptor cell is where the photoreceptor synapses onto the bipolar neurons. Visual pigments within the outer disk segments of the photoreceptor cells are responsible for transducing light into changes in membrane potential. The visual pigment for rod cells is rhodopsin, while cones have three pigments that are similar in structure and function to rhodopsin but respond to different color wavelengths. These colors are red, green, and blue. We recognize color by the wavelengths that excite specific cone receptors (e.g., green grass is transmitted by green wavelengths). Combinations or fusions of colors are interpreted by the brain as a mix of signals that come from the three different cone pigments. The distribution of rods and cones across the retinal epithelium are provided below. 15 Cascade of phototransduction in rods: In rods, rhodopsin is a molecule that becomes activated in response to light. First though, you need to understand that opsin is part of the molecule that is a protein and is embedded within the discs of the outer segment of the rod and retinal is a vitamin A derivative that absorbs light. In the absence of light retinal is bound to opsin. In the presence of light, retinal dissociates from opsin, a processed called bleaching. Photoreceptor cells express potassium channels that move potassium out of the cell. In the absence of light when rhodopsin is inactive, there is an accumulation of cyclic GMP (cGMP) that allows cation channels to allow sodium entry into the cell. This offsets the potassium leak and results in a slightly depolarized potential (-40 mV). The depolarized photoreceptor will release of neurotransmitter (glutamate) onto bipolar cells. When light activates rhodopsin, the opsin molecule interacts with transducin protein. Transducin leads to reductions in cGMP by activating a phosphodiesterase. This ultimately closes cation channels responsible for sodium and calcium influx, but the potassium efflux still remains. The final result is a hyperpolarized cell with a membrane potential of around -70 mV. This hyperpolarized potential results in a reduction of neurotransmitter release onto bipolar neurons. It is important to recognize, however, that these are not all or none processes. Responses can be graded by the intensity of light received by the photoreceptor cells. Also important to recognize is the necessity of these photoreceptors to recover from bleaching. In this way, retinal must be converted back to its inactive form to recombine with opsin. This presents a time delay for the receptors to adjust and is best exemplified by what occurs when you walk into a dark room from a bright sunny day…..you can’t see a thing and it takes a moment to adjust. 16 Signal processing occurs in the bipolar cells following the release of glutamate onto them by photoreceptor cells. These bipolar cells have two distinct responses. The first response is considered the light-on response where the bipolar neuron is inhibited by glutamate release in the dark and activated by release in the light. In light-off bipolar cells, excitation occurs by glutamate release in the dark while the cell is inhibited by this release in the light. This is an example of how one stimulus creates two different responses. Bipolar neurons can either excite or inhibit ganglion cells. 17 Ganglion cell bodies from the retina extend into axons that collectively make up the optic nerve. The optic nerves of each eye cross at the optic chiasm; however, they also pass along information to each side of the brain that represents each eye. From the optic chiasm visual information is sent to the lateral geniculate body of the thalamus and then to the visual cortex, where orientation and mapping within the retina are conserved. Visual information from the lateral geniculate nucleus is also sent to the pretectum (specifically the Edinger Westphal nucleus), a region of the brain responsible for constricting the pupils in response to light. This information leaves thenuclei via the parasympathetic fibers of the oculomotor nerve (CNIII) to reduce the amount of light entering BOTH eyes. This is referred to as the pupillary reflex. 18 19