Part 2: Regulation of Special Senses, and Vision

• Visual perception requires the eye (an organ than focuses on visual images and responds to light), and neural pathways (that interpret signals and transform the visual images into patterns of graded and action potentials).
• The optic disk (blind spot) is where the neurons join to the optic nerve (has no photoreceptors, at the back of the retina).
• The macula lutea is the round area at the center of the retina (at the back of the eyeballs). The macula also contains the fovea centralis.
• The fovea centralis is the central part of the macula lutea that is filled with photoreceptors that are particularly responsible for resolution. Hence, it is the sharpest region of vision.
• The cornea is in charge of refraction (the bending of light rays when passing between compartments of different density (from textbook e16)).
• The lens focuses visual images on the retina.
• When images are taken in, they are upside down on the retina, but then it flips back upright when it gets to the brain (the brain reverses the imagine upright).
• Accommodation is the process where the eye adjusts the shape of the lens to keep objects in focus. This is done by zonular fibers which are attached the ciliary muscle to the lens. Hence, the ciliary muscle moves the lens for accommodation.
• When the eye is focus on a far-away object, the ciliary muscles and zonular fibers are relaxed. They cause the lens to be flat.
• When the eye is out of focus on a close object, the ciliary muscles are still relaxed, so the lens hasn't accommodated to the close object. This causes light rays from the near object to diverge (go everywhere).
• When the eye finally focuses (accommodates) on a close object, parasympathetic nerves are fired to contract the ciliary muscle, which relaxes the zonular fibers and causes the lens to become round.
• Myopia (nearsightedness) is when the eyes have difficulty seeing far objects. The focal point isn't close enough to the retina. This can be solved with concave lenses.
• Hyperopia (farsightedness) is when the eyes have difficulty seeing near objects. The focal point is goes way past the retina (goes behind the eyes almost). This can be corrected with convex lenses.
• Most babies have hyperopia before their sight is fully developed.
• Presbyopia is the loss of lens elasticity which results in being unable to accommodate for near vision. This condition usually occurs in people over the age of 40.
• Astigmatism is where the lens or cornea surface are not smoothly spherical (round). This results in multiple focal points towards the retina, and the image being distorted.
• Glaucoma: damage to the retina because the optic nerve's intraocular pressure is increased. This results in vision loss.
• Cataracts are clouding of the lens. This causes vision to be obstructed and blurry.
• Photoreceptors pass sensory information to the bipolars cells, then those bipolar cells pass information on to the ganglion cells.
• Ganglion cells from the optic nerve leave the eye at the optic disk (the blind spot at the back of the eye).
• Horizontal, bipolar, and amacrine cells are interneurons.
• Horizontal cells are specialized neurons in the retina that combine information from multiple photoreceptor cells, specifically cones and rods (paraphrased from textbook e15).
• Bipolar cells are neurons that have one input branch that's attached to one cell, and one output branch that's attached to another cell (paraphrased from textbook e15). Hence, it acts as an interneuron between two cells.
• Amacrine cells are specialized neurons in the retina that combine information from local photoreceptor cells, specifically ganglion cells (paraphrased from textbook e15).
• Ganglion cells are retinal neurons that are postsynaptic to the bipolar cells (take information from bipolar cells). Axons of ganglion cells form the optic nerve (directly from textbook e15).
• Amacrine cells affect the activity of the ganglion cells.
• Rods are photoreceptors that function/are sensitive in low light (dark) conditions.
• Cones are photoreceptors that function best in bright light. They are responsible for color vision.
• Rods and cone are only sensitive to a narrow range of wavelengths (400-750nm). There are many more wavelengths that human eyes are simply incapable to seeing.
• In dark conditions, an enzyme called guanylyl cyclase converts GTP into cGMP. cGMP opens ligand-gated ion channels, so Na+ and Ca2+ come in and out of the photoreceptor disk's membrane. Hence, the photoreceptor becomes depolarized, resulting in there being more neurotransmitter waves in the dark.
• In bright light conditions, the photoreceptor cell becomes hyperpolarized. This happens due to an enzyme called cGMP-phosphodiesterase (made by tranducin when retinal leaves the opsin in bright light conditions) breaks down cGMP. This causes the ion channels to close, thus making the photoreceptor hyperpolarized.
• Opsins are the are the protein components of photopigment (taken directly from textbook e15).
• Photoreceptors interact with bipolar and ganglion cells in two distinct pathway: "on-pathways" and "off-pathways".
• Photoreceptor and bipolar cells only produces graded responses (meaning that they can't generate action potential). They lack the voltage-gated channels for action potential, so they can only ever be depolarize or hyperpolarized.
• Ganglion cells are the first cells in the pathway where action potentials are initiated.
• Essentially, photoreceptors and bipolar cells can't produce action potentials, while ganglion CAN generate action potentials because they make up the optic nerve.
• In both ON and OFF pathways, photoreceptors are depolarized in the absence of light, causing the release of glutamate onto bipolar cells.
• Glutamate receptors of ON bipolar cells are INHIBITORY (metabotropic receptors), while glutamate receptors of OFF bipolar cells are EXCITATORY (ionotropic receptors).
• GTP is glutamate.
• In dark conditions, there is more GTP which then gets converted to cGMP, which causes the cone (photoreceptor) to be depolarized. This triggers the OFF bipolar cell, which then causes the bipolar cell to be excitatory (ionotropic receptors).