7- Neurophysiology (Vision)- Pt 2.docx

- **Activation of photoreceptors by light: General Info** - Photoreceptors, bipolar cells, horizontal cells, and amacrine cells use graded potentials rather than action potentials. - All the retinal neurons conduct their visual signals along the cell by direct flow of...

- **Activation of photoreceptors by light: General Info** - Photoreceptors, bipolar cells, horizontal cells, and amacrine cells use graded potentials rather than action potentials. - All the retinal neurons conduct their visual signals along the cell by direct flow of electric currents, known as "**electrical conduction**". - The same degree of hyperpolarization in the rod and cone is conducted by direct electric current flow in the cytoplasm all the way to the synaptic body, which is where the neurotransmitters will be released and transmitted to the next cell. - The use of graded potentials rather than action potentials allows for graded conduction of signals strength. - Strength of the hyperpolarizing output signal is directly related to the intensity of the illumination, making the signal "all or nothing". - Ganglionic cells generate action potential. - **Activation of photoreceptors by light: Steps** - Step 1: Activation of rhodopsin by light leads to activation of a G-protein, transducin. - Step 2: The activated transducin splits away and activates PDE6 - Step 3: The activated PDE6 hydrolyzes cGMP to 5'-GMP (inactive form), causing the sodium-calcium channel to close - Step 4: The sodium-calcium channel closure leads to hyperpolarization (membrane potential becomes more negative) - Step 5: The decrease in glutamate released will excite the bipolar cells. - Glutamate has an inhibitory function. - **Photoreceptors in the dark** - In the dark, cGMP phosphodiesterase is inactive (PDE6). - Step 1: cGMP accumulates and binds to ligand gated sodium-calcium ion channels. - Step 2: The flow of cations into the rods keeps the membrane **depolarized**. - Step 3: The calcium is transported back out again by an antiporter. - Step 4: Energy is used by allowing the sodium and potassium to run down their electrochemical gradients - Step 5: Calcium ions are forced to be transported against their gradient. - **Visual Acuity** - Acuity of visual images reflects the following factors: - Population of retinal cells - The ratio of rods to cone photoreceptor cells - The ratio of photoreceptor cells to ganglion cells - Cone cells provide better acuity than rod cells. - Hundreds of rods feed signals via bipolar cells to a single ganglion cell. - Only a few cone cells feed signals to a single ganglion cell. - There are more ganglion cells in the area centralis than there are in the peripheral portion of the retina. - Area centralis provides the highest visual acuity in the retina. - **Image Formation: General Info** - Images are formed by the eye, which is optically equivalent to the usual photographic camera, equipped with a: - Lens system - Pupil acting as a variable aperture system - Retina acting as film - The eye catches the light reflected by the objects and guide its passage until the image is formed. - The **lens** works by converging the light rays to a certain focal point on the retina, where light rays have almost unobstructed access to the photoreceptors. - The central retina is the area of highest visual acuity in most mammals, and there is no distinct fovea formed in most mammals. - Images formed on the retina are real, inverted, and smaller than the object. - The distance between the light rays emanating from the top and the bottom of the object in the image get closer together as they approach the eye, and at some point, they cross and reverse. - By reversing and decreasing the image, the eye is able to visualize objects much larger than it. - **Image Formation: Light Refraction Steps** - Step 1: Light is refracted off of the interface between the air and the anterior surface of the cornea and then is refracted in the following order: - Step 2: The interface between the posterior surface of the cornea and the aqueous humor - Step 3: The interface between the aqueous humor and the anterior surface of the lens of the eye - Step 4: The interface between the posterior surface of the lens and the vitreous humor. - **Visual Pathway** - The images formed on the retina are transformed by cones and rods into nerve impulses. - The brain processes and reorients the image. - Visual signals are carried by optic nerve fibers that form the optic nerve, optic chiasm, and optic tract. - For **visual perception**: - Fibers terminate at the thalamus (lateral geniculate nucleus), and then the optic radiation, then occipital lobe (which is the visual area of the brain). - For **visual reflexes** (such as body and ocular reflex, and pupillary constriction), - Fibers terminate at the midbrain (pretectal nucleus and rostral colliculus), then the tectospinal tract and cranial nerve nuclei. - **Pupillary Light Reflex (PLR)** - The PLR is a reflex arc composed of: - Receptors for light within the retina, such as Photoreceptors - Afferent neurons, such as the Optic nerve (cranial nerve 2) - Brainstem centers (for integration), such as the pretectal nucleus, and the parasympathetic nucleus of the oculomotor nerve (cranial nerve 3). - Efferent neurons, such as the oculomotor nerve (cranial nerve 3). - Effector organ - Pupillary sphincter muscles contract (miosis) under bright light - Direct PLR leads to miosis in illuminated eyes - Consensual/indirect PLR leads to miosis in the eye that is not illuminated, making it weaker than direct PLR. - **Cranial Nerve 2 (optic nerve) Decussation in different species** - Birds and lizards: 100% - Mice: 97% - Large animal: 80-90% - Dogs: 75% - Cats: 65% - Primates: 50%