 In this video I will describe the sensory transduction mechanisms and sensory pathways for the visual modality. Vision is the ability to detect light patterns from the outside environment and interpret them into images. Light is detected by photoreceptor cells in the retina of the eye. There are two major types of photoreceptors, rods and cones. Rods are the photoreceptors that are activated by a broad range of light wavelengths, meaning a broad range of colors of light. Therefore rods give us black and white vision and not colored vision. Cones are the photoreceptors that are activated by specific colors of light. There are three major types of cone photoreceptors. There are cones that respond to red light, cones that respond to green light, and cones that respond to blue light. The combination of those three cones enables us to perceive the wide range of color in our environment. Light is detected by a G-protein coupled receptor protein in the photoreceptor cells known as rhodopsin. The rhodopsin protein contains a small molecule cofactor known as retinal, a form of vitamin A. Light causes a shape change to the retinal molecule, altering its structure from the bent cis isomer to the linear all trans retinal isomer. This isomerization will activate the rhodopsin receptor protein, stimulating an intracellular signaling mechanism that causes hyperpolarization and a decrease in the release of neurotransmitters. This is an unusual transduction mechanism in that detecting the stimulus will lead to a decrease in the release of neurotransmitters, where the other examples we've studied, the stimulus caused depolarization and increased release of neurotransmitter. The opposite case occurs in the visual signal transduction mechanism. When light activates rhodopsin, rhodopsin will activate an enzyme known as phosphodiesterase. Phosphodiesterase catalyzes the chemical reaction that converts cyclic GMP to GMP. So cyclic GMP or cyclic guanosine monophosphate is a second messenger molecule that activates a cyclic nucleotide gated sodium ion channel in the membrane of the photoreceptor cells. As phosphodiesterase breaks down cyclic GMP, the cyclic nucleotide gated sodium ion channels close. This causes a hyperpolarization of the membrane potential in the photoreceptor cells, and hyperpolarization then decreases the release of neurotransmitter from the photoreceptor cells onto the dendrites of bipolar cells. The sensory pathway for the visual modality starts with photoreceptor cells relaying information to bipolar cells, then bipolar cells in the eye relay information to the ganglion cells of the retina in the eye. These retinal ganglion cells then have axons that enter the cranium in the optic nerve, cranial nerve number two. The axons of the retinal ganglion cells travel through the optic nerves to the optic chiasm. At the optic chiasm, axons coming from the medial portion of the retina cross the midline projecting to the contralateral hemisphere of the brain. This midline crossing ensures that the left hemisphere processes the visual information from the right half of the visual field, and the right hemisphere of the brain processes information coming from the left half of the visual field. After the optic chiasm, the axons of the retinal ganglion cells continue into the optic tract. And eventually will form synapses with neurons in a region of the thalamus known as the lateral geniculate nucleus. Then the thalamus neurons will relay information to the primary visual cortex in the occipital logo of the cerebrum where visual perception begins. The perception of the visual modality begins in the occipital lobe with the perception of subtle details identifying edges and shapes and boundaries between light and dark. However, the perception of objects in our visual field requires processing by higher level areas. Information from the primary visual cortex will be sent to higher order visual processing areas. This processing will move through two distinct parallel pathways of visual information processing known as the dorsal stream and the ventral stream. You can see here the dorsal stream processes information in the parietal lobe and the dorsal stream is focused on the location and movement of objects in the visual field. Whereas processing in the ventral stream involves regions of the temporal lobe and is focused on identifying objects in the visual field.