7. Visual System

• Sensation is the detection of stimuli, whereas perception is the interpretation of stimuli. You always sense more than you perceive because you ignore a lot in order to focus
• Photons


  Elementary particle of light that our eyes have evolved to perceive. They are both particles and waves, but neuroscientists focus on their wave forms, because that is how they travel, which directly affects how they are perceived
• The human visible range of light is between 400 and 700 nanometers
• Conditions for light to be seen:

  • Photons need to be present
  • Photons must reflect off of objects
  • Photons have to reach the back of the eye
• Cornea


  Anterior part of the eye that refracts incoming rays of light so they converge precisely at the retina. If it doesn't do that properly, things too close or too far look blurry, so glasses compensate for these issues bending light to get everything where it needs to be
• the Cornea

  
• Pupil


  Hole in the center of the eye that light enters through. It can change its diameter to let in more or less light so that we can see better in various conditions
• In the dark, mydriasis is when the pupil gets bigger using the pupillary dilator muscle (signals from the sympathetic nerve system). In the light, myosis is when the pupil gets smaller using the pupillary sphincter (signals from the parasympathetic nerve system).
• the pupil

  
• Lens


  Part of the eye behind the pupil that refracts light onto the retina, but this time is able to change its own shape in a process called accomodation
• Ciliary muscle


  Circular muscle that surrounds the lens of the eye that bends it to focus on things based on their distance
• The lens flattens if you look far away and thickens in the middle if you look close
• Visual field


  Portion of our surroundings that we can see without moving our eyes
• Extraocular muscles


  The six muscles that control eye movement, plus the one that controls upper eyelid movement, are powered by cranial nerves 3 4 and 6
• Nasal hemiretina


  Medial half of the retina that is closer to the nose
• Temporal hemiretina


  Lateral half of the retina that is closer to the temples
• Fovea


  Small pit in the center of the retina where only 2 degrees of the visual field gets reflected, where we have the most acute vision (because there are less cells with lipids to refract light), and what we perceive as being "focused on"
• Optic disc


  Elliptical disc where information exits the eyeball after going from the back to the front then coming back. Because it's a hole there are no photoreceptors there so we cannot perceive light there, making it our blind spot
• Optic nerve


  Shuttles visual information to the brain
• Photopigments


  Light-sensing components of photoreceptors
• Photoreceptor cells have 2 parts; the outer part has hundreds of billions of photopigments in membranous discs, and the inner part has the organelles and the nucleus. The axon terminal is attached to the inner part
• Photoreceptors


  Detect photons and convert them into neurotransmitter release via phototransduction

• Rod cells


  Most of our peripheral vision goes through these, which are concentrated outside of the fovea. They have high synaptic convergence which means small signals create a large signal, which means blurry peripheral
• Purkinje shift


  When things appear to have a bluish tint in the dark or at night because rod cells (most active in low-light conditions) are much more sensitive to blue shades
• Cone cells


  Represent 1/20 of our photoreceptors but give us our best vision. They are concentrated at the fovea and have very low synaptic convergence, leading to sharp images
• Cone photoreceptors are responsible for seeing color. S-cones best sense 420 nm violet light, M-cones best sense 530 nm green light, and L-cones best sense 560 red light. They can all sense other colors, but to lesser extents. Every color on the visible light spectrum is within these cells
• Duplicity theory of vision


  Idea that photopic cells (cones, high-acuity and color in daytime) and scotopic vision cells (rods, peripheral and for seeing in low-light) work together as two separate systems that complement each other and work together
• Color vision deficiency


  When one of the cone cells doesn't work properly so perception of color is abnormal. Occurs somewhere on the x-chromosome, and men are 15x more likely to get it (8% of men, <1% of women)
• Dark current


  In the dark, photoreceptors have leak sodium channels that let sodium in those neurons, causing depolarization, which releases more neurotransmitter (glutamate)
• cGMP


  Molecule that increases dark current
• When light hits a photon it decreases its stores of cGMP, which decrease dark current, which hypoerpolarizes the cell, inhibiting the release of neurotransmitters (glutamate)
• Retinal


  Chemical synthesized from vitamin A, which is low-level in the dark, but then switches to a high-level configuration when hit with a proton. This is what decreases cGMP levels and causes hyperpolarization
• Opsin


  Proteins that exist in different forms and cause retinal to react differently based on the wavelength of light. These determine what kind of photoreceptor color cell they are (s, m, or l)
• Lateral inhibition


  Horizontal cells inhibit neurons at the same "level" so we can better see edges (why those faint grey dots appear on the Hermann grid)
• Horizontal cells


  Cells that inhibit their adjacent cells when activated (when activated they stop releasing glutamate, causing them to hyperpolarize and release neurotransmitters to their neighbours, which inhibits them). This helps us identify the origin of a light signal by minimizing the noise around it
• Bipolar cells


  Named bc extend in 2 opp directions: dendrites receive signals from photoreceptors and axons give signals to next 2 layers down, the amacrine and retinal ganglion cells. Because they get glutamate from photoreceptors on inhibitory metabotropic receptors, their activity increases as photoreceptory activity decreases, and vv
• Amacrine cells


  Interneurons that synthesize and release GABA and get their signals from bipolar cells so they can modulate the activity of nearby neurons
• Melanopsin


  Sensitive opsin molecule in 1-2% of retinal ganglion cells that help us have a circadian rhythm
• Retinal ganglion cells

  Receive signals from bipolar cells, their axons bundle together to form the optic nerve (unmyelinated so no light is detracted) and actually communicate using action potentials
• Optic nerve (cranial nerve II)