Lecture 16 Lecture Outline:A. Special Sensesa. Gustationb. Visioni. Eye anatomy & physiologyii. Problems with visionGustation: sense of taste- Tastebuds are chemoreceptors on tongue, some in cheeks, mainly at back of tongue- Taste pore opens to Gustatory receptor cells which are connected to basal cell deeper in tongueo Gustatory receptors can be bitter, sweet, sour, salty, or umamio Solid food must be dissolved in saliva to enter taste pore, and bind to gustatory receptor cells, which forma receptor potential using gated ion channels with basal cell, which will send action potential to braino Gustatory cells/3 first order neurons (front of tongue, back of tongue, throat) synapse in medulla synapse in thalamus Gustatory cortexo Medulla triggers reflexes to stimulate salivation, gastric secretion, or gag reflexVision: sense of seeing stuff- Light cornea aqueous humor lens vitreous humor retina- Eye composed of three layerso Fibrous tunic: composed of cornea and sclera Cornea: Avascular (allows for transplantation) Sclera: bundles of connective tissue, attaches to extrinsic eye muscleso Vascular tunic: composed of iris, ciliary body, and choroid Iris: attached to ciliary body to control amount of light that enters eye- Divides anterior segment into posterior and anterior chambers- Glaucoma: aqueous humor does not properly get drained from eyeo Puts pressure on anterior segment, pushes vitreous humor back on sensitive retina Ciliary body: muscle cells that control size of lens for focusing Choroid: very vascular layer of eye, filled with dark pigment molecules Pupil: hole formed by iris that controls amount of light that enters eye and helps focus- Constriction: used for bright light and near visiono Done by circular muscle cells- Dilation: used for dim light, distant vision, stress, fear, or interest in somethingo Done by radial muscle cells that pull iris outwardo Nervous Tunic: Retina: composed of ganglion, bipolar, photoreceptive and pigmented epithelial layer- Light goes deep into retina, electrical signals come back out in opposite direction- Pigmented epithelial layer: contains pigment molecules to help absorb light- light stimulates rods and cones to form receptor potentials with bipolar cells with use of neurotransmitter, which form receptor potential with ganglion layer which fire action potential to the brainBio 312- Optic Disc: point where axons of al ganglion cells converge to form optic nerveo Blind spot because no photoreceptor cells- Photoreceptive cells: rods and coneso Cones: responsible for color vision Provide for high visual acuity Require stronger light intensity to stimulate Greatest density in fovea centralis One cone synapses with one bipolar cell which synapses with one ganglion cell. Results in high acuity, but requires higher intensity of lightto fire action potentialo Rods: most plentiful (30x) responsible for poor resolution and acuity (outline) More sensitive to light, no color discrimination Most common in periphery of eye, away from fovea Participate in converging pathways, many rods synapse with one bipolar cell, multiple bipolar cells converge on one ganglion cell Several rods form weak receptor potentials (low light intensity) which can summate on bipolar layer to fire action potential from ganglion cell Funneling effect prevents brain from knowing exactly where in eye the image is focusedo Extrinsic eye muscles: skeletal muscle that controls the eye, controlled by somatic nervous systemo Lens: Suspended behind iris by suspensory ligaments Transparent and elastic Shape (thickness) can be changed by amount of pull exerted by suspensory ligaments Light waves from a near object are diverging at a greater degree, and require higher degree of convergence to focus on retina Light waves from a distant object are almost parallel and require very little convergence by lens to focus on retina Cornea has constant refractive power due to thickness and tough structure Lens is a convex lens that converges light on the retina Eyes are best adapted for distance vision because of default convergence of relaxed eye- Distance vision does not require much refraction, less work for ciliary muscle Near objects require higher degree of refraction, more work for ciliary muscles Accommodation: to keep an object in focus as it moves closer, the lens becomes more rounded- Flattened weak lens becomes stronger and rounded by contracted ciliary muscles as object moves closer, stimulated by parasympathetic (autonomic) stimulationo Presbyopia: as people age, lens becomes less elastic, lose ability to accommodate Corrected by lenses to increase refractive power for near objectso Myopia: nearsightedness caused by an eyeball that is too long, causing objects to focus in front of retina Requires concave lenso Hyperopia: farsightedness caused by an eyeball that is too short, casing objects to focus behind retina Requires convex lenso Emmetropic eye: normal, with correct amount of refractive power, and shape 99% of refractive problems are due to eyeballs that are misshapen rather than lens problemso Astigmatism: uneven thickness/curvature of lens and/or cornea Causes blurring of some of the image- Photoreception: rods and cones contain light sensitive visual pigments that break down when they absorb lighto Piments made of liht absorbing molecule called retinal that combins with various types of proteins called opsinso 4 types of pigments, each absorb light over different ranges of wavelengths Rhodopsin: found in rods and can absorb visual light across entire spectrum Other three types found in cones (red, blue, green) named after which wavelengths they best
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