1. PowerPoint ® Lecture Slides prepared by Janice Meeking, Mount Royal College C H A P T E R Copyright © 2010 Pearson Education, Inc. 15 The Special Senses: Part A

2. Copyright © 2010 Pearson Education, Inc. Internal Chambers and Fluids Posterior segment contains vitreous humor that: Transmits light Supports the posterior surface of the lens Holds the neural retina firmly against the pigmented layer Contributes to intraocular pressure Anterior segment is composed of two chambers Anterior chamber—between the cornea and the iris Posterior chamber—between the iris and the lens

3. Copyright © 2010 Pearson Education, Inc. Internal Chambers and Fluids Anterior segment contains aqueous humor Plasma like fluid continuously filtered from capillaries of the ciliary processes Drains via the scleral venous sinus (canal of Schlemm) at the sclera-cornea junction Supplies nutrients and oxygen mainly to the lens and cornea but also to the retina, and removes wastes Glaucoma: compression of the retina and optic nerve if drainage of aqueous humor is blocked

4. Copyright © 2010 Pearson Education, Inc. The Retina: Photoreceptors Rods: About 125 million – mostly in the periphery Respond to dim light Are used for peripheral vision Cones: About 6 million mostly concentrated in the center Respond to bright light Have high-acuity color vision Are found in the macula lutea Are concentrated in the fovea centralis

5. Copyright © 2010 Pearson Education, Inc. Photoreceptors - Rods functional characteristics Sensitive to dim light and best suited for night vision Absorb all wavelengths of visible light Perceived input is in gray tones only Sum of visual input from many rods feeds into a single ganglion cell Results in fuzzy and indistinct images http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html

6. Copyright © 2010 Pearson Education, Inc. Photoreceptors - Cones functional characteristics Need bright light for activation (have low sensitivity) Have pigments that furnish a vividly colored view Each cone synapses with a single ganglion cell Vision is detailed and has high resolution http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html

7. Copyright © 2010 Pearson Education, Inc. Other cells in the retina Horizontal cells – extend across the outer portion of the retina at the level of the synapses between photoreceptors and bipolar cells. Amacrine cells found at the level of the synapses between bipolar and ganglion cells. Both can facilitate of inhibit communication between photo receptors and ganglion cells By doing that they can alter the sensitivity of the retina Bipolar cells. Connect the receptor to ganglion cells Ganglion cells encode light information within action potentials to be processed transferred to the visual cortex

8. Copyright © 2010 Pearson Education, Inc. Light Electromagnetic radiation – all energy waves from short gamma rays to long radio waves Our eyes respond to a small portion of this spectrum called the visible spectrum Rods and the different cones in the retina respond to different wavelengths of the visible spectrum

9. Copyright © 2010 Pearson Education, Inc. Cornea Clear window in the anterior part of the eye that allows the entrance of light. It is a major part of the light-bending apparatus of the eye Covered by epithelial sheets on both faces (anterior and posterior) External layer – stratified squamous ET – for protection; merges with the ocular conjuntiva provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears. This layer is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. Moist and being nourished by tears

10. Copyright © 2010 Pearson Education, Inc. Cornea Bowman’s layer a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers Stroma Located behind the external epithelium A thick, transparent middle layer, consisting of regularly-arranged collagen fibers It consists primarily of water and layered protein fibers that give the cornea its strength, elasticity, and form

11. Copyright © 2010 Pearson Education, Inc. Cornea Deep epithelial layer – This single layer of cells is located between the stroma and the aqueous humor. Because the stroma tends to absorb water, the endothelium's primary task is to pump excess water out of the stroma (by having sodium pumps). Without this pumping action, the stroma would swell with water, become cloudy, and ultimately opaque

12. Copyright © 2010 Pearson Education, Inc. http://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg http://www.siumed.edu/~dking2/intro/IN022b.htm

13. Copyright © 2010 Pearson Education, Inc. Lens A biconvex, transparent, flexible, avascular structure that: Allows precise focusing of light onto the retina Is composed of epithelium and lens fibers Lens is avascular because blood vessels interfere with transparency. The lens depends entirely upon the aqueous and vitreous humors for nourishment. Lens has 2 regions Lens epithelium – cuboidal cells found at the anterior surface of the lens. These cells differentiate into lens fibers Lens fibers – cells filled with the transparent protein crystallin. These cells are packed in layers and contain no nuclei. New lens fibers are added continuously the lens enlarges, become denser, less elastic.

14. Copyright © 2010 Pearson Education, Inc. http://www.siumed.edu/~dking2/ssb/EE011b.htm

15. Copyright © 2010 Pearson Education, Inc. Refraction and Lenses When light passes from one transparent medium to another its speed changes and it refracts (bends) Light passing through a convex lens (as in the eye) is bent so that the rays converge (join) to a focal point When a convex lens forms an image, the image is upside down and reversed right to left

16. Copyright © 2010 Pearson Education, Inc. Focusing Light on the Retina Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors Light is refracted: At the cornea Entering the lens Leaving the lens The lens curvature and shape allow for fine focusing of an image

17. Copyright © 2010 Pearson Education, Inc. Focusing Light on the Retina In the normal resting state: our ciliary muscle is relaxed the elastic lens tends to become thick aqueous & vitreous humour push outward on the sclerotic coat ligaments become extended / tensed lens pulled into a thin shape

18. Copyright © 2010 Pearson Education, Inc. Focusing for Distant Vision Light from a distance needs little adjustment for proper focusing Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.) or: The object distance at which the eye is focused with the eye lens in a neutral or relaxed state. Figure 15.13a

19. Copyright © 2010 Pearson Education, Inc. Focusing Light on the Retina - short focal length contraction of ciliary muscle distance between edges of ciliary body decreases relaxation of suspensory ligament lens becomes thicker focal length shortens light rays converge earlier; image formed on retina

20. Copyright © 2010 Pearson Education, Inc. Focusing for Close Vision Close vision requires: Accommodation – changing the lens shape by ciliary muscles to increase refractory power Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye Convergence – medial rotation of the eyeballs toward the object being viewed so both eye are focused on the object

21. Copyright © 2010 Pearson Education, Inc. Problems of Refraction Myopia (nearsightedness) — focal point is in front of the retina, e.g. in a longer than normal eyeball Corrected with a concave lens Hyperopia (farsightedness) — focal point is behind the retina, e.g. in a shorter than normal eyeball Corrected with a convex lens Astigmatism — caused by unequal curvatures in different parts of the cornea or lens Corrected with cylindrically ground lenses, corneal implants, or laser procedures

22. Copyright © 2010 Pearson Education, Inc. Photoreception http://library.thinkquest.org/28030/physio/perceive.htm Photoreceptors are modified neurons They absorb light and generate chemical or electrical signals 2 cell types that produce visual images rods and cones The rods:cones ratio is 20:1 except in the fovea

23. Copyright © 2010 Pearson Education, Inc. Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina) Inner segment – facing the interior 2 segments are separated by a narrow constriction - cilium The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal Process of bipolar cell Outer fiber Apical microvillus Discs containing visual pigments Melanin granules Discs being phagocytized Pigment cell nucleus Inner fibers Rod cell body Cone cell body Synaptic terminals Rod cell body Nuclei Mitochondria Connecting cilia Basal lamina (border with choroid) Pigmented layer Outer segment Inner segment

24. Copyright © 2010 Pearson Education, Inc. Photoreception: Functional Anatomy of Photoreceptors Photoreception – process by which the eye detects light energy Rods and cones contain visual pigments (photopigments) in the outer segment The visual pigment is different in each of the photoreceptors Each of the 3 types of cones contains a unique visula pigment Embedded in areas of the plasma membrane that form discs change shape as they absorb light This foldings increase surface area that is available for trapping light In rods – the discs are discontinuous while in the cones they are continuous

25. Copyright © 2010 Pearson Education, Inc. Chemistry of Visual Pigments Retinal is a light-absorbing molecule Combines with proteins called opsins to form 4 types of visual pigments Retinal is similar to and is synthesized from vitamin A Vitamin A is stored in the liver and transported by the blood to the cells of the pigmented layer (local reservoir of vitamin A) Retinal has two 3D forms/isomers: 11-cis – a bent structure when connected to opsin all-trans – when struck by light and change the shape of opsin to its active form Transforming fro 11-cis to all-trans is the only light dependent stage Isomerization of retinal initiates electrical impulses in the optic nerve

26. Copyright © 2010 Pearson Education, Inc. Excitation of Rods Light phase When light is absorbed, rhodopsin breaks down 11-cis isomer is converted into the all-trans isomer Retinal and opsin separate (bleaching of the pigment) Dark phase In the dark, rhodopsin forms and accumulates Regenerated from all-trans-retinal Formed from vitamin A

27. Copyright © 2010 Pearson Education, Inc. Excitation of Cones Visual pigments in cones are similar to rods (retinal + opsins) Cones are less sensitive – need more light to be activated There are three types of cones: Blue – wave length 420nm, Green – wave length 530nm, Red – 560nm The absorption spectra overlap giving the hues - activation of more than one type of cone Method of excitation is similar to rods but the cones need higher- intensity (brighter) light because they are less sensitive COLOR PRESENTATION FROM WEBSITE

28. Copyright © 2010 Pearson Education, Inc. Phototransduction The outer segments of the photoreceptor has ligand- regulated sodium gates that bind to cGMP on the intracellular side. In the dark, cGMP opens the gate and permits the inflow of sodium and calcium which reduces the membrane potential from -70mv to -40mv This depolarized current is called the dark current and it results in in continuous NT (glutamate) release by the photoreceptors in the synapse with the bipolar cells Light stops the dark current

29. Copyright © 2010 Pearson Education, Inc. Phototransduction Light energy splits rhodopsin into all-trans retinal, releasing activated opsin The freed opsin activates the G protein transducin Transducin catalyzes activation of phosphodiesterase (PDE) PDE hydrolyzes cGMP to GMP and releases it from sodium channels Without bound cGMP, sodium channels close but potassium channels in the outer segment remain open The photoreceptor membrane hyperpolarizes, and neurotransmitter (glutamate) cannot be released. In this case, hyperpolarization is a signal to the bipolar cell to release NT in the synapse with the ganglion cells

30. Copyright © 2010 Pearson Education, Inc. Signal Transmission in the Retina Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) Light hyperpolarizes photoreceptor cells, causing them to stop releasing the inhibitory neurotransmitter glutamate Bipolar cells (no longer inhibited) are then allowed to depolarize and release neurotransmitter onto ganglion cells Ganglion cells generate APs that are transmitted in the optic nerve

31. Copyright © 2010 Pearson Education, Inc. Adaptation to bright light (going from dark to light) As long as the light is low intensity, relatively little amount of rhodopsin is bleached In high intensity light, rhodopsin is bleached as fast as it is re-formed Going from dark/dim light to light - first we see white light because the sensitivity of the retina is “set” to dim light Both rods and cones are strongly stimulated and large amounts of the pigments are broken, producing a flood of signals that are responsible for the white light under the condition of high intensity of light, rods’ system is “turned off” and the cones system adapts By switching from the rod to the cone system – visual acuity is gained

32. Copyright © 2010 Pearson Education, Inc. Adaptation to dark Initially we do not see nothing Cones stop functioning in low light and the beginning rods are still “turned off” Rhodopsin accumulates in the dark and retinal sensitivity is restored

33. Copyright © 2010 Pearson Education, Inc. Visual Pathways Axons of retinal ganglion cells form the optic nerve Medial fibers of the optic nerve decussate at the optic chiasm Most fibers of the optic tracts continue to the thalamus Other optic tract fibers end in superior colliculi in the midbrain (initiating visual reflexes) Optic radiations travel from the thalamus to the visual cortex

34. Copyright © 2010 Pearson Education, Inc. Depth Perception Both eyes view the same image from slightly different angles Each eye view a different angle of the object The nose and eye socket blocks view of opposite side Depth perception (three-dimensional vision) results from cortical fusion of the 2 visual signals

35. Copyright © 2010 Pearson Education, Inc. Visual processing There are few levels of signal processing Retinal – at the level of ganglion cells Thalamic – sorting Cortical - meaning

36. Copyright © 2010 Pearson Education, Inc. Retinal Processing - rodes Each photoreceptor at the retina monitors specific receptive field For the 130,000,000 photoreceptors there are about 6 million bipolar cells and 1 million ganglion cells. Information from photoreceptors converges Ganglions cells that monitor rods are called M cells (Magnus cells – magnus- great) and they can receive signals from as many as 1000 rods They provide information on general form of object, motion and shadow in dim light This arrangement results in loss of specificity

37. Copyright © 2010 Pearson Education, Inc. Retinal Processing - rodes A ganglion cells responds differently to stimuli that arrive in the center of its receptive field than to stimuli that arrive at the edges. On-center neurons – are excited (depolarize) by light arriving at the center and inhibited by light at the edges of the receptive field Off-center neurons – are inhibited by light arriving at the center and excited by light at the edges of the receptive field On and off receptors are result of sub-type of receptors to glutamate This kind of retinal processing improves the detection of the edges of object within the visual field It is not the only type of ganglion cells pattern detection but the easiest to explain – simple spot of light

38. Copyright © 2010 Pearson Education, Inc. Retinal Processing - cones Cones show very little convergence In the fovea, the ration cones to ganglion cells is 1:1 – these ganglion cells are called P cells (Parvo cells: parvus – small) P cells are active in bright light and provide information about edges, fine detail and color. Bipolar cells receiving signals from cones feed directly into excitatory synapse in the ganglion cell The activation of P cells means that the light arrived in specific location and the result is a more precise visual information supplied by the cones.