BI 232 Laboratory General Senses & Special Senses: Vision

1. BI 232 Laboratory General Senses &Special Senses: Vision

2. What is sensation? • Sensation • Interpretation of a sensory nerve impulses by the brain as an awareness of an external or internal event. • Any stimuli the body is aware of. • Not uniformly distributed throughout body, punctate distribution (two point discrimination) • What are we not aware of….. • Radiation (alpha, beta, gamma, X), ultra high frequency sound waves, UV light • No sensory receptorsfor these stimuli.

3. Sensory Modalities • Different types of sensations: • Touch, pain, temperature, vibration, hearing, vision. • Each type of sensory neuron can respond to only one type of stimuli. • Two classes of sensory modalities: • General senses - touch, pain, temp, pressure, vibration • Somatic are sensations from body walls • Visceral are sensations from internal organs • Special senses • Smell, taste, hearing, vision, and balance

4. General Senses vs. Special Senses • Somatosensory or somatic sense. • Distributed over entire body. • Relatively simple, but widely distributed receptors. • Temperature • Pain • Touch • Pressure • Vibration • Proprioception

5. General Senses vs. Special Senses • Have designated, specialized organs. • Eye • Vision • Mouth and nose (Lab 4) • Chemical senses • Gustation (taste) • Olfaction (smell) • Ear (Lab 4) • Hearing • Equilibrium • Information is carried by special somatic afferent and visceral efferent nerve fibers.

6. Classification by Stimulus Detected • Nociceptors • Pain receptors • Sensation produce in pain. • Thermoreceptors • Temperature receptors • Free nerve endings. • Chemoreceptors • Detect small changes in the concentration of specific chemicals or compounds. • Taste, smell and changes in body fluid chemistry. • Mechanoreceptors • Sensitive to stimuli that distort their cell membranes. • Tactile receptors - touch • Baroreceptors – organ pressure • Proprioceptors - position

7. Classes of Mechanoreceptors • Tactile receptors • Sensations of touch, pressure and vibration. • Free nerve endings • Root hair plexus • Tactile discs (Merkel’s discs) • Tactile corpuscles (Meissner’s corpuscles) • Lamellated corpuscles (Pacinian corpuscles) • Baroreceptors • Detect pressure changes in the walls of blood vessels and portions of digestive, reproductive and urinary tracts. • Proprioceptors • Monitor the positions of joints and muscles. • Muscle spindles and Golgi tendon organs (GTO)

8. Lamellated corpuscle (Pacinian corpuscle) • Lie deep in the dermis. • Respond only when deep pressure is first applied. • Monitor high frequency vibrations.

9. Tactile corpuscle (Meissner’s corpuscle) • Lie in the dermal papillae of the skin. • Respond to light touch.

10. Student Activities • Perform the following activities described in the Survival Guide: • Proprioceptor histology (compound microscope and slides) – tactile and lamellated corpuscles. • Plotting the density and location of touch and temperature receptors. • Two-point discrimination. • Testing tactile localization. • Adaptation to temperature receptors.

11. Special Senses: Vision

12. External Anatomy of the Eye • Eyebrows • Provide facial expression, protection from glare and perspiration. • Eyelids (palpebrae) • Block foreign objects, help with sleep, blink to moisten.

13. External Anatomy of the Eye • Conjunctiva • Transparent mucous membrane lines eyelids and covers anterior surface of eyeball except cornea. • Richly innervated and vascular (heals quickly). (Conjunctivitis)

14. Lacrimal Apparatus • Lacrimal gland • Site of tear production and secretion via lacrimal ducts. • Tears • Flow across eyeball. • Wash away foreign particles. • Help with diffusion of O₂ and CO₂. • Contain bactericidal lysozyme. • Drainage of tears conveyed into nasal cavity via: • Lacrimal puncta • Lacrimal canals • Lacrimal sac • Nasolacrimal duct

15. 3 Layers (Tunics) of Eye • Fibrous Tunic - outermost • Vascular Tunic • Neural Tunic

16. Layers of the Eyeball: Fibrous tunic • Sclera • “White” of eye. • Dense irregular connective tissue layer (collagen and fibroblasts). • Provides shape and support. • Posteriorly pierced by Optic Nerve (CNII). • Cornea • Helps focus light(refraction). • Astigmatism • 3 layers • Non-keratinized stratified squamous. • Collagen fibers and fibroblasts. • Simple squamous epithelium. • Nourished by tears and aqueous humor

17. Layers of the Eyeball: Vascular tunic • Description ofiris and pupil: • Colored portion of eye is iris. • Shape of flat donut suspended between cornea and lens. • Attaches to ciliary body. • Hole in center is pupil • Function is to regulate amount of light entering eye. • Autonomic reflexes • Circular muscle fibers contract in bright light to shrink pupil. (parasympathetic) • Radial muscle fibers contract in dim light to enlarge pupil. (sympathetic)

18. Layers of the Eyeball: Vascular tunic • Choroid • Pigmented epithelial cells (melanocytes) and blood vessels. • Provides nutrients to retina. • Black pigment in melanocytes absorb scattered light. • Ciliary body • Ciliary processes • Folds on ciliary body. • Secrete aqueous humor. • Suspensory ligaments • Attach lens to ciliary process. • Ciliary muscle • Controls tension on ligaments and lens.

19. Layers of the Eyeball: Vascular tunic • Lens: • Avascular • Crystalline proteins arranged like layers in onion. • Clear capsule and perfectly transparent. • Lens held in place by suspensory ligaments. • Focuses light on fovea.

20. Layers of the Eyeball: Neural tunic • AKA: inner or nervous tunic, posterior 3/4 of eyeball. • Retina • Made up of an outer layer of pigmented epithelium (pigmented retina). • Inner layer of nervous tissue (sensory retina). • Attached only at optic disc where optic nerve begins and ora serrata (anterior margin). • Pressed against rear of eyeball by vitreous body. • Detached retina • Trauma • Fluid between layers. • Distortion or blindness.

21. Retinal Detachment

23. Layers of the Eyeball: Neural tunic • Sensory retina made up of three layers of neurons: • Photoreceptor cells sensitive to light rays. • Rods (rod shaped) • Shades of gray in dim light. • Most numerous. • Cones (cone shaped) • Sharp, color vision (CC!) • Fovea centralis • Densely packed region of cones. • Sharpest resolution or acuity. • Bipolar cells • Ganglionic cells

24. Layers of the Eyeball: Neural tunic • Optic nerve • Second cranial nerve (CNII). • Extends from eyeball to brain. • Optic disc • Optic nerve exiting back of eyeball. • Creates blind spot (no presence of photoreceptors). • Central retinal artery and vein • Fan out to supply nourishment to retina. • Visible for inspection. • Macula lutea (yellow spot) • Just lateral to center of retina, which constitutes region of maximum visual acuity. • Fovea centralis • Located in center of macula where visual acuity is highest. • High concentration of retinal cones.

26. Macular Degeneration - AMD

29. 2 Cavities of the Interior of Eyeball • Anterior Cavity anterior to lens. • Filled with aqueous humor. • Produced by ciliary body. • Continually drained. • Replaced every 90 minutes. • 2 Chambers of Anterior Cavity • Anterior chamber between cornea and iris. • Posterior chamber between iris and lens. • Posterior Cavity posterior to lens. • Filled with vitreous body/humor (jellylike). • Formed once during embryonic life. • Floaters are debris in vitreous of older individuals. • Glaucoma • Increased intraocular pressure that could produce blindness • Problem with drainage of aqueous humor

30. Extrinsic Eyes Muscles • 6 muscles inserting on external surface of eyeball. • 4 rectus muscles move eye up, down, left and right. • 2 oblique - Superior and inferior oblique more complicated. • All innervated by Oculomotor III except • Trochlear IV – superior oblique • Abducens VI – lateral rectus

31. Extrinsic Eye Muscles • Superior rectus • Moves eye superiorly and medial. • Innervation - Oculomotor III • Medial rectus • Moves eye toward medially. • Innervation – Oculomotor III • Inferior rectus • Moves eye inferiorly (secondarily moves eye laterally). • Innervation – Oculomotor III

32. Extrinsic Eye Muscles • Inferior oblique • Moves eye laterally and superior. • Innervation – Oculomotor III • Superior oblique • Rotates eye medially and inferior. • Trochlea – ligament sling. • Innervation - Trochlear IV • Lateral rectus • Moves eye away from the nose. • Innervation - Abducens VI

34. 3 Intrinsic Muscles of the Eye • Smooth muscles located within eye. • Iris • Regulates size of pupil • Muscles of the iris • Constrictor pupillae (circular) • Innervated by parasympathetic fibers. • Dilator pupillae (radial) • Innervated by sympathetic fibers (ANGRY) • Response varies with different levels of light. • Ciliary muscle • Controls shape of lens.

35. Intrinsic Muscles of the Eye: Iris • Pupils constrict (parasympathetic – sphincter - circular). • Close vision and bright light. • Pupils dilate (sympathetic - radial). • Distant vision and dim light.

36. Student Activities • Perform the following activities described in the Survival Guide: • Histology of the retina (compound microscope and slides) • Demonstrating the blind spot • Afterimages • Testing for astigmatism • Testing for color blindness • Testing visual acuity • Distribution of rods and cones • Eye dissection

37. Activity: Histology of the Retina

38. Activity: Demonstrating the Blind Spot • Optic nerve • Second cranial nerve (CNII) • Extends from eyeball to brain • Optic disc • Optic nerve exiting back of eyeball. • Creates blind spot. • Due to lack of presence of photoreceptors.

39. Afterimages • Negative afterimages • Photoreceptors adapt to overstimulation and lose sensitivity. • Positive afterimages • Appear the same color as the original image • Generally very brief, lasting less than half a second. • In most cases, only very bright stimuli (e.g. sun) produce positive afterimages • Stimulus eliciting positive image usually trigger a negative afterimage quickly via the adaptation process. • Note: cause of positive afterimages is not well understood, but it is suggested that with persisting activity in the brain retinal photoreceptor cells continue to send neural impulses to occipital lobe.

40. Activity: Afterimages • Rhodopsin • Photosensitive pigment of the rods. • Composed of a light-sensitive chemical, retinal, and protein, opsin. • When light strikes retina, rhodopsin splits into its two component parts and becomes pale (bleached). • The time for separation and reassembly of photopigments can be tested by staring at a contrasting image on a car.

41. Color Vision • Primates have well developed color vision. • Nocturnal vertebrates have only rods. • Cones are named for absorption peaks of photopsins. • Blue cones peak sensitivity at 420 nm. • Green cones peak at 531 nm. • Red cones peak at 558 nm (orange-yellow). • Perception of color is based on mixture of nerve signals. • Color blindness is hereditary lack of one photopsin.

42. Activity: Testing for Color Blindness • Color blindness • Inability to distinguish between certain colors. • Absence of certain cone photopigments. • Red-green is common (lack either red or green cones). • Incapable of distinguishing red from green. • Sex-linked recessive (8% of males). • Use Ishihara color plates for test. • Night blindness (nyctalopia) • Difficulty seeing in low light. • Inability to make normal amount of rhodopsin. • Possibly due to deficiency of vitamin A.

43. Refraction • Bending of light rays • Light passing through substance with different refractive index at any angle other than 90 degrees. • Cornea refracts light more than lens does. • Lens fine-tunes the image as shift focus between near and distant objects. • Light passes through the following structures before it strikes retina: • Cornea • Aqueous humor • Lens • Vitreous humor

44. Focal Point and Focal Distance • Focal point • Specific point of intersection on the retina. • Focal distance • Distance between the center of the lens and its focal point. • Determined by two factors: • Distance from the object to the lens • Shape of the lens

45. Focal Distance • Distance from the object to the lens • Closer an object is, the greater the focal distance. • Shape of the lens • Rounder the lens, the more refraction occurs resulting in shorter focal distance.

46. Accommodation • Alteration in the curvature of lens of the eye to focus an image on the retina. • Near objects – lens becomes rounder. • Distance objects – lens becomes flatter.

47. Accommodation: Normal and Dysfunctional • Normal vision • Emmetropia – normal vision • Image will be focused retina’s surface. • Visual dysfunction • Myopia – nearsighted (see near) • Eyeball is too deep or curvature of lens is too great. • Focal point is in front of retina, so distance objects are blurry. • Correction – diverging lens. • Hyperopia – farsighted (see far) • Eyeball is too shallow or curvature of lens is too flat. • Focal point is behind of retina, so near objects are blurry. • Correction – converging lens

48. Accommodation: Normal and Dysfunctional

49. Presbyopia • Reduction in ability to see near small print, cell phone • Age related 40+ • Decrease in flexibility of lens • Magnifiers (converging) or bifocals (both converging and diverging)

50. Activity: Testing for Astigmatism • Astigmatism • Degree of curvature in cornea or lens varies from one axis to another. • Corneal surface wavy. • Causes light to focus on more than one area retina. • Parts of image out of focus.