Sensory Physiology

1. Sensory Physiology Vision, Hearing, and Orientation

2. Light Refraction • Light is refracted whenever it passes between material of different densities • Light passing through the eye is refracted by… • cornea • aqueous humor • lens • vitreous humor • Focus light on fovea centralis Fig 3.17

3. Ciliary Muscles and Lens • Lens • solid but pliable transparent body • used to focus light on the retina • Ciliary Muscle • ring-shaped smooth muscle • linked to lens by suspensory ligaments • adjusts shape of lens to focus light Lecture Text Fig. 10.32

4. Accommodation • Changing lens shape to focus light from objects at different distances • Far objects • light from narrow range of angles • ciliary muscles relax, lens stretched • less convex, less bending of light • Near objects • light from wide range of angles • ciliary muscles contract, lens recoils • more convex, more bending of light Lecture Text Fig. 10.33

5. Refractive Power • Strength by which a lens bends light • In eye, only lens has variable refractive power •  lens convexedness,  refractive power • Focus light from objects different distances on the fovea • Refractive Power (diopters) = 1 / focal length (m) • focal length = 0.25 m • RP = 4 diopters • focal length = 0.50 m • RP = 2 diopters

6. Refractive Power of Eye • Distance from lens to fovea ~1.5 cm • RP = 67 diopters for light from distant objects • RP can be increased to 79 diopters by thickening lens to observe close objects • Focus light on retina • Enhance visual acuityfor objects at different distances • Ability to discriminate between points in the visual field

7. Refractive Power and Visual Disorders • myopia (nearsightedness) • distant object brought into focus in front of the retina • Elongated eyeball • Abnormally high convexedness to cornea or lens • Too much refractive power • corrected w/ concave lenses Lecture Text Fig. 10.34

8. Refractive Power and Visual Disorders • hyperopia (farsightedness) • close object brought into focus behind of the retina • Shortened eyeball • Abnormally low convexedness to cornea or lens • too little refractive power • corrected w/ convex lenses Lecture Text Fig. 10.34

9. Refractive Power and Visual Disorders • Astigmatism • Oblong shape to cornea or lens (not perfect hemisphere) • refraction of light in horizontal plane ≠ that in the vertical plane • Corrective lens prescriptions • +3 (diopters) = convex lens for hyperopia • -2 (diopters) = concave lens for myopia • astigmatisms include strength of lens and axis of defect • e.g. +2 axis 90 = horizontal plane Lecture Text Fig. 10.34

10. Age-related Changes in Accommodation • Throughout, continuous stretching of lens • Lens loses elasticity with age • Remains in “stretched” state • Loses ability to increase refractive power • Presbyopia (aka presbyopta) • Far-sightedness associated with age • Analyzed with near point of vision test • 8 cm at age 10, 100 cm at age 70

11. Experiments:Visual Accommodation • Snellen Eye Chart (myopia) • 20’ from chart • Test one eye at a time • Read smallest font possible • Determine visual acuity based on distance associated with each font size • Astigmatism Chart (astigmatism) • Test one eye at a time • If astigmatism present, one set of lines (axis of astigmatism) will be sharper and darker than the others

12. Experiments:Visual Accomodation • Near Point of Vision (Presbyopia) • Test one eye at a time • Place meter stick on bridge of nose • Focus on pencil tip • Draw tip along meter stick towards eye • Point at which tip just begins to become fuzzy = near point of vision.

13. Retina • Inner layer of the eye • Contains photoreceptors • Rods – light intensity (scotopic) • Cones – color, high acuity (photopic) • Fovea centralis • point where light is focused • high density of cones • High acuity • Optic disk • where optic nerve joins the eye • no photoreceptors - “blind spot” Figs 3.17 and 3.19

14. Blind Spot Experiment • Cover right eye • Hold paper in right hand at arm length, with + sign sticking out to the right • Looking directly at black spot, move paper toward eye • Note that at one point the + sign disappears from peripheral vision

15. Stereoptic Vision and Depth Perception • Visual fields of eyes overlap • Viewing of object in both visual fields allows depth perception • Near objects – lateral projection on retinas • Far objects – projection at center of retinas

16. 3-D Vision • One person holds test tube at arms length • Other holds pencil in arm upright • Try to swing down lower arm to place pencil directly in test tube • Repeat, with one eye closed

17. Sound Conduction and Deafness • Sound can be perceived from vibrations of the skull as well as conducted through the ear • Vibrations to skull can be used to diagnose basic type of deafness • Conductive deafness • damage to conduction system (tympanic membrane, ear bones, etc.) • Can hear skull vibrations • Sensorineural deafness • damage to sensors or nerves (cochlea, auditory nerve, etc.) • Cannot hear skull vibrations

18. Tests • Rhinne Test • Place tuning fork on mastoid process • Webers test • Place tuning fork on midsagittal line • Binaural sound • Follow direction of sound with eyes closed

19. Orientation, Balance and Coordination • Orientation and balance rely on numerous inputs • Vestibular apparatus – detects movement and orientation of head • Touch, pressure and proprioception • Indicate mechanical forces acting on rest of body • Vision

20. Orientation, Balance and Coordination • Experiment – time how long you can stand on one foot without losing your balance while… • Keeping your eyes open • Keeping your eyes closed without touching furniture, counters, etc. • Keeping your eyes closed and touching one finger to a countertop