The Special Senses

1. The Special Senses • Smell, taste, vision, hearing and equilibrium • Housed in complex sensory organs

2. Tunics (Layers) of Eyeball • Fibrous Tunic(outer layer, cornea and sclera) • Vascular Tunic (middle layer) • Nervous Tunic(inner layer)

3. Vascular Tunic -- Iris & Pupil • Colored portion of eye • Contains melanin • Shape of flat donut suspended between cornea & lens • 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 • radial muscle fibers contract in dim light to enlarge pupil

4. Vascular Tunic -- Muscles of the Iris

5. Vascular Tunic -- Description of lens • Avascular • Crystallin proteins arranged like layers in onion • Clear capsule & perfectly transparent • Lens held in place by suspensory ligaments • Focuses light on fovea (back surface of eye)

6. Vascular Tunic -- Suspensory ligament • Suspensory ligaments attach lens to ciliary process • Ciliary muscle controls tension on ligaments & lens

7. Nervous Tunic -- Retina • Posterior 3/4 of eyeball • Optic disc • optic nerve exiting back of eyeball • Central retina BV • fan out to supply nourishment to retina • visible for inspection • hypertension & diabetes • Detached retina • trauma (boxing) • fluid between layers • distortion or blindness View with Ophthalmoscope

8. Layers of Retina • Pigmented epithelium • nonvisual portion • absorbs stray light & helps keep image clear • 3 layers of neurons (outgrowth of brain) • photoreceptor layer • bipolar neuron layer • ganglion neuron layer

9. Rods & Cones--Photoreceptors • Rods----rod shaped • shades of gray in dim light • 120 million rod cells • discriminates shapes & movements • distributed along periphery • Cones----cone shaped • sharp, color vision • 6 million • fovea of macula lutea • densely packed region • at exact visual axis of eye • 2nd cells do not cover cones • sharpest resolution or acuity

10. Pathway of Nerve Signal in Retina • Light penetrates retina • Rods & cones transduce light into action potentials • Rods & cones excite bipolar cells • Bipolars excite ganglion cells • Axons of ganglion cells form optic nerve leaving the eyeball (blind spot) • To thalamus & then the primary visual cortex

11. 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 • anterior chamber between cornea and iris • posterior chamber between iris and lens • Posterior cavity (posterior to lens) • filled with vitreous body (jellylike) • formed once during embryonic life • floaters are debris in vitreous of older individuals

12. Aqueous Humor • Continuously produced by ciliary body • Flows from posterior chamberinto anterior through the pupil • Scleral venous sinus • canal of Schlemm • opening in white of eyeat junction of cornea & sclera • drainage of aqueous humor from eye to bloodstream • Glaucoma • increased intraocular pressure that could produce blindness • problem with drainage of aqueous humor

13. Major Processes of Image Formation • Refraction of light • by cornea & lens • light rays must fall upon the retina • Accommodation of the lens • changing shape of lens so that light is focused • Constriction of the pupil • less light enters the eye

14. Definition of Refraction • Bending of light as it passes from one substance (air) into a 2nd substance with a different density (cornea) • In the eye, light is refracted by the cornea and the lens

15. Refraction by the Cornea & Lens • Image focused on retina is inverted & reversed from left to right • Brain learns to work with that information • 75% of Refraction is done by cornea -- rest is done by the lens • Light rays from > 6 m are nearly parallel and only need to be bent enough to focus on retina • Light rays from < 6 m are more divergent & need more refraction • extra process needed to get additional bending of light is called accommodation

16. Accommodation & the Lens • Convex lens refract light rays towards each other • Lens of eye is convex on both surfaces • View a distant object • lens is nearly flat by pulling of suspensory ligaments • View a close object • ciliary muscle is contracted & decreases the pull of the suspensory ligaments on the lens • elastic lens thickens as the tension is removed from it • increase in curvature of lens is called accommodation

17. Near Point of Vision and Presbyopia • Near point is the closest distance from the eye an object can be & still be in clear focus • 4 inches in a young adult • 8 inches in a 40 year old • lens has become less elastic • 31 inches in a 60 to 80 year old • Reading glasses may be needed by age 40 • presbyopia • glasses replace refraction previously provided by increased curvature of the relaxed, youthful lens

18. Correction for Refraction Problems • Emmetropic eye (normal) • can refract light from 20 ft away • Myopia (nearsighted) • eyeball is too long from front to back • glasses concave • Hypermetropic (farsighted) • eyeball is too short • glasses convex (coke-bottle) • Astigmatism • corneal surface wavy • parts of image out of focus

19. Convergence of the Eyes • Binocular vision in humans has both eyes looking at the same object • Allows for the perception of depth • As you look at an object close to your face, both eyeballs must turn inward. • convergence • required so that light rays from the object will strike both retinas at the same relative point • extrinsic eye muscles must coordinate this action

20. Photoreceptors • Named for shape of outer segment • Transduction of light energy into a receptor potential in outer segment • Photopigment is integral membrane protein of outer segment membrane • photopigment membrane folded into “discs” & replaced at a very rapid rate • Photopigments = opsin (protein) + retinal (derivative of vitamin A) • rods contain rhodopsin • cone photopigments contain 3 different opsin proteins permitting the absorption of 3 different wavelengths (colors) of light

21. Color Blindness & Night Blindness • Color blindness • inability to distinguish between certain colors • absence of certain cone photopigments • red-green color blind person can not tell red from green • Night blindness • difficulty seeing in low light • inability to make normal amount of rhodopsin • possibly due to deficiency of vitamin A

22. Photopigments • Isomerization • light cause cis-retinal to straighten & become trans-retinal shape • lead to production of receptor potential • Bleaching • enzymes separate the trans-retinal from the opsin • colorless final products • Regeneration • in darkness, an enzyme converts trans-retinal back to cis-retinal (resynthesis of a photopigment)

23. Formation of Receptor Potentials • In darkness • Na+ channels are held open and photoreceptor is always partially depolarized (-30mV) • continuous release of inhibitory neurotransmitter onto bipolar cells • In light • enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70mV) • release of inhibitory neurotransmitter is stopped • bipolar cells become excited and a nerve impulse will travel towards the brain

24. Release of Neurotransmitters

25. synapse in thalamus & visual cortex Brain Pathways of Vision

26. Processing of Image Data in the Brain • Visual information in optic nerve travels to • occipital lobe for vision • midbrain for controlling pupil size & coordination of head and eye movements • hypothalamus to establish sleep patterns based upon circadian rhythms of light and darkness

27. Visual fields • Left occipital lobe receives visual images from right side of an object through impulses from nasal 1/2 of the right eye and temporal 1/2 of the left eye • Left occipital lobe sees right 1/2 of the world • Fibers from nasal 1/2 of each retina cross in optic chiasm

28. Anatomy of the Ear Region

29. External Ear • Function = collect sounds • Structures • auricle or pinna • elastic cartilage covered with skin • external auditory canal • curved 1” tube of cartilage & bone leading into temporal bone • ceruminous glands produce cerumen = ear wax • tympanic membrane or eardrum • epidermis, collagen & elastic fibers, simple cuboidal epith. • Perforated eardrum (hole is present) • at time of injury (pain, ringing, hearing loss, dizziness) • caused by explosion, scuba diving, or ear infection

30. Middle Ear Cavity

31. Middle Ear Cavity • Air-filled cavity in the temporal bone • Separated from external ear by eardrum and from internal ear by oval & round window • 3 ear ossicles connected by synovial joints • malleus attached to eardrum, incus, stapes attached to membrane of oval window • Auditory tube leads to nasopharynx • helps to equalize pressure on both sides of eardrum

32. Inner Ear---Bony Labyrinth Vestibule canals • Bony labyrinth = set of tubelike cavities in temporal bone • semicircular canals, vestibule & cochlea lined with periosteum & filled with perilymph • surrounds & protects Membranous Labyrinth ampulla

33. Inner Ear---Membranous Labyrinth • Membranous labyrinth = set of membranous tubes containing sensory receptors for hearing & balance and filled with endolymph • utricle, saccule, ampulla, 3 semicircular ducts & cochlea

34. Cranial nerves of the Ear Region • Vestibulocochlear nerve = CN VIII

35. Cochlear Anatomy • 3 fluid filled channels found within the cochlea • scala vestibuli, scala tympani and cochlear duct • Vibration of the stapes upon the oval window sends vibrations into the fluid of the scala vestibuli

36. Tubular Structures of the Cochlea • Stapes pushes on fluid of scala vestibuli at oval window • At helicotrema, vibration moves into scala tympani • Fluid vibration dissipated at round window which bulges • The central structure is vibrated (cochlear duct)

37. Section thru one turn of Cochlea • Partitions that separate the channels are Y shaped • vestibular membrane above & basilar membrane below form the central fluid filled chamber (cochlear duct) • Fluid vibrations affect hair cells in cochlear duct

38. Anatomy of the Organ of Corti • 16,000 hair cells have 30-100 stereocilia (microvilli) • Microvilli make contact with tectorial membrane (gelatinous membrane that overlaps the spiral organ of Corti) • Basal sides of inner hair cells synapse with 1st order sensory neurons whose cell body is in spiral ganglion

39. Physiology of Hearing • Auricle collects sound waves • Eardrum vibrates • slow vibration in response to low-pitched sounds • rapid vibration in response to high-pitched sounds • Ossicles vibrate since malleus attached to eardrum • Stapes pushes on oval window producing fluid pressure waves in scala vestibuli & tympani • oval window vibration 20X more vigorous than eardrum • Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane • Microvilli are bent producing receptor potentials

40. Overview of Physiology of Hearing

41. Hair Cell Physiology • Hair cells convert mechanical deformation into electrical signals • As microvilli are bent, mechanically-gated channels in the membrane let in K+ ions • This depolarization spreads & causes voltage-gated Ca+2 channels at the base of the cell to open • Triggering the release of neurotransmitter onto the first order neuron • more neurotransmitter means more nerve impulses

42. Cochlear Implants • If deafness is due to destruction of hair cells • Microphone, microprocessor & electrodes translate sounds into electric stimulation of the vestibulocochlear nerve • artificially induced nerve signals follow normal pathways to brain • Provides only a crude representation of sounds

43. Physiology of Equilibrium (Balance) • Static equilibrium • maintain the position of the body (head) relative to the force of gravity • macula receptors within saccule & utricle • Dynamic equilibrium • maintain body position (head) during sudden movement of any type--rotation, deceleration or acceleration • crista receptors within ampulla of semicircular ducts

44. Vestibular Apparatus • Notice: semicircular ducts with ampulla, utricle & saccule

45. Otolithic Organs: Saccule & Utricle • Thickened regions called macula within the saccule & utricle of the vestibular apparatus • Cell types in the macula region • hair cells with stereocilia (microvilli) & one cilia (kinocilium) • supporting cells that secrete gelatinous layer • Gelatinous otolithic membrane contains calcium carbonate crystals called otoliths that move when you tip your head

46. Detection of Position of Head • Movement of stereocilia or kinocilium results in the release of neurotransmitter onto the vestibular branches of the vestibulocochler nerve

47. Crista: Ampulla of Semicircular Ducts • Small elevation within each of three semicircular ducts • anterior, posterior & horizontal ducts detect different movements • Hair cells covered with cupula of gelatinous material • When you move, fluid in canal bends cupula stimulating hair cells that release neurotransmitter

48. Detection of Rotational Movement • When head moves, the attached semicircular ducts and hair cells move with it • endolymph fluid does not and bends the cupula and enclosed hair cells • Nerve signals to the brain are generated indicating which direction the head has been rotated