Special sensory receptors Distinct, localized receptor cells in head Vision Taste Smell Hearing Equilibrium

Transcription

1 Special sensory receptors Distinct, localized receptor cells in head Vision Taste Smell Hearing Equilibrium 1

2 Special sensory receptors Distinct, localized receptor cells in head Vision Taste Smell Hearing Equilibrium 2

3 70% of body's sensory receptors in eye Visual processing by ~ half cerebral cortex Most of eye protected by cushion of fat and bony orbit 3

4 Protect the eye and aid eye function Eyebrows Eyelids (palpebrae) Conjunctiva Lacrimal apparatus Extrinsic eye muscles 4

5 Overlie supraorbital margins Function Shade eye from sunlight Prevent perspiration from reaching eye 5

6 Protect eye anteriorly Separated at palpebral fissure Meet at medial and lateral commissures Lacrimal caruncle At medial commissure Contains oil and sweat glands Tarsal plates supporting connective tissue 6

7 Eyelashes Nerve endings of follicles initiate reflex blinking Lubricating glands associated with eyelids Tarsal (Meibomian) glands Modified sebaceous glands Oily secretion lubricates lid and eye Ciliary glands between hair follicles Modified sweat glands 7

8 Levator palpebrae superioris Gives upper eyelid mobility Blink reflexively every 3-7 seconds Protection Spread secretions to moisten eye 8

9 Transparent mucous membrane Produces a lubricating mucous secretion Palpebral conjunctiva lines eyelids Bulbar conjunctiva covers white of eyes Conjunctival sac between palpebral and bulbar conjunctiva Where contact lens rests 9

10 Lacrimal gland and ducts that drain into nasal cavity Lacrimal gland in orbit above lateral end of eye Lacrimal secretion (tears) Dilute saline solution containing mucus, antibodies, and lysozyme Blinking spreads tears toward 10

11 medial commissure Tears enter paired lacrimal canaliculi via lacrimal puncta Then drain into lacrimal sac and nasolacrimal duct 10

12 Six straplike extrinsic eye muscles Originate from bony orbit; insert on eyeball Enable eye to follow moving objects; maintain shape of eyeball; hold in orbit Four rectus muscles originate from common tendinous ring; names indicate movements 11

13 Superior, inferior, lateral, medial rectus muscles Two oblique muscles move eye in vertical plane and rotate eyeball Superior and inferior oblique muscles 11

14 Six straplike extrinsic eye muscles Originate from bony orbit; insert on eyeball Enable eye to follow moving objects; maintain shape of eyeball; hold in orbit Four rectus muscles originate from common tendinous ring; names indicate movements 12

15 Superior, inferior, lateral, medial rectus muscles Two oblique muscles move eye in vertical plane and rotate eyeball Superior and inferior oblique muscles 12

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17 Wall of eyeball contains three layers Fibrous Vascular Inner Internal cavity filled with fluids called humors Lens separates internal cavity into anterior and posterior segments (cavities) 14

18 Outermost layer; dense avascular connective tissue Two regions: sclera and cornea 1. Sclera Opaque posterior region Protects, shapes eyeball; anchors extrinsic eye muscles Continuous with dura mater of brain posteriorly 15

19 2. Cornea Transparent anterior 1/6 of fibrous layer Bends light as it enters eye Sodium pumps of corneal endothelium on inner face help maintain clarity of cornea Numerous pain receptors contribute to blinking and tearing reflexes 16

20 Middle pigmented layer Three regions: choroid, ciliary body, and iris 1. Choroid region Posterior portion of uvea Supplies blood to all layers of eyeball Brown pigment absorbs light to prevent light scattering and visual confusion 17

21 3. Iris Colored part of eye 2. Ciliary body Ring of tissue surrounding lens Smooth muscle bundles (ciliary muscles) control lens shape Capillaries of ciliary processes secrete fluid Ciliary zonule (suspensory ligament) holds lens in position 18

22 3. Iris Colored part of eye Pupil central opening that regulates amount of light entering eye Close vision and bright light sphincter pupillae (circular muscles) contract; pupils constrict Distant vision and dim light dilator pupillae (radial muscles) contract; pupils dilate sympathetic fibers 19

23 Changes in emotional state pupils dilate when subject matter is appealing or requires problem-solving skills 19

24 Originates as outpocketing of brain Delicate two-layered membrane Outer Pigmented layer Single-cell-thick lining Absorbs light and prevents its scattering Phagocytize photoreceptor cell fragments Stores vitamin A Optic disc (blind spot) Site where optic nerve leaves eye Lacks photoreceptors Quarter-billion photoreceptors of two types Rods Cones 20

25 Inner Neural layer Transparent Composed of three main types of neurons Photoreceptors, bipolar cells, ganglion cells Signals spread from photoreceptors to bipolar cells to ganglion cells Ganglion cell axons exit eye as optic nerve 21

26 Rods Cones Dim light, peripheral vision receptors More numerous, more sensitive to light than cones No color vision or sharp images Numbers greatest at periphery Vision receptors for bright light High-resolution color vision Macula lutea exactly at posterior pole Mostly cones Fovea centralis Tiny pit in center of macula with all cones; best vision 22

27 Two sources of blood supply Choroid supplies outer third (photoreceptors) Central artery and vein of retina supply inner two-thirds Enter/exit eye in center of optic nerve Vessels visible in living person 23

28 The lens and ciliary zonule separate eye into two segments Anterior and posterior segments Posterior segment contains vitreous humor that Transmits light Supports posterior surface of lens Holds neural layer of retina firmly against pigmented layer Contributes to intraocular pressure 24

29 Forms in embryo; lasts lifetime Anterior segment composed of two chambers Anterior chamber between cornea and iris Posterior chamber between iris and lens 24

30 Anterior segment contains aqueous humor Plasma like fluid continuously formed by capillaries of ciliary processes Drains via scleral venous sinus (canal of Schlemm) at scleracornea junction Supplies nutrients and oxygen 25

31 mainly to lens and cornea but also to retina, and removes wastes Glaucoma: blocked drainage of aqueous humor increases pressure and causes compression of retina and optic nerve blindness 25

32 Biconvex, transparent, flexible, and avascular Changes shape to precisely focus light on retina Two regions Lens epithelium anteriorly; Lens fibers form bulk of lens Lens fibers filled with transparent 26

33 protein crystallin Lens becomes more dense, convex, less elastic with age cataracts (clouding of lens) consequence of aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight 26

34 Eyes respond to visible light Light Small portion of electromagnetic spectrum Wavelengths of nm Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds Color of light objects reflect determines color eye perceives 27

35 Refraction Bending of light rays Due to change in speed when light passes from one transparent medium to another Occurs when light meets surface of different medium at an oblique angle Curved lens can refract light 28

36 Light passing through convex lens (as in eye) is bent so that rays converge at focal point Image formed at focal point is upside-down and reversed right to left Concave lenses diverge light Prevent light from focusing 29

37 Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, entire neural layer of retina, photoreceptors Light refracted three times along pathway Entering cornea Entering lens 30

38 Leaving lens Majority of refractory power in cornea Change in lens curvature allows for fine focusing 30

39 Eyes best adapted for distant vision Far point of vision Distance beyond which no change in lens shape needed for focusing 20 feet for emmetropic (normal) eye Cornea and lens focus light precisely on retina Ciliary muscles relaxed Lens stretched flat by tension in ciliary zonule 31

40 Light from close objects (<6 m) diverges as approaches eye Requires eye to make active adjustments using three simultaneous processes Accommodation of lenses Constriction of pupils Convergence of eyeballs 32

41 Accommodation Changing lens shape to increase refraction Near point of vision Closest point on which the eye can focus Presbyopia loss of accommodation over age 50 Constriction 33

42 Accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye Convergence Medial rotation of eyeballs toward object being viewed 33

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44 Myopia (nearsightedness) Focal point in front of retina, e.g., eyeball too long Corrected with a concave lens Hyperopia (farsightedness) Focal point behind retina, e.g., eyeball too short Corrected with a convex lens 35

45 Astigmatism Unequal curvatures in different parts of cornea or lens Corrected with cylindrically ground lenses or laser procedures 35

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47 Rods and cones Modified neurons Receptive regions called outer segments Contain visual pigments (photopigments) Molecules change shape as absorb light Inner segment of each joins cell body 37

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49 Vulnerable to damage Degenerate if retina detached Destroyed by intense light Outer segment renewed every 24 hours Tips fragment off and are phagocytized 39

50 Vulnerable to damage Degenerate if retina detached Destroyed by intense light Outer segment renewed every 24 hours Tips fragment off and are phagocytized 40

51 rods Functional characteristics Very sensitive to light Best suited for night vision and peripheral vision Contain single pigment Perceived input in gray tones only Pathways converge, causing fuzzy, indistinct images 41

52 Cones Functional characteristics Need bright light for activation (have low sensitivity) React more quickly Have one of three pigments for colored view Nonconverging pathways result in detailed, high-resolution vision Color blindness lack of one or more cone pigments 42

53 rods Functional characteristics Very sensitive to light Best suited for night vision and peripheral vision Contain single pigment Perceived input in gray tones only Pathways converge, causing fuzzy, indistinct images 43

54 Cones Functional characteristics Need bright light for activation (have low sensitivity) React more quickly Have one of three pigments for colored view Nonconverging pathways result in detailed, high-resolution vision Color blindness lack of one or more cone pigments 44

55 Retinal Light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Synthesized from vitamin A Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form) Bent form straight form when pigment absorbs light Conversion of bent to straight initiates reactions electrical impulses along optic nerve 45

56 Deep purple pigment of rods rhodopsin 11-cis-retinal + opsin rhodopsin Three steps of rhodopsin formation and breakdown Pigment synthesis Pigment bleaching Pigment regeneration 46

57 Pigment synthesis Rhodopsin forms and accumulates in dark Pigment bleaching When rhodopsin absorbs light, retinal changes to all-trans isomer Retinal and opsin separate (rhodopsin breakdown) Pigment regeneration All-trans retinal converted to 11-cis isomer Rhodopsin regenerated in outer segments 47

58 Light-activated rhodopsin activates G protein transducin Transducin activates PDE, which breaks down cyclic GMP (cgmp) In dark, cgmp holds channels of outer segment open Na + and Ca 2+ depolarize cell In light cgmp breaks down, channels close, cell hyperpolarizes Hyperpolarization is signal! 48

59 Similar as process in rods Cones far less sensitive to light Takes higher-intensity light to activate cones 49

60 Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) 50

61 When light hyperpolarizes photoreceptor cells Stop releasing inhibitory neurotransmitter glutamate Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells Ganglion cells generate APs transmitted in optic nerve to brain 51

62 Move from darkness into bright light Both rods and cones strongly stimulated Pupils constrict Large amounts of pigments broken down instantaneously, producing glare Visual acuity improves over 5 10 minutes as: Rod system turns off Retinal sensitivity decreases Cones and neurons rapidly adapt 52

63 Move from bright light into darkness Cones stop functioning in low-intensity light Rod pigments bleached; system turned off Rhodopsin accumulates in dark Transducin returns to outer segments Retinal sensitivity increases within minutes Pupils dilate 53

64 Axons of retinal ganglion cells form optic nerve Medial fibers of optic nerve decussate at optic chiasma Most fibers of optic tracts continue to lateral geniculate body of thalamus Fibers from thalamic neurons form optic radiation and project to primary visual cortex in occipital lobes 54

65 Fibers from thalamic neurons form optic radiation Optic radiation fibers connect to primary visual cortex in occipital lobes 55

66 Fibers from thalamic neurons form optic radiation Optic radiation fibers connect to primary visual cortex in occipital lobes Other optic tract fibers send branches to midbrain, ending in superior colliculi (initiating visual reflexes) 56

67 A small subset of ganglion cells in retina contain melanopsin (circadian pigment), which projects to: Pretectal nuclei (involved with pupillary reflexes) Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms 57

68 Both eyes view same image from slightly different angles Depth perception (three-dimensional vision) results from cortical fusion of slightly different images Requires input from both eyes 58

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71 Retinal cells split input into channels Color, brightness, angle, direction, speed of movement of edges (sudden changes of brightness or color) Lateral inhibition decodes "edge" information Job of amacrine and horizontal cells 61

72 Lateral geniculate nuclei of thalamus Process for depth perception, cone input emphasized, contrast sharpened Primary visual cortex (striate cortex) Neurons respond to dark and bright edges, and object orientation Provide form, color, motion inputs to visual association areas (prestriate cortices) 62

73 Occipital lobe centers (anterior prestriate cortices) continue processing of form, color, and movement Complex visual processing extends to other regions "What" processing identifies objects in visual field Ventral temporal lobe 63

74 "Where" processing assesses spatial location of objects Parietal cortex to postcentral gyrus Output from both passes to frontal cortex Directs movements 63