1 Chapter 16 The Special Senses

2 Chemical senses  Taste (gustation)  Smell (olfaction) Vision The ear  Hearing  Equilibrium

3 Touch The sense of touch is part of the General somatic senses____ This chapter deals with the Special category of the two left sensory boxes

4 TASTE Taste buds: mostly on tongue Two types  Fungiform papillae (small, on entire surface of tongue)  Circumvallate papillae (inverted “V” near back of tongue)

5 Taste buds of epithelial cells each Taste receptor cells (gustatory cells) Microvilli through pore, bathed in saliva Disolved molecules bind & induce receptor cells to generate impulses in sensory nerve fibers Taste involves chemicals binding to microvilli

6 Types of taste  Sweet  Sour  Salty  Bitter  Glutamate (MSG) Gustatory (taste) pathway to brainstem & cerebral cortex via two cranial nerves:  VII (Facial n.) – anterior 2/3 of tongue  IX (Glossopharyngeal n.) – posterior 1/3 tongue and pharynx

7 Olfactory epithelium in roof of nasal cavity  Pseudostratified columnar epithelium  Has millions of bipolar neurons = olfactory receptor cells Only neurons undergoing replacement throughout adult life Olfactory cilia bind odor molecules  Mucus captures & dissolves odor molecules Each receptor cell has an axon - are bundled into “filaments” of olfactory nerve  Penetrate cribriform plate of ethmoid bone & enter olfactory bulb Smell (olfaction)

8 Olfactory bulb is in forebrain In bulb nerve axons branch and synapse with mitral cells (neurons in clusters of “glomeruli”) Mitral cells send signals via olfactory tract Olfactory bulb__ _______Olfactory tract ___Filaments of Olfactory nerve (CN I) * *

9

types of smell receptors (approx.) Convergence of many receptor cell signals onto one glomerulus registers a signature pattern Brain recognizes the pattern: sent to unclus (olfactory center) and limbic area Anosmia: absence of the sense of smell  Trauma  Colds or allergies producing excessive mucus  Polyps causing blockage  1/3 are from zinc deficiency

11 The Eye and Vision Vision is the dominant sense in humans 70% of sensory receptors in humans are in the eyes 40% of the cerebral cortex is involved in processing visual information The eye (or eyeball) is the visual organ  Diameter 2.5 cm (1 inch)  Only anterior 1/6 visible  Lies in bony orbit  Surrounded by a protective cushion of fat

12 Accessory structures of the eye Eyebrows Eyelids or palpebrae  Upper & lower separated by palpebral fissure  Corners: medial & lateral canthi  Eyelashes

13 Eyelid tarsal plates give structure  Where orbicularis oculi muscles attach (close eyes) Levator palpebrae superioris muscle  Lifts upper lid voluntarily (inserts on tarsal plate) Tears drain into your nose.

14 Tarsal glands – modified sebaceous (oil) glands in tarsal plates Conjunctiva - transparent mucus membrane of stratified columnar epithelium  Palpebral conjunctiva  Bulbar conjunctiva Covers white of eye but not the cornea (transparent tissue over the iris and pupil)

15 Lacrimal apparatus Responsible for tears  The fluid has mucus, antibodies and lysozyme Lacrimal gland in orbit superolateral to eye Tears pass out through puncta into canaliculi into sac into nasolacrimal duct Empty into nasal cavity (sniffles)

16 The Eye

17 3 Layers form the external wall of the eye 1. (outer) Fibrous: dense connective tissue  Sclera – white of the eye  Cornea 100s of sheets of collagen fibers between sheets of epithelium and endothelium Clear because regular alignment Role in light bending Avascular but does have pain receptors Regenerates 2. (middle) Vascular: uvea  Choroid – posterior, pigmented  Ciliary body  Iris (colored part: see next slide) 3. (inner) Sensory  Retina and optic nerve

18 1. (outer layer) Fibrous: dense connective tissue  Sclera – white of the eye  Cornea 2. (middle) Vascular: uvea  Choroid – posterior, pigmented  Ciliary body Muscles – control lens shape Processes – secrete aqueous humor Zonule (attaches lens)  Iris 3. (inner layer) Sensory  Retina and optic nerve

19 Layers of external wall of eye continued 1. (outer) Fibrous: dense connective tissue  Sclera – white of the eye  Cornea 2. (middle) Vascular: uvea  Choroid – posterior, pigmented  Ciliary body  Iris Pigmented put incomplete: pupil lets in light Sphincter of pupil: circularly arranged smooth muscle - parasympathetic control for bright light and/or close vision Dilator of pupil: radiating smooth muscle – sympathetic control for dim light and/or distance vision 3. (inner) Sensory  Retina

20 Layers of external wall of eye continued 1. (outer) Fibrous: dense connective tissue  Sclera – white of the eye  Cornea 2. (middle) Vascular: uvea  Choroid – posterior, pigmented  Ciliary body  Iris 3. (inner) Sensory  Retina is the only true pigmented structure of the eye

21 some pictures…

22 Chambers and fluids Vitreous humor in posterior segment  Jellylike  Forms in embryo and lasts life-time Anterior segment filled with aqueous humor – liquid, replaced continuously  Anterior chamber between cornea and iris  Posterior chamber between iris and lens  Glaucoma when problem with drainage resulting in increased intraocular pressure (see previous pics)

23 Lens: thick, transparent biconvex disc Changes shape for precise focusing of light on retina Onion-like avascular fibers, increase through life Cataract if becomes clouded Note lens below, but in life it is clear Cataract below: the lens is milky and opaque, not the cornea

24 Cataract (opaque lens)

25 Retina: develops as part of the brain Retina is 2 layers Outer thin pigmented layer:  Melanocytes (prevent light scattering) Inner thicker neural layer  Plays a direct role in vision  Three type of neurons: 1. (outer layer) Fibrous: dense connective tissue Sclera – white of the eye Cornea 2. (middle layer) Vascular: uvea Choroid – posterior, pigmented Ciliary body Iris 3. (inner layer) Sensory Retina and optic nerve Remember the 3 layers of the external eye? 1.Photoreceptors 2.Bipolor cells 3.Ganglion cells

26 Light passes through pupil in iris, through vitreous humor, through axons, ganglion cells and bipolar cells, to photoreceptors next to pigmented layer Know the names of these cells!

27 Photoreceptor neurons signal bipolar cells, which signal ganglion cells to generate (or not) action potentials: axons run on internal surface to optic nerve which runs to brain *Know that axons from the retina form the optic nerve, CN II

28 Photoreceptors: 2 types Rod cells  More sensitive to light - vision permitted in dim light but only gray and fuzzy  Only black and white and not sharp Cone cells  High acuity in bright light  Color vision  3 sub-types: blue, red and green light cones *Know that rods are for B & W and cones are for color

29 One of the Ishihara charts for color blindness Commonly X-linked recessive: 8% males and 0.4% females

30

31 If you want more detail, it’s fascinating…

32 Retina through ophthalmoscope Macula: at posterior pole Fovea: maximal visual acuity (most concentrated cones) Optic disc: optic nerve exits Vessels

33 Visual pathways Green is area seen by both eyes, and is the area of stereoscopic vision At optic chiasm, medial fibers from each eye (which view lateral fields of vision) cross to opposite side. Optic tracts (of crossed and uncrossed, sensing opposite side of visual field of both eyes) synapse with neurons in lateral geniculate of thalamus. These axons form the optic radiation and terminate in the primary visual cortex in the occipital lobe. Left half of visual field perceived by right cerebral cortex, and vice versa.

34 Visual field defects print this out and follow from the fields to the visual cortex using 4 colors remember: fields are reversed and upside down 1. Optic nerve ipsilateral (same side) blind eye 2. Chiasmatic (pituitary tumors classically) lateral half of both eyes gone 3. Optic tract opposite half of visual field gone 4. & 5. Distal to geniculate ganglion of thalamus: homonymous superior field (4) or homonymous inferior field (5) defect Visual cortex Visual fields Location of lesion:

35 Terminology, remember… Optic – refers to the eye Otic – refers to the ear Getting eyedrops and ear drops mixed up is probably not a good idea

36 The Ear Parts of the ear Outer (external) ear Middle ear (ossicles) for hearing) Inner ear (labyrinth) for hearing & equilibrium Sound waves enter the EAM through air.

37 Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates Pressure is equalized by the pharyngotympanic tube (AKA eustachian or auditory tube) Receptors (hair cells) for hearing are in the cochlea The middle ear cavity is filled with air Hair cell receptors are responsible for equilibrium

38 TM causes ossicles in air filled middle ear to move:  Malleus (hammer)  Incus (anvil)  Stapes (stirrup) Ossicles articulate to form a lever system that amplifies and transmits the vibratory motion of the TM to fluids of inner ear cochlea via oval window These are 3 of the smallest bones of the body

39 Skeletal muscles of middle ear When loud, muscles contract, limiting vibration and dampening the noise

40 Inner ear = bony “labyrinth” of 3 parts 1. Cochlea - hearing 2. Vestibule - equilibrium 3. Semicircular canals - equilibrium Filled with perilymph and endolymph fluids Cochlea_______________________ Vestibule___________ Semicircular canals____ In petrous part of the temporal bone

41 Spiral organ of Corti in cochlea contains receptors for hearing (amplifies sound 100 times) Vibration of stirrup at oval window starts traveling waves displacing basilar membrane Sensory hair cells stimulated Nerve ending of cochlear nerve division of VIII (Vestibulo- cochlear n.)

42 Auditory pathway

43 Equilibrium pathway Via vestibular nerve branch of VIII (Vestibulocochlear n.) to the brain stem Only special sense for which most of the information goes to lower brain centers

44 Vestibule contains utricle and saccule Each contains a macula Senses static equilibrium and linear acceleration of the head (not rotational movements) Tips of hairs imbedded in otolithic membrane (calcium carbonate “stones”) Vestibular nerve branches of VIII (Vestibulocochlear n.)

45 Semicircular canals Each of the 3 lies in one of the 3 planes of space Sense rotational acceleration of the head Duct with ampulla housing a small crest: crista ampulla Hairs project into jellylike cupula & basilar cells synapse with fibers of vestibular nerve

46 VIII Vestibulocochlear nerve