The Special Senses

The Special Senses Chemical senses Vision The ear Taste (gustation) Smell (olfaction) Vision The ear Hearing Equilibrium

re: 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

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

Taste buds of 50-100 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

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

Smell (olfaction) 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)

* 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__ * ___Filaments of Olfactory nerve (CN I) _______Olfactory tract *

1000 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

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

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

Eyelid tarsal plates give structure Where orbicularis oculi muscles attach (close eyes) Levator palpebrae superioris muscle Lifts upper lid voluntarily (inserts on tarsal plate)

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)

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)

Extraocular (extrinsic) eye muscles: 6 in # “EOMs intact” means they all work right Four are rectus muscles (straight) Originate from common tendinous or anular ring, at posterior point of orbit Two are oblique: superior and inferior

Extraocular (extrinsic) eye muscles Cranial nerve innervations: Lateral rectus: VI (Abducens n.) – abducts eye outward Medial, superior, inferior rectus & inf oblique: III (Oculomotor n.) – able to look up and in if all work Superior oblique: IV (Trochlear n.) – moves eye down and out

Innervation

Double vision: diplopia (what the patient experiences) Eyes do not look at the same point in the visual field Misalignment: strabismus (what is observed when shine a light: not reflected in the same place on both eyes) – can be a cause of diplopia Cross eyed Gaze & movements not conjugate (together) Medial or lateral, fixed or not Many causes Weakness or paralysis of extrinsic muscle of eye Surgical correction necessary Oculomotor nerve problem, other problems Lazy eye: amblyopia Cover/uncover test at 5 yo If don’t patch good eye by 6, brain ignores lazy eye and visual pathway degenerates: eye functionally blind NOTE: some neurological development and connections have a window of time - need stimuli to develop, or ability lost

3 Layers form the external wall of the eye (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 (middle) Vascular: uvea Choroid – posterior, pigmented Ciliary body Iris (colored part: see next slide) (inner) Sensory Retina and optic nerve

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

Layers of external wall of eye continued (outer) Fibrous: dense connective tissue Sclera – white of the eye Cornea (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 (inner) Sensory Retina

Layers of external wall of eye continued (outer) Fibrous: dense connective tissue Sclera – white of the eye Cornea (middle) Vascular: uvea Choroid – posterior, pigmented Ciliary body Iris (inner) Sensory Retina -------will cover after the chambers and lens

some pictures…

Chambers and fluids Vitreous humor in posterior segment (see previous pics) 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

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

Cataract (opaque lens)

The eye is an optical device: predominantly the lens (to a lesser degree, not shown here, the cornea also) Note: images are upside down and reversed from left to right, like a camera Resting eye set for distance vision: parallel light focused on retina Resting eye doesn’t see near objects because divergent rays are focused behind retina Lens accommodates (becomes rounder) so as to bend divergent rays more sharply, thereby allowing convergence on the retina

Retina: develops as part of the brain 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? 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: Photoreceptors Bipolor cells Ganglion cells

Light passes through pupil in iris, through vitreous humor, through axons, ganglion cells and bipolar cells, to photoreceptors next to pigmented layer

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

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

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

If you want more detail, it’s fascinating…

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

Green is area seen by both eyes, and is the area of stereoscopic vision Visual pathways 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.

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 Visual fields Location of lesion: 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 1. 2. 1. 3. 3. 2. 4. 5. 5. 4. Visual cortex

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

The Ear Parts of the ear Outer (external) ear Middle ear (ossicles) for hearing) Inner ear (labyrinth) for hearing & equilibrium

Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates Pressure is equalized by the pharyngotympanic tube (AKA eustachian or auditory tube)

TM causes ossicles in air filled middle ear to move: Malleus (hammer) Incus (anvil) Stapes (stirrup) These are 3 of the smallest bones of the body 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

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

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

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.)

Auditory pathway

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

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.)

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

VIII Vestibulocochlear nerve