Chapter 16 The Special Senses

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

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 Dissolved molecules bind & induce receptor cells to generate impulses in sensory nerve fibers

Types of taste Sweet Sour Salty Bitter Umami “beef taste”- elicited by Glutamine 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 Has millions of bipolar neurons = olfactory receptor cells Only neurons undergoing replacement throughout adult life Olfactory hair (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 Filaments of Olfactory nerve (CN I) * Olfactory bulb__ _______Olfactory tract *

Anosmia: absence of the sense of smell Trauma Colds or allergies producing excessive mucus Polyps causing blockage 1/3 are from zinc deficiency Head injury Aging

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 # Four are rectus muscles (straight) Lateral rectus, medial rectus, superior rectus, and inferior rectus. Two are oblique: superior and inferior

When Extrinsic Eye Muscles Contract Superior oblique- eyes look out and down Superior rectus- eyes looks up Lateral rectus- eyes look outward Medial rectus- eyes look inward Inferior rectus- eyes looks down Inferior oblique- eyes look in and up

Extraocular (extrinsic) eye muscles Cranial nerve innervations: Lateral rectus: VI (Abducens nerve) Medial, superior, inferior rectus & inferior oblique: III (Oculomotor nerve.) Superior oblique: IV (Trochlear n.)

3 Layers form the external wall of the eye (outer) Fibrous: dense connective tissue Sclera – white of the eye Cornea Clear because regular alignment Role in light bending Avascular but DOES have pain receptors Regenerates (middle) Vascular: Choroid – blood rich, dark pigmented Ciliary body – attaches lens 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 – blood rich, has dark pigmented that prevents light scattering 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 Opening is called PUPIL: lets in light Acts like the diaphragm of a camera lens. Regulates the amount of light that enters by contracting or dilating to see clearly. Dark to dim light = dilation Bright light and close vision = contraction (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

Lens Accommodation Light must be focused to a point on the retina for optimal vision The eye is set for distance vision (over 20 ft away) 20/20 vision- at 20 feet, you see what a normal eye would see at 20 feet (20/100- at 20, normal person would see at 100) The lens must change shape to focus for closer objects

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

http://www.yorku.ca/eye/rod-cone.gif http://www.secretbeyondmatter.com/ourbrains/theworldinourbrains_files/11-1.jpg

Cone Sensitivity There are three types of cones Different cones are sensitive to different wavelengths - red- long - green- medium - blue- short Color blindness is the result of lack of one or more cone type

COLORBLINDNESS Comes from a lack of one or more types of color receptors. Most are green or red or both and that is due to a lack of red receptors. Another possibility is to have the color receptors missing entirely, which would result in black and white vision. An inherited trait that is transferred on the sex chromosomes (23rd pair)- sex-linked trait Occurs more often in males Can not be cured or corrected

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 The Macula is a highly pigmented yellow spot at the center is the fovea. The macula contains the highest concentration of cone cells and high resolution vision. The yellow pigment absorbs blue and UV light which acts as a natural sunblock.

Images Formed on the Retina If the image is focused at the spot where the optic disk is located, nothing will be seen. This is known as the blind spot. There are no photoreceptors there, as nerves and blood vessels pass through this point.

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 of the brain. Optic tracts (of crossed and uncrossed fibers, sensing opposite side of visual field of both eyes) synapse with neurons in the 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 Pathway Photoreceptors of the retina Optic nerve Optic nerve crosses at the optic chiasma Optic tract Thalamus Visual Cortex of Occipital Lobe

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

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

Illusions Geometrical illusions

Illusions Successive contrast : afterimages ... fixate the black dot in the center for 60 seconds ... … and then look at a the black dot in the right panel ! afterimages are the consequence of encoding stimulus change in time: intensity changes are encoded rather than intensity levels adaptation leads to the loss of sensitivity to a particular feature after disappearance of the stimulus the opposite feature is perceived due to opponent processing                                             what do you see?

Auditory System

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

Anatomy of the Ear The ear is divided into three areas Outer (external) ear Middle ear Inner ear (Add C. “INNER EAR” to notes)

The External Ear Involved in hearing only Structures of the external ear Pinna (auricle)- collects sound External auditory canal- channels sound inward

The External Auditory Canal Narrow chamber in the temporal bone- through the external auditory meatus Lined with skin Ceruminous (wax) glands are present Ends at the tympanic membrane (eardrum)

The Middle Ear or Tympanic Cavity Air-filled cavity within the temporal bone Only involved in the sense of hearing

The Middle Ear or Tympanic Cavity Two tubes are associated with the inner ear The opening from the auditory canal is covered by the tympanic membrane (eardrum) The auditory tube connecting the middle ear with the throat (also know as the eustacian tube) Allows for equalizing pressure during yawning or swallowing This tube is otherwise collapsed

Bones of the Tympanic Cavity Three bones span the cavity Malleus (hammer) Incus (anvil) Stapes (stirrip)

http://medicine.wustl.edu/~oto/bbears/images/ossic.jpg http://www.ghorayeb.com/files/STAPES_on_a_Penny_375_SQ.jpg

Bones of the Tympanic Cavity Vibrations from eardrum move the malleus These bones transfer sound to the inner ear

Inner Ear or Bony Labyrinth Also known as osseous labyrinth- twisted bony tubes Includes sense organs for hearing and balance Filled with perilymph

Inner Ear or Bony Labryinth 3 Subdivisions Cochlea Upper chamber is the scala vestibuli Lower chamber is the scala tympani Vestibule Semicircular canals

Chochlea Spiral organ of Corti Receptors = hair cells on the basilar membrane Scala vestibuli Scala tympani

Organ of Corti Gel-like tectorial membrane is capable of bending hair cells (endolymph in the membranous labyrinth of the cochlear duct flows over it and pushes on the membrane) Scala vestibuli Scala tympani

Organs of Hearing Organ of Corti Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe Scala vestibuli Scala tympani

Mechanisms of Hearing Vibrations from sound waves move tectorial membrane (pass through the endolymph fluid filling the membranous labyrinth in the cochlear duct) Hair cells are bent by the membrane

Mechanisms of Hearing An action potential starts in the cochlear nerve The signal is transmitted to the midbrain (for auditory reflexes and then directed to the auditory cortex of the temporal lobe)

Mechanisms of Hearing Continued stimulation can lead to adaptation (over stimulation to the brain makes it stop interpreting the sounds)

Organs of Equilibrium Receptor cells are in two structures Vestibule Semicircular canals

Organs of Equilibrium Equilibrium has two functional parts Static equilibrium- in the vestibule Dynamic equilibrium- in the semicircular canals

Static Equilibrium Maculae – receptors in the vestibule Report on the position of the head Send information via the vestibular nerve

Static Equilibrium Anatomy of the maculae Hair cells are embedded in the otolithic membrane Otoliths (tiny stones) float in a gel around the hair cells

Function of Maculae Movements cause otoliths to bend the hair cells (gravity moves the “rocks” over and pulls the hairs)

http://neuromedia. neurobio. ucla http://neuromedia.neurobio.ucla.edu/campbell/eyeandear/wp_images/177_macula_HP.gif

Dynamic Equilibrium Whole structure is the ampulla Crista ampullaris – receptors in the semicircular canals Tuft of hair cells Cupula (gelatinous cap) covers the hair cells

Dynamic Equilibrium Action of angular head movements The cupula stimulates the hair cells Movement of endolymph pushes the cupula over and pulls the hairs An impulse is sent via the vestibular nerve to the cerebellum

DYNAMIC EQUILIBRIUM STRUCTURES http://www.faculty.une.edu/com/abell/histo/CristaAmp.jpg

http://neuromedia. neurobio. ucla http://neuromedia.neurobio.ucla.edu/campbell/eyeandear/wp_images/177_macula_crista.gif

Deafness Hearing loss- due to disease (ex. meningitus), damage, or age related Conduction deafness- prevention or blocking sounds from entering inner ear. Ex. ear wax, ruptured ear drum, middle ear inflammation (otis media), and otosclerosis (hardening of the ossicles of the ear) Sensoneural deafness- damage to the neural structures from any point from the cochlear hair cells to and including the auditory cortical cells Partial or complete deafness, or gradual loss over time

Deafness Tinnitus- ringing or clicking sound in the absence of auditory stimuli; 1st symptom of cochlear nerve degeneration may result from inflammation of the inner or middle ear side effect from medicine such as aspirin Symptoms- vertigo, nausea, hearing loss Meniere's Syndrome- labyrinth disorder; effects both semicircular canals and cochlea