1

Chapter 16

The Special Senses

2

The Special Senses

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 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

6

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

7

Olfactory epithelium in roof of nasal cavity Has millions of bipolar neurons = olfactory receptor cells

Only neurons undergoing replacement throughout adult lifeOlfactory hair (cilia) bind odor molecules

Mucus captures & dissolves odor moleculesEach 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

10

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

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)

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

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 ContractWhen 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

18

19

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.)

20

3 Layers form the external wall of the eye

1. (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

2. (middle) Vascular: Choroid – blood rich, dark pigmented Ciliary body – attaches lens Iris (colored part: see next slide)

3. (inner) Sensory Retina and optic nerve

21

1. (outer layer) Fibrous: dense connective tissue

Sclera – white of the eye Cornea

2. (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

3. (inner layer) Sensory Retina and optic nerve

22

Layers of external wall of eye continued1. (outer) Fibrous: dense connective tissue

Sclera – white of the eye Cornea

2. (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

3. (inner) Sensory Retina

23

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 -------will cover after the chambers

and lens

24

some pictures…

25

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)

26

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

27

Cataract (opaque lens)

28

(to a lesser degree, not shown here, the cornea also)

a. Resting eye set for distance vision: parallel light focused on retina

b. Resting eye doesn’t see near objects because divergent rays are focused behind retina

c. Lens accommodates (becomes rounder) so as to bend divergent rays more sharply, thereby allowing convergence on the retina

Note: images are upside down and reversed from left to right, like a camera

The eye is an optical device: predominantly the lens

Lens AccommodationLens 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

30

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 tissueSclera – white of the eyeCornea

2. (middle layer) Vascular: uveaChoroid – posterior, pigmentedCiliary bodyIris

3. (inner layer) SensoryRetina and optic nerve

Remember the 3 layers of the external eye?

1. Photoreceptors2. Bipolor cells3. Ganglion cells

31

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

32

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

33

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 SensitivityCone 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

- 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

•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.

37

One of the Ishihara charts for color blindness

Commonly X-linked recessive: 8% males and 0.4% females

38

39

If you want more detail, it’s fascinating…

40

Retina through ophthalmoscope

Macula: at posterior pole Fovea: maximal

visual acuity (most concentrated cones)

Optic disc: optic nerve exits

Vessels

Images Formed on the RetinaImages 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.

42

Visual pathwaysGreen 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 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 PathwayVisual Pathway Photoreceptors of

the retina

Optic nerve

Optic nerve crosses at the optic chiasma

Optic tract

Thalamus

Visual Cortex of Occipital Lobe

44

Visual field defectsprint this out and follow from the fields to the visual cortex using 4 colors

remember: fields are reversed and upside down

1. Optic nerveipsilateral (same side) blind eye

2. Chiasmatic (pituitary tumors classically)lateral half of both eyes gone

3. Optic tractopposite 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

1.

2.

3.

5.4.

Location of lesion:

1.

2.

3.

4.

5.

45

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

Geometrical illusions

Successive contrast : afterimages ...

                                            what do you see?

fixate the black dot in the center for 60 seconds ...

… and then look at a the black dot in the right panel !

50

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 EarAnatomy 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 EarThe External Ear Involved in

hearing only

Structures of the external ear Pinna (auricle)-

collects sound

External auditory canal- channels sound inward

The External Auditory CanalThe 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 CavityThe Middle Ear or Tympanic Cavity

Air-filled cavity within the temporal bone

Only involved in the sense of hearing

The Middle Ear or Tympanic CavityThe 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 CavityBones of the Tympanic Cavity

Three bones span the cavity

Malleus (hammer)

Incus (anvil)

Stapes (stirrip)

http://www.ghorayeb.com/files/STAPES_on_a_Penny_375_SQ.jpg

http://medicine.wustl.edu/~oto/bbears/images/ossic.jpg

Bones of the Tympanic CavityBones of the Tympanic Cavity

Vibrations from eardrum move the malleus

These bones transfer sound to the inner ear

Inner Ear or Bony LabyrinthInner 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 LabryinthInner Ear or Bony Labryinth 3 Subdivisions

Cochlea

Upper chamber is the scala vestibuli

Lower chamber is the scala tympani

Vestibule

Semicircular canals

ChochleaChochlea

Spiral organ of Corti

Receptors = hair cells on the basilar membrane

Scala tympani

Scala vestibuli

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)

Organ of CortiOrgan of Corti

Scala tympani

Scala vestibuli

Organs of HearingOrgans of Hearing Organ of Corti

Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

Scala tympani

Scala vestibuli

Mechanisms of HearingMechanisms 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

65

Mechanisms of HearingMechanisms 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)

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

Mechanisms of HearingMechanisms of Hearing

Organs of EquilibriumOrgans of Equilibrium

Receptor cells are in two structures

Vestibule

Semicircular canals

Organs of EquilibriumOrgans of Equilibrium Equilibrium has two functional parts

Static equilibrium- in the vestibule

Dynamic equilibrium- in the semicircular canals

Static EquilibriumStatic Equilibrium Maculae –

receptors in the vestibule Report on

the position of the head

Send information via the vestibular nerve

Static EquilibriumStatic 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 MaculaeFunction of MaculaeMovements cause otoliths to bend the hair cells (gravity moves the “rocks” over and pulls the hairs)

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

Dynamic EquilibriumDynamic 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 EquilibriumDynamic 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.edu/campbell/eyeandear/wp_images/177_macula_crista.gif

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

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