1

Chapter 16

The Special Senses

2

The Special Senses

Chemical senses Taste (gustation) Smell (olfaction)

Vision The ear

Hearing Equilibrium

3

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

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

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

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

10

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

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 # “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

17

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

18

Innervation

19

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

20

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

21

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

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

IrisPigmented put incomplete: pupil lets in lightSphincter of pupil: circularly arranged smooth muscle -

parasympathetic controlfor bright light and/or close vision

Dilator of pupil: radiating smooth muscle – sympathetic controlfor dim light and/or distance vision

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

29

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

30

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

31

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

32

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

33

One of the Ishihara charts for color blindness

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

34

35

If you want more detail, it’s fascinating…

36

Retina through ophthalmoscope

Macula: at posterior pole

Fovea: maximal visual acuity (most concentrated cones)

Optic disc: optic nerve exits

Vessels

37

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. 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.

38

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.

39

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

40

The Ear Parts of the ear

Outer (external) ear

Middle ear (ossicles) for hearing)

Inner ear (labyrinth) for hearing & equilibrium

41

Sound in external acoustic meatus hits tympanic membrane (eardrum) – it vibrates

Pressure is equalized by the pharyngotympanic tube (AKA eustachian or auditory tube)

42

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

43

Skeletal muscles of middle ear

When loud, muscles contract, limiting vibration and dampening the noise

44

Inner ear = bony “labyrinth” of 3 parts

1. Cochlea - hearing

2. Vestibule - equilibrium

3. Semicircular canals - equilibrium

Filled with perilymph and endolymphfluids

Cochlea_______________________

Vestibule___________

Semicircular canals____

In petrous part of the temporal bone

45

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.)

46

Auditory pathway

47

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

48

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.)

49

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

50

VIII Vestibulocochlear nerve