The Senses

The Senses• General senses of touch

– Temperature– Pressure– Pain

• Special senses– Smell– Taste– Sight– Hearing– Equilibrium

The Eye and Vision

• 70 percent of all sensory receptors are in the eyes

• Each eye has over a million nerve fibers• The eye is a sphere about 1 inch in

diameter…like a ping pong• Only 1/6 of the eye is seen

– Most of the eye is surrounded by bone & cushioned by fat

Accessory Structures of the Eye

• Eyelids• Eyelashes• Muscles

Figure 8.1b

Accessory Structures of the Eye

• Conjunctiva– Membrane that lines the eyelids– Connects to the surface of the eye & secretes

mucus for lubrication

• Homeo Imbalance– Conjunctivitis =

reddened irritated eyes. “Pinkeye” is the HIGHLY infectious form caused by bacteria or virus

Accessory Structures of the Eye

• Lacrimal apparatus– Lacrimal gland –

produces diluted salt solution (tears)

– Lacrimal canals – drains tears from eyes

– Lacrimal sac – provides passage of tears towards nasal cavity

Figure 8.1a

Properties of lacrimal fluid (AKA TEARS!)– Dilute salt solution which contains lysozyme an

anti-bacterial protein• Protects, moistens, and lubricates the eye• Empties into the nasal cavity

• Nasolacrimal duct – empties tears into the nasal cavity (connects eye with nose)– Crying makes you

sniffle

• Homeo Imbalance– A cold or allergies can cause the lacrimal duct

to swell shut. This stops drainage of tears and you get watery eyes.

Structure of the Eye

• The wall is composed of three tunics (layers)– Fibrous tunic –

outside layer– Choroid –

middle layer

– Sensory tunic – inside layer

Figure 8.3a

The Fibrous Tunic• Sclera

– White connective tissue layer• Cornea

– Transparent allows light to pass through– Vulnerable to damage but repairs itself easily– The only human tissue that can be

transplanted without fear of rejection (no blood=no antibodies)

Choroid Layer• Blood-rich nutritive tunic• Pigment dark in color prevents light from

scattering• Modified interiorly into two structures

– Cilliary body – smooth muscle, focuses lens for clear vision

– Iris- smooth muscle, regulates amount of light that enters• Pigmented layer that gives eye color• Pupil – rounded opening in the iris

Sensory Tunic (Retina)

• Contains millions of receptor cells called photoreceptors– Rods & Cones

• Signals pass from photoreceptors to retina• Signals leave the retina toward the brain

through the optic nerve

• Homeo Imbalance– Retinal Detachment: Retina separates from

choroid. Retina cannot get nutrients and can die. Easily fixed with laser surgery.

– Caused by violent motion of the head, genetics

Lens• Biconvex crystal-like structure• Held in place by a ligament attached to the

ciliary body

Figure 8.3a

• The lens divides the eye into 2 segments or chambers

• Homeo Imbalance– Cataracts: Occur as we age. The lens

becomes hard and opaque

Internal Eye Chamber Fluids

• Aqueous humor– Watery fluid found in chamber between the

lens and cornea– Maintains intraocular pressure– Provides nutrients for the lens and cornea

• Homeo Imbalance– If the aqueous humor cannot drain, pressure

in the eye increases dramatically.– This leads to glaucoma, which will become

painful and possibly lead to loss of sight.– Early detection is key since a lot of damage

can be done w/o pain.• The machine that blows on your eye!

Internal Eye Chamber Fluids

• Vitreous humor– Gel-like substance behind the lens that fills

the eyeball– Lasts a lifetime and is not replaced (can be

floaters)

Neurons of the Retina and Vision

• Rods– Most are found towards the edges of the

retina– Allow dim light & peripheral vision– all in gray tones

• Cones– Allow for detailed color vision– Densest in the center of the retina

Cone Sensitivity• There are 3 types of

cones each sensitive to different wavelengths

• Total Color blindness is the result of lack all cone types. Partial is due to lack of 1 or 2 types.

Figure 8.6

Lens Accommodation• Light is bent or refracted

as it enters the eye• The lens changes shape

so that the light is focused on the retina

• The ability to focus on objects closer is called accommodation

• As objects come closer the lens bulges

Vision Problems

• Perfect vision is called emmetropia or “harmonious vision”

• Nearsightedness- you can see up close but not far away. Picture focuses in front of retina

• Farsightedness- you can see far away but not close up. Picture focuses behind retina.

Images Formed on the Retina

• The image on the retina is reversed, upside-down and smaller

Figure 8.10

Visual Pathway• Optic nerve: carry

impulses from retina to brain,

• Part of each optic nerve crosses at the optic chiasma– Each side of brain

receives info from both eyes

– Allows for binocular vision & depth perception

Figure 8.11

The Ear• Houses two senses

– Hearing – Equilibrium (balance)

Anatomy of the Ear

• The ear is divided into three areas– Outer (external)

ear (Hearing)– Middle ear

(Hearing)– Inner ear

(Hearing & Balance)

Figure 8.12

The External EarStructures of the external ear

– Pinna (auricle)- collects and directs sound waves into the auditory canal

– External auditory canal-short chamber in the temporal bone. The walls are covered in ceruminous glands which produce earwax (cerumen).

– Tympanic membrane- end of external ear. Vibrates when sound hit it.

Figure 8.12

The Middle Ear or Tympanic Cavity

• Air-filled cavity within the temporal bone

• Two tubes are associated with the inner ear

– The opening from the auditory canal is covered by the tympanic membrane

– The auditory tube connecting the middle ear with the throat

• Allows for equalizing pressure (popping of ear)

• This tube is otherwise collapsed

• Homeo Imbalance– Otitis Media-Inflammation of the middle ear is

a common result of a sore throat. In acute forms the eardrum bulges and becomes inflamed. Fluid needs to be drained with a semi-permanent ear tube and antibiotics. If untreated it can lead to hearing loss.

Bones of the Tympanic Cavity• The 3 bones in the

cavity collectively called the Ossicles.

• Smallest bones in the body

– Malleus (hammer)– Incus (anvil)– Stapes (stirrup)

• Transfer and amplify the vibration from the ear drum to the fluid in the inner ear!

Figure 8.12

Inner Ear or Bony Labyrinth• Maze of bony

chambers in the temporal bone behind the eye socket.

• The chambers are filled with perilymph fluid (plasma-like)

• Suspended in the perilymph are fluid filled sacs called endolymph

Figure 8.12

Inner Ear or Bony Labrynth• 3 subdivisions

– Cochlea– Vestibule– Semicircular

canals

Figure 8.12

Organs of Hearing• Organ of Corti

– Located within the cochlea

– Contain receptors = hair cells

– Above and below cochlear duct contain perilymph. Sound waves set this fluid into motion.

– Waves hit the basilar membrane & the hairs on it are bent by the movement of the gel-like tectorial membrane above them.

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

Mechanisms of Equilibrium• Our brains compensate for

disturbances in balance– This is a reflex that depends

on sensory receptors with in the vestibule and semi-circular canals.

– Receptors called vestibular apparatus is divided into 2 functional parts: Static and dynamic equilibrium

Static Equilibrium- “at rest”• Maculae – receptors in the vestibule

– Report on the position of the head in respect to gravity

• Anatomy of the maculae– Each maculae is a patch of hair cells embedded in

the otolithic membrane (gel-like)– Otoliths (tiny stones) float in the gel around the hair

cells– Movements cause otoliths to roll and pull the gel

which bends the hair cells (creates signal)– The information is sent via the vestibular nerve and

then to the cerebellum

Function of Maculae

Figure 8.13a–b

Dynamic Equilibrium• Occur in the Semicircular

canals. Orientated in 3 planes so no matter the movement, it is detectable.

• Responds to angular movements. When dancing or rocking on a boat it goes into over drive.

• Within each each semi-circular there is a region called:

– Crista ampullaris: Tuft of hair cells covered by the Cupula (gelatinous cap)

– Drag and motion of the endolymph is transmitted to the brain

Figure 8.14c

neat

• When you hear the same tone…– Your auditory receptors tune it out (hum of air

conditioning, car motor, etc)– BUT hearing is the last sense to leave our

awareness when we fall asleep or receive anesthesia (or even when we die) and the first to come back as we awaken.

Chemical Senses – Taste and Smell

• Both senses use chemoreceptors– Stimulated by chemicals in solution– Taste has four types of receptors– Smell can differentiate a large range of

chemicals• Both senses complement each other and

respond to many of the same stimuli

Olfaction – The Sense of Smell• Olfactory receptors are in the roof of each

nasal cavity– Neurons with long cilia (hairs) covered in

mucus– Chemicals dissolve into mucus & are

detected• Impulses are transmitted via the olfactory nerve

and interpretation of smell is made in the brain• Smells create “smell snapshots” which are

often linked with emotions.– Reaction to smell is rarely neutral

Olfactory Epithelium

Figure 8.17

The Sense of Taste

• Taste buds house taste receptor organs– Most are on the tongue (~10K)– Few on soft palate & cheeks

The Tongue and Taste

• The tongue is covered with projections called papillae. Give tongue it’s texture– Fungifiorm papillae – rounded projection

with taste buds, most numerous– Circumvallate papillae – large papillae with

taste buds, few in back• Taste buds are found on the sides of

papillae

Structure of Taste Buds

• Gustatory cells are the receptors that respond to chemicals in saliva– Have gustatory hairs (long microvilli)– The long microvilli protrude through the taste

pore and when stimulated, depolarize and the impulses go to the brain and you taste

Anatomy of Taste Buds

Figure 8.18

Taste Sensations

• Sweet receptors– Sugars

• Sour receptors– Acids

• Bitter receptors– Alkaloids

• Salty receptors– Metal ions

• http://www.youtube.com/watch?v=38PGB9dcr4c

• Perspective on the 5 senses

• http://www.pbs.org/wgbh/nova/body/tongue-taste.html

• Tour of the tongue: web interactive• http://www.pbs.org/wgbh/nova/body/scienc

e-picky-eaters.html• The science of picky eaters (12:45)• http://www.pbs.org/wgbh/nova/body/bitter-t

aste.html• Genetically modified mouse that cannot

taste bitter