THE SPECIAL SENSES Taste, smell, touch, sight, hearing, & balance.

THE SPECIAL SENSES

Taste, smell, touch, sight, hearing, & balance

How sensory structures gather information:

sensory receptors detect stimuli (chemicals, light, sound, cold, heat, touch, etc) and convert that signal to an electrical signal

the conversion is called sensory transduction, and it is accomplished as a change in the membrane potential of the receptor cell

unlike action potentials, receptor potentials vary in intensity (NOT all-or-none)

How sensory structures gather information:

the signal then enters the CNS and forms a synapse with a sensory neuron

the brain interprets the intensity of the stimulus by the rate at which it is receiving action potentials (more signals = more intense stimulus)

sensory adaptation = the tendency of some sensory receptors to become less sensitive when they are stimulated repeatedly. This keeps the body from continuously reacting to normal background stimuli & becoming overloaded

Human senses are made possible by 5 types of sensory receptors:

Mechanoreceptors = for touch, pressure, motion, sound

Thermoreceptors = temperature receptors Nociceptors = pain receptors Chemoreceptors = for chemicals electromagnetic receptors = for energy such

as light & electricity (ex. photoreceptors)

THE CHEMICAL SENSES: TASTE & SMELL

Taste = gustitation, smell = olfaction Their receptors are classified as

chemoreceptors because they are responding to chemicals dissolved in fluid

These receptors compliment each other & respond to many of the same stimuli

THE SENSE OF TASTE

Taste buds = sensory receptors for taste ~10,000 in a young adult, lessen as we age located primarily on the tongue, also on the

soft palate, inner cheeks, pharanyx, epiglottis

One cannibal to another while eating a clown: "Does this taste funny to you?"

THE SENSE OF TASTE

2 main types of tongue papillae containing taste buds:Fungiform papillae cover the tongue,

especially the tip & sidesCircumvallate papillae are larger, form an

inverted “V” at the back of the tongue replaced every 7-10 days

The taste buds themselves are too small to see without a microscope, but papillae are visible by close inspection of the tongue's surface

THE PHYSIOLOGY OF TASTE Taste buds are composed of groups of

between 50 and 150 columnar taste receptor cells bundled together like a cluster of bananas.

The taste receptor cells within a bud are arranged such that their tips form a small taste pore, and through this pore extend microvilli from the taste cells.

The microvilli of the taste cells bear taste receptors.

taste receptor cells:

THE PHYSIOLOGY OF TASTE

Interwoven among the taste cells in a taste bud is a network of dendrites of sensory nerves called "taste nerves".

When taste cells are stimulated by binding of chemicals to their receptors, they depolarize

this depolarization is transmitted to the taste nerve fibers resulting in an action potential that is ultimately transmitted to the brain.

Once taste signals are transmitted to the brain, several motor neural pathways are activated that are important to digestive function: increased salivation secretion of gastric juices in the stomach.

Among humans, there is substantial difference in taste sensitivity.

one in four people is a "supertaster" = several times more sensitive to bitter and other tastes than those that taste poorly.

such differences are heritable and reflect differences in the number of fungiform papillae and taste buds on the tongue.

receptors for a large number of specific chemicals have been identified that contribute to the reception of taste.

five types of tastes are commonly recognized by humans: Sweet sugars, alcohols, amino acids;

usually indicates energy rich nutrients Salty metal ions, sodium chloride;

allows modulating diet for electrolyte balance

Sour acids such as oranges, tomatoes; typically foods rich in vitamin C

Bitter alkaloids (nicotine, caffeine) + aspirin; allows sensing of diverse natural toxins therefore protective in nature

Umami = beefy; enhanced by monosodium glutamate (a food additive)

TASTE & SMELL

it is clear that the sense of smell profoundly affects the sensation of taste.

Taste may be up to 80% smell Taste is also influenced by the

temperature & texture of food, due to thermoreceptors and mechanoreceptors

THE SENSE OF SMELL

Specialized smell receptor cells are located in a small yellowish patch of mucus membrane lining the roof of the nose

Axons of these sensory cells pass through perforations in the overlying bone and enter 2 elongated olfactory bulbs lying in the top of the bone

The portion of the sensory cell that is exposed to odors possesses hair-like cilia

These cilia contain the receptor sites that are stimulated by odorants carried by airborne molecules

odorants dissolve in the mucus lining in order to start the smell response

Olfactory receptor cells are unique in that they are one of the few neurons to renew themselves (every 60 days)

An odorant acts on many receptors to different degrees

Similarly, a receptor interacts with many different odorants to varying degrees

substances that are volatile (easily turns into a gas), will give off molecules, or odorants

Temperature and humidity affect odor because they increase molecular volatility

The pattern of activity set up in the receptor cells is projected to the olfactory bulb, where it forms a spatial image of the odor

Impulses created by this stimulation pass to other smell regions, giving rise to conscious perceptions of odor in the frontal lobe and emotional responses in the limbic system of the brain

SMELL & MEMORY

A smell can bring on a flood of memories, influence people's moods and even affect their work performance. Because the olfactory bulb is part of the brain's limbic system, an area so closely associated with memory and feeling it's sometimes called the "emotional brain," smell can call up memories and powerful responses almost instantaneously.

Humans have seven primary odors that help them determine objects:

Odor Example

Camphoric Mothballs

Musky Perfume/Aftershave

Roses Floral

Pepperminty Mint Gum

Etheral Dry Cleaning Fluid

Pungent Vinegar

Putrid Rotten Eggs

Anosmia

Anosmia is the inability to smell. Just as the deaf cannot hear and the blind cannot see, anosmics cannot perceive odor and so can barely perceive taste. Sinus disease, growths in the nasal passage, viral infections and head trauma can all cause the disorder. Children born with anosmia often have difficulty recognizing and expressing the disability.

TOUCH & PAIN

Our skin has 3 types of sensory receptors:Mechanoreceptors = for touch, pressure,

motion, sound Thermoreceptors = temperature receptorsNociceptors = pain receptors

Mechanoreceptors are those activated by mechanical stimuli. The tactile senses of touch and pressure are

served by a variety of these receptors. Meissner's corpuscles are abundant in areas with

great sensitivity to touch such as finger tips and lips. They conduct nerve impulses very quickly.

Mechanoreceptors are those activated by mechanical stimuli.

Merkel’s discs adapt to stimuli slowly and give the sensation of continuous pressure against the body surface.

Ruffini's end-organs adapt slowly. They are thought to yield sensations of continuous touch and deep pressure on skin.

Pacinian corpuscles adapt very rapidly and are stimulated by high frequency stimuli to give the sense of vibration.

Free nerve endings associated with hair follicles respond to touch stimuli that move hairs. (ex. cat’s whiskers)

Cutaneous Receptors

In addition to mechanoreceptors, there are receptors for pain and the thermal sensations of hot and cold. These sensations are also served by free nerve endings Thermoreceptors:

Detect hot or cold in the skin Other sensors deep in the body regulate temp of the

blood, helping the hypothalamus regulate body temp

"nociceptors" (noci- as in noxious): two kinds for temperature (hot or cold)Two for pain (rapid pricking pain or slow burning

pain)

"important for a number of reasons:often indicates dangercan make us aware of injury or disease

pain receptors respond to excessive heat or pressure

pain receptors respond to chemicals released by damaged or inflamed tissues

prostaglandins increase pain by sensitizing pain receptors

aspirin & ibuprofen reduce pain by inhibiting prostaglandins

Summary of Sensory receptors in the skin:

Thermoreceptors = temperature receptors Nociceptors = pain receptors Mechanoreceptors:

Free nerve endings - heat, light pressure, pain Pacinian corpuscles – vibration Meissner corpuscles- onset and end of continuous

light pressure Ruffini endings - continuous touch & deep pressure Merkels discs - continuous pressure

The pathway of communication to the CNS:

Touch receptors sensory nerves spinal cord thalamus sensory cortex of cerebrum

Some areas of the body are more sensitive than others:Hands & lips are VERY sensitiveChest & back are less sensitive

TOUCH & PAIN

Our sense of touch can be tested using calipers to test our two-point threshold

Acuity is greatest in the most densely nerve-packed areas of the body

The touch homunculus

EFFECTS OF AGING:

• people lose sensory receptors as they age. • At age 10, most people have about 50 touch

receptors per square millimeter of skin. • At 50, we have about 10. Nevertheless, there

is little midlife loss of sensitivity (there are still more receptors than nerve fibers)

• preliminary results show a 50 percent loss in touch acuity by age 70 or so.

EFFECTS OF AGING:

• People also lose acuity in their sense of taste & smell

• This contributes to malnutrition in the elderly, because food doesn’t taste as good

THE EAR: HEARING & BALANCE

The ear is divided into 3 major areas:Outer earMiddle ear Inner ear

The outer & middle ear are for hearing only The inner ear is for hearing & balance The inner ear is the most complex


Parts of the outer ear:Pinna or auricle = shell-shaped flap that

collects sound waves & channels them in Helix (the rim) + lobe


Parts of the outer ear:Auditory canal = a curved tube that extends

from the auricle to the eardrum (2.5 cm long) Passes through the temporal bone via the external

auditory meatus Lined with ceruminous glands that make cerumen,

or earwax which traps foreign bodies & repels bugs


The boundary between the outer & middle ear is the eardrum or tympanic membrane

The middle ear is an air-filled cavity in the temporal bone

Parts of the middle ear: 3 bones or ossicles:Hammer / malleusAnvil / incusStirrup / stapes


Eustachian tube / pharyngotympanic tube equalizes pressure on either side of the eardrum to allow it to vibrate freely

Swallowing or yawning helps to equalize the pressure & open the eustachian tubes

The eustachian tubes of children are short & nearly horizontal, which permits bacteria to spread from the pharynx to the middle ear = otitis media


The boundary between the middle & inner ear is the oval window

The inner ear is a cavity called the bony labyrinth which is lined with a membrane called the membranous labyrinth

Perilymph is the fluid between the bone & membrane

Endolymph is the fluid within the membranes


Parts of the inner ear / labyrinth:Semicircular canals

Anterior, posterior, & lateral canals oriented in the 3 planes of space

Vestibule = central egg-shaped cavity 2 membranous sacs called the saccule & utricle house

equilibrium receptor regions that respond to the pull of gravity & report on changes in head position


Parts of the inner ear / labyrinth:Cochlea = “snail”

Houses the organ of Corti, the actual organ for hearing The organ of Corti consists of hair cells (actually

specialized microvilli) embedded in the basilar membrane.

The hairs are the receptor cells of the ear sensory neurons at the base of the hairs carry action

potentials from here to the brain via the auditory / vestibulocochlear nerve

Filled with perilymph, fluid similar to and continuous with cerebrospinal fluid

MECHANISMS OF HEARING:

A vibrating object creates pressure waves in the surrounding air

These waves are picked up by the outer ear & channeled inward, causing the eardrum to vibrate

Vibrations pass through the hammer, anvil & stirrup in the middle ear

The stirrup transmits the vibrations to the inner ear through the oval window

MECHANISMS OF HEARING:

As the stirrup transmits the vibrations to the inner ear, pressure waves pass through the cochlea

This causes the basilar membrane to vibrate These vibrations cause the hair-like

projections on the hair cells to bend A receptor potential results, and

neurotransmitters are released

Neurotransmitters cross the synapse & cause an action potential in a sensory neuron

The sensory neuron sends the action potential through the auditory nerve to the brain

Eardrum malleus incus stapes oval window of inner ear perilymph & endolymph of the cochlea 8th cranial nerve brain (auditory area in the temporal lobe of the cerebral cortex)

HOW IS VOLUME DETERMINED?

The louder the sound, the greater the pressure wave it generates

greater amplitude of the wave = more vigorous vibrations of the fluid in the cochlea = more pronounced bending of the hair cells = more action potentials generated in the sensory neurons

LOSS OF HEARING:

Caused by the inability to conduct sounds Tinnitus = ringing or buzzing in ears after

exposure to a very loud environment Possible causes of hearing loss:

Middle ear infectionsRuptured eardrumsStiffening of the middle ear bones (old age)Damage to receptor cells or neuronsDamage or destruction of hair cells

BALANCE & EQUILIBRIUM:

The utricle & saccule are membranous sacs in the vestibule, between the cochlea & the semicircular canals

they provide information about the position of the body at rest

Within the utricle & saccule are hair cells embedded in a gelatinous membrane with tiny crystals of calcium carbonate called otoliths


Gravity pulls on the otoliths & bends the hair cells as the position of the head changes

The impulses generated by the hair cells are carried by the 8th cranial nerve to the cerebellum, the midbrain, & the temporal lobes of the cerebrum


The semicircular canals provide information about the body in motion

These fluid-filled membranous ovals are oriented in 3 different planes

At the base of each is an enlarged area called the ampulla which contains hair cells called crista


As the body moves, the hair cells are bent & then they straighten

The bending generates impulses carried by the 8th cranial nerve to the cerebellum, midbrain, and temporal lobes of the cerebrum

These impulses are interpreted as starting or stopping, or changing direction

EFFECTS OF AGING:

By age 60, the deterioration of the organ of Corti becomes noticeable

We are born with ~40,000 hair cells but they are lost due to loud noises, disease, or drugs & they do not regenerate

We lose the ability to hear high-pitched sounds first

Our sense of equilibrium diminishes too – we react more slowly to tilting & fall more

THE SENSE OF VISION:

Our eyes are the sense organs of visionAble to detect many colorsAble to form images of objects near & farAble to respond to minute amounts of light

energy

The eyeball is protected by 6 bones:Lacrimal, maxilla, zygomatic, frontal, sphenoid,

ethmoid Movement of the eyeball is accomplished by

6 muscles:4 rectus muscles2 oblique musclesThese muscles are innervated

by cranial nerves 3,4,& 6

THE ANATOMY OF VISION:

The outer surface of the eyeball is a tough whitish layer of ct called the sclera

At the front of the eye, the sclera becomes the transparent cornea

The sclera surrounds a pigmented layer called the choroid

At the front of the eye, the choroid forms the iris, which gives the eye its color


The muscles of the iris regulate the size of the pupil, the opening that lets light into the interior of the eye

Light passes through the pupil to the lens which is held in position by ligaments

THE ANATOMY OF VISION: The lens focuses images on the retina,

which contains photoreceptor cells to transduce light energyPhotoreceptor cells are highly concentrated at

the retina’s center of focus, the foveaThere are no photoreceptor cells at the area

where the optic nerve passes through the back of the eye = “blind spot”


Action potentials pass via sensory neurons in the optic nerve to the visual centers of the brain

2 fluid-filled chambers make up the bulk of the eye & helps maintain its shape:Vitreous humor

Fills the large chamber behind the eye

THE ANATOMY OF VISION:Aqueous humor

Fills the small chamber in front of the lens The fluid circulates supplying nutrients & O2; blockage

of the ducts that drain this fluid can cause glaucoma


Conjunctiva = a thin mucous membrane that keeps the outside of the eye moist

Conjunctivitis = “pink eye” = infection of the conjunctiva; can be bacterial or viral

Lacrimal gland = secretes tears to cleanse & moisten the eye surface

THE PHYSIOLOGY OF VISION:

The lens of the eye focuses light onto the retina by bending (refracting) light rays

Focusing is accomplished by changing the shape of the lensThe thicker the lens, the more sharply it

bends lightThe lens becomes thicker & rounder to focus

on nearby objectsThe lens becomes flatter to focus on distant

objects

THE PHYSIOLOGY OF VISION:color vision

In the retina, there are 125 million rod cells & 6 million cone cells

These two photoreceptor cells are named for their shape: Cones

Stimulated by bright light Can distinguish color Do not function in night vision Found in a high concentration in the fovea Contain visual pigments called photopsin 3 types of cones = blue, green, red (deficiency = color blindness)

THE PHYSIOLOGY OF VISION:color vision

Rods Extremely sensitive to light Enable us to see in dim light Only give shades of gray Found in the greatest density at the outer edges of

the retina, completely absent from the fovea Contain visual pigments called rhodopsins which

absorb dim light

THE PHYSIOLOGY OF VISION:

Rods & cones are stimulus transducers When rhodopsin & photopsin absorb light,

they change chemically, which alters the permeability of the cell membrane

The resulting receptor potentials trigger a complex integration process that begins in the retina

Action potentials carry the partly integrated information into the brain via the optic nerve

3-D perceptions result from further integration in several processing centers of the cerebral cortex

We have stereoscopic vision. Each eye receives a slightly different view of the visual field. These views are fused to provide depth perception

VISION PROBLEMS & CORRECTIONS: Errors of refraction

Nearsightedness = myopiaCannot focus well on distant objects, but can

see well at short distancesThe eyeball is longer than normal & the lens

cannot flatten enough to compensateDistant objects are focused in front of the

retina instead of on itCorrected by glasses that are thinner at the

middle than at the edges


Farsightedness = hyperopiaCannot focus well at short distances, but can

see distant objects normally The eyeball is shorter than normal nearby objects are focused behind the retinaCorrected by glasses that are thicker at the

middle than at the edgesPresbyopia = age-related farsightedness due

to a less elastic lens.


Astigmatism = blurred vision caused by a misshapen lens or corneaMakes light rays converge unevenly & not

focus at any one point on the retinaCorrective lenses are asymmetrical in a way

that compensates for the asymmetry in the eye


Surgical procedures to correct vision disorders:RK = radial keratotomy = a knife is used to cut

slits in the cornea to change its shapePRK (photorefractive keratectomy) & LASIK

(laser-assisted in situ keratomileusis) = a laser is used to reshape the cornea and change its focusing power

MORE VISION PROBLEMS:

Night blindnessThe inability to see well in dim light or at nightUsually caused by a deficiency of vitamin A,

which is necessary to synthesize rhodopsin Color blindness

An X-linked genetic disorder in which one of the three sets of cones is lacking or nonfunctional

Total colorblindness is rare, the most common form is red-green colorblindness


Cataracts = cloudy or opaque lensesMost common among the elderlyProteins in the lens break down and lose their

transparencyContributing factors include smoking & long-

term exposure to UV lightSmall cataracts can be destroyed by lasers,

artificial lenses can be surgically implanted


Glaucoma =Normal = the presence of aqueous humor

creates a pressure in the eye called intraocular pressure

When the intraocular pressure increases to levels that cause compression of the retina and optic nerve, it can lead to blindness

Can be caused by high blood pressure & diabetes

Can be controlled by medication if caught early

THE SPECIAL SENSES Taste, smell, touch, sight, hearing, & balance.

Documents

Transcript of THE SPECIAL SENSES Taste, smell, touch, sight, hearing, & balance.