Chapter 12

Nervous System III:

Senses

General vs. Special

• General Senses

• receptors that are widely distributed

throughout the body

• Special Senses

• specialized receptors confined to

structures in the head

Receptor Types

Chemoreceptors

• respond to changes

in chemical

concentrations

Pain receptors

(Nociceptors)

• respond to tissue

damage

Thermoreceptors

• respond to changes

in temperature

Mechanoreceptors

• respond to

mechanical forces

Photoreceptors

• respond to light

• Sensation – brain becomes aware of sensory

impulse

• Perception – brain interprets sensory

information

• Projection – brain projects the sensation back

to its apparent source

• Sensory Adaptation – ability to ignore

unimportant stimuli to prevent sensory overload

General Senses

senses associated with skin, muscles, joints,

and viscera:

three groups

• exteroceptive senses – senses associated with

body surface; touch, pressure, temperature, pain

• visceroceptive senses – senses associated with

changes in viscera; blood pressure stretching blood

vessels, ingesting a meal

• proprioceptive senses – senses associated with

changes in muscles and tendons

Touch and Pressure Senses

Free nerve endings

• common in

epithelial tissues

• simplest receptors

• sense itching

Meissner’s corpuscles

• abundant in hairless portions

of skin; lips

• detect fine touch; distinguish

between two points on the skin

Pacinian corpuscles

• common in deeper subcutaneous

tissues, tendons, and ligaments

• detect heavy pressure and

vibrations

Touch and Pressure

Receptors

8

Temperature Senses

Warm receptors

• sensitive to temperatures above 25oC (77o F)

• unresponsive to temperature above 45oC (113oF)

Cold receptors • sensitive to temperature between 10oC (50oF) and 20oC

(68oF)

Pain receptors

• respond to temperatures below 10oC

• respond to temperatures above 45oC

9

Sense of Pain

• free nerve endings

• widely distributed

• nervous tissue of brain lacks pain receptors

• stimulated by tissue damage, chemical,

mechanical forces, or extremes in temperature

• adapt very little, if at all

10

Visceral Pain

• pain receptors are the only receptors in viscera

whose stimulation produces sensations

• pain receptors respond differently to stimulation

• not well localized

• may feel as if coming from some other part of

the body (known as referred pain) because of

common nerve pathways

•See page 445 Figure 12.2

11

Referred Pain

Pain Nerve Pathways

Acute pain fibers •Well localized

•Occurs rapidly

•Not felt in deep tissues

•Sharp, fast, pricking pain

•Conducted on

myelinated fibers

•Ceases when stimulus is

removed

•OTC pain relief usually

adequate

Chronic pain fibers

• begins slowly and

increase in intensity

•Dull, aching, burning,

throbbing pain

•Can occur anywhere

•Conducted on

unmyelinated fibers

•May continue after

stimulus is removed

•Narcotics or other

prescriptions needed

Regulation of Pain Impulses

Thalamus

• allows person to be

aware of pain

Cerebral Cortex

• judges intensity of pain

• locates source of pain

• produces emotional and

motor responses to pain

Pain Inhibiting

Substances (Analgesics)

Natural:

•enkephalins

• serotonin

• endorphins

Artificial:

• OTC drugs – aspirin,

Tylenol, Motrin

•Narcotics – morphine,

vicodin, demerol

Special Senses

• Taste

• Smell

• Vision

• Hearing/Balance

TASTE: how does it work?

• Taste buds on tongue on

fungiform papillae

(“mushroom-like projections)

• Chemical must be dissolved

in saliva to be detected

• Taste hairs protruding out of

taste pore absorb dissolved

chemicals

Organ = taste bud

Type of receptor = chemoreceptors

Five taste sensations

• Sweet— tip

• Sour— sides

• Salty— perimeter

• Bitter — posterior

• “umami”— throughout

• Taste buds undergo

rapid sensory

adaptation

Cranial Nerves

of Taste

Anterior 2/3 tongue: Facial

Posterior 1/3 tongue: Glossopharyngeal

Pharynx: Vagus

Gustatory center

in parietal lobe of

cerebral cortex

Smell: How does it work?

• Organ = Olfactory epithelium in nasal cavity

• Type of receptors = chemoreceptors

• Chemicals must be dissolved in mucus before being detected

• Receptor cells have endings that respond to unique proteins

Smell: How does it work?

• Every odor has particular signature that triggers a certain combination of receptor cells

• Undergoes rapid sensory adaptation • Olfactory receptor cells are continually replaced throughout life. Only nerve cells in body that are replaced.

Olfactory epithelium just under cribiform plate (of ethmoid bone) in superior nasal epithelium at midline

Vision

• Organ – Retina

• Receptor type – photoreceptors

• Visual Accessory Organs

• eyelids

• lacrimal apparatus

• extrinsic eye muscles

Eyelid

eyelids- palpebra

•composed of four layers

1. skin

2. muscle

• orbicularis oculi – closes

•levator palperbrae superioris – opens

3. connective tissue

4. conjunctiva - mucous membrane; lines

eyelid and covers portion of eyeball

•tarsal glands – secrete oil onto eyelashes

Human Anatomy, Frolich, Head II: Throat/Larynx

Movement

of eye

Eye movement simulator

(http://cim.ucdavis.edu/ey

es/version1/eyesim.htm)

Lacrimal Apparatus

• lacrimal gland

• lateral to eye

• secretes tears

•Tears contain mucous,

antibodies, lysozyme

(anti-bacterial)

• nasolacrimal duct

• collects from lacrimal

sac

• empties tears into

nasal cavity

External Eye Structures

27

Structure of the Eye

• hollow

• spherical

• wall has 3 layers

• outer fibrous tunic

• middle vascular tunic

• inner nervous tunic

Outer Tunic

Cornea

• anterior portion

• transparent

• light transmission

• light refraction

Sclera

• posterior portion

• opaque

• protection

Middle Tunic

Iris

• anterior portion

• pigmented

• controls light

intensity

Ciliary body

• anterior portion

• pigmented

• holds lens

• moves lens for

focusing

Choroid coat

• provides blood supply

• pigments absorb extra light

30

Anterior Portion of Eye

• filled with aqueous humor

Lens

• transparent

• biconvex

• lies behind iris

• elastic

• held in place by

suspensory ligaments of

ciliary body

32

Ciliary Body

• forms internal ring around front of eye

• ciliary processes – radiating folds

• ciliary muscles – contract and relax to move lens

Accommodation

• changing of lens shape to view objects

Iris

• composed of

connective tissue

and smooth muscle

• pupil is hole in iris

• dim light - radial

muscles cause pupil

to dilate

• bright light - circular

muscles cause pupil

to constrict

Aqueous Humor

• fluid in anterior cavity of eye

• provides nutrients

• maintains shape of anterior portion of eye

36

Inner Tunic

• retina

•contains visual receptors

•continuous with optic nerve

•fovea centralis – center of macula lutea;

produces sharpest vision

• optic disc – blind spot; contains no visual

receptors

• vitreous humor – thick gel that holds retina

flat against choroid coat

•http://www.eschoolonline.com/company/examples/eye/eyedissect.html

Posterior Cavity

• contains vitreous humor – thick gel that

holds retina flat against choroid coat

Cataract – Lens

becomes cloudy

Macular Degeneration

- macula lutea in retina

deteriorates

Eye as lens/optical device

Light path: Cornea Aqueous humor Pupil

Lens Vitreous humor Retina

FOCUS

• Ciliary muscles in ciliary body pull on lens to focus far away

• Elasticity of lens brings back to close focus

• Thus, with age, less elasticity, no close focusfar-sighted

Changes in shape of pupil and lens are due to autonomic NS controlled muscles

(animation of lens

http://artsci.shu.edu/biology/Student%20Pages/Kyle%20Keenan/eye/lensmovementnrve.html)

Visual Nerve Pathway

Light Refraction

Refraction

• bending of light

• occurs when light waves pass at an oblique

angle into mediums of different densities

Types of Lenses

Convex lenses cause

light waves to

converge

Concave lenses

cause light waves to

diverge

Clinical Application

Refraction Disorders

• concave lens corrects

nearsightedness (Myopia)

• convex lens corrects

farsightedness (Hyperopia)

Stereoscopic Vision

• provides

perception of

distance and

depth (3D)

• results from

formation of two

slightly different

retinal images

Focusing On Retina

•image focused on retina is upside down and

reversed from left to right

•Processing by the brain results in the image

appearing in correct position

Human Anatomy, Frolich, Head II: Throat/Larynx

Retina and

photoreceptors

M&M, fig. 16.10

Visual Receptors

Rods

• long, thin projections

• contain light sensitive

pigment called rhodopsin

• hundred times more

sensitive to light than cones

• provide vision in dim light

• produce colorless vision

• produce outlines of objects

Neurons have specialized receptors at end with “photo

pigment” proteins

Visual Receptors

Cones

• short, blunt projections

• contain light sensitive

pigments called erythrolabe,

chlorolabe, and cyanolabe

• provide vision in bright light

• produce sharp images

• produce color vision

Neurons have specialized receptors at end with “photo

pigment” proteins

Visual Pigments

Rhodopsin

• light-sensitive pigment in rods

• decomposes in presence of light

erythrolabe – responds to red

chlorolabe – responds to green

cyanolabe – responds to blue

Hearing

Ear – organ of hearing

Receptor type - Mechanoreceptors

Three Sections

• External – collects sound

• Middle – transmits and amplifies sound

• Inner – senses sound and equilibrium

External Ear

• auricle

• collects sounds waves

• external auditory meatus

• lined with ceruminous

glands

• carries sound to

tympanic membrane

• tympanic membrane

• “ear drum”

•vibrates in response to

sound waves

Middle Ear

• tympanic cavity

•air-filled space in temporal

bone

• auditory ossicles (bones)

• vibrate in response to tympanic

membrane

• malleus, incus, and stapes are

smallest bones in the body

• oval window

• opening in wall of tympanic

cavity

• stapes vibrates against it to

move fluids in inner ear

Auditory Tube

• eustachian tube

• connects middle ear

to throat

• helps maintain equal

pressure on both sides

of tympanic membrane

• usually closed by

valve-like flaps in

throat

Inner Ear

• complex system of

labyrinths (maze) • osseous labyrinth

• bony canal in temporal

bone

• filled with perilymph

• membranous labyrinth

• tube within osseous

labyrinth

• filled with endolymph

Inner Ear

Three Parts of Labyrinths

1. cochlea

• functions in

hearing

2. semicircular canals

• functions in

equilibrium

3. vestibule

• functions in

equilibrium

• Spiral organ is receptor epithelium for hearing

• Stapes makes contact with oval window on cochlea

• Vibrations from stapes transferred to fluid within the

cochlea

Cochlea

how it works

In figure shown

uncoiled, in life

is spiral in

shape

• Basilar membrane running down middle

– different frequencies of vibration move different parts of

basilar membrane

– Thicker at start, vibrates with lower sounds

– Thinner at end, vibrates at higher sound

Cochlea

how it works

In figure shown

uncoiled, in life

is spiral in

shape

Organ of Corti

Organ of Corti is on

upper surface of basilar

membrane

• Contains hearing

receptor cells called hair

cells

• Particular sound

frequencies cause hairs

of receptor cells to bend

• Bending of hairs causes

nerve impulse generated

Equilibrium

2 Types: Static Equilibrium

• sense position of head when body is

not moving

•Ex: nodding head yes or no

Dynamic Equilibrium

•sense rotation and movement of head

and body

•Ex: Cheerleader turning flips

Static Equilibrium

Detected in Utricle

and Saccule • Contains gelatin

material that contains

rock-like crystals of

calcium carbonate called

Otoliths

• Has hair cells called

Mucula

• Movement of otoliths in

fluid against macula

stimulates nerve

impulses

Dynamic Equilibrium

Detected by

Semicircular Canals

•Crista ampullaris-

sensory organ in

bulging part called

ampulla

• Contains hair cells

• Rapid turns of

head or body moves

fluid in canals and

stimulates hair cells

ONE FINAL QUESTION?

What similar feature do

all the special sensory

organs have in

common?

71

Life-Span Changes

Age related hearing loss due to

• damage of hair cells in organ of Corti

• degeneration of nerve pathways to the brain

• tinnitus

Age-related visual problems include

• dry eyes

• floaters (crystals in vitreous humor)

• loss of elasticity of lens

• glaucoma

• cataracts

• macular degeneration

EXTRAS

Human Anatomy, Frolich, Head II: Throat/Larynx

Human Anatomy, Frolich, Head II: Throat/Larynx

Support of Eye--conjunctiva • Mucous membrane that coats inner

surface of eyelid (palpebral part) and then folds back onto surface of eye (ocular part)

• Thin layer of connective tissue covered with stratified columnar epithelium

• Very thin and transparent, showing blood vessels underneath (blood-shot eyes)

• Goblet cells in epithelium secrete mucous to keep eyes moist

• Vitamin A necessary for all epithelial secretions—lack leads to conjunctiva drying up—”scaly eye”

Details: Retina and photoreceptors

• Retina is outgrowth of brain

• Neurons have specialized receptors at end with “photo pigment”

proteins (rhodopsins)

– Rod cells function in dim light, not color-tuned

– Cone cells have three types: blue, red, green

– In color blindness, gene for one type of rhodopsin is deficient, usually red

or green

• Photoreceptors sit on pigmented layer of choroid. Pigment from

melanocytes--melanoma possible in retina!!

• Axons of photoreceptors pass on top or superficial to photoreceptor

region

• Axons congregate and leave retina at optic disc (blind spot)

• Fovea centralis is in direct line with lens, where light is focused most

directly, and has intense cone cell population (low light night vision

best from side of eye)

• Blood vessels superficial to photoreceptors (retina is good sight to

check for small vessel disease in diabetes)

Human Anatomy, Frolich, Head II: Throat/Larynx

• Outer Ear: auricle is elastic cartilage attached to dermis, gathers sound

• Middle ear: ear ossicles transmit and modulate sound

• Inner ear: cochlea, ampullae and semicircular canals sense sound and

equilibrium

Ear/Hearing

M&M, fig. 16.17

Human Anatomy, Frolich, Head II: Throat/Larynx

Middle Ear • External auditory canal ends

at tympanic membrane

which vibrates against

malleus on other side

• Inside middle ear chamber

– malleusincus stapes

which vibrates on oval window

of inner ear

• Muscles that inhibit vibration

when sound is too loud

– Tensor tympani m. (inserts on

malleus)

– Stapedius m. (inserts on

stapes)

M&M, fig. 16.19

Human Anatomy, Frolich, Head II: Throat/Larynx

Inner Ear/Labyrinth

• Static equilibrium, linear motion

– Utricle, saccule are egg-shaped sacs in center (vestibule) of labyrinth

• 3-D motion, angular acceleration

– 3 semicircular canals for X,Y,Z planes

• Sound vibrations

– Cochlea (“snail”)

M&M, fig. 16.20

Auditory Nerve (Acoustic) VIII receives stimulus from all to brain

Vestibular n.—equilibrium Cochlear n.—hearing