To perceive a sensation
Stimulation must be strong enough to initiate response
Receptor must convert stimulus to impulse
Interpretation of impulse must occur in brain
Receptors
Cells that receive information from the environment & then send impulses via conductors to
CNS
Converts stimulus to impulse
Sensory Categories
Somatic receptors- in skin, muscles & joints
Visceral receptors- in internal organs
Special sense receptors- in head
Types of receptors
Photoreceptors- sensitive to light stimuli
Chemoreceptors- sensitive to chemical stimuli
Mechanoreceptors- sensitive to movement
Sense organs receive stimulation & receptor converts stimulus into electrical impulse that is sent to
specific region of brain
All impulses generated by different sense organs are electrically similar but region of brain where they
go varies depending on which brain lobe is in charge of those stimuli
Taste & Smell
Special chemoreceptors allow for senses of taste & smell
Taste buds- clusters of chemoreceptors for taste; found on tongue, roof of mouth & in throat
Papillae- small projections on tongue that house most of the 10,000 taste buds; gives tongue
velvet-like appearance
Substance must be in solution before its flavor can be detected
Solution enters taste buds through small opening & stimulates nerve endings that line inner
surface
Nerve endings begin impulse that travels to parietal lobe of brain to be interpreted as taste
Humans detect salt, sweet, sour & bitter flavors
Other flavors perceived by receptors in nasal passages & as result of smelling food rather than
tasting them
Olfactory receptors- ciliated chemoreceptors located in olfactory epithelium of nasal passage; lie within
mucus lining; short life span (~60 days)
Special olfactory receptors respond to different chemicals
Chemicals as gas or particles enter nose & dissolve in watery mucus before they stimulate
olfactory receptors
Olfactory receptors generate impulses that travel through olfactory nerve to temporal lobe
Sight
Wall of eyeball has 3 layers
Sclera- sclerotic coat
Outer layer of white opaque tough fibrous membrane
Helps maintain shape
Forms transparent cornea where the light waves pass into the eyeball
Cornea is extremely sensitive to touch so has many nerve endings but lacks
blood vessels (this lack of blood vessels is why they can be transplanted)
Uvea- middle layer of eye
Choroid coat
Supplies blood vessels (carry nutrients/wastes) to layers of eye
Contains pigment granules that prevent reflection of light within eye & absorb
stray light that would otherwise blur image
Ciliary body
Two smooth muscles that support & modify shape of lens
Part of anterior chamber of eye
Contraction of muscles causes light to bend more
Iris
Regulates amount of light entering eye
Two muscle layers: sphincter (circular fibers which decrease size of pupil)
& radiate (fibers which increase size of pupil)
Papillary dilator muscles contract & allow more light into eye
Colored portion of eye due to pigment found in muscle layers
Back layer contains large granules of blue pigment & front layer will
either have no pigment or contain melanin
Eyes of all newborns are blue since it takes time for melanin to form
Pupil Opening for light to enter eye
Retina- innermost layer
Completely lines inner surface & contains cells responsible for converting light into
sensory impulses
Vein-like structure is nerve ganglia
Optic nerve leaves here to send messages to brain; blind spot results from lack of
neurons where optic nerve leaves the eye
Photoreceptor cells are sensitive to damage from heat/light
Rods- nerve cells containing light sensitive molecules for vision in dim light
Outer segment contains photosensitive chemical rhodopsin or visual
purple; 120 million/retina; several rods share one neuron to brain;
detect light in shades of gray
Cones- conical shaped; responsible for color vision; 3 kinds- red (L), green (M),
& blue (S); names of cones based on light wavelength; each cone
contains photochemical substance sensitive to that wavelength; each
cone has its own neuron to brain; 6 million/retina; sensitive to color &
bright light; center of retina only has these while edges of retina contain
mixture of rods/cones
Visible color is produced by combining the various stimulations of cones
in proper proportions & these signals are relayed to brain for
color analysis to take place
Lens- behind pupil & iris
Disc shaped elastic & transparent
Lacks blood vessels
Held in place by ligament attached to ciliary muscle which controls amount of tension exerted
on lens
Job is to converge or focus light rays
Vision is most acute when light rays focus on fovea centralis
Only a limited part of the electromagnetic spectrum can excite our photoreceptors
When light energy stimulates rods/cones, nerve impulses are produced/transmitted to visual
center in occipital lobe of brain to be interpreted
Photoreceptors are activated when light produces chemical change in pigment molecules in the
receptor cells
Lens focuses image on retina where image will always be upside down/reversed but brain
corrects this
Bright light dazzles eyes & causes pain because the intense light bleaches the rhodopsin
Dark adaptation- rhodopsin must accumulate in rods for vision in dimmer light; gradually increases &
max sensitivity is reached in 20 min
Positive afterimage- carryover of visual impressions that are exactly the same as the original stimulus
pattern; ex. Movie
Negative afterimage- some visual image remains even though external stimulus has stopped; probably
due to fatigue of originally stimulated receptors
Some Disorders of Vision
Astigmatism- caused by imperfect curvature of cornea or lens so light doesn’t come to a single sharp
focal point on retina; eyestrain & headaches common; corrected by prescription lens
Myopia- nearsightedness; image focuses in front of retina usually because eye is too long for refraction
of lens; can see up close but not far away without help of concave lens to change focal point to
fall on retina
Hypermetropia- hyperopia; farsightedness; image focuses behind the retina because eye is too short
for refraction of lens; can see near objects only with help of convex lens
Presbyopia- ciliary muscles that change shape of lens lose their contractive power; eyeglasses are
needed for reading; loss of elasticity of lens; common in individuals over 50
Colorblindness- deficiency in color perception; usually absence of red or green cones; may be inherited
(8% of males) or result of injury or disease
Monochromats- people who see only black/white; total colorblindness; rare; lack of cones or
have defect in visual cortex of brain
Dichromats- red/green or blue/yellow (rarer) colorblind due to problem in cone receptors or
neurological confusion of colors; genes for M&L cones are found on X chromosome
Trichromats- normal color vision
Conjunctiviitis- pink eye; viral or bacterial infection; can be an allergic reaction, or due to chemical
irritants or leaving contacts in too long; results in dilation of blood vessels in conjunctiva
Cataracts- clouding of the normally transparent lens of eye; proteins in lens change shape & lens
becomes cloudy; result from injury, heredity, birth defects, dehydration, cigarette smoking,
sunlight exposure; can be surgically removed
Glaucoma- increased fluid pressure within eye which can damage the optic nerve & cause gradual
blindness; damage is irreversible but medication/surgery can help decrease pressure;
degenerative disease; 2-3 million US afflicted; begins with decrease in peripheral vision; optic
nerve enlarges & becomes assymetrical
Retinal detachment- retina separates from back of eye which can lead to vision loss or blindness;
common cause is shrinking of vitreous fluid in eye which is normal & expected with age;
requires surgery & >90% can be repaired; torn retinas are treated by lasers or freezing to
prevent detachment; nearsighted people at greater risk since retinal layer is thinner
Macular degeneration- loss of clarity of vision in center of visual field; risk factors: age, smoking,
genetic predisposition
Hearing & Balance
Ear
Sensitive mechanical receptor capable of detecting physical movement of air in form of
sound vibrations & converting them into neural signals sent to brain for interpretation
External ear- pinna
Outside lateral surface of head
Made of flexible cartilage & skin
Serves to gather sound waves into auditory canal which extends into temporal bone
to tympanic membrane
Ceruminous glands in auditory canal produce sticky earwax that keeps foreign objects
out
Middle ear
Small airfilled chamber within temporal bone, walls lined by mucus membrane
Ossicles
Malleus attaches to inner surface of eardrum & incus
Incus attaches to stapes
Stapes base vibrates against membrane on inner wall of middle ear (oval
window)
Sound energy is concentrated/amplified here
Carries sound waves from outer ear to inner ear
Must maintain equal pressure between middle ear & outside or eardrum will not
vibrate properly & hearing will be impaired
If the outside pressure decreases, the middle ear pressure is higher than outside so the
eardrum bulges outward; if the outside pressure increases, the middle ear
pressure is lower than outside so the eardrum bulges inward
Eustachian tube- tube leading from middle ear to pharynx that equalizes pressure
between middle ear & outside
Swallowing or yawning causes muscle to open throat opening of tube allowing air to
move through it & equalize pressure
Inner ear
Bony labyrinth divided into 3 fluid filled canals, membranous labyrinth separated
from body by liquid called perilymph & is filled by endolymph
Vestibule- connects to middle ear by oval window, contains 2 fluid filled sacs which have
hair cells in them; otoliths are small particles of calcium carbonate that entangle
in the hairs
Cochlea- snail shell which has branch of auditory nerve leaving its middle; oval window
begins vibrations in fluid here; functional unit of hearing in inner ear
Organ of Corti- sense receptors are cilia on columnar epithelium; movement of fluid
stimulates cilia & initiates nerve impulses to auditory nerve; changes sound
waves to nerve impulses
How we hear
Stapes transmits vibrations via oval window to fluid in vestibular canal
Cilia in organ of Corti vibrate & waves pass on to cochlea
Auditory nerve picks up impulses which travel to hearing center of temporal lobe of brain
Conductive hearing loss occurs when a problem is in outer or middle ear; may result from blockage of
external ear canal, perforated eardrum, impacted wax, otosclerosis or middle ear infection;
usually corrected by surgery or hearing aid
Nerve hearing impairment is when inner ear has problem; caused by disturbances in inner ear
circulation or fluid pressure or from problems in nerve conduction; not correctable but rarely
total deafness
Semicircular canals
3 looped tubes each lying in a different plane In temporal bone
Where canals meet there is a group of cilia that can detect changes in acceleration/deceleration
Impulses go to medulla oblongata of brain
Helps to maintain proper balance
Static equilibrium- orientation of body relative to ground; when head tilts the otoliths in vestibule
contact hair cells which send nerve impulses to brain
Dynamic equilibrium- orientation of body in relation to movement along horizontal plane; when
accelerating the fluid in semicircular canals cause cilia to bend in direction opposite to motion