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Senses
Sensation: response to environment (i.e. stimulus) via generation of
nerve impulse
-nerve impulses from the body are sent via ascending tracts in
spinal cord to the brain
-from the special senses – in through the brain stem
-sensation occurs upon arrival of nerve impulse at cerebral cortex
-perception is the interpretation of these sensations after cortex
processing
Senses
• can be classified in several ways
• based on:
– 1. what they respond to
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1. Chemoreceptors
2. Mechanoreceptors
3. Nociceptors/pain receptors
4. Thermoreceptors
5. Photoreceptors
– 2. how they are structured
• e.g. corpuscles, free nerve endings
– 3. their location
• e.g. exteroceptors, interoceptors, proprioceptors, cutaneous
receptors
General vs. Special Senses
• General
– Touch, pressure, itch, tickle, heat/cold = cutaneous
receptors (touch)
– Proprioception - proprioceptors
• Special
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Sight
Hearing
Vision
Smell
Proprioceptors
-located in muscles, joints and tendons (and the inner ear)
-provide position of limbs and degree of muscle relaxation
-proprioceptor types:
1. sensory neuron wrapped around a muscle fiber (i.e. muscle spindle
2. found embedded in a tendon
3. found in a joint
-stretching of muscle fiber, tendon or flexing of joint stimulates the receptor
Patellar reflex:
-muscle stretch, proprioceptor
fires impulse to spinal cord,
reflex arc results, muscle fiber
response
Cutaneous receptors – General Sensation of
Touch
-located in skin
-receptors can be classified as cutaneous
-can also be classified as thermoreceptors,mechanoceptors, nociceptors
etc….
-can also be classified as free nerve endings (exposed dendrites) or
corpuscles (covered dendrites)
-impulses first sent to primary somatosensory area of brain – postcentral gyrus of parietal lobe
-association areas of the parietal lobe continue the processing
Cutaneous receptors – General Sensation of Touch
-touch receptors: Meissner’s corpuscles(fingertips, lips, tongue, nipples, penis/clitoris)
Merkel disks (epidermis/dermis)
Root hair plexus (root of hair)
-pressure receptors: Pacinian corpuscles (lamellated corpuscle)
-temp receptors: free nerve endings that respond to cold OR warmth
-Krause end bulbs, Ruffini endings
Taste
-taste requires dissolving of substances in saliva
-taste buds: salty, sweet, bitter and sour – more????
-accumulation of neurons called gustatory neurons
-10,000 taste buds found scattered over tongue, soft palate & larynx
-one taste bud responds to one kind of chemical called a tastant
-many taste buds are clustered at specific locations
– e.g. tip of tongue = sweet taste buds
salty
bitter
sour
-taste buds are found on projections called papillae
-buds project gustatory “hairs” into canals found in between adjacent papillae
-canal opens onto the surface of the tongue
-saliva flows over surface of tongue, into the canals and over the gustatory “hairs”
Cranial
Nerve I
-four types of papillae
-only three types of papilla
will bear taste buds
taste papillae:
1. foliate
2. fungiform
3. circumvallate
4. filiform (texture) – no
taste buds
Anatomy of Taste
Buds
• An oval body consisting of
50 receptor cells (gustatory
or taste cells) surrounded by
supporting cells
• each taste neuron projects a
single gustatory hair
projects upward through the
taste pore
• taste pore is exposed to
saliva flowing over tongue
• Basal cells develop into new
receptor cells every 10 days.
Physiology of Taste
• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4 primary tastes
– most sensitive to bitter (poisons)
– least sensitive to salty and sweet
• Mechanism
– dissolved substance binds ligand-gated receptors on the
gustatory hairs  action potential
– action potential results in neurotransmitter release and
stimulation of the 1st order neuron forming either cranial
nerves, VII, IX and X
Gustatory Pathway
• gustatory fibers/axons found in three cranial nerves
– VII (facial) serves anterior 2/3 of tongue
– IX (glossopharyngeal) serves posterior 1/3 of tongue
– X (vagus) serves palate & epiglottis
• Signals travel to thalamus - extend from the thalamus to
the primary gustatory area on parietal lobe of the cerebral
cortex
• provide conscious perception of taste
• Taste fibers also extend to limbic system – association of
taste with a memory
Smell
-requires dissolving of chemicals in mucus
-neurons = olfactory cells - located within olfactory epithelium in the nasal cavity
-olfactory epithelium covers superior nasal conchae and cribriform plate
-contains neurons (olfactory cells) and supportive cells and basal cells
Olfactory epithelium
• Olfactory receptors
– bipolar neurons with cilia or
olfactory hairs
• Supporting cells
– columnar epithelium
• Basal cells = stem cells
– replace receptors monthly
• Olfactory glands
– produce mucus
• Both epithelium & glands
innervated by cranial nerve
VII.
Eustacian tube
With tubal tonsil
Olfaction: Sense of Smell
• Odorants bind to
receptors
• Na+ channels open
• Depolarization occurs
• Nerve impulse is
triggered
• each olfactory neuron
can respond to
multiple odorants
Olfactory Pathway
• Olfactory neurons pass through the foramina in the
cribriform plate
• Synapse with the neurons of the olfactory bulb
• Axons of these neurons form the olfactory tract
• bulb + tract = Cranial nerve I
• CNI synapses on the primary olfactory area of
temporal lobe
– conscious awareness of smell begins
• Other pathways lead to association areas in frontal lobe
(e.g. Brodmann area 11) where identification of the odor
occurs
Adaptation & Odor Thresholds
• Adaptation = decreasing sensitivity of repeated
stimuli over time
• Olfactory adaptation is very rapid
– 50% in 1 second
– complete in 1 minute
• Low threshold
– only a few molecules need to be present
e.g. methyl mercaptan added to natural gas as warning
Hearing & Equilibrium
-outer ear: pinna - cartilage and skin
-for collection of sound waves
-middle ear: tympanic membrane and 3 ossicles (malleus, incus, stapes)
-transmission of sound waves to inner ear
-inner ear: cochlea (hearing), saccule, utricle & three semicircular canals (balance)
External Ear
• Function = collect sounds
• Structures
– auricle or pinna
• elastic cartilage covered with
skin
– external auditory canal
• curved 1” tube of cartilage & bone
leading into temporal bone
• ceruminous glands produce cerumen
= ear wax
– tympanic membrane or eardrum
• epidermis, collagen & elastic fibers, simple cuboidal epith.
• Perforated eardrum (hole is present)
– at time of injury (pain, ringing, hearing loss, dizziness)
– caused by explosion, scuba diving, or ear infection
Middle Ear Cavity
• Air filled cavity in the temporal
bone
• Separated from the external ear by
the tympanic membrane
• Contains 3 ear ossicles connected
by synovial joints
– malleus attached to tympanic
membrane, incus &
stapes attached to the membrane of
oval window
– stapedius and tensor tympani
muscles attach to these ossicles –
prevent large vibrations that would
damage the inner ear
Middle Ear Cavity
Oval window
• Separated from external ear by
eardrum and from internal ear by
oval & round windows
– These windows have membranes
similar to the tympanic membrane
– Will vibrate with the same frequency
as the tympanic membrane
– As the oval window pushes in the
round window must bulge outwards
to keep the pressure within the ear in
a safe zone
Round
window
Middle Ear Cavity
• Eustacian tube leads
to nasopharynx
– helps to equalize
pressure on both sides
of eardrum – better
vibration and better
sound
Inner Ear---Bony & Membranous Labyrinth
• Bony labyrinth = set of
tube-like chambers
carved into the petrous
part of temporal bone
– surrounds & protects
Membranous
Labyrinth
– filled with a fluid called
perilymph
• Membranous labyrinth = set of fluid-filled tubes within the bony
labyrinth
– filled with a fluid called endolymph
– contains sensory receptors for hearing & balance
– comprised of: utricle, saccule, ampulla, 3 semicircular ducts & cochlea
Inner ear in the temporal bone
Cochlear Anatomy
• Cochlea is made of 3 fluidfilled channels
• scala vestibuli, scala tympani
and scala media (cochlear
duct)
• Partitions that separate the channels are Y shaped
– vestibular membrane above & basilar membrane below form a
central fluid filled chamber = scala media (cochlear duct)
Cochlear Physiology
• Vibration of the stapes
upon the oval window
pushes it in  sends
vibrations into the fluid
of the scala vestibuli and
scala tympani
• The movement of the fluid within these scala stimulates the neurons
of the scala media  HEARING
• Fluid pressure is dissipated at round window - which bulges out as
the oval window pushes in
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within the cochlear duct is the organ of hearing = Organ of Corti
Organ of Corti made up of a top Tectorial membrane and a bottom Basilar membrane
“sandwiching” a collection of sensory neurons
Sensory neurons are called hair cells –bear large cilia called stereocilia
Stereocilia embed themselves in the tectorial membrane
Fluid vibrations moving through the scala vestibuli and tympani push in on the fluid in the
cochlear duct
This shifts the basilar membrane back and forth and bends the hair cells that are “stuck” in
the tectorial membrane
Results in an action potential – stimulates the sensory neurons of the 8th cranial nerve
(acoustic or vestibulocochlear nerve)  processing in temporal lobe
Scala vestibuli
Vestibular membrane
Tectorial membrane
Basilar membrane
Scala tympani
Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head) relative to the force of gravity
– macula receptors within saccule & utricle
• Dynamic equilibrium
– maintain body position (head) during
sudden movement of any type--rotation,
deceleration or acceleration
– crista receptors within ampulla
of semicircular ducts
Saccule & Utricle
• Located between the
semicircular canals and the
cochlea = area known as
vestibule
• Oval window positioned
over utricle
• Round window positioned
over saccule
• Both contain receptors
known as the macula
Static equilibrium: Saccule & Utricle
• Thickened regions called
macula within the saccule &
utricle
• macula made up of:
– hair cells (neurons with
stereocilia)
– supporting cells that secrete
gelatinous otolithic membrane
• otolithic membrane contains
calcium carbonate crystals
called otoliths that move
when you tip your head
• head movement and otolith
movement bends the hair
cells and results in action
potentials
• 3 SC canals: anterior, posterior &
horizontal/lateral
• detect different movements
• small elevation within each of three
semicircular ducts = ampulla
Dynamic equilibrium: Semicircular Ducts
• Hair cells in the ampulla are
covered with cupula of
gelatinous material
• When you move, fluid in the
canal bends cupula
stimulating hair cells to
produce an action potential
• The AP stimulates the
neurons of the
vestibulocochlear nerve
(CNVIII)
• Signals run from CNVIII to
the temporal lobe, motor
cortex, cerebellum and to the
medulla
Vision
-sight is generated by the bending and focusing of light onto the
retina
• Anterior cavity (anterior to lens)
– filled with a liquid aqueous humor
• produced by the ciliary body
• continually drained
• replaced every 90 minutes
– 2 chambers
• anterior chamber between cornea and iris
• posterior chamber between iris and lens
• Posterior cavity (posterior to lens)
– filled with vitreous body (jellylike)
– formed once during embryonic life
Accessory Structures of Eye
• Eyelids or palpebrae
– protect & lubricate
– epidermis, dermis, CT,
orbicularis oculi m., tarsal
plate, tarsal glands &
conjunctiva
• Tarsal glands
– oily secretions keep lids
from sticking together
• Conjunctiva
– palpebral & bulbar
– stops at corneal edge
– dilated BV--bloodshot
Extraocular Muscles
• Six muscles that insert
on the exterior surface
of the eyeball
• Innervated by CN III,
IV or VI.
• 4 rectus muscles -superior, inferior,
lateral and medial
• 2 oblique muscles -inferior and superior
Lacrimal Apparatus
• about 1 ml of tears produced per day. Spread over eye by
blinking. Contains bactericidal enzyme called lysozyme.
Tunics (Layers) of Eyeball
• Fibrous Tunic
(outer layer)
– Sclera and
Cornea
• Vascular Tunic
(middle layer)
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Choroid layer
Iris
Ciliary body
Lens
• Nervous Tunic
(inner layer)
– retina
CORNEA
• Transparent & curved
• Helps focus light by bending
(refraction)
• 3 layers of epithelia, collagen fibers
and fibroblasts
• Shape has a “memory”
• Nourished by tears & aqueous
humor
SCLERA
• “White” of the eye
• Dense irregular connective tissue layer
• Provides shape & support, attachment for
eye muscles
• Posteriorly pierced by Optic Nerve (CNII)
Fibrous Tunic
Vascular Tunic
• Choroid
– majority of vascular layer
– found on top of the sclera
and under the retina
– pigmented epithelial cells
that make melanin
(melanocytes)
– rich in blood vessels that
provide nutrients to retina
– melanin absorbs scattered
light and ensures light
passes through the choroid
and into the retina
Lens:
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Avascular
Crystallin proteins arranged
like layers in onion
Clear capsule & perfectly
transparent
Lens held in place by
suspensory ligaments which
attach to the ciliary bodies
Focuses light on fovea
(center of the retina)
Vascular Tunic
Vascular Tunic
•Ciliary body
–choroid extends to the front of
the eye as ciliary muscles and
ciliary processes
– for controlling the shape of
the lens
–ciliary processes
•folds from the ciliary body
•secrete aqueous humor
–ciliary muscle
• Suspensory ligaments attach lens to ciliary process
• Ciliary muscle controls tension on ligaments & lens
•smooth muscle that alters
shape of lens
•attach to the ciliary
processes
Vascular Tunic
Aqueous Humor
• Continuously produced
by ciliary body
• Flows from posterior
chamber into anterior
through the pupil
• Scleral venous sinus
– canal of Schlemm
– opening in white of eye
at junction of cornea &
sclera
– drainage of aqueous
humor from eye to
bloodstream
Vascular Tunic -- The Iris
• is a colored extension off the
ciliary processes
• Constrictor pupillae
muscles (circular muscles)
– Contraction results in pupil
constriction
– are innervated by
parasympathetic fibers
while
• Dilator pupillae muscles
(radial muscles)
Constriction of the Pupil
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Sharpens vision by preventing blurry edges
Protects retina very excessively bright light
– are innervated by
sympathetic fibers
– Contraction results in pupil
dilation
• Response varies with
different levels of light
Nervous Tunic = Retina
• posterior 3/4 of eyeball
– anterior edge = ora
serrata
• central depression in
retina is the fovea
centralis (inside the
macula lutea)
View with Ophthalmoscope
• accumulation of colorsensitive photoceptors
(cones)
Nervous Tunic = Retina
• Optic disc
– attachment of retina to
the optic nerve
– also known as the blind
spot (no photoreceptors)
Layers of Retina
CHOROID
• Pigmented epithelium
– nonvisual portion
– absorbs stray light &
helps keep image clear
• Retina:
• 3 layers of neurons
– photoreceptor layer
– bipolar neuron layer
– ganglion neuron layer
• 2 other cell types
(modify the signal)
Posterior cavity
– horizontal cells
– amacrine cells
Photoreceptors
-rod and cones
-rod cells: black and white, bright
and dark
-cone cells: color vision
-visual pigment is rhodopsin: opsin and retinal
-visual pigment of a photoreceptor is found in
“discs” in the outer segment of the photoreceptor
-inner segment is the cell body
-synaptic endings for neurotransmitter release
Photoreceptors
•Rods----rod shaped outer segment
–low light photoreceptor
–images in dim light
–90 to 120 million rod cells at the edge of the retina
–peripheral & night vision
•Cones----cone shaped outer segment
–4.5 to 6 million cones concentrated in the fovea centralis
– sharp, color vision
–less sensitive to light than cones but they operate faster (detect changes in
shape, give fine details)
–three types – red, blue and green
Physiology of Vision
• In darkness
– Na+ channels are held open and photoreceptor is always partially
depolarized - continuous release of an inhibitory neurotransmitter onto
bipolar cells
– The bipolar cells are prevented from creating their action potential
– No nerve impulse leaves the retina = NO IMAGE
• In light
– Light is absorbed by the rhodopsin pigment - activates enzymes in the
photoreceptor
– These enzymes cause the closing of Na+ channels – hyperpolarizing the
photoreceptor – no action potential!!
– release of inhibitory neurotransmitter is stopped
– bipolar cells can now spontaneously generate their action potential
– this creates an action potential in the ganglion cells – via an electrical synapse
– a nerve impulse will travel towards the brain = IMAGE
Major Processes of Image Formation
• Refraction of light
– light rays must fall upon the retina –
so they must be bent
– refraction = bending of light as it passes
from one substance (air) into a 2nd
substance with a different density
(cornea)
– in the eye, light is refracted by the
cornea and the lens
• Accommodation of the lens
– changing shape of lens so that light is
focused
• Constriction of the pupil
– less light enters the eye
Refraction by the Cornea & Lens
• image focused on retina is inverted &
reversed from left to right
• brain learns to work with that
information – switches it back in the
occipital lobe
• For objects far away - 75% of
refraction is done by cornea -- rest is
done by the lens
• light rays from more than 20’ are
nearly parallel and only need to be
bent enough to focus on retina –
mainly by the cornea
• For objects close up - light rays from
less than 6’ are more divergent & need
more refraction – cornea and lens
Corneal refraction
Corneal
& lens
refraction
Accommodation & the Lens
• View a distant object
– lens is nearly flat by pulling of suspensory ligaments on the lens
– this requires relaxation of the ciliary muscles!!!!!
• View a close object
– ciliary muscle contracts & decreases the pull on the lens by the
suspensory ligaments
– elastic lens thickens as the tension is removed from it
– this causes a rounding of the lens – more refraction
Accommodation – rounding of the
lens to bend light rays for close up
vision
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Emmetropic eye
(normal)
– can refract light from
20 ft away
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Myopia (nearsighted)
– eyeball is too long
from front to back
– glasses concave
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Hypermetropic
(farsighted)
– eyeball is too short
– glasses convex (cokebottle)
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Astigmatism
– corneal surface wavy
– parts of image out of
focus