Download Lecture notes for Chapter 13

Provides links from and to world outside body
All neural structures outside brain
Sensory receptors
Peripheral nerves and associated ganglia
Efferent motor endings
Input to nervous system
[Chapter 13]
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SENSORY RECEPTORS
Specialized to respond to changes in environment (stimuli)
Activation results in graded potentials that trigger nerve impulses
Sensation (awareness of stimulus) and perception (interpretation of meaning of
stimulus) occur in brain
Classification of receptors:
Based on
Type of stimulus they detect
Location in body
Structural complexity
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Classification by Stimulus Type
Mechanoreceptors—respond to touch, pressure, vibration, and stretch
Thermoreceptors—sensitive to changes in temperature
Photoreceptors—respond to light energy (e.g., retina)
Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood
chemistry)
Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive
pressure, inflammatory chemicals)
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Classification by Location
Exteroceptors
Respond to stimuli arising outside body
Receptors in skin for touch, pressure, pain, and temperature
Most special sense organs
Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
Sometimes cause discomfort but usually unaware of their workings
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Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
Sometimes cause discomfort but usually unaware of their workings
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Proprioceptors
Respond to stretch in skeletal muscles, tendons, joints, ligaments, and
connective tissue coverings of bones and muscles
Inform brain of one's movements
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Simple receptors for general senses
Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and
muscle sense
Modified dendritic endings of sensory neurons
Either nonencapsulated (free) or encapsulated
Nonencapsulated (free) nerve endings
Abundant in epithelia and connective tissues
Most nonmyelinated, small-diameter group C fibers; distal endings have
knoblike swellings
Respond mostly to temperature and pain; some to pressure-induced tissue
movement; itch
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Thermoreceptors
Cold receptors (10–40ºC); in superficial dermis
Heat receptors (32–48ºC); in deeper dermis
Outside those temperature ranges  nociceptors activated  pain
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Nociceptors
Detection protein in nerve ending membranes – vanilloid receptor (transient
receptor potential cation channel subfamily V member 1 – TrpV1)
Ion channel opened by heat, low pH, chemicals, e.g., capsaicin (red
peppers)
Respond to:
Pinching, chemicals from damaged tissue, capsaicin
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Light touch receptors
Tactile (Merkel) discs
Hair follicle receptors
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Note that this part of the textbook graph is about UNENCAPSULATED receptors
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Encapsulated Dendritic Endings
All mechanoreceptors in connective
tissue capsule
Tactile (Meissner's) corpuscles—
discriminative touch
Lamellar (Pacinian) corpuscles—deep
pressure and vibration
Bulbous corpuscles (Ruffini endings)—
deep continuous pressure
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Encapsulated Dendritic Endings
Muscle spindles—muscle stretch
Tendon organs—stretch in tendons
Joint kinesthetic receptors—joint
position and motion
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Somatosensory system – part of sensory system serving body wall and limbs (in the
above schema separate from Special sensory and Visceral sensory)
Receives inputs from
Exteroceptors, proprioceptors, and interoceptors
Input relayed toward head, but processed along way
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Sensation - the awareness of changes in the internal and external environment
To produce a sensation:
Receptors have specificity for stimulus energy
Stimulus must be applied in receptive field
And then transduction occurs (change of energy information – from mechanical
energy [kinetic energy] to electrical [graded potentials] in above example)
Stimulus changed to graded potential
[vocabulary: theses sensory induced graded potentials are called
Generator potential or receptor potential]
Graded potentials must reach threshold  Action Potential
In general sense receptors where the nerve cell itself is the sensory cell, graded
potential called generator potential
Stimulus  Generator potential in afferent neuron  Action potential
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In special sense organs - (b) - right side of picture:
Graded potential in receptor cell called receptor potential (instead of generator
potential) affects amount of neurotransmitter released
Release is proportional to potential. And then graded potential in the afferent neuron
is proportional to the neurotransmitter bound.
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Adaptation is change in sensitivity in presence of constant stimulus
Receptor membranes become less responsive
Receptor potentials decline in frequency or stop
Phasic (fast-adapting) receptors signal beginning or end of stimulus
Examples - receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all
Examples - nociceptors and most proprioceptors
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Phasic (fast-adapting) receptors signal beginning or end of stimulus
Examples - receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all
Examples - nociceptors and most proprioceptors
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Pathways of three neurons conduct
sensory impulses upward to
appropriate cortical regions
First-order sensory neurons
Conduct impulses from receptor level
to spinal reflexes or second-order
neurons in CNS
Second-order sensory neurons
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Transmit impulses to third-order
sensory neurons
Third-order sensory neurons
Conduct impulses from thalamus to
the somatosensory cortex (perceptual
level)
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Survival depends upon sensation and perception
Sensation - the awareness of changes in the internal and external environment
[Stowens modification – sensation is the activity of the cortical brain cells in
response to sensory stimulation]
Perception - the conscious interpretation of those stimuli
[Stowens: perception is the awareness of the sensation]
Levels of neural integration in sensory systems:
1.
Receptor level—sensory receptors
2.
Circuit level—processing in ascending pathways
3.
Perceptual level—processing in cortical sensory areas
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Interpretation of sensory input depends on specific location of target neurons in
sensory cortex
Aspects of sensory perception:
Perceptual detection—ability to detect a stimulus (requires summation of impulses)
Magnitude estimation—intensity coded in frequency of impulses
Spatial discrimination—identifying site or pattern of stimulus (studied by two-point
discrimination test)
Feature abstraction—identification of more complex aspects and several stimulus
properties
Quality discrimination—ability to identify submodalities of a sensation (e.g., sweet
or sour tastes)
Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g.,
melody in piece of music)
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Per relative involvement in innervating somatic and visceral regions of body
Somatic sensory (SS)
Visceral sensory (VS)
Visceral (autonomic) motor (VM)
Somatic motor (SM)
[Chapter 12]
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Myelinated and nonmyelinated nerve fibers allow communication between parts of
spinal cord, and spinal cord and brain
Run in three directions
Ascending – up to higher centers (sensory inputs)
Descending – from brain to cord or lower cord levels (motor outputs)
Transverse – from one side to other (commissural fibers)
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Divided into three white columns (funiculi) on each side
Dorsal (posterior), lateral, and ventral (anterior)
Each spinal tract composed of axons with similar destinations and functions
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Major spinal tracts part of multineuron pathways
Decussation – Pathways cross to other side
Relay – Consist of two or three neurons
Somatotopy – precise spatial relationship
Symmetry – pathways paired symmetrically
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Ascending pathways consist of three neurons:
First-order neuron
Conducts impulses from cutaneous receptors and proprioceptors
Branches diffusely as enters spinal cord or medulla
Synapses with second-order neuron
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Ascending pathways
Second-order neuron
Interneuron
Cell bodies in dorsal horn of spinal cord or medullary nuclei
Axons extend to thalamus or cerebellum
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Third-order neuron
Interneuron
Cell body in thalamus
Axon extends to somatosensory cortex
[No third-order neurons in cerebellum]
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Three main pathways, two transmit somatosensory information to sensory cortex via
thalamus:
1) Dorsal column–medial lemniscal pathways - Axons of second-order neurons cross
to other side, decussate, in medulla
Provide discriminatory touch and conscious proprioception
2) Spinothalamic pathways
3) Spinocerebellar tracts terminate in the cerebellum
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Dorsal column-medial lemniscal pathways
Transmit input to somatosensory cortex for discriminative touch and
vibrations
Composed of paired fasciculus cuneatus and fasciculus gracilis in spinal cord
and medial lemniscus in brain (medulla to thalamus)
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Lateral and ventral spinothalamic tracts – axons of second-order neurons decussate in
spinal cord
Transmit pain, temperature, coarse touch, and pressure impulses within lateral
spinothalamic tract
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Lateral and ventral spinothalamic tracts
Transmit pain, temperature, coarse touch, and pressure impulses within lateral
spinothalamic tract
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Spinocerebellar tracts - ventral and dorsal
Convey information about muscle or tendon stretch to cerebellum
Used to coordinate muscle activity
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[Chapter 15]
Special sensory receptors
Distinct, localized receptor cells in head
Vision
Taste
Smell
Hearing
Equilibrium
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Special sensory receptors
Distinct, localized receptor cells in head
Vision
Taste
Smell
Hearing
Equilibrium
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70% of body's sensory receptors in eye
Visual processing by ~ half cerebral cortex
Most of eye protected by cushion of fat and bony orbit
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Protect the eye and aid eye function
Eyebrows
Eyelids (palpebrae)
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
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Overlie supraorbital margins
Function
Shade eye from sunlight
Prevent perspiration from reaching eye
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Protect eye anteriorly
Separated at palpebral fissure
Meet at medial and lateral commissures
Lacrimal caruncle
At medial commissure
Contains oil and sweat glands
Tarsal plates—supporting connective tissue
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Eyelashes
Nerve endings of follicles initiate reflex blinking
Lubricating glands associated with eyelids
Tarsal (Meibomian) glands
Modified sebaceous glands
Oily secretion lubricates lid and eye
Ciliary glands between hair follicles
Modified sweat glands
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Levator palpebrae superioris
Gives upper eyelid mobility
Blink reflexively every 3-7 seconds
Protection
Spread secretions to moisten eye
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Conjunctiva - transparent mucous membrane
Produces a lubricating mucous secretion
Palpebral conjunctiva lines eyelids
Bulbar conjunctiva covers white of eyes
Conjunctival sac between palpebral and bulbar conjunctiva
Where contact lens rests
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Lacrimal gland and ducts that drain
into nasal cavity
Lacrimal gland in orbit above lateral
end of eye
Lacrimal secretion (tears)
Dilute saline solution containing
mucus, antibodies, and lysozyme
Blinking spreads tears toward medial
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commissure
Tears enter paired lacrimal canaliculi
via lacrimal puncta
Then drain into lacrimal sac and
nasolacrimal duct
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Six straplike extrinsic eye muscles
Originate from bony orbit; insert on
eyeball
Enable eye to follow moving objects;
maintain shape of eyeball; hold in orbit
Four rectus muscles originate from
common tendinous ring; names
indicate movements
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Superior, inferior, lateral, medial rectus
muscles
Two oblique muscles move eye in
vertical plane and rotate eyeball
Superior and inferior oblique muscles
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Six straplike extrinsic eye muscles
Originate from bony orbit; insert on
eyeball
Enable eye to follow moving objects;
maintain shape of eyeball; hold in orbit
Four rectus muscles originate from
common tendinous ring; names
indicate movements
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Superior, inferior, lateral, medial rectus
muscles
Two oblique muscles move eye in
vertical plane and rotate eyeball
Superior and inferior oblique muscles
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Wall of eyeball contains three layers
Fibrous
Vascular
Inner
Internal cavity filled with fluids called humors
Lens separates internal cavity into anterior and posterior segments (cavities)
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Outermost layer; dense avascular connective tissue
Two regions: sclera and cornea
1. Sclera
Opaque posterior region
Protects, shapes eyeball; anchors extrinsic eye muscles
Continuous with dura mater of brain posteriorly
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2. Cornea
Transparent anterior 1/6 of fibrous layer
Bends light as it enters eye
Sodium pumps of corneal endothelium on inner face help maintain
clarity of cornea
Numerous pain receptors contribute to blinking and tearing reflexes
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Middle pigmented layer = uvea
Three regions: choroid, ciliary body, and iris
First region of pigmented layer:
Choroid region [outlined in green on slide]
Posterior portion of uvea
Supplies blood to all layers of eyeball
Brown pigment absorbs light to prevent light scattering and visual confusion
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Second region of pigmented layer:
Ciliary body
Ring of tissue surrounding lens
Smooth muscle bundles (ciliary muscles) control lens shape
Capillaries of ciliary processes secrete fluid
Ciliary zonule (suspensory ligament) holds lens in position
Third region of pigmented layer:
Iris - colored part of eye
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Iris - colored part of eye
Pupil—central opening that regulates amount of light entering eye
Close vision and bright light—sphincter
pupillae (circular muscles) contract;
pupils constrict
Distant vision and dim light—dilator
pupillae (radial muscles) contract; pupils
dilate – sympathetic fibers
Changes in emotional state—pupils
dilate when subject matter is appealing
or requires problem-solving skills
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The lens and ciliary zonule separate eye into two segments
Anterior and posterior segments
Posterior segment contains vitreous
humor that
Transmits light
Supports posterior surface of lens
Holds neural layer of retina firmly
against pigmented layer
Contributes to intraocular pressure
Forms in embryo; lasts lifetime
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Anterior segment composed of two
chambers
Anterior chamber—between cornea
and iris
Posterior chamber—between iris and
lens
Anterior segment contains aqueous
humor
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Plasma like fluid continuously formed
by capillaries of ciliary processes
Drains via scleral venous sinus (canal of
Schlemm) at sclera-cornea junction
Supplies nutrients and oxygen mainly
to lens and cornea but also to retina,
and removes wastes
Glaucoma: blocked drainage of
aqueous humor increases pressure
and causes compression of retina
and optic nerve  blindness
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Biconvex, transparent, flexible, and
avascular
Changes shape to precisely focus
light on retina
Two regions
Lens epithelium anteriorly; Lens fibers
form bulk of lens
Lens fibers are living cells – nucleus
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and most organelles lost during
differentiation
Lens fibers filled with transparent
protein crystallin
Lens becomes more dense, convex,
less elastic with age
cataracts (clouding of lens) consequence of aging,
diabetes mellitus, heavy smoking, frequent
exposure to intense sunlight
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Retina
Originates as outpocketing of brain
Delicate two-layered membrane
first layer = outer Pigmented layer
Single-cell-thick lining
Absorbs light and prevents its scattering
Phagocytize photoreceptor cell fragments
Stores vitamin A
Optic disc (blind spot)
Site where optic nerve leaves eye
Lacks photoreceptors
Quarter-billion photoreceptors of two types
Rods
Cones
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second layer = inner Neural layer (of retina)
Transparent
Composed of three main types of neurons
Photoreceptors, bipolar cells, ganglion cells
Signals spread from photoreceptors to bipolar cells to ganglion cells
Ganglion cell axons exit eye as optic nerve
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Rods
Dim light, peripheral vision receptors
More numerous, more sensitive to light than cones
No color vision or sharp images
Numbers greatest at periphery
Cones
Vision receptors for bright light
High-resolution color vision
Macula lutea exactly at posterior pole
Mostly cones
Fovea centralis
Tiny pit in center of macula with all cones; best vision
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Two sources of blood supply
Choroid supplies outer third (photoreceptors)
Central artery and vein of retina supply inner two-thirds
Enter/exit eye in center of optic nerve
Vessels visible in living person
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Eyes respond to visible light
Small portion of electromagnetic spectrum
Wavelengths of 400-700 nm [wavelength inverse of frequency]
Light
Packets of energy (photons or quanta) that travel in wavelike fashion at high
speeds
Color of light objects reflect determines color eye perceives
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Refraction
Bending of light rays
Due to change in speed when light passes from one transparent
medium to another
Occurs when light meets surface of different medium at an oblique
angle
Curved lens can refract light
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Light passing through convex lens (as in eye) is bent so that rays converge at focal
point
Image formed at focal point is upside-down and reversed right to left
Concave lenses diverge light
Prevent light from focusing
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Pathway of light entering eye:
cornea, aqueous humor, lens,
vitreous humor, entire neural layer
of retina, photoreceptors
Light refracted three times along
pathway
Entering cornea
Entering lens
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Leaving lens
Majority of refractory power in
cornea
Change in lens curvature allows for
fine focusing
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Eyes best adapted for distant vision
Far point of vision
Distance beyond which no change in lens shape needed for focusing
20 feet for emmetropic (normal) eye
Cornea and lens focus light precisely on retina
Ciliary muscles relaxed
Lens stretched flat by tension in ciliary zonule
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Light from close objects (<6 m) diverges as approaches eye – so “needs” to be bent
more to get into eye, lens becomes rounder, more convex
Requires eye to make active adjustments using three simultaneous processes
Accommodation of lenses
Constriction of pupils
Convergence of eyeballs
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Accommodation
Changing lens shape to increase
refraction
Near point of vision
Closest point on which the eye can focus
Presbyopia—loss of accommodation
over age 50
Constriction
Accommodation pupillary reflex
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constricts pupils to prevent most
divergent light rays from entering eye
Convergence
Medial rotation of eyeballs toward
object being viewed
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Myopia (nearsightedness)
Focal point in front of retina, e.g.,
eyeball too long
Corrected with a concave lens
Hyperopia (farsightedness)
Focal point behind retina, e.g., eyeball
too short
Corrected with a convex lens
Astigmatism
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Unequal curvatures in different parts
of cornea or lens
Corrected with cylindrically ground
lenses or laser procedures
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Rods and cones
Modified neurons
Receptive regions called outer segments
Contain visual pigments (photopigments)
Molecules change shape as absorb light
Inner segment of each joins cell body
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Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
Tips fragment off and are phagocytized
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Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
Tips fragment off and are phagocytized
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rods
Functional characteristics
Very sensitive to light
Best suited for night vision and peripheral vision
Contain single pigment
Perceived input in gray tones only
Pathways converge, causing fuzzy, indistinct images
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Cones
Functional characteristics
Need bright light for activation (have low sensitivity)
React more quickly
Have one of three pigments for colored view
Nonconverging pathways result in detailed, high-resolution vision
Color blindness–lack of one or more cone pigments
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rods
Functional characteristics
Very sensitive to light
Best suited for night vision and peripheral vision
Contain single pigment
Perceived input in gray tones only
Pathways converge, causing fuzzy, indistinct images
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Cones
Functional characteristics
Need bright light for activation (have low sensitivity)
React more quickly
Have one of three pigments for colored view
Nonconverging pathways result in detailed, high-resolution vision
Color blindness–lack of one or more cone pigments
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Retinal
Light-absorbing molecule that combines with one of four proteins (opsins) to
form visual pigments
Synthesized from vitamin A
Retinal isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form)
Bent form  straight form when pigment absorbs light
Conversion of bent to straight initiates reactions  electrical impulses
along optic nerve
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Deep purple pigment of rods–rhodopsin
11-cis-retinal + opsin  rhodopsin
Three steps of rhodopsin formation and breakdown
Pigment synthesis
Pigment bleaching
Pigment regeneration
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Pigment synthesis
Rhodopsin forms and accumulates in dark
Pigment bleaching
When rhodopsin absorbs light, retinal changes to all-trans isomer
Retinal and opsin separate (rhodopsin breakdown)
Pigment regeneration
All-trans retinal converted to 11-cis isomer
Rhodopsin regenerated in outer segments
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Light-activated rhodopsin activates G protein transducin
Transducin activates PDE, which breaks down cyclic GMP (cGMP)
In dark, cGMP holds channels of outer segment open  Na+ and Ca2+ depolarize cell
In light cGMP breaks down, channels close, cell hyperpolarizes
Hyperpolarization is signal!
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Similar as process in rods
Cones far less sensitive to light
Takes higher-intensity light to activate cones
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Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)
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When light hyperpolarizes photoreceptor cells
Stop releasing inhibitory neurotransmitter glutamate
Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto
ganglion cells
Ganglion cells generate APs transmitted in optic nerve to brain
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Move from darkness into bright light
Both rods and cones strongly stimulated
Pupils constrict
Large amounts of pigments broken down instantaneously, producing glare
Visual acuity improves over 5–10 minutes as:
Rod system turns off
Retinal sensitivity decreases
Cones and neurons rapidly adapt
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Move from bright light into darkness
Cones stop functioning in low-intensity light
Rod pigments bleached; system turned off
Rhodopsin accumulates in dark
Transducin returns to outer segments
Retinal sensitivity increases within 20–30 minutes
Pupils dilate
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Axons of retinal ganglion cells form optic nerve
Medial fibers of optic nerve decussate at optic chiasma
Most fibers of optic tracts continue to lateral geniculate body of thalamus
Fibers from thalamic neurons form optic radiation and project to primary visual
cortex in occipital lobes
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Fibers from thalamic neurons form optic radiation
Optic radiation fibers connect to primary visual cortex in occipital lobes
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Fibers from thalamic neurons form optic radiation
Optic radiation fibers connect to primary visual cortex in occipital lobes
Other optic tract fibers send branches to midbrain, ending in superior colliculi
(initiating visual reflexes)
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A small subset of ganglion cells in retina contain melanopsin (circadian pigment),
which projects to:
Pretectal nuclei (involved with pupillary reflexes)
Suprachiasmatic nucleus of hypothalamus, timer for daily biorhythms
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Both eyes view same image from slightly different angles
Depth perception (three-dimensional vision) results from cortical fusion of slightly
different images
Requires input from both eyes
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Retinal cells split input into channels
Color, brightness, angle, direction, speed of movement of edges (sudden
changes of brightness or color)
Lateral inhibition decodes "edge" information
Job of amacrine and horizontal cells
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Lateral geniculate nuclei of thalamus
Process for depth perception, cone input emphasized, contrast sharpened
Primary visual cortex (striate cortex)
Neurons respond to dark and bright edges, and object orientation
Provide form, color, motion inputs to visual association areas (prestriate
cortices)
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Occipital lobe centers (anterior
prestriate cortices) continue
processing of form, color, and
movement
Complex visual processing extends
to other regions
"What" processing identifies objects in
visual field
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Ventral temporal lobe
"Where" processing assesses spatial
location of objects
Parietal cortex to postcentral gyrus
Output from both passes to frontal
cortex
Directs movements
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Three major areas of ear
1.
External (outer) ear – hearing only
2.
Middle ear (tympanic cavity) – hearing only
3.
Internal (inner) ear – hearing and equilibrium
Receptors for hearing and balance respond to separate stimuli
Are activated independently
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External Ear
Auricle (pinna)Composed of
Helix (rim); Lobule (earlobe)
Funnels sound waves into auditory canal
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External acoustic meatus (auditory canal)
Short, curved tube lined with skin bearing hairs, sebaceous glands, and
ceruminous glands
Transmits sound waves to eardrum
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Tympanic membrane (eardrum)
Boundary between external and middle ears
Connective tissue membrane that vibrates in response to sound
Transfers sound energy to bones of middle ear
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A small, air-filled, mucosa-lined cavity in temporal bone
Flanked laterally by eardrum
Flanked medially by bony wall containing oval (vestibular) and round
(cochlear) windows
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Epitympanic recess—superior portion of middle ear
Mastoid antrum
Canal for communication with mastoid air cells
Pharyngotympanic (auditory) tube—connects middle ear to nasopharynx
Equalizes pressure in middle ear cavity with external air pressure
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Three small bones in tympanic cavity: the malleus, incus, and stapes
Suspended by ligaments and joined by synovial joints
Transmit vibratory motion of eardrum to oval window
Tensor tympani and stapedius muscles contract reflexively in response to loud
sounds to prevent damage to hearing receptors
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Bony labyrinth
Tortuous channels in temporal bone
Three regions: vestibule, semicircular canals, and cochlea
Filled with perilymph – similar to CSF
Membranous labyrinth
Series of membranous sacs and ducts
Filled with potassium-rich endolymph
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Blue structure – series of ducts
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Central egg-shaped cavity of bony labyrinth
Contains two membranous sacs
1.
Saccule is continuous with cochlear duct
2.
Utricle is continuous with semicircular canals
These sacs
House equilibrium receptor regions (maculae)
Respond to gravity and changes in position of head
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Three canals (anterior, lateral, and posterior) that each define ⅔ circle
Lie in three planes of space
Membranous semicircular ducts line each canal and communicate with utricle
Ampullae of each duct houses equilibrium receptor region called the crista
ampullaris
Receptors respond to angular (rotational) movements of the head
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A spiral, conical, bony chamber
Size of split pea
Extends from vestibule
Coils around bony pillar (modiolus)
Contains cochlear duct, which houses spiral organ (organ of Corti) and ends
at cochlear apex
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Cavity of cochlea divided into three chambers
Scala vestibuli—abuts oval window, contains perilymph
Scala media (cochlear duct)—contains endolymph
Scala tympani—terminates at round window; contains perilymph
Scalae tympani and vestibuli are continuous with each other at helicotrema (apex)
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The "roof" of cochlear duct is vestibular membrane
External wall is stria vascularis – secretes endolymph
"Floor" of cochlear duct composed of
Bony spiral lamina
Basilar membrane, which supports spiral organ
The cochlear branch of nerve VIII runs from spiral organ to brain
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EM viewed from tectorial membrane
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Sound is
Pressure disturbance (alternating areas of high and low pressure) produced by
vibrating object
Sound wave
Moves outward in all directions
Illustrated as an S-shaped curve or sine wave
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Frequency
Number of waves that pass given point in given time
Pure tone has repeating crests and troughs
Wavelength
Distance between two consecutive crests
Shorter wavelength = higher frequency of sound
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Amplitude
Height of crests
Amplitude perceived as loudness
Subjective interpretation of sound intensity
Normal range is 0–120 decibels (dB)
Severe hearing loss with prolonged exposure above 90 dB
Amplified rock music is 120 dB or more
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Pitch
Perception of different frequencies
Normal range 20–20,000 hertz (Hz)
Higher frequency = higher pitch
Quality
Most sounds mixtures of different frequencies
Richness and complexity of sounds (music)
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Sound waves vibrate tympanic membrane
Ossicles vibrate and amplify pressure at oval window
Cochlear fluid set into wave motion
Pressure waves move through perilymph of scala vestibuli
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Waves with frequencies below threshold of hearing travel through helicotrema and
scali tympani to round window
Sounds in hearing range go through cochlear duct, vibrating basilar membrane at
specific location, according to frequency of sound
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Waves with frequencies below threshold of hearing travel through helicotrema and
scali tympani to round window
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Sounds in hearing range go through cochlear duct, vibrating basilar membrane at
specific location, according to frequency of sound
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Fibers near oval window short and stiff
Resonate with high-frequency pressure waves
Fibers near cochlear apex longer, more floppy
Resonate with lower-frequency pressure waves
This mechanically processes sound before signals reach receptors
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Cells of spiral organ
Supporting cells
Cochlear hair cells
One row of inner hair cells
Three rows of outer hair cells
Have many stereocilia and one kinocilium
Afferent fibers of cochlear nerve coil about bases of hair cells
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Stereocilia
Protrude into endolymph
Longest enmeshed in gel-like tectorial membrane
Sound bending these toward kinocilium
Opens mechanically gated ion channels
Inward K+ and Ca2+ current causes graded potential and release
of neurotransmitter glutamate
Cochlear fibers transmit impulses to brain
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Stereocilia
Protrude into endolymph
Longest enmeshed in gel-like tectorial membrane
Sound bending these toward kinocilium
Opens mechanically gated ion channels
Inward K+ and Ca2+ current causes graded potential and release
of neurotransmitter glutamate
Cochlear fibers transmit impulses to brain
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Impulses from cochlea pass via
spiral ganglion to cochlear nuclei of
medulla
From there, impulses sent
To superior olivary nucleus
Via lateral lemniscus to Inferior
colliculus (auditory reflex center)
From there, impulses pass to medial
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geniculate nucleus of thalamus,
then to primary auditory cortex
Auditory pathways decussate so that
both cortices receive input from
both ears
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Pitch perceived by impulses from specific hair cells in different positions along basilar
membrane
Loudness detected by increased numbers of action potentials that result when hair
cells experience larger deflections
Localization of sound depends on relative intensity and relative timing of sound
waves reaching both ears
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Vestibular apparatus
Equilibrium receptors in semicircular canals and vestibule
Vestibular receptors monitor static equilibrium
Semicircular canal receptors monitor dynamic equilibrium
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Sensory receptors for static equilibrium
One in each saccule wall and one in each utricle wall
Monitor the position of head in space, necessary for control of posture
Respond to linear acceleration forces, but not rotation
Contain supporting cells and hair cells
Stereocilia and kinocilia are embedded in the otolith membrane studded with otoliths
(tiny CaCO3 stones)
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Maculae in utricle respond to horizontal movements and tilting head side to side
Maculae in saccule respond to vertical movements
Hair cells synapse with vestibular nerve fibers
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Hair cells release neurotransmitter continuously
Movement modifies amount they release
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Bending of hairs in direction of kinocilia
Depolarizes hair cells
Increases amount of neurotransmitter release
More impulses travel up vestibular nerve to brain
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Bending away from kinocilium
Hyperpolarizes receptors
Less neurotransmitter released
Reduces rate of impulse generation
Thus brain informed of changing position of head
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Sensory receptor for rotational acceleration
One in ampulla of each semicircular canal
Major stimuli are rotational movements
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Each crista has supporting cells and hair cells that extend into gel-like mass called
ampullary cupula
Dendrites of vestibular nerve fibers encircle base of hair cells
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Hair Cell Transduction
Mechanosensitive Ion Channels are gated by Cilia displacement; they are associated
with tonic release of Glutamate at rest but levels can either increase or decrease
depending on direction of Cilia deflection.
TOWARD TALLEST CILIA: Channels open when tip links are stretched causing influx of
K⁺ and Depolarization. This opens Voltage-Gated Ca²⁺ channels at the Basolateral
surface of the Hair Cell triggering ↑ Glutamate release
TOWARD SHORTEST CILIA: Tip link relax = ↓ Glutamate release
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Sensory receptor for rotational acceleration
One in ampulla of each semicircular canal
Major stimuli are rotational movements
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In picture, physical spin produces movement of ampulla from left to right
Because the endolymph doesn’t instantly move with the ampulla, the cupula
bends to the left;
When the endolymph is moving with the ampulla, there is no sensation of
movement;
When physical spin slows and stops, endolymph keeps moving briefly, bending
cupula to right.]
Bending of hairs in the opposite direction causes hyperpolarizations, and
fewer impulses reach the brain
Thus brain informed of rotational movements of head
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Equilibrium information goes to
reflex centers in brain stem
Allows fast, reflexive responses to
imbalance
Impulses travel to vestibular nuclei
in brain stem or cerebellum, both of
which receive other input
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Equilibrium information goes to
reflex centers in brain stem
Allows fast, reflexive responses to
imbalance
Impulses travel to vestibular nuclei
in brain stem or cerebellum, both of
which receive other input
Three modes of input for balance
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and orientation:
Vestibular receptors
Visual receptors
Somatic receptors
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Smell (olfaction) and taste (gustation)
Chemoreceptors respond to chemicals in aqueous solution
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Olfactory epithelium in roof of nasal cavity
Covers superior nasal conchae
Contains olfactory sensory neurons
Bipolar neurons with radiating olfactory cilia
Supporting cells surround and cushion olfactory receptor cells
Olfactory stem cells lie at base of epithelium
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Bundles of nonmyelinated axons of olfactory receptor cells form olfactory nerve
(cranial nerve I)
Unusual bipolar neurons
Thin apical dendrite terminates in knob
Long, largely nonmotile cilia (olfactory cilia) radiate from knob
Covered by mucus (solvent for odorants)
Olfactory stem cells differentiate to replace them
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Humans can distinguish ~10,000 odors
~400 "smell" genes active only in nose
Each encodes unique receptor protein
Protein responds to one or more odors
Each odor binds to several different receptors
Each receptor has one type of receptor protein
Pain and temperature receptors also in nasal cavities
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Gaseous odorant must dissolve in fluid of olfactory epithelium
Activation of olfactory sensory neurons
Dissolved odorants bind to receptor proteins in olfactory cilium membranes
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Odorant binds to receptor  activates G protein
G protein activation  cAMP (second messenger) synthesis
cAMP  Na+ and Ca2+ channels opening
Na+ influx  depolarization and impulse transmission
Ca2+ influx  olfactory adaptation
Decreased response to sustained stimulus
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Olfactory receptor cells synapse with mitral cells in glomeruli of olfactory bulbs
Axons from neurons with same receptor type converge on given type of glomerulus
Mitral cells amplify, refine, and relay signals
Amacrine granule cells release GABA to inhibit mitral cells
Only highly excitatory impulses transmitted
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Impulses from activated mitral cells
travel via olfactory tracts to piriform
lobe of olfactory cortex
Some information to frontal lobe
Smell consciously interpreted and
identified
Some information to hypothalamus,
amygdala, and other regions of
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limbic system
Emotional responses to odor elicited
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Receptor organs are taste buds
Most of 10,000 taste buds on tongue papillae
On tops of fungiform papillae
On side walls of foliate and vallate papillae
Few on soft palate, cheeks, pharynx, epiglottis
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Taste buds:
On tops of fungiform papillae
On side walls of foliate and vallate papillae
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50–100 flask-shaped epithelial cells of 2 types
Gustatory epithelial cells—taste cells
Microvilli (gustatory hairs) are receptors
Three types of gustatory epithelial cells
One releases serotonin; others lack synaptic vesicles but one
releases ATP as neurotransmitter
Basal epithelial cells—dynamic stem cells that divide every 7-10 days
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There are five basic taste sensations
1.
Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts
2.
Sour—hydrogen ions in solution
3.
Salty—metal ions (inorganic salts)
4.
Bitter—alkaloids such as quinine and nicotine; aspirin
5.
Umami—amino acids glutamate and aspartate
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Possible sixth taste
Growing evidence humans can taste long-chain fatty acids from lipids
Perhaps explain liking of fatty foods
Taste likes/dislikes have homeostatic value
Guide intake of beneficial and potentially harmful substances
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To taste, chemicals must
Be dissolved in saliva
Diffuse into taste pore
Contact gustatory hairs
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Binding of food chemical (tastant) depolarizes taste cell membrane 
neurotransmitter release
Initiates a generator potential that elicits an action potential
Different thresholds for activation
Bitter receptors most sensitive
All adapt in 3-5 seconds; complete adaptation in 1-5 minutes
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Gustatory epithelial cell depolarization caused by
Salty taste due to Na+ influx (directly causes depolarization)
Sour taste due to H+ (by opening cation channels)
Unique receptors for sweet, bitter, and umami coupled to G protein gustducin
Stored Ca2+ release opens cation channels  depolarization 
neurotransmitter ATP release
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Cranial nerves VII and IX carry impulses from taste buds to solitary nucleus of medulla
Impulses then travel to thalamus and from there fibers branch to
Gustatory cortex in the insula
Hypothalamus and limbic system (appreciation of taste)
Vagus nerve transmits from epiglottis and lower pharynx
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Triggers reflexes involved in digestion
Increase secretion of saliva into mouth
Increase secretion of gastric juice into stomach
May initiate protective reactions
Gagging
Reflexive vomiting
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Taste is 80% smell
Thermoreceptors, mechanoreceptors, nociceptors in mouth also influence tastes
Temperature and texture enhance or detract from taste
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