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Chapter 16
16-1
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Sense Organs
• Sensory receptors
– properties and
types
• General senses
• Chemical senses
• Hearing and
equilibrium
• Vision
16-2
Properties of Receptors
• Sensory transduction
– convert stimulus energy into nerve energy
• Receptor potential
– local electrical change in receptor cell
• Adaptation
– conscious sensation declines with continued
stimulation
16-3
Receptors Transmit Information
1. Modality - type of stimulus
2. Location
– each sensory receptor receives input from its
receptive field
– sensory projection - brain identifies site of
stimulation
3. Intensity
– frequency, number of fibers and which fibers
4. Duration - change in firing frequency over time
– phasic receptor - burst of activity and quickly adapt
(smell and hair receptors)
– tonic receptor - adapt slowly, generate impulses
continually (proprioceptor)
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Receptive Fields
16-5
Classification of Receptors
• By modality:
– chemo-, thermo-, mechano-, photo- receptors
and nociceptors
• By origin of stimuli
– interoceptors - detect internal stimuli
– proprioceptors - sense body position and
movements
– exteroceptors - detect external stimuli
• By distribution
– general senses - widely distributed
– special senses - limited to head
16-6
Unencapsulated Nerve Endings
• Dendrites not wrapped
in connective tissue
• General sense receptors
– for pain and temperature
• Tactile discs
– associated with cells at
base of epidermis
• Hair receptors
– monitor movement of hair
16-7
Encapsulated Nerve Endings
• Dendrites wrapped by glial
cells or connective tissue
– tactile corpuscles - phasic
• light touch and texture
– krause end bulb - phasic
• tactile; in mucous membranes
– lamellated corpuscles - phasic
• deep pressure, stretch, tickle and
vibration
– ruffini corpuscles - tonic
• heavy touch, pressure, joint
movements and skin stretching
16-8
Somesthetic Projection Pathways
• 1st order neuron (afferent neuron)
– from body, enter the dorsal horn of spinal cord via
spinal nerves
– from head, enter pons and medulla via cranial
nerve
– touch, pressure and proprioception on large, fast,
myelinated axons
– heat and cold on small, unmyelinated, slow fibers
• 2nd order neuron
– decussation to opposite side in spinal cord or
medulla/pons
– end in thalamus, except for proprioception
(cerebellum)
• 3rd order neuron
– thalamus to primary somesthetic cortex of
cerebrum
16-9
Pain
• Nociceptors – allow awareness of tissue injuries
– found in all tissues except the brain
• Fast pain travels in myelinated fibers at 30 m/sec
– sharp, localized, stabbing pain perceived with injury
• Slow pain travels unmyelinated fibers at 2 m/sec
– longer-lasting, dull, diffuse feeling
• Somatic pain from skin, muscles and joints
• Visceral pain from stretch, chemical irritants or
ischemia of viscera (poorly localized)
• Injured tissues release chemicals that stimulate pain
fibers (bradykinin, histamine, prostaglandin)
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Projection Pathway for Pain
• General pathway – conscious pain
– 1st order neuron cell bodies in dorsal root ganglion of
spinal nerves or cranial nerves V, VII, IX, and X
– 2nd order neurons decussate and send fibers up
spinothalamic tract or through medulla to thalamus
• gracile fasciculus carries visceral pain signals
– 3rd order neurons from thalamus reach primary
somesthetic cortex as sensory homunculus
• Spinoreticular tract
– pain signals reach reticular formation, hypothalamus
and limbic
– trigger visceral, emotional, and behavioral reactions
16-11
Pain Signal Destinations
16-12
Referred Pain
• Misinterpreted pain
– brain “assumes” visceral pain is coming from
skin
– heart pain felt in shoulder or arm because
both send pain input to spinal cord segments
T1 to T5
16-13
Referred Pain
16-14
CNS Modulation of Pain
• Intensity of pain - affected by state of
mind
• Endogenous opiods (enkephalins,
endorphins and dynorphins)
– produced by CNS and other organs under
stress
– in dorsal horn of spinal cord (spinal gating)
– act as neuromodulators block
transmission of pain
16-15
Spinal Gating
• Stops pain signals at dorsal horn
– descending analgesic fibers from reticular
formation travel down reticulospinal tract to
dorsal horn
• secrete inhibitory substances that block pain
fibers from secreting substance P
• pain signals never ascend
– dorsal horn fibers inhibited by input from
mechanoreceptors
• rubbing a sore arm reduces pain
16-16
Spinal Gating of Pain Signals
16-17
Chemical Sense - Taste
• Gustation - sensation of taste
– results from action of chemicals on taste
buds
• Lingual papillae
– filiform (no taste buds)
• important for texture
– foliate (no taste buds)
– fungiform
• at tips and sides of tongue
– vallate (circumvallate)
• at rear of tongue
• contains 1/2 of taste buds
16-18
Taste Bud Structure
• Taste cells
– apical microvilli serve
as receptor surface
– synapse with sensory
nerve fibers at their
base
• Supporting cells
• Basal cells
16-19
Physiology of Taste
•
•
Molecules must dissolve in saliva
5 primary sensations - throughout tongue
1.
2.
3.
4.
5.
•
Sweet - concentrated on tip
Salty - lateral margins
Sour - lateral margins
Bitter - posterior
Umami - taste of amino acids (MSG)
Influenced by food texture, aroma,
temperature, and appearance
– mouthfeel - detected by lingual nerve in papillae
•
Hot pepper stimulates free nerve endings
(pain)
16-20
Physiology of Taste
• Mechanisms of action
– activate 2nd messenger systems
• sugars, alkaloids and glutamates bind to
receptors
– depolarize cells directly
• sodium and acids penetrate cells
16-21
Projection Pathways for Taste
• Innervation of taste buds
– facial nerve (VII) - anterior 2/3’s of tongue
– glossopharyngeal nerve (IX) - posterior 1/3
– vagus nerve (X) - palate, pharynx, epiglottis
16-22
Projection Pathways for Taste
• To solitary nucleus in medulla
• To hypothalamus and amygdala
– activate autonomic reflexes
• e.g. salivation, gagging and vomiting
• To thalamus, then postcentral gyrus of
cerebrum
– conscious sense of taste
16-23
Chemical Sense - Smell
• Olfactory mucosa
– contains receptor cells
for olfaction
– highly sensitive
• up to 10,000 odors
– on 5cm2 of superior
concha and nasal
septum
16-24
Olfactory Epithelial Cells
• Olfactory cells
– olfactory hairs
neurons with 20
cilia
• bind odor
molecules in thin
layer of mucus
– axons pass
through cribriform
plate
– survive 60 days
• Supporting
cells
• Basal cells
– divide
16-25
Physiology of Smell
• Molecules bind to receptor on olfactory
hair
– hydrophilic - diffuse through mucus
– hydrophobic - transport by odorant-binding
protein
• Activate G protein and cAMP system
• Opens ion channels for Na+ or Ca2+
– creates a receptor potential
• Action potential travels to brain
• Receptors adapt quickly
– due to synaptic inhibition in olfactory bulbs
16-26
Olfactory Pathway
• Olfactory cells synapse in olfactory bulb
– on mitral and tufted cell dendrites
– in spherical clusters called glomeruli
• each glomeruli dedicated to single odor
16-27
Olfactory Pathway
• Output from bulb forms olfactory tracts
– end in primary olfactory cortex and thalamus
– travel to insula and frontal cortex
– identify odors
– integrate taste and smell into flavor
– travel to hypocampus, amygdala, and
hypothalamus
• memories, emotional and visceral reactions
16-28
Olfactory Pathway
• Feedback
– granule cells in olfactory cortex synapse in
glomeruli
• food smells better when hungry
16-29
Olfactory Projection Pathways
16-30
The Nature of Sound
• Sound - audible vibration of molecules
– vibrating object pushes air molecules
16-31
Pitch and Loudness
• Pitch - frequency vibrates specific parts of ear
– hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec)
– speech is 1500-4000 where hearing is most sensitive
• Loudness – amplitude; intensity of sound energy
16-32
Outer Ear
16-33
Outer Ear
• Fleshy auricle (pinna) directs air vibrations
down external auditory meatus
– cartilagenous and bony, S-shaped tunnel
ending at eardrum
– glandular secretions and dead cells form
cerumen (earwax)
16-34
Anatomy of Middle Ear
16-35
Middle Ear
• Air-filled tympanic cavity in temporal bone
between tympanic membrane and oval window
– continuous with mastoid air cells
• Contains
– auditory tube (eustachian tube) connects to
nasopharynx
• equalizes air pressure on tympanic membrane
– ear ossicles
• malleus
• incus
• stapes
– stapedius and tensor tympani muscles attach to
stapes and malleus
16-36
Anatomy of Inner Ear
16-37
Inner Ear
• Bony labyrinth - passageways in temporal bone
• Membranous labyrinth - fleshy tubes lining bony
tunnels
– filled with endolymph (similar to intracellular fluid)
– floating in perilymph (similar to cerebrospinal fluid)
16-38
Details of Inner Ear
Fig. 16.12c
16-39
Details of Inner Ear
16-40
Anatomy of Cochlea
• Scala media (cochlear duct)
– separated from
• scala vestibuli by vestibular membrane
• scala tympani by basilar membrane
• Spiral organ (organ of corti)
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Spiral Organ
16-42
Spiral Organ
• Stereocilia of hair cells attach to
gelatinous tectorial membrane
• Inner hair cells
– hearing
• Outer hair cells
– adjust cochlear responses to different
frequencies
– increase precision
16-43
SEM of Cochlear Hair Cells
16-44
Physiology of Hearing - Middle Ear
• Tympanic membrane
– has 18 times area of oval window
– ossicles transmit enough force/unit area at
oval window to vibrate endolymph in scala
vestibuli
• Tympanic reflex – muscle contraction
– tensor tympani m. tenses tympanic
membrane
– stapedius m. reduces mobility of stapes
• best response to slowly building loud sounds
• occurs while speaking
16-45
Stimulation of Cochlear Hair Cells
• Vibration of ossicles causes vibration of
basilar membrane under hair cells
– as often as 20,000 times/second
16-46
Cochlear Hair Cells
• Stereocilia of OHCs
– bathed in high K+
• creating electrochemical gradient
– tips embedded in tectorial membrane
– bend in response to movement of basilar
membrane
• pulls on tip links and opens ion channels
• K+ flows in – depolarization causes release of
neurotransmitter
• stimulates sensory dendrites at base
16-47
Potassium Gates
16-48
Sensory Coding
• Vigorous vibrations excite more inner hair
cells over a larger area
– triggers higher frequency of action potentials
– brain interprets this as louder sound
• Pitch depends on which part of basilar
membrane vibrates
– at basal end, membrane narrow and stiff
• brain interprets signals as high-pitched
– at distal end, 5 times wider and more flexible
• brain interprets signals as low-pitched
16-49
Basilar Membrane Frequency Response
Notice high and low frequency ends
16-50
Cochlear Tuning
• Increases ability of cochlea to receive
some sound frequencies
• Outer hair cells contract reducing
basilar membranes freedom to vibrate
– fewer signals from that area allows brain
to distinguish between more and less
active areas of cochlea
• Pons has inhibitory fibers that synapse
near the base of IHCs
– increases contrast between regions of
cochlea
16-51
Innervation of Internal Ear
• Vestibular ganglia - visible in vestibular nerve
• Spiral ganglia - buried in modiolus of cochlea 16-52
Auditory Pathway
16-53
Auditory Projection Pathway
• Spiral ganglion formed by cell bodies of
sensory neurons
• Axons form cochlear nerve portion of CN VIII
• Synapse in cochlear nuclei
• Binaural hearing
– superior olivary nucleus compares sounds from
both sides to identify direction
• Inferior colliculus helps
– locate origin of sound
– process fluctuations in pitch during speech
– produce startle response; head turning to loud
sound
• Fibers from inferior colliculus go to primary
auditory cortex – temporal lobe
16-54
Auditory Processing Centers
• Damage to either auditory cortex does not cause
unilateral deafness (extensive decussation)
16-55
Equilibrium
• Control of coordination and balance
• Receptors in vestibular apparatus
– semicircular ducts contain crista
– saccule and utricle contain macula
• Static equilibrium – perceived by macula
– perception of head orientation
• Dynamic equilibrium
– perception of motion or acceleration
• linear acceleration perceived by macula
• angular acceleration perceived by crista
16-56
Saccule and Utricle
• Contain macula
– hair cells with stereocilia and one kinocilium
buried in a gelatinous otolithic membrane
– otoliths add to the density and inertia and
enhance the sense of gravity and motion
16-57
Macula
•
•
Static equilibrium - when head is tilted, weight of membrane bends the
stereocilia
Dynamic equilibrium – in car, linear acceleration detected as otoliths lag
behind
16-58
Crista ampullaris
• Consists of hair cells buried in a mound of gelatinous
membrane (one in each duct)
• Orientation causes ducts to be stimulated by rotation in
16-59
different planes
Crista Ampullaris - Head Rotation
• As head turns, endolymph lags behind, pushes cupula,
16-60
stimulates hair cells
Vestibular Projection Pathways
16-61
Equilibrium Projection Pathways
• Hair cells of macula sacculi, macula
utriculi and semicircular ducts synapse
on vestibular nerve
• Fibers end in vestibular nucleus in
pons and medulla
16-62
Equilibrium Projection Pathways
• Information sent to 5 targets
– cerebellar control of head and eye
movements and posture
– nuclei of CN III, IV, and VI to produce
vestibulo ocular reflex
– reticular formation control of blood
circulation and posture
– vestibulospinal tracts innervate
antigravity muscles
– thalamic relay to cerebral cortex for
awareness of position and movement
16-63
Vision and Light
• Vision - perception of light emitted or
reflected from objects in the environment
• Visible light
– electromagnetic radiation with wavelengths
from 400 to 750 nm
– must cause a photochemical reaction to
produce a nerve signal
• radiation below 400 nm; energetic, kills cells
• radiation above 750 nm; too little energy to cause
photochemical reaction
16-64
External Anatomy of Eye
16-65
Eyebrows and Eyelids
• Eyebrows provide facial expression
• Eyelids (palpebrae)
– block foreign objects, help with sleep,
blink to moisten
– meet at corners (commissures)
– consist of orbicularis oculi muscle and
tarsal plate covered with skin outside
and conjunctiva inside
– tarsal glands secrete oil that reduces
tear evaporation
– eyelashes help keep debris from eye
16-66
Conjunctiva
• Transparent mucous membrane lines eyelids and
covers anterior surface of eyeball except cornea
• Richly innervated and vascular (heals quickly)
16-67
Lacrimal Apparatus
• Tears flow across eyeball help to wash away
foreign particles, help with diffusion of O2 and
CO2 and contain bactericidal enzyme
16-68
Extrinsic Eyes Muscles
• 6 muscles inserting on eyeball
– 4 rectus, superior and inferior oblique muscles
• Innervated by cranial nerves III, IV and VI
16-69
Innervation of Extrinsic Eye Muscles
16-70
Tunics of the Eyeball
• Fibrous layer - sclera and cornea
• Vascular layer - choroid, ciliary body and iris
• Internal layer - retina and optic nerve
16-71
Optical Components
• Structures refract light to focus on retina
– cornea
• transparent cover on anterior surface of eyeball
– aqueous humor
• serous fluid posterior to cornea, anterior to lens
– lens
• changes shape to help focus light
– rounded with no tension
– flattened due to pull of suspensory ligaments
– vitreous humor
• jelly fills space between lens and retina
16-72
Aqueous Humor
• Produced by ciliary body, flows to posterior
chamber through pupil to anterior chamber reabsorbed into canal of Schlemm
16-73
Cataracts and Glaucoma
• Cataract - clouding of lens
– aging, diabetes, smoking, and UV light
• Glaucoma
– death of retinal cells due to elevated
pressure within the eye
• obstruction of scleral venous sinus
• colored halos and dimness of vision
16-74
Neural Components
• Includes retina and optic nerve
• Retina
– forms as an outgrowth of the diencephalon
– attached at optic disc and at ora serrata
– pressed against rear of eyeball by vitreous
16-75
Detached retina
• Blow to head or lack of vitreous
• Blurry areas in field of vision
• Disrupts blood supply, leads to blindness
16-76
Ophthalmoscopic Exam of Eye
• Macula lutea - cells on visual axis of eye (3 mm)
– fovea centralis - center of macula; finely detailed
images due to packed receptor cells
• Direct evaluation of blood vessels
16-77
Test for Blind Spot
• Optic disk = blind spot
– optic nerve exits posterior surface of eyeball
– no receptor cells
• Blind spot - use test illustration above
– close eye, stare at X and red dot disappears
• Visual filling - brain fills in green bar
across blind spot area
16-78
Formation of an Image
• Light passes through lens to form inverted
image on retina
• Pupillary constrictor - smooth muscle
encircling the pupil
– parasympathetic stimulation narrows pupil
• Pupillary dilator - spokelike myoepithelial cells
– sympathetic stimulation widens pupil
• Active when light intensity changes or gaze
shifts from distant object to nearby object
– photopupillary reflex -- both pupils constrict if one
eye is illuminated (type of consensual reflex)
16-79
Principle of Refraction
Light striking the lens or cornea at a 90 degree
angle is not bent.
16-80
Refraction
• Bending of light rays occurs
when light passes through
substance with different
refractive index at any angle
other than 90 degrees
– refractive index of air
is arbitrarily set to n = 1
– refractive index
• cornea is n = 1.38
• lens is n = 1.40
• Cornea refracts light more than
lens does
– due to shape of cornea
– lens becomes rounder to
increase refraction for near vision
16-81
Near Response
•
Allows eyes to focus on nearby object
(that sends oblique light waves to eyes)
1. convergence of eyes
•
eyes orient their visual axis towards object
2. constriction of pupil
•
blocks peripheral light rays and reduces
spherical aberration (blurry edges)
3. accomodation of lens
•
ciliary muscle contracts, lens takes convex
shape
– light refracted more strongly and focused onto retina
16-82
Emmetropia and Near Response
Fig. 16.31a
Distant object
Close object
16-83
Emmetropia and Near Response
16-84
Accommodation of Lens
16-85
Effects of Corrected Lenses
• Hyperopia - farsighted (eyeball too short)
– correct with convex lenses
• Myopia - nearsighted (eyeball too long)
– correct with concave lenses
16-86
Photoreceptor Cells
• Posterior layer of retina pigment
epithelium
– purpose is to absorb stray light
and prevent reflections
• Photoreceptors
– rod cells (night - scotopic vision)
• outer segment - stack of coinlike
membranous discs studded with
rhodopsin pigment molecules
– cone cells (color - photopic
vision)
• outer segment tapers to a point
16-87
Histology - Layers of Retina
16-88
Location of Visual Pigments
16-89
Nonreceptor Retinal Cells
• Bipolar cells (1st order neurons)
– synapse on ganglion cells
– large amount of convergence
• Ganglion cells (2nd order neurons)
– axons of these form optic nerve
– more convergence occurs (114 receptors
to one optic nerve fiber)
• Horizontal and amacrine cells form
connections between other cells
– enhance perception of contrast, edges of
objects and changes in light intensity
16-90
Schematic Layers of the Retina
16-91
Visual Pigments
• Rod cells have rhodopsin
– has absorption peak at wavelength of 500 nm
– 2 major parts of molecule
• opsin - protein portion
• retinal - a vitamin A derivative
• Cones contain photopsin (iodopsin)
– opsin moieties contain different amino acids
that determine wavelengths of light absorbed
– 3 kinds of cones absorbing different
wavelengths of light produce color vision
16-92
Rhodopsin Bleaching/Regeneration
• Rhodopsin absorbs light, converted from bent
shape (cis-retinal) to straight (trans-retinal)
– retinal dissociates from opsin (bleaching)
– 5 minutes to regenerate 50% of bleached rhodopsin
16-93
Generating Visual Signals
16-94
Generating Nerve Signals - Rods
• In the dark, rods exhibit a dark signal
– flow of Na+ and release of neurotransmitter
(glutamate)
• depolarization by Na+ stimulates glutamate release
• In the light, dark current and glutamate
release stops
– bleached rhodopsin molecule acts like an
enzyme and breaks down cGMP molecules
– Na+ gates close and dark current ceases
(inhibition stops), nerve signal results
16-95
Bipolar Cell Function
• Two kinds of bipolar cells
– inhibited (hyperpolarized) by glutamate
• excited by rising light intensity
– excited (depolarized) by glutamate
• excited by falling light intensity
• As your eye scans a scene, areas of light
and dark cause a changing pattern of
bipolar cell responses
• Variable pattern of stimulation of ganglion
cells and nerve signals sent to the brain
16-96
Light and Dark Adaptation
• Light adaptation (walk out into sunlight)
– pupil constriction and pain from over
stimulated retinas
– color vision and acuity below normal for 5 to
10 minutes
• Dark adaptation (turn lights off)
– dilation of pupils occurs
– 20 to 30 minutes required for regeneration of
rhodopsin
16-97
Duplicity Theory
• Explains why we have both rods and
cones
• Single type of receptor cell incapable
of providing high sensitivity and high
resolution
– sensitive night vision = one type of cell
and neural circuitry
– high resolution daytime vision = different
cell type and neuronal circuitry
16-98
Duplicity Theory
16-99
Scotopic System (Night Vision)
• Rods sensitive – react even in dim light
– extensive neuronal convergence
– 600 rods converge on 1 bipolar cell
– many bipolar converge on each ganglion cell
– results in high degree of spatial summation
• one ganglion cells receives information from 1
mm2 of retina producing only a coarse image
• Edges of retina have widely-spaced rod
cells, act as motion detectors
16-100
Photopic System (Day Vision)
• Fovea contains only 4000 tiny cone cells
(no rods)
– no neuronal convergence
– each foveal cone cell has “private line to
brain”
• High-resolution color vision
– little spatial summation so less sensitivity
to dim light
16-101
Color Vision
• Primates have well
developed color vision
– nocturnal vertebrates
have only rods
• Cones named for
absorption
peaks of photopsins
– blue cones peak sensitivity
at 420 nm
– green cones peak at 531 nm
– red cones peak at 558 nm
(orange-yellow)
• Color perception based on
mixture of nerve signals
16-102
Color Blindness
• Hereditary lack of one photopsin
– red-green is common (lack either red or
green cones)
• incapable of distinguishing red from green
• sex-linked recessive (8% of males)
16-103
Stereoscopic Vision (Stereopsis)
• Depth perception - ability to
judge distance to objects
– requires 2 eyes with
overlapping visual fields
– panoramic vision has eyes
on sides of head (horse)
• Fixation point
– farther away requires image
focus medial to fovea
– closer results in image focus lateral to fovea
16-104
Visual Projection Pathway
16-105
Visual Projection Pathway
• Bipolar and ganglion cells in retina - 1st and 2nd order
neurons (ganglion cell axons of form CN II)
• Hemidecussation in optic chiasm
– 1/2 of fibers decussate so that images of all objects in left
visual field fall on right half of each retina
– each side of brain sees what is on side where it has motor
control over limbs
• 3rd order neurons in lateral geniculate nucleus of
thalamus form optic radiation to 1 visual cortex
where conscious visual sensation occurs
• Few fibers project to superior colliculi and midbrain
for visual reflexes (photopupillary and
accomodation)
16-106
Visual Information Processing
• Some processing occurs in retina
– adjustments for contrast, brightness, motion
and stereopsis
• Visual association areas in parietal and
temporal lobes process visual data
– object location, motion, color, shape,
boundaries
– store visual memories (recognize printed
words)
16-107