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Flashcards in 0-1 Chapter 16 - sense Organs Deck (275):
1

sense organs

nerve tissue surrounded by other tissues that enhance response to certain type of stimulus
•added epithelium, muscle or connective tissue

2

transduction

the conversion of one form of energy to another
–fundamental purpose of any sensory receptor

3

receptor potential

small, local electrical change on a receptor cell brought about by an initial stimulus
•results in release of neurotransmitter or a volley of action potentials that generates nerve signals to the CNS

4

sensation

a subjective awareness of the stimulus
–most sensory signals delivered to the CNS produce no conscious sensation

5

Receptors Transmit Four Kinds of Information

Modality
Location
Intensity
Duration

6

Modality

type of stimulus or the sensation it produces
–vision, hearing, taste

7

labeled line code

all action potentials are identical. Each nerve pathway from sensory cells to the brain is labeled to identify its origin, and the brain uses these labels to interpret what modality the signal represents

8

Location

encoded by which nerve fibers are issuing signals to the brain

9

receptive field

area that detects stimuli for a sensory neuron

10

sensory projection

brain identifies site of stimulation

11

projection pathways

the pathways followed by sensory signals to their ultimate destination in the CNS

12

Intensity

encoded in 2 ways

Strength
frequency

13

Duration

how long the stimulus lasts

14

sensory adaptation

if stimulus is prolonged, the firing of the neuron gets slower over time, and we become less aware of the stimulus

15

phasic receptor

generate a burst of action potentials when first stimulated, then quickly adapt and sharply reduce or stop signaling even though the stimulus continues

16

tonic receptor

adapt slowly, generate nerve signals more steadily

17

Classification of Receptors by

modality
origin of stimuli
distribution

18

by modality

–thermoreceptors, photoreceptors, nociceptors, chemoreceptors, and mechanoreceptors

19

origin of stimuli

–exteroceptors -detect external stimuli
–interoceptors -detect internal stimuli
–proprioceptors -sense body position and movements

20

by distribution

–general (somesthetic) senses -widely distributed
–special senses -limited to head
•vision, hearing, equilibrium, taste, and smell

21

General Senses

structurally simple receptors
–one or a few sensory fibers and a little connective tissue

22

unencapsulated nerve endings

•dendrites not wrapped in connective tissue
–free nerve endings
–tactile (Merkel) discs
–hair receptors (peritrichial endings

23

free nerve endings

–for pain and temperature
–skin and mucous membrane

24

tactile discs

–for light touch and texture
–associated with Merkel cells at base of epidermis

25

hair receptors

–wrap around base of hair follicle
–monitor movement of hair

26

encapsulated nerve endings

•dendrites wrapped by glial cells or connective tissue
•connective tissue enhances sensitivity or selectivity of response

27

encapsulated nerve endings

types

–tactile (Meissner) corpuscles
–Krause end bulbs
–bulbous (Ruffini) corpuscles
–lamellar (pacinian) corpuscles
–muscle spindles
–golgi tendon organs

28

tactile (Meissner) corpuscles

–light touch and texture
–dermal papillae of hairless skin

29

Krause end bulb

–tactile; in mucous membranes

30

lamellated (pacinian) corpuscles

phasic
–deep pressure, stretch, tickle and vibration
–periosteum of bone, and deep dermis of skin

31

bulbous (Ruffini) corpuscles

tonic
–heavy touch, pressure, joint movements and skin stretching

32

Sound receptors are

mechanoreceptors

33

Somesthetic Projection Pathways

from receptor to final destination in the brain, most somesthetic signals travel by way of three neurons

34

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

35

2nd order neuron

–decussation to opposite side in spinal cord, medulla, or pons
–end in thalamus, except for proprioception, which ends in cerebellum

36

3rd order neuron

–thalamus to primary somesthetic cortex of cerebrum

37

pain

discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action
–important since helps protect us

38

nociceptors

two types providing different pain sensations

39

fast pain

travels in myelinated fibers at 12 -30 m/sec
•sharp, localized, stabbing pain perceived with injury

40

slow pain

travels unmyelinated fibers at 0.5 -2 m/sec
•longer-lasting, dull, diffuse feeling

41

somatic pain

from skin, muscles and joints

42

visceral pain

from the viscera
–stretch, chemical irritants or ischemia of viscera (poorly localized

43

bradykinin

most potent pain stimulus known
–makes us aware of injury and activates cascade or reactions that promote healing

44

Projection Pathway for Pain

two main pain pathways to brain, and multiple subroutes

45

first-order neuron cell bodies

in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X

46

spinothalamic tract

most significant pain pathway
–carries most somatic pain signals

47

spinoreticular tract

carries pain signals to reticular formation
–activate visceral, emotional and behavioral reactions to pain

48

referred pain

pain in viscera often mistakenly thought to come from the skin or other superficial site

49

analgesic

(pain-relieving) mechanisms of CNS just beginning to be understood

50

enkephalins

two analgesic oligopeptides with 200 times the potency of morphine

51

endogenous opioids

internally produced opium-like substances
•enkephalins, endorphins, and dynorphins

52

neuromodulators

neuromodulators that can block the transmission of pain signals and produce feelings of pleasure and euphoria

53

spinal gating-

stops pain signals at the posterior horn of the spinal cord

SEE DIAGRAM

54

spinal gating-

rubbing or massaging injury

•pain-inhibiting neurons of the posterior horn receive input from mechanoreceptors in the skin and deeper tissues
–rubbing stimulates mechanoreceptors which stimulates spinal interneurons to secrete enkephalins that inhibit second-order pain neurons

55

gustation

(taste) –sensation that results from action of chemicals on taste buds

MUST BE LIQUID TO TASTE

56

taste buds - location

4000 -taste buds mainly on tongue
–inside cheeks, and on soft palate, pharynx, and epiglottis

57

lingual papillae

4 areas

filiform
foliate
fungiform
vallate (circumvallate)

58

filiform

no taste buds
•important for food texture

59

foliate

no taste buds
•weakly developed in humans

60

fungiform

•at tips and sides of tongue

61

vallate (circumvallate)

•at rear of tongue
•contains 1/2 of all taste buds

62

taste cells

synapse with and release neurotransmitters onto sensory neurons at their base
Have: taste hairs, taste pores,

63

taste hairs

have tuft of apical microvilli(taste hairs) that serve as receptor surface for taste molecules
taste hairs are epithelial cells not neurons

64

taste pores

pit in which the taste hairs project

65

basal cells

stem cells that replace taste cells every 7 to 10 days

66

supporting cells

resemble taste cells without taste hairs, synaptic vesicles, or sensory role

67

Physiology of Taste

to be tasted, molecules must dissolve in saliva and flood the taste pore

68

five primary sensations

salty
–sweet
–sour
–bitter
–umami

69

mouthfeel

detected by branches of lingual nerve in papillae

70

two mechanisms of action

activate 2nd messenger systems
depolarize cells directly
either mechanism results in release of neurotransmitters that stimulate dendrites at base of taste cells

71

activate 2nd messenger systems

•sugars, alkaloids, and glutamate bind to receptors which activates G proteins and second-messenger systems within the cell

72

depolarize cells directly

sodium and acids penetrate cells and depolarize it directly

73

Projection Pathways for Taste

- facial nerve, glossopharyngeal nerve, vagus nerve
-all fibers reach solitary nucleus in medulla oblongata
-signals sent two destinations: hypothalamus and amygdala or Thalamus

74

facial nerve

collects sensory information from taste buds over anterior two-thirds of tongue

75

glossopharyngeal nerve

from posterior one-third of tongue

76

vagus nerve

from taste buds of palate, pharynx and epiglottis

77

hypothalamus and amygdala

control autonomic reflexes –salivation, gagging and vomiting

78

thalamus

relays signals to postcentral gyrus of cerebrum for conscious sense of taste

79

orbitofrontal cortex

sent on to orbitofrontal cortex to be integrated with signals from nose and eyes -form impression of flavor and palatability of food

80

olfaction

sense of smell

81

olfactory mucosa

–contains 10 to 20 million olfactory cells, which are neurons, as well as epithelial supporting cells and basal stem cells
–mucosa of superior concha, nasal septum, and roof of nasal cavity covering about 5 cm2

82

olfactory cells

–are neurons
–shaped like little bowling pins
only neurons in the body directly exposed to the external environment
–have a lifespan of only 60 days

83

olfactory hairs

head bears 10 –20 cilia called olfactory hairs
–have binding sites for odorant molecules and are nonmotile
–lie in a tangled mass in a thin layer of mucus

84

axons collect into small fascicles and leave cranial cavity through

the cribriform foramina in the ethmoid bone

85

fascicles are collectively regarded as

Cranial Nerve I

86

olfactory receptors adapt

quickly
–due to synaptic inhibition in olfactory bulbs
PHASIC

87

Human Pheromones

–human body odors may affect sexual behavior

88

olfactory cells synapse in

olfactory bulb
–on dendrites of mitral and tufted cells

89

glomeruli

dendrites meet in spherical clusters called glomeruli
•each glomeruli dedicated to single odor because all fibers leading to one glomerulus come from cells with same receptor type

90

tufted and mitral cell axons form

olfactory tracts
–reach primary olfactory cortex in the inferior surface of the temporal lobe

91

Hearing and Equilibrium

both senses reside in the inner ear, a maze of fluid-filled passages and sensory cells
•fluid is set in motion and how the sensory cells convert this motion into an informative pattern of action potentials

92

hearing

a response to vibrating air molecules

93

equilibrium

the sense of motion, body orientation, and balance

94

sound

any audible vibration of molecules
–a vibrating object pushes on air molecules
–in turn push on other air molecules
–air molecules hitting eardrum cause it to vibration

95

pitch

our sense of whether a sound is „high‟ or „low‟
–determined by the frequency

96

infrasonic

infrasonic frequencies below 20 Hz

97

ultrasonic

ultrasonic frequencies above 20,000 Hz

98

loudness

the perception of sound energy, intensity, or amplitude of the vibration
–expressed in decibels (dB)
–prolonged exposure to sounds > 90dB can cause damage

99

ear has three sections

outer, middle, and inner ear
–first two are concerned only with the transmission of sound to the inner ear
–inner ear –vibrations converted to nerve signals

100

outer ear

a funnel for conducting vibrations to the tympanic membrane (eardrum)

101

auricle

(pinna) directs sound down the auditory canal
•shaped and supported by elastic cartilage

102

auditory canal

passage leading through the temporal bone to the tympanic membrane

103

external acoustic meatus

slightly s-shaped tube that begins at the external opening and courses for about 3 cm

104

guard hairs

protect outer end of canal

105

cerumen

earwax) –mixture of secretions of ceruminous and sebaceous glands and dead skin cells
–sticky and coats guard hairs
–contains lysozyme with low pH that inhibits bacterial growth
–water-proofs canal and protects skin
–keeps tympanic membrane pliable

106

middle ear

located in the air-filled tympanic cavity in temporal bone

107

tympanic membrane

(eardrum) –closes the inner end of the auditory cana
•innervated by sensory branches of the vagus and trigeminal nerves
–highly sensitive to pain

108

tympanic cavity

is continuous with mastoid air cells

109

auditory (eustachian) tube

connects middle ear cavity to nasopharynx
•equalizes air pressure on both sides of tympanic membrane

110

auditory ossicles

malleus
incus
stapes

111

malleus

attached to inner surface of tympanic membrane

112

incus

articulates in between malleus and stapes

113

stapes

footplate rests on oval window –inner ear begins

114

stapedius and tensor tympani muscles attach to

stapes and malleus

115

Otitis media

(middle ear infection) is common in children
–auditory tube is short and horizontal
–infections easily spread from throat to tympanic cavity and mastoid air cells

116

tympanostomy

lancing tympanic membrane and draining fluid from tympanic cavity
–inserting a tube to relieve the pressure and allow infection to heal

116

bony labyrinth

passageways in temporal bone

117

membranous labyrinth

fleshy tubes lining the bony labyrinth

118

Inner (Internal) Ear fleshy tubes filled with

endolymph-similar to intracellular fluid

120

Inner (Internal) Ear fleshy tubes floating in

perilymph-similar to cerebrospinal fluid

121

labyrinth

vestibule and three semicircular ducts

122

cochlea

organ of hearing
–2.5 coils around an screwlike axis of spongy bone, the modiolus
–threads of the screw form a spiral platform that supports the fleshy tube of the cochlea

123

cochlea has three fluid-filled chambers separated by membranes:

scala vestibule
scala tympani
scala media

124

scala vestibuli

superior chamber
•filled with perilymph
•begins at oval window and spirals to apex

125

scala tympani

inferior chamber
•filled with perilymph
•begins at apex and ends at round window
–secondary tympanic membrane –membrane covering round window

126

scala media

(cochlear duct) –triangular middle chamber
•filled with endolymph

127

scala media separated from scala vestibuli by

vestibular membrane

128

scala media separated from scala tympani by

thicker basilar membrane

129

scala media contains

spiral organ -organ of Corti -acoustic organ –converts vibrations into nerve impulses

130

spiral organ

spiral organ has epithelium composed of hair cells and supporting cells

131

stereocilia

hair cells have long, stiff microvilli called stereocilia on apical surface

132

tectorial membrane

gelatinous tectorial membrane rests on top of stereocilia

133

spiral organ has four rows of hair cells spiraling along its length

inner hair cells
outer hair cells

134

inner hair cells

single row of about 3500 cells
•provides for hearing

135

outer hair cells

three rows of about 20,000 cells
•adjusts response of cochlea to different frequencies
•increases precision

136

tympanic membrane

–has 18 times area of oval window
–ossicles concentrate the energy of the vibrating tympanic membrane on an area 1/18the size

137

tympanic reflex

–during loud noise, the tensor tympani pulls the tympanic membrane inward and tenses it
–stapedius muscle reduces the motion of the stapes

138

vibration of ossicles causes

vibration of basilar membrane under hair cells
–as often as 20,000 times per second
–hair cells move with basilar membrane

139

stereocilia of outer hair cells

–bathed in high K+fluid, the endolymph
•creating electrochemical gradient
•outside of cell is +80 mV and inside about –40 mV
–tip embedded in tectorial membrane

140

stereocilium on inner hair cells

–single transmembrane protein at tip that functions as a mechanically gated ion channel
K+flows in –depolarization causes release of neurotransmitter
•stimulates sensory dendrites and generates action potential in the cochlear nerve

141

Sensory Coding

for sounds to carry meaning, we must distinguish between loudness and pitch

142

loudness

for sounds to carry meaning, we must distinguish between loudness and pitch
•variations in loudness(amplitude) cause variations in the intensity of cochlear vibrations

143

pitch

depends on which part of basilar membrane vibrates

144

at basal end

membrane attached, narrow and stiff
•brain interprets signals as high-pitched

145

at distal end

5 times wider and more flexible
•brain interprets signals as low-pitched

145

deafness

hearing loss

146

conductive deafness

conditions interfere with transmission of vibrations to inner ear
•damaged tympanic membrane, otitis media, blockage of auditory canal, and otosclerosis

147

otosclerosis

fusion of auditory ossicles that prevents their free vibration

148

sensorineural (nerve) deafness

death of hair cells or any nervous system elements concerned with hearing
•factory workers, musicians and construction workers

149

vestibular ganglia

visible lump in vestibular nerve

150

spiral ganglia

buried in modiolus of cochlea

151

Auditory Projection Pathway

sensory fibers begin at the bases of the hair cells
–somas form the spiral ganglion around the modiolus
–axons lead away from the cochlea as the cochlear nerve
–joins with the vestibular nerve to form the vestibulocochlear nerve, Cranial Nerve VIII

152

each ear sends nerve fibers to

both sides of the pons
–end in cochlear nuclei
–synapse with second-order neurons that ascend to the nearby superior olivary nucleus
–superior olivary nucleus issues efferent fibers back to the cochlea
•involved with cochlear tuning

153

binaural hearing

comparing signals from the right and left ears to identify the direction from which a sound is coming
–function of the superior olivary nucleus

154

fibers ascend to the

inferior colliculi of the midbrain
–helps to locate the origin of the sound, processes fluctuation in pitch, and mediate the startle response and rapid head turning in response to loud noise

155

third-order neurons begin

in the inferior colliculi and lead to the thalamus

156

fourth-order neurons

complete the pathway from thalamus to primary auditory complex
–involves four neurons instead of three unlike most sensory pathways

157

primary auditory cortex

lies in the superior margin of the temporal lobe
–site of conscious perception of sound

158

because of extensive decussation of the auditory pathway

damage to right or left auditory cortex does not cause unilateral loss of hearing

159

equilibrium

coordination, balance, and orientation in three-dimensional space

160

vestibular apparatus

constitutes receptors for equilibrium
three semicircular ducts
two chambers

161

three semicircular ducts

detect only angular acceleration

162

two chambers

•anterior saccule and posterior utricle
•responsible for static equilibrium and linear acceleration

163

static equilibrium

the perception of the orientation of the head when the body is stationary

164

dynamic equilibrium

perception of motion or acceleration

165

linear acceleration

change in velocity in a straight line (elevator)

166

angular acceleration

change in rate of rotation (car turns a corner)

167

macula

2 by 3 mm patch of hair cells and supporting cells in the saccule and utricle

168

macula sacculi

lies vertically on wall of saccule
•because the macula sacculi is nearly vertical, it responds to vertical acceleration and deceleration

169

macula utriculi

lies horizontally on floor of utricle

170

each hair cell has

40 to 70 stereocilia and one true cilium -kinocilium embedded in a gelatinous otolithic membrane

171

otoliths

calcium carbonate-protein granules that add to the weight and inertia and enhance the sense of gravity and motion

172

static equilibrium

when head is tilted, heavy otolithic membrane sags, bending the stereocilia, and stimulating the hair cells

173

dynamic equilibrium

in car, linear acceleration detected as otoliths lag behind, bending the stereocilia, and stimulating the hair cells

174

rotary movements detected by the

three semicircular ducts
•bony semicircular canals of temporal bone hold membranous semicircular ducts
•each duct filled with endolymphand opens up as a dilated sac (ampulla) next to the utricle
•each ampulla contains crista ampullaris, mound of hair cells and supporting cells

175

crista ampullaris

•consists of hair cells with stereocilia and a kinocilium buried in a mound of gelatinous membrane called the cupula(one in each duct)
•orientation causes ducts to be stimulated by rotation in different planes

176

Equilibrium Projection Pathways

hair cells of macula sacculi, macula utriculi and semicircular ducts synapse on vestibular nerve (part of CN VIII)
•fibers end in a complex of four vestibular nuclei on each side of the pons and medulla
–left and right nuclei receive input from both ears

177

Equilibrium Projection Pathways

information sent to

-cerebellum

178

cerebellum

integrates vestibular information into its control of head and eye movements, muscle tone, and posture

179

vision

(sight) –perception of objects in the environment by means of the light that they emit or reflect

180

light

visible electromagnetic radiation
–light must cause a photochemical reaction to produce a nerve signal

181

ultraviolet radiation

-< 400 nm; has too much energy and destroys macromolecules

182

infrared radiation

-> 750 nm; too little energy to cause photochemical reaction, but does warm the tissues

183

eyebrows

provide facial expression
–protect eyes from glare and perspiration

184

eyelids

(palpebrae)
–block foreign objects, help with sleep, blink to moisten
–meet at corners (commissures)

185

eyelids consist of

–consist of orbicularis oculi muscle and tarsal plate covered with skin outside and conjunctiva inside
–tarsal glands secrete oil that reduces tear evaporation
–eyelasheshelp keep debris from eye

186

conjunctiva

a transparent mucous membrane that lines eyelids and covers anterior surface of eyeball, except cornea
•richly innervated and vascular (heals quickly)
–secretes a thin mucous film that prevents the eyeball from drying

187

Lacrimal Apparatus

•tears flow across eyeball help to wash away foreign particles, deliver O2and nutrients, and prevent infection with a bactericidal lysozyme
•tears flow through lacrimal punctum (opening on edge of each eyelid) to the lacrimal sac, then into the nasolacrimal duct emptying into nasal cavity

188

Extrinsic Eyes Muscles

•6 muscles attached to exterior surface of eyeball
–superior, inferior, lateral, and medial rectus muscles, superior and inferior oblique muscles
•innervated by cranial nerves III, IV and VI

189

superior, inferior, medial and lateral rectus muscles move the eye

up, down, medially & laterally
Oculomotor nerve (III)

190

superior and inferior oblique

mm. turn the “twelve o‟clock pole” of each eye toward or away from the nose
superior - Trochlear nerve (IV)
lateral - Abducens nerve (VI)

191

orbital fat

surrounds sides and back of eye, cushions eye and allows free movement, protects blood vessels, and nerves

192

three principal components of the eyeball

–three layers (tunics) that form the wall of the eyeball
–optical component –admits and focuses light
–neural component –the retina and optic nerve

193

Tunics of the Eyeball

tunica fibrosa
tunica vasculosa
tunica interna

194

tunica fibrosa

outer fibrous layer
–sclera–dense, collagenous white of the eye
–cornea-transparent area of sclera that admits light into eye

195

tunica vasculosa

(uvea) –middle vascular layer
–choroid
–ciliary body
–iris

196

choroid

highly vascular, deeply pigmented layer behind retina

197

ciliary body

extension of choroid that forms a muscular ring around lens
•supports lens and iris
•secretes aqueous humor

198

iris

colored diaphragm controlling size of pupil, its central opening
•melanin in chromatophores of iris -brown or black eye color
•reduced melanin –blue, green, or gray color

199

tunica interna

retina and beginning of optic nerve

200

Optical Components

•transparent elements that admit light rays, refract (bend) them, and focus images on the retina

201

Optical Components 4

–cornea
–aqueous humor
–lens
–vitreous body (humor)

202

cornea

•transparent cover on anterior surface of eyeball

203

aqueous humor

•serous fluid posterior to cornea, anterior to lens
•reabsorbed by scleral venous sinus (canal of Schlemm)
•produced and reabsorbed at same rate

204

lens

•lens fibers –flattened, tightly compressed, transparent cells that form lens
•suspended by suspensory ligaments from ciliary body
•changes shape to help focus light
–rounded with no tension or flattened with pull of suspensory ligaments

205

vitreous body

(humor) fills vitreous chamber
•jelly fills space between lens and retina

206

Aqueous Humor

released by ciliary body into posterior chamber, passes through pupil into anterior chamber -reabsorbed into canal of Schlemm

207

Neural Components

includes retina and optic nerve

208

retina

–forms as an outgrowth of the diencephalon
–attached to the rest of the eye only at optic disc and at ora serrata
–pressed against rear of eyeball by vitreous humor
–detached retina causes blurry areas in field of vision and leads to blindness

209

examine retina with

opthalmoscope

210

macula lutea

patch of cells on visual axis of eye

211

fovea centralis

pit in center of macula lutea

212

blood vessels

of the retina

213

fovea centralis

center of macula; finely detailed images due to packed receptor cells

214

optic disk

blind spot
–optic nerve exits posterior surface of eyeball
–no receptor cells at that location

215

visual filling

brain fills in green bar across blind spot area

216

cataract

clouding of lens
–lens fibers darken with age, fluid-filled bubbles and clefts filled with debris appear between the fibers

217

glaucoma

elevated pressure within the eye due to obstruction of scleral venous sinus and improper drainage of aqueous humor

218

intraocular pressure measured with

tonometer

219

Formation of an Image

light passes through lens to form tiny inverted image on retina

220

iris diameter

–pupillary constrictor -smooth muscle encircling the pupil
•parasympathetic stimulation narrows pupil
–pupillary dilator -spokelike myoepithelial cells
•sympathetic stimulation widens pupil

221

pupillary constrictor

smooth muscle encircling the pupil

222

pupillary dilator

spokelike myoepithelial cells

223

pupillary constriction and dilation occurs in two situations

–when light intensity changes
–when our gaze shifts between distant and nearby objects

224

photopupillary reflex

pupillary constriction in response to light

225

consensual light reflex

because both pupils constrict even if only one eye is illuminated

226

refraction

the bending of light rays

227

Refraction in the Eye

•light passing through the center of the cornea is not bent
•light striking off-center is bent towards the center
•aqueous humor and lens do not greatly alter the path of light

228

cornea refracts light more than

lens does
–lens merely fine-tunes the image
–lens becomes rounder to increase refraction for near vision

229

emmetropia

state in which the eye is relaxed and focused on an object more than 6 m (20 ft) away
–light rays coming from that object are essentially parallel
–rays focused on retina without effort

230

near response

adjustments to close range vision requires three processes
convergence of eyes
constriction of pupil

231

convergence of eyes

eyes orient their visual axis towards object

232

constriction of pupil

•blocks peripheral light rays and reduces spherical aberration (blurry edges)

233

accommodation of lens

change in the curvature of the lens that enables you to focus on nearby objects
•ciliary muscle contracts, lens takes convex shape

234

near point of vision

closest an object can be and still come into focus

235

emmetropia

distant object
relatively dilated pupil
relatively thin lens
lens flatter

236

Convergence

close object
relatively constricted pupil
relatively thick lens
lens thicker

237

Hyperopia

(farsightedness)

238

Myopia

(nearsightedness)

239

Sensory Transduction in the Retina

•conversion of light energy into action potentials occurs in the retina

240

structure of retina

pigment epithelium
neural components

241

pigment epithelium

most posterior part of retina
•absorbs stray light so visual image is not degraded

242

neural components of the retina from the rear of the eye forward

photoreceptor cells
bipolar cells
ganglion cells

243

photoreceptor cells

absorb light and generate a chemical or electrical signal
–rods, cones, and certain ganglion cells
–only rods and cones produce visual images

244

bipolar cells

synapse with rods and cones and are first-order neurons of the visual pathway

245

ganglion cells

largest neurons in the retina and are the second-order neurons of the visual pathway

246

light absorbing cells

derived from same stem cells as ependymal cells of the brain
–rod cells
-cone cells

247

rod cells

(night -scotopic vision or monochromatic vision)
•outer segment –modified cilium specialized to absorb light
–stack of 1,000 membranous discs studded with globular proteins, the visual pigment, rhodopsin
•inner segment –contains organelles sitting atop cell body with nucleus

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cone cells

(color, photopic, or day vision)
•similar except outer segment tapers
•outer segment tapers to a point
•plasma membrane infoldings form discs

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neuronal convergence

and information processing in retina before signals reach brain
–multiple rod or cone cells synapse on one bipolar cell
–multiple bipolar cells synapse on one ganglion cell

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rods contain visual pigment

rhodopsin (visual purple)
–two major parts of molecule
•opsin -protein portion embedded in disc membrane of rod‟s outer segment
•retinal(retinene) -a vitamin A derivative

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cones contain

photopsin(iodopsin)
–retinal moiety same as in rods
–opsin moiety contain different amino acid sequences that determine wavelengths of light absorbed
–3 kinds of cones, identical in appearance, but absorb different wavelengths of light to produce color vision

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Rhodopsin Bleaching/Regeneration

rhodopsin absorbs light, converted from bent shape in dark (cis-retinal) to straight (trans-retinal)
–retinal dissociates from opsin (bleaching)
–5 minutes to regenerate 50% of bleached rhodopsin
•cones are faster to regenerate their photopsin –90seconds for 50%

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Generating Optic Nerve Signals

•in dark, rods steadily release the neurotransmitter, glutamate from basal end of cell
•when rods absorb light, glutamate secretion ceases
•bipolar cells sensitive to these on and off pulses of glutamate secretion
•these cells excited by rising light intensities
•when bipolar cells detect fluctuations in light intensity, they stimulate ganglion cells directly or indirectly
•ganglion cells are the only retinal cells that produce action potentials
•ganglion cells respond to the bipolar cells with rising and falling firing frequencies
•via optic nerve, these changes provide visual signals to the brain

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light adaptation

(walk out into sunlight)
–pupil constriction and pain from over stimulated retinas
–pupils constrict to reduce pain & intensity
–color vision and acuity below normal for 5 to 10 minutes
–time needed for pigment bleaching to adjust retinal sensitivity to high light intensity
–rod vision nonfunctional

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dark adaptation

(turn lights off)
–dilation of pupils occurs
–rod pigment was bleached by lights
–in dark, rhodopsin regenerates faster than it bleaches
–in a minute or two night (scotopic) vision begins to function
–after 20 to 30 minutes the amount of regenerated rhodopsin is sufficient for your eyes to reach maximum sensitivity

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duplicity theory of vision

explains why we have both rods and cones
–a single type of receptor can not produce both high sensitivity and high resolution
•it takes one type of cell and neural circuit for sensitive night vision
•it takes a different cell type and neuronal circuit for high resolution daytime vision

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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 mm2of retina producing only a coarse image
•edges of retina have widely-spaced rod cells, act as motion detectors
–low resolution system only
–cannot resolve finely detailed images

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Color Vision 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

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Color Vision

•primates have well developed color vision
–nocturnal vertebrates
have only rods

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three types of cones

are named for absorption peaks of their photopsins
–short-wavelength(S) cones peak sensitivity at 420 nm
–medium-wavelength(M) cones peak at 531 nm
–long-wavelength(L) cones peak at 558 nm

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color perception

based on mixture of nerve signals representing cones of different absorption peaks

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color blindness

have a hereditary alteration or lack of one photopsin or another
•most common is red-green color blindness
–results from lack of either L or M cones

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stereoscopic vision

is depth perception -ability to judge distance to objects
–requires two eyes with overlapping visual fields which allows each eye to look at the same object from different angles

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panoramic vision

has eyes on sides of head (horse or rodents –alert to predators but no depth perception)

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fixation point

point in space in which the eyes are focused
–looking at object within 100 feet, each eye views from slightly different angle
–provides brain with information used to judge position of objects relative to fixation point

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Visual Projection Pathway

first-order neurons

bipolar cells of retina

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Visual Projection Pathway

second-order neurons

retinal ganglion cells are second-order neurons whose axons form optic nerve

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optic chiasm

two optic nerves combine to form optic chiasm
–half the fibers cross over to the opposite side of the brain (hemidecussation) and chiasm splits to form optic tracts

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optic tracts

•right cerebral hemisphere sees objects in the left visual field because their images fall on the right half of each retina
•each side of brain sees what is on side where it has motor control over limbs

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optic tracts pass

laterally around the hypothalamus with most of their axons ending in the lateral geniculate nucleus of the thalamus

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Visual Projection Pathway

third-order neurons

third-order neurons arise in geniculate nucleus of the thalamus and form the optic radiation of fibers in the white matter of the cerebrum
-project to primary visual cortex of occipital lobe

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conscious visual sensation occurs

primary visual cortex of occipital lobe

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Visual Information Processing

some processing begins in retina
–adjustments for contrast, brightness, motion and stereopsis

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primary visual cortex is connected by

association tracts to visual association areas in parietal and temporal lobes which process retinal data from occipital lobes
–object location, motion, color, shape, boundaries
–store visual memories (recognize printed words)