0-1 Chapter 16 - sense Organs Flashcards
sense organs
nerve tissue surrounded by other tissues that enhance response to certain type of stimulus
•added epithelium, muscle or connective tissue
transduction
the conversion of one form of energy to another
–fundamental purpose of any sensory receptor
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
sensation
a subjective awareness of the stimulus
–most sensory signals delivered to the CNS produce no conscious sensation
Receptors Transmit Four Kinds of Information
Modality
Location
Intensity
Duration
Modality
type of stimulus or the sensation it produces
–vision, hearing, taste
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
Location
encoded by which nerve fibers are issuing signals to the brain
receptive field
area that detects stimuli for a sensory neuron
sensory projection
brain identifies site of stimulation
projection pathways
the pathways followed by sensory signals to their ultimate destination in the CNS
Intensity
encoded in 2 ways
Strength
frequency
Duration
how long the stimulus lasts
sensory adaptation
if stimulus is prolonged, the firing of the neuron gets slower over time, and we become less aware of the stimulus
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
tonic receptor
adapt slowly, generate nerve signals more steadily
Classification of Receptors by
modality
origin of stimuli
distribution
by modality
–thermoreceptors, photoreceptors, nociceptors, chemoreceptors, and mechanoreceptors
origin of stimuli
–exteroceptors -detect external stimuli
–interoceptors -detect internal stimuli
–proprioceptors -sense body position and movements
by distribution
–general (somesthetic) senses -widely distributed
–special senses -limited to head
•vision, hearing, equilibrium, taste, and smell
General Senses
structurally simple receptors
–one or a few sensory fibers and a little connective tissue
unencapsulated nerve endings
•dendrites not wrapped in connective tissue
–free nerve endings
–tactile (Merkel) discs
–hair receptors (peritrichial endings
free nerve endings
–for pain and temperature
–skin and mucous membrane
tactile discs
–for light touch and texture
–associated with Merkel cells at base of epidermis
hair receptors
–wrap around base of hair follicle
–monitor movement of hair
encapsulated nerve endings
- dendrites wrapped by glial cells or connective tissue
* connective tissue enhances sensitivity or selectivity of response
encapsulated nerve endings
types
–tactile (Meissner) corpuscles –Krause end bulbs –bulbous (Ruffini) corpuscles –lamellar (pacinian) corpuscles –muscle spindles –golgi tendon organs
tactile (Meissner) corpuscles
–light touch and texture
–dermal papillae of hairless skin
Krause end bulb
–tactile; in mucous membranes
lamellated (pacinian) corpuscles
phasic
–deep pressure, stretch, tickle and vibration
–periosteum of bone, and deep dermis of skin
bulbous (Ruffini) corpuscles
tonic
–heavy touch, pressure, joint movements and skin stretching
Sound receptors are
mechanoreceptors
Somesthetic Projection Pathways
from receptor to final destination in the brain, most somesthetic signals travel by way of three neurons
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, medulla, or pons
–end in thalamus, except for proprioception, which ends in cerebellum
3rd order neuron
–thalamus to primary somesthetic cortex of cerebrum
pain
discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action
–important since helps protect us
nociceptors
two types providing different pain sensations
fast pain
travels in myelinated fibers at 12 -30 m/sec
•sharp, localized, stabbing pain perceived with injury
slow pain
travels unmyelinated fibers at 0.5 -2 m/sec
•longer-lasting, dull, diffuse feeling
somatic pain
from skin, muscles and joints
visceral pain
from the viscera
–stretch, chemical irritants or ischemia of viscera (poorly localized
bradykinin
most potent pain stimulus known
–makes us aware of injury and activates cascade or reactions that promote healing
Projection Pathway for Pain
two main pain pathways to brain, and multiple subroutes
first-order neuron cell bodies
in dorsal root ganglion of spinal nerves or cranial nerves V, VII, IX, and X
spinothalamic tract
most significant pain pathway
–carries most somatic pain signals
spinoreticular tract
carries pain signals to reticular formation
–activate visceral, emotional and behavioral reactions to pain
referred pain
pain in viscera often mistakenly thought to come from the skin or other superficial site
analgesic
(pain-relieving) mechanisms of CNS just beginning to be understood
enkephalins
two analgesic oligopeptides with 200 times the potency of morphine
endogenous opioids
internally produced opium-like substances
•enkephalins, endorphins, and dynorphins
neuromodulators
neuromodulators that can block the transmission of pain signals and produce feelings of pleasure and euphoria
spinal gating-
stops pain signals at the posterior horn of the spinal cord
SEE DIAGRAM
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
gustation
(taste) –sensation that results from action of chemicals on taste buds
MUST BE LIQUID TO TASTE
taste buds - location
4000 -taste buds mainly on tongue
–inside cheeks, and on soft palate, pharynx, and epiglottis
lingual papillae
4 areas
filiform
foliate
fungiform
vallate (circumvallate)
filiform
no taste buds
•important for food texture
foliate
no taste buds
•weakly developed in humans
fungiform
•at tips and sides of tongue
vallate (circumvallate)
- at rear of tongue
* contains 1/2 of all taste buds
taste cells
synapse with and release neurotransmitters onto sensory neurons at their base
Have: taste hairs, taste pores,
taste hairs
have tuft of apical microvilli(taste hairs) that serve as receptor surface for taste molecules
taste hairs are epithelial cells not neurons
taste pores
pit in which the taste hairs project
basal cells
stem cells that replace taste cells every 7 to 10 days
supporting cells
resemble taste cells without taste hairs, synaptic vesicles, or sensory role
Physiology of Taste
to be tasted, molecules must dissolve in saliva and flood the taste pore
five primary sensations
salty –sweet –sour –bitter –umami
mouthfeel
detected by branches of lingual nerve in papillae
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
activate 2nd messenger systems
•sugars, alkaloids, and glutamate bind to receptors which activates G proteins and second-messenger systems within the cell
depolarize cells directly
sodium and acids penetrate cells and depolarize it directly
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
facial nerve
collects sensory information from taste buds over anterior two-thirds of tongue
glossopharyngeal nerve
from posterior one-third of tongue
vagus nerve
from taste buds of palate, pharynx and epiglottis
hypothalamus and amygdala
control autonomic reflexes –salivation, gagging and vomiting
thalamus
relays signals to postcentral gyrus of cerebrum for conscious sense of taste
orbitofrontal cortex
sent on to orbitofrontal cortex to be integrated with signals from nose and eyes -form impression of flavor and palatability of food
olfaction
sense of smell
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
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
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
axons collect into small fascicles and leave cranial cavity through
the cribriform foramina in the ethmoid bone
fascicles are collectively regarded as
Cranial Nerve I
olfactory receptors adapt
quickly
–due to synaptic inhibition in olfactory bulbs
PHASIC
Human Pheromones
–human body odors may affect sexual behavior
olfactory cells synapse in
olfactory bulb
–on dendrites of mitral and tufted cells
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
tufted and mitral cell axons form
olfactory tracts
–reach primary olfactory cortex in the inferior surface of the temporal lobe
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
hearing
a response to vibrating air molecules
equilibrium
the sense of motion, body orientation, and balance
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
pitch
our sense of whether a sound is „high‟ or „low‟
–determined by the frequency
infrasonic
infrasonic frequencies below 20 Hz
ultrasonic
ultrasonic frequencies above 20,000 Hz
loudness
the perception of sound energy, intensity, or amplitude of the vibration
–expressed in decibels (dB)
–prolonged exposure to sounds > 90dB can cause damage
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
outer ear
a funnel for conducting vibrations to the tympanic membrane (eardrum)
auricle
(pinna) directs sound down the auditory canal
•shaped and supported by elastic cartilage
auditory canal
passage leading through the temporal bone to the tympanic membrane
external acoustic meatus
slightly s-shaped tube that begins at the external opening and courses for about 3 cm
guard hairs
protect outer end of canal
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
middle ear
located in the air-filled tympanic cavity in temporal bone
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
tympanic cavity
is continuous with mastoid air cells
auditory (eustachian) tube
connects middle ear cavity to nasopharynx
•equalizes air pressure on both sides of tympanic membrane
auditory ossicles
malleus
incus
stapes
malleus
attached to inner surface of tympanic membrane
incus
articulates in between malleus and stapes
stapes
footplate rests on oval window –inner ear begins
stapedius and tensor tympani muscles attach to
stapes and malleus
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
tympanostomy
lancing tympanic membrane and draining fluid from tympanic cavity
–inserting a tube to relieve the pressure and allow infection to heal
bony labyrinth
passageways in temporal bone
membranous labyrinth
fleshy tubes lining the bony labyrinth
Inner (Internal) Ear fleshy tubes filled with
endolymph-similar to intracellular fluid
Inner (Internal) Ear fleshy tubes floating in
perilymph-similar to cerebrospinal fluid
labyrinth
vestibule and three semicircular ducts
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
cochlea has three fluid-filled chambers separated by membranes:
scala vestibule
scala tympani
scala media
scala vestibuli
superior chamber
•filled with perilymph
•begins at oval window and spirals to apex
scala tympani
inferior chamber
•filled with perilymph
•begins at apex and ends at round window
–secondary tympanic membrane –membrane covering round window
scala media
(cochlear duct) –triangular middle chamber
•filled with endolymph
scala media separated from scala vestibuli by
vestibular membrane
scala media separated from scala tympani by
thicker basilar membrane
scala media contains
spiral organ -organ of Corti -acoustic organ –converts vibrations into nerve impulses
spiral organ
spiral organ has epithelium composed of hair cells and supporting cells
stereocilia
hair cells have long, stiff microvilli called stereocilia on apical surface
tectorial membrane
gelatinous tectorial membrane rests on top of stereocilia
spiral organ has four rows of hair cells spiraling along its length
inner hair cells
outer hair cells
inner hair cells
single row of about 3500 cells
•provides for hearing
outer hair cells
three rows of about 20,000 cells
•adjusts response of cochlea to different frequencies
•increases precision
tympanic membrane
–has 18 times area of oval window
–ossicles concentrate the energy of the vibrating tympanic membrane on an area 1/18the size
tympanic reflex
–during loud noise, the tensor tympani pulls the tympanic membrane inward and tenses it
–stapedius muscle reduces the motion of the stapes
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
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
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
Sensory Coding
for sounds to carry meaning, we must distinguish between loudness and pitch
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
pitch
depends on which part of basilar membrane vibrates
at basal end
membrane attached, narrow and stiff
•brain interprets signals as high-pitched
at distal end
5 times wider and more flexible
•brain interprets signals as low-pitched
deafness
hearing loss
conductive deafness
conditions interfere with transmission of vibrations to inner ear
•damaged tympanic membrane, otitis media, blockage of auditory canal, and otosclerosis
otosclerosis
fusion of auditory ossicles that prevents their free vibration
sensorineural (nerve) deafness
death of hair cells or any nervous system elements concerned with hearing
•factory workers, musicians and construction workers
vestibular ganglia
visible lump in vestibular nerve
spiral ganglia
buried in modiolus of cochlea
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
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
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
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
third-order neurons begin
in the inferior colliculi and lead to the thalamus
fourth-order neurons
complete the pathway from thalamus to primary auditory complex
–involves four neurons instead of three unlike most sensory pathways
primary auditory cortex
lies in the superior margin of the temporal lobe
–site of conscious perception of sound
because of extensive decussation of the auditory pathway
damage to right or left auditory cortex does not cause unilateral loss of hearing
equilibrium
coordination, balance, and orientation in three-dimensional space
vestibular apparatus
constitutes receptors for equilibrium
three semicircular ducts
two chambers
three semicircular ducts
detect only angular acceleration
two chambers
- anterior saccule and posterior utricle
* responsible for static equilibrium and linear acceleration
static equilibrium
the perception of the orientation of the head when the body is stationary
dynamic equilibrium
perception of motion or acceleration
linear acceleration
change in velocity in a straight line (elevator)
angular acceleration
change in rate of rotation (car turns a corner)
macula
2 by 3 mm patch of hair cells and supporting cells in the saccule and utricle
macula sacculi
lies vertically on wall of saccule
•because the macula sacculi is nearly vertical, it responds to vertical acceleration and deceleration
macula utriculi
lies horizontally on floor of utricle
each hair cell has
40 to 70 stereocilia and one true cilium -kinocilium embedded in a gelatinous otolithic membrane
otoliths
calcium carbonate-protein granules that add to the weight and inertia and enhance the sense of gravity and motion
static equilibrium
when head is tilted, heavy otolithic membrane sags, bending the stereocilia, and stimulating the hair cells
dynamic equilibrium
in car, linear acceleration detected as otoliths lag behind, bending the stereocilia, and stimulating the hair cells
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
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
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
Equilibrium Projection Pathways
information sent to
-cerebellum
cerebellum
integrates vestibular information into its control of head and eye movements, muscle tone, and posture
vision
(sight) –perception of objects in the environment by means of the light that they emit or reflect
light
visible electromagnetic radiation
–light must cause a photochemical reaction to produce a nerve signal
ultraviolet radiation
-< 400 nm; has too much energy and destroys macromolecules
infrared radiation
-> 750 nm; too little energy to cause photochemical reaction, but does warm the tissues
eyebrows
provide facial expression
–protect eyes from glare and perspiration
eyelids
(palpebrae)
–block foreign objects, help with sleep, blink to moisten
–meet at corners (commissures)
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
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
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
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
superior, inferior, medial and lateral rectus muscles move the eye
up, down, medially & laterally Oculomotor nerve (III)
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)
orbital fat
surrounds sides and back of eye, cushions eye and allows free movement, protects blood vessels, and nerves
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
Tunics of the Eyeball
tunica fibrosa
tunica vasculosa
tunica interna
tunica fibrosa
outer fibrous layer
–sclera–dense, collagenous white of the eye
–cornea-transparent area of sclera that admits light into eye
tunica vasculosa
(uvea) –middle vascular layer
–choroid
–ciliary body
–iris
choroid
highly vascular, deeply pigmented layer behind retina
ciliary body
extension of choroid that forms a muscular ring around lens
•supports lens and iris
•secretes aqueous humor
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
tunica interna
retina and beginning of optic nerve
Optical Components
•transparent elements that admit light rays, refract (bend) them, and focus images on the retina
Optical Components 4
–cornea
–aqueous humor
–lens
–vitreous body (humor)
cornea
•transparent cover on anterior surface of eyeball
aqueous humor
- serous fluid posterior to cornea, anterior to lens
- reabsorbed by scleral venous sinus (canal of Schlemm)
- produced and reabsorbed at same rate
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
vitreous body
(humor) fills vitreous chamber
•jelly fills space between lens and retina
Aqueous Humor
released by ciliary body into posterior chamber, passes through pupil into anterior chamber -reabsorbed into canal of Schlemm
Neural Components
includes retina and optic nerve
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
examine retina with
opthalmoscope
macula lutea
patch of cells on visual axis of eye
fovea centralis
pit in center of macula lutea
blood vessels
of the retina
fovea centralis
center of macula; finely detailed images due to packed receptor cells
optic disk
blind spot
–optic nerve exits posterior surface of eyeball
–no receptor cells at that location
visual filling
brain fills in green bar across blind spot area
cataract
clouding of lens
–lens fibers darken with age, fluid-filled bubbles and clefts filled with debris appear between the fibers
glaucoma
elevated pressure within the eye due to obstruction of scleral venous sinus and improper drainage of aqueous humor
intraocular pressure measured with
tonometer
Formation of an Image
light passes through lens to form tiny inverted image on retina
iris diameter
–pupillary constrictor -smooth muscle encircling the pupil
•parasympathetic stimulation narrows pupil
–pupillary dilator -spokelike myoepithelial cells
•sympathetic stimulation widens pupil
pupillary constrictor
smooth muscle encircling the pupil
pupillary dilator
spokelike myoepithelial cells
pupillary constriction and dilation occurs in two situations
–when light intensity changes
–when our gaze shifts between distant and nearby objects
photopupillary reflex
pupillary constriction in response to light
consensual light reflex
because both pupils constrict even if only one eye is illuminated
refraction
the bending of light rays
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
cornea refracts light more than
lens does
–lens merely fine-tunes the image
–lens becomes rounder to increase refraction for near vision
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
near response
adjustments to close range vision requires three processes
convergence of eyes
constriction of pupil
convergence of eyes
eyes orient their visual axis towards object
constriction of pupil
•blocks peripheral light rays and reduces spherical aberration (blurry edges)
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
near point of vision
closest an object can be and still come into focus
emmetropia
distant object
relatively dilated pupil
relatively thin lens
lens flatter
Convergence
close object
relatively constricted pupil
relatively thick lens
lens thicker
Hyperopia
(farsightedness)
Myopia
(nearsightedness)
Sensory Transduction in the Retina
•conversion of light energy into action potentials occurs in the retina
structure of retina
pigment epithelium
neural components
pigment epithelium
most posterior part of retina
•absorbs stray light so visual image is not degraded
neural components of the retina from the rear of the eye forward
photoreceptor cells
bipolar cells
ganglion cells
photoreceptor cells
absorb light and generate a chemical or electrical signal
–rods, cones, and certain ganglion cells
–only rods and cones produce visual images
bipolar cells
synapse with rods and cones and are first-order neurons of the visual pathway
ganglion cells
largest neurons in the retina and are the second-order neurons of the visual pathway
light absorbing cells
derived from same stem cells as ependymal cells of the brain
–rod cells
-cone cells
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
cone cells
(color, photopic, or day vision)
•similar except outer segment tapers
•outer segment tapers to a point
•plasma membrane infoldings form discs
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
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
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
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%
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
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
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
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
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
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
Color Vision
•primates have well developed color vision
–nocturnal vertebrates
have only rods
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
color perception
based on mixture of nerve signals representing cones of different absorption peaks
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
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
panoramic vision
has eyes on sides of head (horse or rodents –alert to predators but no depth perception)
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
Visual Projection Pathway
first-order neurons
bipolar cells of retina
Visual Projection Pathway
second-order neurons
retinal ganglion cells are second-order neurons whose axons form optic nerve
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
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
optic tracts pass
laterally around the hypothalamus with most of their axons ending in the lateral geniculate nucleus of the thalamus
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
conscious visual sensation occurs
primary visual cortex of occipital lobe
Visual Information Processing
some processing begins in retina
–adjustments for contrast, brightness, motion and stereopsis
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)
