Transcript eye

General Senses
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Six general senses
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Temperature
Pain
Touch
Pressure
Vibration
Proprioception (body position)
Receptors for each sense, classified
according to the nature of the stimulus
that excites them
Nociceptors (pain receptors)- Free nerve
endings that are found in the superficial
portions of the skin, joint capsules, and
around blood vessel walls
 Three types of pain
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 Fast pain: very localized sensations, reach CNS
very quick, and relayed to primary sensory cortex
 Slow pain: only allow you to determine the
general area involved
 Referred pain: perception of pain coming from
parts of the body that are not actually stimulated
Referred
Pain
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Thermoreceptors (Temperature receptors)free nerve scattered right below the surface
of the skin, in skeletal muscles, liver, and
hypothalamus
 Very active when the temperature is changing but
quickly adapts to a stable temperature
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Mechanoreceptors- sensitive to stimuli that
distort their cell membranes
 Tactile: sensation of touch, pressure, and
vibration
 Baroreceptors: changes in pressure
 Proprioceptors: monitors position of joints,
tension in tendons and ligaments, and the state of
muscular contractions
Tactile receptors
 Fine touch and pressure receptors: detailed
information
 Crude touch and pressure receptors: poor
localization and little information about stimulus
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Six types of tactile receptors:
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Free nerve endings
Root hair plexus
Tactile discs (Merkel’s disc)
Tactile corpuscles (Meissner’s corpuscles)
Lamellated corpuscles
Ruffini corpuscles
Tactile Receptors in the Skin
Baroreceptors
 Free nerve endings that branch within the
elastic tissues in the wall of distensible
organs
 Respond immediately to changes in pressure
but adapt rapidly to the change and return to
a state or “normal”
Proprioceptors
 Do not adapt to constant stimulation and
each receptor continuously sends
information to CNS
 How you know your current body position
Chemoreceptors
 Only respond to water-soluble and lipidsoluble substances that dissolve in
surrounding fluid
 Sends information to the brain stem centers
that deal with autonomic control of
respiratory and cardiovascular functions
 Monitor the bloods pH, CO₂, and oxygen
levels
Concept Check
What would happen to you if the
information from proprioceptors in your
legs were blocked from reaching the CNS?
 Proprioceptors relay information about limb
position and movement to the CNS,
especially the cerebellum. Blockage of this
information would result in uncoordinated
movements, and the individual probably
would be unable to walk.
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The Special Senses
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The five special senses
 Smell
 Taste
 Vision
 Equilibrium
 Hearing
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Receptors from these senses are
distributed to specific areas of the
cerebral cortex and brain stem
Smell (Olfaction)
 Olfactory organs are located in the nasal
cavity and contain the olfactory receptors
plus epithelial supporting cells
 Receptor cells are bipolar neurons with
hair-like cilia covering that dendrites. The
cilia project into the nasal cavity
 Inhaled air reaches the olfactory organs,
water and lipid soluble chemicals
(Chemoreceptors) must diffuse into the
mucus in order for the olfactory receptors to
stimulate
Information from the receptors are relayed to
the CNS, which interprets the smell on the
basis of the particular pattern of receptor
activity
Olfactory Pathway
1. When olfactory receptors are stimulated,
their fibers synapse with neurons in the
olfactory bulbs
2. Sensory impulses are first analyzed in the
olfactory blubs, then travel along the
olfactory tracts to the limbic system, and
then arrive to the olfactory cortex
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Taste (Gustatory)
 Taste buds: sensory structures formed
from taste receptors and specialized
epithelial cells
 Papillae: epithelial projections
 Circumvallate papillae
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Taste cells (gustatory cells) are modified
epithelial cells that function as receptors
 Contain the taste hairs that are the portions
sensitive to taste. These hairs protrude from
openings called taste pores
Circumvallate papillae
Taste Sensations
 Specific taste receptors are concentrated
on different areas of the tongue
 Sweet- tip of the tongue
 Sour- along lateral edges of the tongue
 Salt- abundant in the tip and upper portion of
the tongue
 Bitter- back of the tongue
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Two additional tastes discovered
 Umami- taste of chicken and beef broth
 Water
Taste Pathways
 Taste impulses travel on the facial,
glossopharyngeal, and vagus nerves to
the medulla oblongata and then to the
gustatory cortex of the cerebrum
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Olfactory receptors play an important
role in taste perception
Concept Check
If you completely dry the surface of your
tongue and then place salt or sugar
crystals on it, you cannot taste them. Why
not??
 Drying the surface of the tongue removes
moisture needed to dissolve the sugar
molecules or salt ions. Because taste bud
(receptors) are sensitive only to molecules
and ions that are in a solution, the taste
buds will not be stimulated.
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Vision
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Accessory organs: lacrimal
apparatus, eyelids, and extrinsic
muscles aid the eye in its function
 Provide protection, lubrication, and
support
Visual Accessory Organs
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The eyelid: protects the eye from foreign
objects and is made up of the thinnest
skin of the body lined with conjunctiva
 Eyelashes: very robust hairs that also help
prevent foreign particles from reaching the
eye surface
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The lacrimal apparatus produces
tears that lubricate and cleanse the
eye
 Two small ducts (nasolacrimal ducts)
drain tears into the nasal cavity
 Tears also contain lysozyme, which is a
antibacterial enzyme
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The extrinsic muscles of the eye
attach to the sclera and allow the eye
to move in all directions
Structure of the Eye
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The eye is a fluid-filled hallow
sphere with three distinct layers
or tunics
 The fibrous tunic (Outer)
 The vascular tunic (Middle)
 The Neural tunic (Inner)
The Fibrous Tunic (Outermost)
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Functions as some physical protection,
attachment site for extrinsic muscles, and
assists in the focusing process
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Consists of the transparent cornea at the
front of the eye, and the white sclera of
the anterior eye.
The Vascular Tunic
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The middle, vascular tunic includes the
iris, ciliary body, and choroid
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The choroid coat is vascular and darkly
pigmented. Performs two functions:
nourish other tissues of the eye and
keep the inside of the eye dark.
 Separates the fibrous and neural tunics
posterior to the ciliary body
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The ciliary body is a ring of smooth
muscle around the front of the eye,
contains suspensory ligaments that
position the lens so light passing
through the pupil passes through the
center of the lens
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The lens can adjust its shape to facilitate
focusing by accommodation
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The iris is a thin, smooth muscle that
adjusts to the amount of light that is
coming through the pupil, a hole in its
center
 Has a set of muscles that either contract or
dilate depending on light intensity
○ Pupillary dilator muscles (radial)
○ Pupillary constrictor muscles (sphincter)
Eye Color
Eye color is determined by the amount of
melonocytes that the iris contains and the
pigmented epithelium on its posterior
surface
 When melonocytes are absent, light can
pass through which makes the eye appear
blue
 The more melonocytes the iris has, less
light can pass throught, which makes the
eye become darker like gray, brown, or
black
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The Neural Tunic (Innermost)
The inner tunic consists of the retina,
which contains photoreceptors. It
covers the back side of the eye to the
ciliary body
 Retina can be divided into two layers:
the pigmented and neural part
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 Pigmented part- absorbs light
 Neural part- photoreceptors that respond to
light, preliminary processing and integration
of visual information
In the center of the retina is the macula
lutea with the fovea centralis in its
center. The center for color blindness
and sharpest vision
 Medial to the fovea centralis is the optic
disc, where nerve fibers leave the eye
and there is a blind spot
 The large cavity of the eye is filled with
vitreous humor. Which helps maintain
the shape of the eye and holds the
retina against choroid.
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Blind spot
The Lens
 The function of the lens to focus the
visual image on the photoreceptors. This
occurs by changing the shape of the
lens called accommodation
 The lens can either become rounder to
focus on nearby images or flattens to
focus on distant images
Concept Check
A man in his early 60s comes to the clinic
complaining of fuzzy vision. An eye
examination reveals clouding of his lens.
What is his problem and what factors might
have contributed to it?
 The man would be diagnosed with
cataracts. This could be caused from
smoking, UV radiation, aging, or an injury.
As aging occurs, the lens becomes less
elastic, and begins to lose its transparency.
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Light Refraction
When light waves pass from one
medium to another they bend because
of the different densities. This
occurrence is called refraction.
 The cornea and lens both bend the light
waves
 The lens provides the extra refraction
needed to focus the light rays from an
object to a focal point, a specific point
of interaction on the retina.
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Image Formation
The Brain compensates for the upside down
and backwards image,
without our
Conscious
awareness
Visual Receptors
 Two kinds of modified neurons comprise
the visual receptors; elongates rods and
blunt-shaped cones
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Rods are more sensitive to light and are
able to function in dim light; they do not
discriminate among colors of light
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Cones are less sensitive than rods,
which allow them only to function in
bright light. They also allow us to see
images in color
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To see things in detail, a person moves
the eyes so the image falls on the fovea
centralis, which contains the greatest
amount of cones
Visual Pigments
 The discs of the outer segment in rods
and cones contain the visual pigments
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Light sensitive pigment in rods is
rhodopsin, which breaks down into a
protein, opsin, and retinal (from vitamin
A) in the presence of light
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Breaking down of rhodopsin activates an
enzyme that initiates changes in the rod
cell membrane, creating a nerve impulse
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These nerve impulses travel away from
the retina and are interpreted as vision
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Light sensitive pigments in cones are
also proteins. There are three different
types of cones, each containing a
different visual pigment.
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Color-blindness occurs if one or more
classes of cones are absent or not
functional
Concept Check
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A man claiming to have difficulty seeing at night
decides to go to his eye doctor. What is the name
of the disorder that he has? What dietary
supplement will be recommended? If this condition
progressed too far, what retinal structures will be
deteriorated?
The man is suffering from night blindness, a
condition when an insufficient amount of vitamin A
is in the body. Which effects the amount of visual
pigment in the photoreceptors, causes it to drop.
He will need to intake more vitamin A. If he does
not get an adequate amount, his rods will become
affected.
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The wavelength of light determines the
color perceived from it; each of the three
pigments is sensitive to different
wavelengths of light
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The color perceived depends on which
set of cones the light stimulates; if all
three sets are stimulated, the color is
white; if none are stimulated, the color
is black.
Visual Pathway
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The axons of ganglion cells converge
on the optic disc, penetrate the wall of
the eye, and go to the optic nerve
The two optic nerves, one from each
eye, cross over in the optic chiasm
Impulses are transmitted to the
thalamus and then to the visual cortex
Hearing
The ear is divided into three regions: the
external, middle, and inner ear. These
three parts allow us to have the sense of
hearing and equilibrium.
 External ear: consists of the auricle
which collects the sound and travels
down the external auditory meatus.
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 Auricle- supported by elastic cartilage, that
protects the opening of the canal.
Middle ear: begins with the tympanic
membrane, and is an air-filled space
that houses the auditory ossicles.
 There are three auditory ossicles, the
malleus, the incus, and the stapes.
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The tempanic membrane converts
arriving sound energy into mechanical
movements of the auditory ossicles.
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The stapes vibrates the fluid inside the
oval window of the inner fluid
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The auditory ossicles both transmit and
amplify sound waves
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Inner ear: consists of the membranous
labyrinth, bony labyrinth, endolymph,
and perilymph
 Between the bony and membranous
labyrinths is a fluid called, perilymph.
 Inside the membranous labyrinth is a fluid
called, endolymph.
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The bony labyrinth can be divided into
3 parts:
 Vestibule- consists of the saccule and
utricle. Receptors in these sacs provide
sense of gravity and acceleration.
 Semicircular canals- enclose semicircular
ducts, the receptors are stimulated by
rotation of the head.
 Cochlea- contains the cochlear duct which
provides the sense of hearing
○ the cochlea duct lies between 2 chambers
called the vestibular duct and the tympanic
duct
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The organ of Corti consists of hair cells
of the cochlea duct, which lie on the
basilar membrane
 The hair cells come into contact with the
tectorial membrane, which is attached to the
inner wall of the cochlear duct
 Vibrations in the fluid of the inner ear cause
the hair cells to bend
 This stimulates sensory neurons nearby
Concept Check
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How would the loss of stereocilia from
the hair cells of the organ of Corti affect
hearing?
Loss of stereocilia (as a result from
constant exposure to loud sounds)
would reduce hearing sensitivity and
could eventually result in deafness.
The Hearing Process:
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Sound waves arrive at the tympanic
membrane.
The vibration of the tympanic
membrane causes movement of the
auditory ossicles.
The movement of the stapes at the oval
window establish pressure waves in the
perilympth of the vestibular duct.
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The pressure waves distort the basilar
membrane on their way to the round
window of the tympanis duct.
The vibration of the basilar membrane
cause the vibration of hair cells against
the tectorial membrane.
Information about the region and
intensity of stimulation is relayed to the
CNS.
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Auditory Nerve Pathway:
 Nerve fibers carry impulses to the auditory
cortices of the temporal lobes where they
are interpreted.
Equilibrium
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There are two aspects of equilibrium,
static and dynamic equilibrium.
 Static equilibrium- these organs help us
maintain the position of the head when the
head and body are still
 Dynamic equilibrium- these organs help us
maintain balance when the head and body
suddenly move and rotate.
Static Equilibrium
 These organs are located within the
bony vestibule of the inner ear, inside
the utricle and saccule
 Receptors in the utricle and saccule
respond to gravity and linear
acceleration.
 A macula, consists of hair cells and
supporting cells
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The hair cells contact gelatinous
material holding otoliths, which is the
whole complex consisting of gelatinous
mass and crystals.
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Gravity causes the gelatin and crystals
to shift, bending hair cells and generates
a nervous impulse.
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Impulses travel to the brain by a
vestibule branch of the
vestibulocochlear nerve, indicating the
position of the head.
Dynamic Equilibrium
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The three semicircular ducts detect
motion of the head, and aid in balancing
the head and sudden movements of the
body.
 The three ducts are; anterior, posterior, and
lateral semicircular.
 Each duct contains an ampulla, which is a
swollen region containing sensory receptors.
Hair cells attached to the wall of the
ampulla form a raised structure known
as crista.
 The stereocilia of the hair cells are
embedded in cupula, which is a
gelatinous structure.
 When the head rotates, movement of
the endolymph pushes against the
structure and stimulates the hair cells.
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Aging and the Senses
The general lack of replacement of
neurons in the nervous system leads to
a decline in sensory function with age.
 Some of this decline can be
compensated by increase in stimuli
strength or concentration, but the loss of
axons cannot be increased in a like
manner.
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Smell: olfactory receptor cells are replaced
regularly but the total number does decline
with age. These receptors also become
less sensitive. This is why elderly
individuals have trouble detecting odors in
small concentrations.
 Taste: Mucous membranes begin to thin
and the number and sensitivity of taste
buds decline. Elderly people find their food
to be bland, whereas children find their
food to be too spicy.
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Vision: there are various disorders that
are associated with aging
 The lens loses its elasticity with age and
stiffens. They also become farsighted, a
condition called presbyopia.
 Cataracts develop
 Gradual loss of rods
 Macular degeneration-growth and
proliferation of blood vessels in the retina
which causes blindness. Loss of
photoreceptors and retinal scarring.
SS
Macular Degeneration
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Hearing: the tympanic membrane loses
some of its elasticity, making it more
difficult to hear high pitched sounds.
The progressive loss of hearing is
called, presbycusis.
Concept Check
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
Why does it increasingly get more
difficult to see close object as we get
older?
With age, the lens loses it elasticity and
stiffens. These changes result in a flatter
lens that cannot round up to focus the
image of a close object on the retina.
This can be corrected with a converging
lens.