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Introduction
Vision
The Special Senses• Specialized structures designed to respond to specific
stimuli
• Variable complexity
Receptors
• Transducers
– Stimulus energy is converted to an AP
• 2 types of graded potentials generated by receptors
– Receptor potential
• Occurs in a separate receptor cell (not the neuron)
• Example: Rods and cones use phototransduction
– Generator potential
• Occurs when either:
– The neuron is the receptor
» Example: neurons in olfactory epithelium
– The receptor releases NT and stimulates the sensory neuron
General Properties of Receptors
Stimulus causing receptor potentials
⇓
Generator potential in afferent neuron
⇓
Nerve impulse
General Properties of Receptors
Sensation and Perception
Stimulatory input
Conscious level = perception
Detection of stimulus = sensation
• Information conveyed by receptors
– Modality
• Type of stimulus or sensation – hearing, vision, taste, etc.
– Location
• Brain refers pain to its point of origin
– Intensity
• Determined by impulse transmission rate, variations in sensory
thresholds in a group of sensory neurons
– Duration
• Encoded by changes in the pattern of impulse transmission over time
General properties of receptors
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Sensory Adaptation
• Reduction in rate of impulse transmission when stimulus is prolonged
• Occurs in all receptors except those for pain
• Let’s try it…
• Stimulus Modality
� Chemoreceptors
� Thermoreceptors
� Nociceptors
� Mechanoreceptors
� Photoreceptors
Classification of Receptors
• Stimulus Modality
� Chemoreceptors
� Respond to chemical
concentrations
� Odors, tastes, dissolved
chemicals, etc
� Thermoreceptors
� Nociceptors
� Mechanoreceptors
� Photoreceptors
Classification of Receptors
• Stimulus Modality
� Chemoreceptors
� Thermoreceptors
� Respond to heat and cold
� Nociceptors
� Mechanoreceptors
� Photoreceptors
Classification of Receptors
• Stimulus Modality
� Chemoreceptors
� Thermoreceptors
� Nociceptors
� Free nerve endings
� Respond to tissue damage
� Give the perception of pain
� Mechanoreceptors
� Photoreceptors
Classification of Receptors
• Stimulus Modality
� Chemoreceptors
� Thermoreceptors
� Nociceptors
� Mechanoreceptors
� Respond to physical
deformity
� Touch pressure, stretch,
vibration, tension
� Photoreceptors
Classification of Receptors
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• Stimulus Modality
� Chemoreceptors
� Thermoreceptors
� Nociceptors
� Mechanoreceptors
� Photoreceptors
� Respond to light
� Rods, cones, certain
retinal ganglion cells
Classification of Receptors
• Origin of stimuli
� Exteroceptors
� Interoceptors
� Proprioceptors
Classification of Receptors
• Origin of stimuli
� Exteroceptors
� Respond to stimuli that originate
outside the body
• Examples: light, smell, sound
� Interoceptors
� Proprioceptors
Classification of Receptors
• Origin of stimuli
� Exteroceptors
� Interoceptors
� Respond to stimuli that originate
inside the body
• Examples: pressure, hunger, thirst
� Proprioceptors
Classification of Receptors
• Origin of stimuli
� Exteroceptors
� Interoceptors
� Proprioceptors
� Respond to stimuli that
originate due to the
positions and tensions
of joints and muscles
Classification of Receptors
• Vision
• Hearing
• Olfaction
• Gustation
Special senses
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• 70% of all sensory receptors are in the eye
• Nearly half of the cerebral cortex is involved in processing
visual information
• Optic nerve is one of body’s largest nerve tracts
Vision Introduction
• The eye is a photoreceptor organ
• Refraction
– Light bends when it passes from one medium to another
• Conversion (transduction) of light into AP’s
• Information is interpreted in cerebral cortex
Vision Introduction
Figure 15.1a
Eyelashes
Site where
conjunctiva
merges with
cornea
Lateral
commissure
Medial
commissure
Eyelid
Eyelid
Eyebrow
Palpebral
fissure
(a) Surface anatomy of the right eye
Figure 15.1b
(b) Lateral view; some structures shown in sagittal section
Orbicularis oculi muscle
Eyebrow
Palpebral conjunctiva(covers inside of eyelids)
Cornea
Eyelashes
Bulbar conjunctiva(covers outer surface of eye)
Conjunctival sac
Orbicularis oculi muscle
• Lacrimal apparatus
– Series of structures that produce tears, direct their flow, and
drain them
• Lacrimal gland and ducts
– Connect to nasal cavity
• Lacrimal secretion (tears)
– Dilute saline solution
» Mucus, antibodies, and lysozyme
– Blinking spreads the tears toward medial commissure
– Drain into the nasolacrimal duct
Accessory Structures
Figure 15.2
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Lacrimal sac
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• Six extrinsic eye muscles
Accessory Structures
Figure 15.3a
Inferior rectus
muscle
Inferior oblique
muscle
Superior oblique
muscle
Superior oblique
tendon
Superior rectus
muscle
Lateral rectus
muscle
(a) Lateral view of the right eye
Figure 15.3c
(c) Summary of muscle actions and innervating cranial nerves
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique
Moves eye laterally
Moves eye medially
Elevates eye and turns it medially
Depresses eye and turns it medially
Elevates eye and turns it laterally
Depresses eye and turns it laterally
VI (abducens)
III (oculomotor)
III (oculomotor)
III (oculomotor)
III (oculomotor)
IV (trochlear)
Muscle ActionControlling
cranial nerve • Wall of eyeball contains three layers (tunics)
–Fibrous
–Vascular
–Sensory (retinal)
the Eyeball
• Three layers
1) Fibrous tunic
• Sclera – “white” of the eye
• Cornea – clear continuation over the iris and pupil
the eyeball
Figure 15.4a
Sclera
Cornea
Fibrous Tunic
Scleral venous sinus
(AKA canal of Schlemm)
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• Cornea must be actively dehydrated to remain
transparent
• Sodium pumps remove sodium from the
endothelium – water follows
the eyeball
Corneal Edema
• Three layers
2) Vascular tunic (uvea) – 3 components
–Choroid
–Ciliary body
– Iris
the eyeball
• Three layers
2) Vascular tunic (uvea) – 3 components
–Choroid
» Highly vascular
» Darkly pigmented
- Absorbs stray
light
–Ciliary body
– Iris
the eyeball
• Three layers
2) Vascular tunic (uvea)
–Choroid
–Ciliary body
» Ciliary processes
- Arranged in a ring
around the lens
» Ciliary muscle
- Used to focus the
eye
– Iris
the eyeball
• Three layers
2) Vascular tunic (uvea) – 3 components
–Choroid
–Ciliary body
– Iris
- Radial muscle fibers contract = pupil dilation
- Circular muscle fibers contract = pupil constriction
the eyeball
Figure 15.4a
Choroid
Sclera
Ciliary bodyCiliary zonule
(suspensory
ligament)
IrisPupil
Lens
Scleral venous
sinus
VascularTunic
Cornea
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• Three layers
3) Retina (innermost layer)
• Photoreceptor neurons
– Rods and Cones
• Bipolar neurons
• Ganglion neurons
– Axons form the optic nerve
– Exit eyeball at optic disc
the eyeball
• Rods and cones
– Specialized photoreceptors
– Rods = black and white vision
– Cones = daylight color vision
– Fovea centralis
• Cones most concentrated here
• Point of highest visual acuity
• Other retinal layers are displaced (forms a depression)
the eyeball
Figure 15.4a
Central artery
and vein of
the retina
Optic disc
(blind spot)
Optic nerve
Fovea centralis
Macula luteaRetinaChoroid
Sclera
Ciliary body
Ciliary zonule
Cornea
Iris
Pupil
Lens
Scleral venous
sinus
Retina
Figure 15.6b
Pigmented
layer of retina
(b) Cells of the neural layer of the retina
Amacrine cellHorizontal cell
• Rod
Photoreceptors
• Cone
Bipolar
cellsGanglion
cells
Pathway of lightPathway of signal output
• Outer 1/3 of retina
– Choroid
• Inner 2/3 of retina
– Central artery and vein
Retinal Blood Supply
Figure 15.6a
(a) Posterior aspect of the eyeball
Neural layer of retinaPigmented
layer of
retina
Central artery
and vein of retina Optic
nerve
Sclera
Choroid
Optic disc
Pathway of light
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Blood supply to the retina
• Photoreceptors
– Rods
• More numerous at peripheral region
• Dim light
– Indistinct, fuzzy, non-color peripheral vision
• About 100 million per eye
The Eyeball
• Photoreceptors
– Cones
• Red, blue, and green
– Highest density in macula lutea
» Concentrated in fovea centralis
• Operate in bright light
• High-acuity color vision
The eyeball
Figure 15.7
Macula
lutea
Centralarteryand veinemergingfrom theoptic disc
Optic disc
Retina
• Lens
– Biconvex, transparent, flexible
– Attached to ciliary body by suspensory ligaments
– Allows precise focusing of light on the retina
– Forms a partition
• Creates an anterior and posterior cavity
The eyeball
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Figure 15.14 (1 of 3)
Focal
plane
Focal point is on retina.
Emmetropic eye (normal)• Two Cavities
– Anterior cavity
• Filled with aqueous humor
– Modified plasma
– Constantly produced, in constant circulation
– Accumulation may lead to glaucoma
The eyeball
Figure 15.8
Sclera
Bulbar
conjunctiva
Scleral venoussinus
Posteriorchamber
Anteriorchamber
Anteriorsegment(containsaqueoushumor)
Corneal-scleral junction
Cornea
Cornea
Corneal epithelium
Corneal endothelium
Aqueous humor
Iris
Lens
Lens epithelium
Lens
Posteriorsegment(contains vitreoushumor)
Ciliary zonule
(suspensory
ligament)
Ciliary
processes
Ciliary
muscle
Ciliary body
1 Aqueous humor is formed by filtration from the capillaries in the ciliary processes.
2 Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber. Some also flows through the vitreous humor (not shown).
3 Aqueous humor is reabsorbed into the venous blood by the scleral venous sinus.
1
2
3
• Two Cavities
– Posterior cavity
• Filled with vitreous humor
– Gelatinous mass
– Avascular
– Very few cells
» A few phagocytes that remove debris from the field of vision
– Responsible for eye holding its spherical shape
The eyeball
• Visual acuity – ability to discern detail
– Ratio of distances
• 20/20
• 20/40
– Less visual acuity
• 20/15
– More visual acuity
The Physiology of Vision
• 5 processes produce a visual image
1. Refraction
2. Accommodation
3. Pupil constriction
4. Convergence
5. Photoreception
The Physiology of vision
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• The bending of light rays
Refraction
• Most of the light we see is reflected off of objects
Refraction
• This light begins as a single point, but spreads as
distance from the object increases
Refraction
• The eye must collect and re-bend the light to a single
point in order to construct an image
• Since focal length is fixed, the eye changes the shape
of the lens to accomplish this
Refraction
Figure 15.14 (1 of 3)
Focal
plane
Focal point is on retina.
Emmetropic eye (normal)
Focal length is fixed in the eye
• Light passing through a convex lens (as in the eye) is bent so
that the rays converge at a focal point
• The image formed at the focal point is upside-down and
reversed right to left
Refraction
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Figure 15.13a
Lens
Inverted
image
Ciliary zonule
Ciliary muscle
Nearly parallel raysfrom distant object
(a) Lens is flattened for distant vision. Sympathetic
input relaxes the ciliary muscle, tightening the ciliary
zonule, and flattening the lens.
Sympathetic activation
• Refracting media
• Light is refracted by
– Cornea
– Aqueous humor
– Lens
– Vitreous humor
• Change in lens curvature
– Allows for fine focusing of an image
Refraction
• Eye is adapted for distance vision
• Ciliary muscle is relaxed = lens is flat
� As light moves closer greater curvature required
• Ciliary muscle contracts = lens is curved
Accommodation
Figure 15.13a
Lens
Inverted
image
Ciliary zonule
Ciliary muscle
Nearly parallel raysfrom distant object
(a) Lens is flattened for distant vision. Sympathetic
input relaxes the ciliary muscle, tightening the ciliary
zonule, and flattening the lens.
Sympathetic activation
Figure 15.13b
Divergent raysfrom close object
(b) Lens bulges for close vision. Parasympathetic
input contracts the ciliary muscle, loosening the
ciliary zonule, allowing the lens to bulge.
Invertedimage
Parasympathetic activation
• Near vision
– Light rays from near objects require extensive refraction
– Accommodation
• Ciliary muscles contract
• Lens shape changes
• Eye strain
Accommodation
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• Near point
– Closest point at which an object can be brought into focus
• Typically 25cm
– Maximum bulge
– Presbyopia - Aging
Accommodation
• Close vision requires
–Accommodation
–Pupil constriction• Prevents most divergent light rays from entering the eye
–Convergence• Medial rotation of the eyeballs toward the object being viewed
(binocular vision)
Close Vision
• Contraction of the circular muscles = constriction
• Contraction of the radial muscles = dilation
Pupil Manipulation
• Sensory transduction
– Light energy is converted into nerve impulses
• Takes place in retina
– Photoreceptors
Photoreception
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• Rods and cones
–Outer segment of each contains visual pigments
• Molecules that change shape as they absorb light
Photoreception
Figure 15.15a
Process of
bipolar cell
Outer fiber
Apical microvillus
Discs containing
visual pigments
Melanin
granules
Discs being
phagocytized
Pigment cell nucleus
Inner fibers
Rod cell body
Cone cell body
Synaptic terminals
Rod cell body
Nuclei
Mitochondria
Connecting
cilia
Basal lamina (border
with choroid)
The outer segmentsof rods and cones are embedded in the pigmented layer of the retina.
Pig
mente
d l
ayer
Oute
r se
gm
ent
Inner
se
gm
ent
Figure 15.15b
Rod discs
Visualpigmentconsists of• Retinal• Opsin
(b) Rhodopsin, the visual pigment in rods, is embedded in
the membrane that forms discs in the outer segment.
Note that they
are connected
to each other
here
Figure 15.18 (1 of 2)
Na+
Ca2+
Ca2+
Photoreceptor
cell (rod)
Bipolar
cell
Ganglion
cell
In the dark
In the dark, cGMP opens sodium
channels and depolarizes
photoreceptors
Inhibitory neurotransmitter is released
(glutamic acid)
Bipolar cells are
constantly inhibited
in the dark
Figure 15.16
11-cis-retinal
Bleaching of
the pigment:
Light absorption
by rhodopsin
triggers a rapid
series of steps
in which retinal
changes shape
(11-cis to all-trans)
and eventually
releases from
opsin.
1
Rhodopsin
Opsin and
Regeneration
of the pigment:
Enzymes slowly
convert all-trans
retinal to its
11-cis form in the
pigmented
epithelium;
requires ATP.
Dark Light
All-trans-retinal
Oxidation
2H+
2H+
Reduction
Vitamin A
2
11-cis-retinal
All-trans-retinal
Figure 15.18 (2 of 2)
1 cGMP-gated channels
are closed, so Na+ influx
stops; the photoreceptor
hyperpolarizes.
Voltage-gated Ca2+
channels close in synaptic
terminals.
2
No neurotransmitter
is released.
3
Lack of IPSPs in bipolar
cell results in depolarization.
4
Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
5
EPSPs occur in ganglion
cell.
6
Action potentials
propagate along the
optic nerve.
7
Photoreceptor
cell (rod)
Bipolar
cell
Ganglion
cell
Light
Ca2+
In the light
In the presence of light
bipolar cells are not
inhibited
Breakdown of
rhodopsin causes closing
of sodium channels
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• Excitation of cones
– Method of excitation is similar to that of rods
– There are three types of cones
• Named for the colors of light absorbed: blue, green, & red
• Light absorbed depends upon opsins present
Photoreception
• Light adaptation– Occurs when moving from darkness into bright light
• Large amounts of pigments are broken down instantaneously
– Produces glare
• Pupils constrict
• Dramatic changes in retinal sensitivity
– Rod function ceases
• Cones and neurons rapidly adapt
– Visual acuity improves over 5–10 minutes
Photoreception
• Dark adaptation
– Occurs when moving from bright light into darkness
• The reverse of light adaptation
• Cones stop functioning in low-intensity light
• Pupils dilate
• Rhodopsin accumulates in the dark
– Retinal sensitivity increases within 20–30 minutes
Photoreception
• Duplicity theory
– Why do we have 2 kinds of photoreceptors?
• Neural circuits for night are not suited for day
– Rods:Bipolar cell
• 600:1
• Spatial summation of large field = poor detail
– Cones:Bipolar cell
• 1:1
• Best for detail in small receptive fields
Photoreception
• Axons from medial portion of each retina cross at optic
chiasma
– Continue as optic tracts
• Optic radiation fibers connect to the primary visual cortex in
the occipital lobes
– Visual input from both eyes is interpreted by brain
• Distance
• Depth perception
Visual Pathway to the brain
Figure 15.19a
Pretectal nucleus
Fixation point
Optic radiation
Optic tract
Optic chiasma
Uncrossed (ipsilateral) fiber
Crossed (contralateral) fiber
Optic nerve
Lateral geniculatenucleus ofThalamus – primary connection of optic nerve to occipital lobe
Superior colliculus Occipital lobe (primary visual cortex)
The visual fields of the two eyes overlap considerably.Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma.
Suprachiasmatic
nucleus
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• Myopia
(nearsightedness)
– Close objects seen clearly
– Image is focused in front
of the retina
– Correction = concave lens
Abnormalities of Vision
• Hyperopia
(farsightedness)
– Distant objects seen
clearly
– Image is focused behind
the retina
– Correction = convex lens
Abnormalities of Vision
• Astigmatism
• Detached retina
• Conjunctivitis
• Glaucoma
• Cataract
• Diplopia
• Night blindness
• Color blindness
• Macular Degeneration
Abnormalities of vision
Abnormalities of vision
• Astigmatism
– Abnormal curvature of eye
– Light rays don’t meet at a common focus
– Causes visual distortion
• Detached retina
– Retina becomes separated from underlying choroid
– Causes loss of vision in affected area
Abnormalities of vision
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• Conjunctivitis
– Inflammation of the conjunctiva
– Causes by bacteria, viruses, irritants, allergies
– “Pink eye”
Abnormalities of vision
• Glaucoma
– Increased pressure in the eyeball
– Leads to optic nerve damage
– Causes gradual loss of sight
– Damage is irreversible, but may be slowed or stopped
Abnormalities of vision
• Cataract
– Lens becomes progressively opaque
– Results in blurred vision
– Can be surgically removed and replaced with an artificial
lens
Abnormalities of vision
• Diplopia
– Double vision
– At least 20 different causes
• Cranial nerve III palsy
• Aneurysm
• High blood pressure
• Diabetes
• Inflammatory conditions
• Guillain-Barre syndrome
• Problems with the lid,
pupil, or eye muscles
• Etc.
Abnormalities of vision
• Nightblindness (nyctalopia)
– Loss of low light vision
– Most common cause: retinitis pigmentosa
• Rods lose ability to respond to light
Abnormalities of vision
• Colorblindness
– Defect in perception of colors
– Defect in cones
– Can be acquired; usually genetic
Abnormalities of vision
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• Macular degeneration
– Damage to macula and retina
– Risk factors
• Genetic
• Lifestyle
Abnormalities of vision
• Bonus: amblyopia
– “Lazy eye”
– Decreased vision in eye that appears normal
– Developmental problem of the brain
– Brain doesn’t recognize image from one eye
Abnormalities of vision