Download The Special Senses Receptors General Properties of Receptors

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Receptors
The Special Senses
Introduction
Vision
General Properties of Receptors
• Specialized structures designed to respond to specific
stimuli
• Variable complexity
General Properties of 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
Stimulus causing receptor potentials
⇓
Generator potential in afferent neuron
⇓
Nerve impulse
• Occurs when either:
– The neuron is the receptor
» Example: neurons in olfactory epithelium
– The receptor releases NT and stimulates the sensory neuron
Sensation and Perception
Stimulatory input
General properties of receptors
• 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
Conscious level = perception
– Duration
• Encoded by changes in the pattern of impulse transmission over time
Detection of stimulus = sensation
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Sensory Adaptation
Classification of Receptors
• Stimulus Modality
Chemoreceptors
Thermoreceptors
Nociceptors
Mechanoreceptors
Photoreceptors
• Reduction in rate of impulse transmission when stimulus is prolonged
• Occurs in all receptors except those for pain
• Let’s try it…
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
Nociceptors
Free nerve endings
Respond to tissue damage
Give the perception of pain
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
Mechanoreceptors
Respond to physical
deformity
Touch pressure, stretch,
vibration, tension
Photoreceptors
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Classification of Receptors
• Stimulus Modality
Classification of Receptors
• Origin of stimuli
Chemoreceptors
Thermoreceptors
Nociceptors
Mechanoreceptors
Photoreceptors
Exteroceptors
Interoceptors
Proprioceptors
Respond to light
Rods, cones, certain
retinal ganglion cells
Classification of Receptors
• Origin of stimuli
Classification of Receptors
• Origin of stimuli
Exteroceptors
Exteroceptors
Interoceptors
Respond to stimuli that originate
outside the body
• Examples: light, smell, sound
Respond to stimuli that originate
inside the body
• Examples: pressure, hunger, thirst
Interoceptors
Proprioceptors
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
Special senses
•
•
•
•
Vision
Hearing
Olfaction
Gustation
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Vision Introduction
Vision Introduction
• 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
• 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
Eyebrow
Orbicularis oculi muscle
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Palpebral conjunctiva
(covers inside of eyelids)
Cornea
Lateral
commissure
Eyelashes
Bulbar conjunctiva
(covers outer surface of eye)
Eyelid
Conjunctival sac
Medial
commissure
Orbicularis oculi muscle
(a) Surface anatomy of the right eye
(b) Lateral view; some structures shown in sagittal section
Figure 15.1a
Figure 15.1b
Accessory Structures
Lacrimal sac
• 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
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Figure 15.2
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Accessory Structures
Superior oblique
muscle
Superior oblique
tendon
Superior rectus
muscle
• Six extrinsic eye muscles
Lateral rectus
muscle
Inferior rectus
Inferior oblique
muscle
muscle
(a) Lateral view of the right eye
Figure 15.3a
the Eyeball
Muscle
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique
Action
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
Controlling
cranial nerve
VI (abducens)
III (oculomotor)
III (oculomotor)
III (oculomotor)
III (oculomotor)
IV (trochlear)
• Wall of eyeball contains three layers (tunics)
–Fibrous
–Vascular
–Sensory (retinal)
(c) Summary of muscle actions and innervating cranial nerves
Figure 15.3c
the eyeball
• Three layers
1) Fibrous tunic
Sclera
Cornea
• Sclera – “white” of the eye
• Cornea – clear continuation over the iris and pupil
Scleral venous sinus
(AKA canal of Schlemm)
Fibrous Tunic
Figure 15.4a
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the eyeball
• Cornea must be actively dehydrated to remain
transparent
• Sodium pumps remove sodium from the
endothelium – water follows
the eyeball
• Three layers
2) Vascular tunic (uvea) – 3 components
–Choroid
–Ciliary body
–Iris
Corneal Edema
the eyeball
• Three layers
the eyeball
• Three layers
2) Vascular tunic (uvea) – 3 components
2) Vascular tunic (uvea)
–Choroid
–Choroid
–Ciliary body
» Highly vascular
» Darkly pigmented
- Absorbs stray
light
» Ciliary processes
- Arranged in a ring
around the lens
» Ciliary muscle
- Used to focus the
eye
–Ciliary body
–Iris
–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
Ciliary body
Ciliary zonule
(suspensory
ligament)
Sclera
Choroid
Iris
Pupil
Cornea
Lens
Scleral venous
sinus
VascularTunic
Figure 15.4a
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the eyeball
the eyeball
• Three layers
• Rods and cones
3) Retina (innermost layer)
–
–
–
–
• Photoreceptor neurons
– Rods and Cones
• Bipolar neurons
• Ganglion neurons
– Axons form the optic nerve
– Exit eyeball at optic disc
Ciliary body
Ciliary zonule
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)
Ganglion
cells
Sclera
Choroid
Bipolar
cells
Photoreceptors
• Rod
• Cone
Retina
Cornea
Iris
Pupil
Macula lutea
Fovea centralis
Optic nerve
Lens
Scleral venous
sinus
Central artery
and vein of
the retina
Optic disc
(blind spot)
Amacrine cell
Horizontal cell
Pigmented
Pathway of signal output
layer of retina
Pathway of light
(b) Cells of the neural layer of the retina
Retina
Figure 15.4a
Retinal Blood Supply
Figure 15.6b
Pathway of light
• Outer 1/3 of retina
Neural layer of retina
– Choroid
• Inner 2/3 of retina
Pigmented
layer of
retina
Choroid
Sclera
– Central artery and vein
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
Figure 15.6a
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Blood supply to the retina
The Eyeball
The eyeball
• Photoreceptors
• Photoreceptors
– Rods
– Cones
• Red, blue, and green
– Highest density in macula lutea
• More numerous at peripheral region
• Dim light
» Concentrated in fovea centralis
– Indistinct, fuzzy, non-color peripheral vision
• Operate in bright light
• High-acuity color vision
• About 100 million per eye
The eyeball
Central
artery
and vein
emerging
from the
optic disc
• Lens
–
–
–
–
Macula
lutea
Optic disc
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
Retina
Figure 15.7
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The eyeball
Emmetropic eye (normal)
• Two Cavities
– Anterior cavity
Focal
plane
• Filled with aqueous humor
– Modified plasma
– Constantly produced, in constant circulation
– Accumulation may lead to glaucoma
Focal point is on retina.
Figure 15.14 (1 of 3)
Iris
Lens epithelium
Lens
Cornea
Corneal epithelium
Corneal endothelium
Aqueous humor
Anterior Anterior
segment chamber
Posterior
(contains
chamber
aqueous
humor)
Scleral venous
sinus
1 Aqueous humor is
Cornealformed by filtration
from the capillaries in
scleral junction
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.
The eyeball
Posterior
segment
(contains
vitreous
humor)
2
• Two Cavities
Ciliary zonule
(suspensory
ligament)
Ciliary body
3
Bulbar
conjunctiva
Sclera
1
Ciliary
processes
• 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
Ciliary
muscle
Cornea
– Posterior cavity
Lens
Figure 15.8
The Physiology of Vision
• 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
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Refraction
Refraction
• The bending of light rays
• Most of the light we see is reflected off of objects
Refraction
Refraction
• This light begins as a single point, but spreads as
distance from the object increases
• 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
Focal length is fixed in the eye
Refraction
Emmetropic eye (normal)
Focal
plane
• 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
Focal point is on retina.
Figure 15.14 (1 of 3)
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Refraction
Sympathetic activation
Nearly parallel rays
from distant object
•
Lens
Refracting media
• Light is refracted by
– Cornea
– Aqueous humor
– Lens
– Vitreous humor
Ciliary zonule
Inverted
image
Ciliary muscle
•
Change in lens curvature
– Allows for fine focusing of an image
(a) Lens is flattened for distant vision. Sympathetic
input relaxes the ciliary muscle, tightening the ciliary
zonule, and flattening the lens.
Figure 15.13a
Accommodation
Sympathetic activation
Nearly parallel rays
from distant object
• Eye is adapted for distance vision
Lens
• Ciliary muscle is relaxed = lens is flat
As light moves closer greater curvature required
• Ciliary muscle contracts = lens is curved
Ciliary zonule
Inverted
image
Ciliary muscle
(a) Lens is flattened for distant vision. Sympathetic
input relaxes the ciliary muscle, tightening the ciliary
zonule, and flattening the lens.
Figure 15.13a
Parasympathetic activation
Divergent rays
from close object
Accommodation
Inverted
image
• Near vision
– Light rays from near objects require extensive refraction
– Accommodation
• Ciliary muscles contract
• Lens shape changes
• Eye strain
(b) Lens bulges for close vision. Parasympathetic
input contracts the ciliary muscle, loosening the
ciliary zonule, allowing the lens to bulge.
Figure 15.13b
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Accommodation
• Near point
– Closest point at which an object can be brought into focus
• Typically 25cm
– Maximum bulge
– Presbyopia - Aging
Close Vision
• 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)
Pupil Manipulation
• Contraction of the circular muscles = constriction
• Contraction of the radial muscles = dilation
Photoreception
• Sensory transduction
– Light energy is converted into nerve impulses
• Takes place in retina
– Photoreceptors
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Process of
bipolar cell
Photoreception
• Rods and cones
–Outer segment of each contains visual pigments
Synaptic terminals
Rod cell body
Rod cell body
Cone cell body
Nuclei
Outer fiber
Mitochondria
Pigmented layer
Outer segment
Inner
segment
• Molecules that change shape as they absorb light
Inner fibers
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
Connecting
cilia
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Melanin
granules
Basal lamina (border
with choroid)
Figure 15.15a
In the dark
Na+
Ca2+
Rod discs
In the dark, cGMP opens sodium
channels and depolarizes
photoreceptors
Photoreceptor
cell (rod)
Ca2+
Inhibitory neurotransmitter is released
(glutamic acid)
Note that they
are connected
to each other
here
Bipolar cells are
constantly inhibited
in the dark
Visual
pigment
consists of
• Retinal
• Opsin
Bipolar
cell
Ganglion
cell
(b) Rhodopsin, the visual pigment in rods, is embedded in
the membrane that forms discs in the outer segment.
Figure 15.15b
Breakdown of
rhodopsin causes closing
of sodium channels
11-cis-retinal
2H+
Oxidation
Vitamin A
11-cis-retinal
Rhodopsin
Reduction
2H+
2 Regeneration
of the pigment:
Enzymes slowly
convert all-trans
retinal to its
11-cis form in the
pigmented
epithelium;
requires ATP.
Figure 15.18 (1 of 2)
Dark
Light
In the light
1 cGMP-gated channels
are closed, so Na+ influx
stops; the photoreceptor
hyperpolarizes.
Light
1 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.
Photoreceptor
cell (rod)
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
In the presence of light
bipolar cells are not
inhibited
4 Lack of IPSPs in bipolar
cell results in depolarization.
Bipolar
cell
Ca2+
Ganglion
cell
Opsin and
All-trans-retinal
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
All-trans-retinal
Figure 15.16
Figure 15.18 (2 of 2)
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Photoreception
• 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
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
• 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
Visual Pathway to the brain
Fixation point
• 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
Optic nerve
Suprachiasmatic
nucleus
Pretectal nucleus
Lateral geniculate
nucleus of
Thalamus – primary
connection of optic
nerve to occipital lobe
Superior colliculus
Optic chiasma
Optic tract
Uncrossed (ipsilateral) fiber
Crossed (contralateral) fiber
Optic radiation
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.
Figure 15.19a
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Abnormalities of Vision
• Myopia
(nearsightedness)
Abnormalities of Vision
• Hyperopia
(farsightedness)
– Close objects seen clearly
– Image is focused in front
of the retina
– Correction = concave lens
– Distant objects seen
clearly
– Image is focused behind
the retina
– Correction = convex lens
Abnormalities of vision
•
•
•
•
•
•
•
•
•
Abnormalities of vision
• Astigmatism
– Abnormal curvature of eye
– Light rays don’t meet at a common focus
– Causes visual distortion
Astigmatism
Detached retina
Conjunctivitis
Glaucoma
Cataract
Diplopia
Night blindness
Color blindness
Macular Degeneration
Abnormalities of vision
• Detached retina
– Retina becomes separated from underlying choroid
– Causes loss of vision in affected area
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Abnormalities of vision
• 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
• Nightblindness (nyctalopia)
– Loss of low light vision
– Most common cause: retinitis pigmentosa
• Rods lose ability to respond to light
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
• Colorblindness
– Defect in perception of colors
– Defect in cones
– Can be acquired; usually genetic
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Abnormalities of vision
• 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
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