Download Light - Collin College Faculty Website Directory

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
15
The Special
Senses:
Part A
Copyright © 2010 Pearson Education, Inc.
Internal Chambers and Fluids
• Posterior segment contains vitreous humor that:
• Transmits light
• Supports the posterior surface of the lens
• Holds the neural retina firmly against the pigmented layer
• Contributes to intraocular pressure
• Anterior segment is composed of two chambers
• Anterior chamber—between the cornea and the iris
• Posterior chamber—between the iris and the lens
Copyright © 2010 Pearson Education, Inc.
Internal Chambers and Fluids
• Anterior segment contains aqueous humor
• Plasma like fluid continuously filtered from capillaries of
the ciliary processes
• Drains via the scleral venous sinus (canal of Schlemm) at
the sclera-cornea junction
• Supplies nutrients and oxygen mainly to the lens and cornea
but also to the retina, and removes wastes
• Glaucoma: compression of the retina and optic nerve if
drainage of aqueous humor is blocked
Copyright © 2010 Pearson Education, Inc.
The Retina: Photoreceptors
• Rods:
• About 125 million – mostly in the periphery
• Respond to dim light
• Are used for peripheral vision
• Cones:
• About 6 million mostly concentrated in the center
• Respond to bright light
• Have high-acuity color vision
• Are found in the macula lutea
• Are concentrated in the fovea centralis
Copyright © 2010 Pearson Education, Inc.
Photoreceptors - Rods functional characteristics
• Sensitive to dim light and best
suited for night vision
• Absorb all wavelengths of
visible light
• Perceived input is in gray
tones only
• Sum of visual input from
many rods feeds into a single
ganglion cell
• Results in fuzzy and indistinct
images
Copyright © 2010 Pearson Education, Inc.
http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html
Photoreceptors - Cones functional characteristics
• Need bright light for
activation
(have
low
sensitivity)
• Have pigments that furnish
a vividly colored view
• Each cone synapses with a
single ganglion cell
• Vision is detailed and has
high resolution
Copyright © 2010 Pearson Education, Inc.
http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html
Other cells in the retina
•
Horizontal cells – extend across the outer portion of the retina
at the level of the synapses between photoreceptors and bipolar
cells.
•
Amacrine cells found at the level of the synapses between
bipolar and ganglion cells.
•
Both can facilitate of inhibit communication between photo
receptors and ganglion cells
•
By doing that they can alter the sensitivity of the retina
•
Bipolar cells. Connect the receptor to ganglion cells
•
Ganglion cells encode light information within action potentials
to be processed transferred to the visual cortex
Copyright © 2010 Pearson Education, Inc.
Light
• Electromagnetic radiation – all energy waves from short
gamma rays to long radio waves
• Our eyes respond to a small portion of this spectrum called
the visible spectrum
• Rods and the different cones in the retina respond to
different wavelengths of the visible spectrum
Copyright © 2010 Pearson Education, Inc.
Cornea
• Clear window in the anterior part of the eye that allows the
entrance of light.
• It is a major part of the light-bending apparatus of the eye
• Covered by epithelial sheets on both faces (anterior and
posterior)
• External layer – stratified squamous ET – for protection;
merges with the ocular conjuntiva
• provide a smooth surface that absorbs oxygen and other
needed cell nutrients that are contained in tears.
• This layer is filled with thousands of tiny nerve endings
that make the cornea extremely sensitive to pain when
rubbed or scratched.
• Moist and being nourished by tears
Copyright © 2010 Pearson Education, Inc.
Cornea
• Bowman’s layer
• a tough layer that protects the corneal stroma,
consisting of irregularly-arranged collagen fibers
• Stroma
• Located behind the external epithelium
• A thick, transparent middle layer, consisting of
regularly-arranged collagen fibers
• It consists primarily of water and layered protein
fibers
• that give the cornea its strength, elasticity, and form
Copyright © 2010 Pearson Education, Inc.
Cornea
• Deep epithelial layer – This single layer of cells is
located between the stroma and the aqueous humor.
• Because the stroma tends to absorb water, the
endothelium's primary task is to pump excess water
out of the stroma (by having sodium pumps).
• Without this pumping action, the stroma would swell
with water, become cloudy, and ultimately opaque
Copyright © 2010 Pearson Education, Inc.
http://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg
Copyright © 2010 Pearson Education, Inc.
http://www.siumed.edu/~dking2/intro/IN022b.htm
Lens
• A biconvex, transparent, flexible, avascular structure that:
• Allows precise focusing of light onto the retina
• Is composed of epithelium and lens fibers
• Lens is avascular because blood vessels interfere with transparency.
• The lens depends entirely upon the aqueous and vitreous humors for
nourishment.
• Lens has 2 regions
• Lens epithelium – cuboidal cells found at the anterior surface of
the lens. These cells differentiate into lens fibers
• Lens fibers – cells filled with the transparent protein crystallin.
These cells are packed in layers and contain no nuclei.
• New lens fibers are added continuously the lens enlarges,
become denser, less elastic.
Copyright © 2010 Pearson Education, Inc.
http://www.siumed.edu/~dking2/ssb/EE011b.htm
Copyright © 2010 Pearson Education, Inc.
Refraction and Lenses
• When light passes from one
transparent
medium
to
another its speed changes
and it refracts (bends)
• Light passing through a
convex lens (as in the eye)
is bent so that the rays
converge (join) to a focal
point
• When a convex lens forms
an image, the image is
upside down and reversed
right to left
Copyright © 2010 Pearson Education, Inc.
Focusing Light on the Retina
• Pathway of light entering the eye: cornea, aqueous
humor, lens, vitreous humor, and the neural layer of
the retina to the photoreceptors
• Light is refracted:
• At the cornea
• Entering the lens
• Leaving the lens
• The lens curvature and shape allow for fine focusing
of an image
Copyright © 2010 Pearson Education, Inc.
Focusing Light on the Retina
• In the normal resting state:
• our ciliary muscle is relaxed
• the elastic lens tends to become
thick
• aqueous & vitreous humour push
outward on the sclerotic coat
• ligaments become extended /
tensed
• lens pulled into a thin shape
Copyright © 2010 Pearson Education, Inc.
Focusing for Distant Vision
• Light from a distance needs
little adjustment for proper
focusing
• Far point of vision – the
distance beyond which the lens
does not need to change shape
to focus (20 ft.) or:
The object distance at which
the eye is focused with the
eye lens in a neutral or
relaxed state.
Copyright © 2010 Pearson Education, Inc.
Figure 15.13a
Focusing Light on the Retina - short focal length
• contraction of ciliary muscle
• distance between edges of ciliary body decreases
• relaxation of suspensory ligament
• lens becomes thicker
• focal length shortens
• light rays converge earlier; image formed on retina
Copyright © 2010 Pearson Education, Inc.
Focusing for Close Vision
• Close vision requires:
• Accommodation – changing the lens shape by
ciliary muscles to increase refractory power
• Constriction – the pupillary reflex constricts the
pupils to prevent divergent light rays from entering
the eye
• Convergence – medial rotation of the eyeballs
toward the object being viewed so both eye are
focused on the object
Copyright © 2010 Pearson Education, Inc.
Problems of Refraction
• Myopia (nearsightedness) — focal point is in front of the retina,
e.g. in a longer than normal eyeball
• Corrected with a concave lens
• Hyperopia (farsightedness) — focal point is behind the retina,
e.g. in a shorter than normal eyeball
• Corrected with a convex lens
• Astigmatism — caused by unequal curvatures in different parts
of the cornea or lens
• Corrected with cylindrically ground lenses, corneal
implants, or laser procedures
Copyright © 2010 Pearson Education, Inc.
Photoreception
• Photoreceptors are modified neurons
• They absorb light and generate
chemical or electrical signals
• 2 cell types that produce visual
images
• rods and cones
• The rods:cones ratio is 20:1
except in the fovea
Copyright © 2010 Pearson Education, Inc.
http://library.thinkquest.org/28030/physio/perceive.htm
• 2 segments are separated
by a narrow constriction
- cilium
• The inner segment
connects to the cell body
which is continuous of
the inner fiber that has
the synaptic terminal
Copyright © 2010 Pearson Education, Inc.
Inner fibers
Synaptic terminals
Rod cell body
Rod cell body
Cone cell body
Nuclei
Outer fiber
Mitochondria
Inner
segment
• Inner segment – facing
the interior
Process of
bipolar cell
Pigmented layer
Outer segment
• Outer segment - points
towards the wall of the
eye (towards the
pigmented layer of the
retina)
Melanin
granules
Connecting
cilia
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Basal lamina (border
with choroid)
Photoreception: Functional Anatomy of Photoreceptors
• Photoreception – process by which the eye detects light energy
• Rods and cones contain visual pigments (photopigments) in the
outer segment
• The visual pigment is different in each of the photoreceptors
• Each of the 3 types of cones contains a unique visula pigment
• Embedded in areas of the plasma membrane that form discs
• change shape as they absorb light
• This foldings increase surface area that is available for trapping
light
• In rods – the discs are discontinuous while in the cones they are
continuous
Copyright © 2010 Pearson Education, Inc.
Chemistry of Visual Pigments
• Retinal is a light-absorbing molecule
• Combines with proteins called opsins to form 4 types of visual
pigments
• Retinal is similar to and is synthesized from vitamin A
• Vitamin A is stored in the liver and transported by the blood to
the cells of the pigmented layer (local reservoir of vitamin A)
• Retinal has two 3D forms/isomers:
• 11-cis – a bent structure when connected to opsin
• all-trans – when struck by light and change the shape of opsin to
its active form
• Transforming fro 11-cis to all-trans is the only light dependent
stage
• Isomerization of retinal initiates electrical impulses in the optic nerve
Copyright © 2010 Pearson Education, Inc.
Excitation of Rods
• Light phase
• When light is absorbed, rhodopsin breaks down
• 11-cis isomer is converted into the all-trans isomer
• Retinal and opsin separate (bleaching of the
pigment)
• Dark phase
• In the dark, rhodopsin forms and accumulates
• Regenerated from all-trans-retinal
• Formed from vitamin A
Copyright © 2010 Pearson Education, Inc.
Excitation of Cones
• Visual
pigments
(retinal + opsins)
in
cones
are
similar
to
rods
• Cones are less sensitive – need more light to be activated
• There are three types of cones:
• Blue – wave length 420nm,
• Green – wave length 530nm,
• Red – 560nm
• The absorption spectra overlap giving the hues - activation of more
than one type of cone
• Method of excitation is similar to rods but the cones need higherintensity (brighter) light because they are less sensitive
COLOR PRESENTATION FROM WEBSITE
Copyright © 2010 Pearson Education, Inc.
Phototransduction
• The outer segments of the photoreceptor has ligandregulated sodium gates that bind to cGMP on the
intracellular side.
• In the dark, cGMP opens the gate and permits the inflow
of sodium and calcium which reduces the membrane
potential from -70mv to -40mv
• This depolarized current is called the dark current and it
results in in continuous NT (glutamate) release by the
photoreceptors in the synapse with the bipolar cells
• Light stops the dark current
Copyright © 2010 Pearson Education, Inc.
Phototransduction
• Light energy splits rhodopsin into all-trans retinal, releasing
activated opsin
• The freed opsin activates the G protein transducin
• Transducin catalyzes activation of phosphodiesterase (PDE)
• PDE hydrolyzes cGMP to GMP and releases it from sodium
channels
• Without bound cGMP, sodium channels close but potassium
channels in the outer segment remain open
• The
photoreceptor
membrane
hyperpolarizes,
neurotransmitter (glutamate) cannot be released.
and
• In this case, hyperpolarization is a signal to the bipolar cell to
release NT in the synapse with the ganglion cells
Copyright © 2010 Pearson Education, Inc.
Signal Transmission in the Retina
• Photoreceptors and bipolar cells only generate graded potentials
(EPSPs and IPSPs)
• Light hyperpolarizes photoreceptor cells, causing them to stop
releasing the inhibitory neurotransmitter glutamate
• Bipolar cells (no longer inhibited) are then allowed to
depolarize and release neurotransmitter onto ganglion cells
• Ganglion cells generate APs that are transmitted in the optic
nerve
Copyright © 2010 Pearson Education, Inc.
Adaptation to bright light (going from dark to light)
• As long as the light is low intensity, relatively little amount of
rhodopsin is bleached
• In high intensity light, rhodopsin is bleached as fast as it is re-formed
• Going from dark/dim light to light - first we see white light because
the sensitivity of the retina is “set” to dim light
• Both rods and cones are strongly stimulated and large amounts of the
pigments are broken, producing a flood of signals that are responsible
for the white light
• under the condition of high intensity of light, rods’ system is “turned
off” and the cones system adapts
• By switching from the rod to the cone system – visual acuity is
gained
Copyright © 2010 Pearson Education, Inc.
Adaptation to dark
• Initially we do not see nothing
• Cones stop functioning in low light and the beginning rods
are still “turned off”
• Rhodopsin accumulates in the dark and retinal sensitivity
is restored
Copyright © 2010 Pearson Education, Inc.
Visual Pathways
• Axons of retinal ganglion cells form the optic nerve
• Medial fibers of the optic nerve decussate at the optic
chiasm
• Most fibers of the optic tracts continue to the thalamus
• Other optic tract fibers end in superior colliculi in the
midbrain (initiating visual reflexes)
• Optic radiations travel from the thalamus to the visual
cortex
Copyright © 2010 Pearson Education, Inc.
Depth Perception
• Both eyes view the same image from slightly different
angles
• Each eye view a different angle of the object
• The nose and eye socket blocks view of opposite
side
• Depth perception (three-dimensional vision) results
from cortical fusion of the 2 visual signals
Copyright © 2010 Pearson Education, Inc.
Visual processing
• There are few levels of signal processing
• Retinal – at the level of ganglion cells
• Thalamic – sorting
• Cortical - meaning
Copyright © 2010 Pearson Education, Inc.
Retinal Processing - rodes
• Each photoreceptor at the retina monitors specific receptive field
• For the 130,000,000 photoreceptors there are about 6 million bipolar
cells and 1 million ganglion cells.
• Information from photoreceptors converges
• Ganglions cells that monitor rods are called M cells (Magnus cells
– magnus- great) and they can receive signals from as many as
1000 rods
• They provide information on general form of object, motion and
shadow in dim light
• This arrangement results in loss of specificity
Copyright © 2010 Pearson Education, Inc.
Retinal Processing - rodes
• A ganglion cells responds differently to stimuli that arrive in the
center of its receptive field than to stimuli that arrive at the edges.
• On-center neurons – are excited (depolarize) by light arriving at
the center and inhibited by light at the edges of the receptive
field
• Off-center neurons – are inhibited by light arriving at the center
and excited by light at the edges of the receptive field
• On and off receptors are result of sub-type of receptors to
glutamate
• This kind of retinal processing improves the detection of the edges
of object within the visual field
• It is not the only type of ganglion cells pattern detection but the
easiest to explain – simple spot of light
Copyright © 2010 Pearson Education, Inc.
Retinal Processing - cones
• Cones show very little convergence
• In the fovea, the ration cones to ganglion cells is 1:1 – these
ganglion cells are called P cells (Parvo cells: parvus – small)
• P cells are active in bright light and provide information
about edges, fine detail and color.
• Bipolar cells receiving signals from cones feed directly into
excitatory synapse in the ganglion cell
• The activation of P cells means that the light arrived in
specific location and the result is a more precise visual
information supplied by the cones.
Copyright © 2010 Pearson Education, Inc.