D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
VISUAL SYSTEM, NEUROLOGY
Lecture 14: Retina, Visual Pathways
1.
Retina: (chapter 50)
• organization: (fig. 50 – 1 & ppts. 1 & 2)
- layers: 1) pigment layer (minimizes flair, stores vitamin A)
2) layer of rods and cones – outer segments (contain photosensitive
pigment) and inner segments (contain mitochondria) of the photoreceptors
3) outer nuclear layer – cell bodies of rods & cones
4) outer plexiform layer – synaptic processes from photoreceptors with
dendrites of bipolar cells and of horizontal cells
5) inner nuclear layer – bipolar and horizontal cells
6) inner plexiform layer – synaptic processes of bipolar cells + amacrine cells
and dendrites of ganglion cells
7) ganglionic layer – cell bodies of ganglion cells
8) layer of optic nerve fibers contains axons of ganglion cells
9) inner limiting membrane
- light (from lens) must pass through seven inner layers to reach outer
segments of rods & cones – an arrangement that interferes with sharpness of the
image (visual acuity)
- fovea: small area in center of retina (directly behind lens); site of visual
acuity; inner layers & blood capillaries are displaced permitting light to reach
photoreceptors unobstructed (fig. 50 – 2 & ppt. 3); fovea is the center of the macula
lutea & contains mostly cones with few rods, (except central fovea, which is
exclusively cones)
• rods & cones: (figs. 50 – 3, 50 – 4 & ppts. 4 to 6)
- outer segment: contains photosensitive pigments (rhodopsin in rods, color
pigments in cones); long & narrow (rod-shaped) in rods; wider & shorter (coneshaped) in cones; pigments are conjugated proteins, arrayed in a series of membrane
layers (lamellae) of outer segments
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- p. 2 -
- distribution: rods more abundant peripherally; cones more abundant
centrally; central fovea is exclusively cones; rods are 20x more numerous than cones
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- p. 3 -
• rhodopsin ‘bleaching’: rhodopsin (visual purple) is a conjugated protein
(contains a derivative of vitamin A {cis retinal} linked to the protein scotopsin)
- the pigment undergoes a rapid, stepwise conversion when it absorbs light
energy; the cis retinal is converted to all-trans retinal, which pulls away from the protein;
active rhodopsin (metarhodopsin) is formed when all-trans retinal separates from the
protein (fig. 50 – 5 & ppts. 7 to 10)
- all-trans retinal may be reconverted to cis retinal to combine with
(scot)opsin and restore rhodopsin or it may be converted to a form of vitamin A
- vitamin A is stored (in pigment epithelium) or converted to cis retinal
- vitamin A deficiency leads to night blindness
• transduction: transduction process involves a series of enzyme-catalyzed steps
(G protein pathway), each step introducing amplification, which gives rods extreme visual
sensitivity
- active rhodopsin inhibits sodium leakage into the outer segment; concurrent
sodium pump removal of sodium leads to a drop in membrane potential (hyperpolarization)
(ppt. 11), which passes as a graded potential to the synaptic region of the rod cell
- receptor potential (graded potential) is prolonged (> 1 sec.) and is
proportional to the log of light intensity; these properties enable rods to respond to wide
range of light intensities
• transmission: in darkness, photoreceptors are depolarized, releasing an
inhibitory neurotransmitter (likely glutamate) upon bipolar cells, which hyperpolarize
(IPSP) and fail to excite ganglion cells
- in light, hyperpolarization of photoreceptors permits bipolar cells to
develop EPSPs, which can excite ganglion cells (ppt. 12)
• cone pigments & color vision: three different types: red-sensitive, greensensitive & blue-sensitive (fig. 50 – 8 & ppt. 13)
- transduction is similar to that for rhodopsin (proteins are photopsins
combined with cis retinal)
- three different pigments absorb at blue, green-yellow & orange-red
regions of spectrum, corresponding with peak light sensitivities for the respective
cone types; interpretation of colors depends on combined degrees of absorbance at a
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- p. 4 -
particular wavelength of light by different cones, e.g. orange light (580 nm.) excites redsensitive 99%, green-sensitive 42% & blue-sensitive not at all) (fig. 50 – 10 & ppt. 14)
- much lower sensitivity than rods (1/30 to 1/300)
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- p. 5 -
• sensory adaptation (light adaptation & dark adaptation):
- light adaptation results from depletion of light-sensitive pigments in rods
and cones exposed to prolonged intense light; pupillary reflex (constriction) also
contributes to this reduction of sensitivity to light
- dark adaptation results from a buildup to saturation of the light-sensitive
pigments exposed to prolonged poor illumination; pupillary reflex (dilation) also
contributes increased sensitivity to light
• sensitivity & acuity (ppt. 15):
- sensitivity (discussed above) was seen to be a property of transduction by
the photochemical systems, especially for rods, due to amplification steps
- neural circuits for rods contribute to visual sensitivity through
convergent circuitry that maximizes summation (as many as 300 rods may excite
one ganglion cell)
- acuity (associated with fovea) is also promoted by neural circuitry of
cones in central retina – one-for-one circuits from cone to bipolar to ganglion cell
ensure point-to-point discrimination of detail
• lateral inhibition: photoreceptors synapse (glutamate) with bipolar cells
(vertical path) & with horizontal cells (horizontal path), which are inhibitory
- lateral inhibition circuits prevent spread of excitation, heightening contrast &
sharpening excitation signals; depolarizing bipolar cells & adjacent hyperpolarizing
bipolar cells provide more refined lateral inhibition; some amacrine cells may also
contribute to lateral inhibition
• ganglion cells: axons of these cells form optic nerve; they transmit via action
potentials whereas most other cells of retinal pathways transmit via graded
potentials
- W cells transmit from rods largely from periphery of retina & are likely
responsible for visual sensitivity signals to brain; slower transmission (A fibers)
- X cells transmit mainly from cones (responsible for acuity and color vision)
(A fibers)
- Y cells transmit from wide areas of retina (believed responsible for rapid
changes in visual image) (A fibers)
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- p. 6 -
- tonic signal (resting frequency of APs) characterizes ganglion cell activity;
excitation – increased signal strength, inhibition – decreased signal strength
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2.
- p. 7 -
Neural pathways:
• optic chiasma & optic tracts (fig. 51 – 1 & ppt. 16): optic nerves converge to
meet in midline at optic chiasma (anterior to diencephalon of brain); each optic nerve
carries entire retinal signals from one eye
- at optic chiasma, nerve fibers from each lateral retina continue to same
side of brain (ipsilateral distribution) whereas fibers from medial retina cross over
to opposite side (contralateral distribution)
- redistribution of fibers constitutes optic tracts, which carry signals for visual
field of opposite side (e.g. right optic tract ‘sees’ left visual field)
- optic tract fibers pass to lateral geniculate area of thalamus
• thalamus: (new system) optic tract fibers synapse with second order cells in
lateral geniculate body of thalamus; point-for-point fidelity of retinal signals is maintained
(e.g., neural layers of lateral geniculate area process inputs from lateral retina of one
eye separately from those of medial retina of the other eye) and inhibitory signals from
visual cortex and from reticular areas of midbrain control what information is passed onward
• visual cortex (fig. 51 – 2 & ppts. 17 & 18): fibers from thalamus follow optic
radiation (geniculocalcarine tract) to visual cortex in the occipital lobe
- primary visual cortex straddles a prominent fissure (calcarine fissure) of
medial side of occipital lobe; six layers emulate organization of somatosensory cortex
with layer IV receiving input from optic radiation
- alternating columns of neurons (striate cortex) process information
from each eye, comparing information (facilitates stereopsis)
- layers I – III and V – VI appear to play a role in color interpretation
(facilitated by specific cortical regions called 'color blobs') as well as relaying signals to
other areas of brain (fig. 51 - 4 & ppt. 19)
- macular region of retina (contains fovea) occupies largest portion of
cortical area with peripheral retina occupying progressively less
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- p. 8 -
- secondary visual cortex surrounds primary on all sides except medial
side; interpretation of three dimensional aspects and objects within the visual field
(moving or stationary) are analyzed in superior-anterior area (near somatosensory
association area); visual detail and color are analyzed in anterior-lateral area (fig. 51 –
3 & ppt. 20)
• midbrain & eye movements (old system):
- fibers from cortex communicate with hypothalamic nuclei, midbrain
nuclei (area of superior colliculus) and nuclei of ventral lateral thalamus
- cranial nerves III, IV & VI control the muscles responsible for eye
movements (fig. 51 – 7 & ppt. 21)
- extrinsic muscles: rectus muscles (superior, inferior & medial) and inferior
oblique (cranial n. III); superior oblique (cranial n. IV); lateral rectus (cranial n. VI)
contribute to movements of eye in different directions (ppt. 22)
- various brainstem nuclei receive input from primary and secondary
cortices to control oculomotor system, e.g., secondary cortical fibers to the midbrain
(superior colliculus) are responsible for involuntary fixation of the eyes upon a
component of the visual field (fig. 51 - 8 & ppt. 23)
• neural control of accommodation & pupillary reflex: visual signals to the
brainstem activate the autonomic nerves to ciliary & iris muscles; neural pathway for
accommodation is not clearly understood but pupillary reflex is known to involve
parasympathetic fibers via cranial nerve III to the ciliary ganglion (constricts pupil)
& sympathetic fibers via upper thoracic nerves to superior cervical ganglion (dilates
pupil) (ppt. 24)