Lecture 11 - Retinal phototransduction and signal processing Flashcards
Sclera:
A relatively spherical and avascular, white dense connective tissue that covers the globe posterior to the cornea.
What does the sclera provide?
a strong tough external framework to protect the delicate optic and neural structures.
The sclera maintains what?
the shape of the globe so that the retinal image is undisturbed and provides attachment for the extraocular muscles to rotate the globe and the ciliary muscle to accommodate the lens.
Cornea:
The window of the eye, it is a mechanically strong and transparent connective tissue that covers anterior 1/6 surface of the eye.
What is the most powerful focusing element of the eye?
Cornea, roughly twice as powerful as the lens.
Lens:
Specialized epithelial tissue that is responsible for fine-tuning the image that is projected on the retina.
Where is the lens?
Lies inside the eye surrounded by aqueous humor. It is transparent and has high refractive power.
What stabilizes the lens and allow accommodation to occur?
Elastin based zonular fibrils
Uveal tract consists of what three structures
the choroid, the ciliary body and the iris.
What is the Choroid?
Capillary bed nourishing the photoreceptors and outer retina
What are the parts of the Ciliary body?
1) ciliary muscle 2) vascular component
the ciliary muscle does what?
controlling the refractive power of the lens
a vascular component of the ciliary body does what?
that produces the aqueous humor filling the anterior chamber.
Iris:
Colored portion of the eye seen through the cornea.
What does the iris contain?
two sets of muscles with opposing actions that allow the size of the pupil (opening at the center) to be adjusted by neural control.
Anterior chamber:
Volume behind the cornea and in front of the lens. Filled with aqueous humor.
Posterior chamber:
Region between the vitreous and the lens.
Aqueous humor:
Clear watery liquid that nourishes the cornea and lens. It is produced by the vascular component of the ciliary body.
Vitreous humor:
Thick gelatinous substance filling the space between the back of the lens and the surface of the retina.
Retina:
Contains neurons that absorb light and process visual information in the images and send that information to the brain.
Macula:
Oval spot containing a yellowish pigment (xantophyl). Supports high acuity.
Fovea:
Small depression at the center of the macula - has highest spatial acuity.
Optic disk:
Whitish circular area where retinal axons leave the eye and travel through the optic nerve to targets in the midbrain and thalamus.
What is the site where blood vessels supply the inner retina enter the eye?
Optic disk
Blood supply for the eye
The ocular vessels are all derived from the ophthalmic artery (OA), a branch of the internal carotid artery.
What are the two distinct vascular systems that supply the eye?
a) the anterior segment (iris and ciliary body) and b) the retinal systems.
Vascularization of the anterior segment
originates from the anterior ciliary arteries and the long posterior ciliary arteries. Penetrating vessels through the sclera vascularize the iris and the ciliary body.
Retinal supply
Delivery of metabolic substrates and oxygen to the retina is accomplished by two separate vascular systems
What are the two separate vascular systems of retinal supply?
the inner retinal and choroidal. The retinal and choroidal vessels differ morphologically and functionally from each other.
View with an ophthalmoscope shows what?
the surface of retina and the vasculature of the inner retina
What is cataracts?
Clouding of the lens that affects vision
Most cataracts are related to what?
Aging
Cataracts are the leading cause of what?
Blindness worldwide
Cataracts by age 80
more than half of all Americans have a cataract or have had cataract surgery
What are the risk factors for cataracts?
Aging, diabetes, sunlight and smoking
What are the symptoms for cataracts?
Hazy vision, poor night vision, glare and faded colors
What is the treatment for cataracts?
Surgical removal of the cloudy lens and replacement with an artificial lens. Very little recovery time after surgery
What is the process known as cataract formation?
Disruption of the order of the organization of the lens cell fibers or aggregation of the proteins within them can destroy transparency of the cell
Presbyopia causes what?
Loss of lens elasticity with age causes this inability to focus on near objects
Total refractive power =
power emmetropic eye + power of accommodation
What is glaucoma?
Group of diseases that damage the eye’s optic nerve and can result in vision problems
What are the types of glaucoma?
1) normal tension 2) open angle 3) closed angle
What are the risk factors for glaucoma?
Elevated eye pressure, thin cornea, abnormal optic nerve anatomy, high blood pressure
What is the relationship between intraocular pressure and glaucoma?
Not all subjects with high IOP develop glaucoma
What are the symptoms for glaucoma?
None until its too late, loss of peripheral visual fields
What is the treatment for glaucoma?
Eye drops to decrease aqueous production and/or increase drainage. Surgery.
The circulating aqueous humor does what?
It nourishes the cornea and lens structures that must be transparent and therefore devoid of blood vessels.
Aqueous humor is secreted by what?
the ciliary epithelium lining the ciliary processes
Where does the aqueous humor flow?
around the lens and through the pupil into the anterior chamber
How does the aqueous humor leave the eye?
by passive flow at the anterior chamber angle
What are the risk factors for glaucoma?
High eye pressure resulting from poor drainage of aqueous humor.
Open angle glaucoma:
Slow development of pathology. Caused by obstruction of drainage canals.
-Close angle:
Sudden increase in intraocular pressure. Closed or narrow angle between the iris and cornea.
How is intraocular pressure regulated?
At the front of the eye
Retina
A ‘window to the brain’, is part of the Central Nervous System. It is a thin neural tissue (200 microns thick) that lines the back of the eye.
What does incoming light first hit?
the ganglion cell layer, the rest of the inner retina, then the photoreceptor (PR) nuclei and inner segments before the PR outer segments, where phototransduction occurs.
What must light travel through before striking and activating the rods and cones?
the thickness of the retina
Subsequently the absorption of photons by the visual pigment of the photoreceptors does what?
It is translated into first a biochemical message and then into an electrical message that can stimulate all the succeeding neurons of the retina.
How is the retinal message concerning the photic input transmitted?
to the brain by the spiking discharge pattern of the ganglion cells via the optic nerve.
What are the layers of the retina?
3 nuclear (cell somas), 2 plexiform (synaptic connections), 1 nerve fiber layer (NFL).
What are the neuron cell types?
1) photoreceptors 2) horizontal cells 3) bipolar cells 4) amacrine cells 5) ganglion cells
What are the glia cells?
1) Muller glia (radial) 2) microglia
Layers and neuron types:
1) Photoreceptor outer segments (OS), 2) Outer nuclear layer (ONL) 3) Outer plexiform layer (OPL) 4) Inner nuclear layer (INL) 5) Inner plexiform layer (IPL) 6) Ganglion cell layer (GCL) 7) Nerve fiber layer (NFL)
Outer nuclear layer (ONL):
photoreceptor somas
Outer plexiform layer (OPL):
photoreceptor / bipolar /horizontal cell synapses
Inner nuclear layer (INL):
horizontal, bipolar and amacrine cell somas
Inner plexiform layer (IPL):
bipolar/amacrine/ganglion cell synapses
Ganglion cell layer (GCL):
ganglion cell somas
What are Pigmented epithelium (RPE):
melanin-containing cells behind the photoreceptors,
What is the function of pigmented epithelium?
acts as backstop for light, maintains phototransduction machinery of photoreceptors by recycling of PR discs, pigment regeneration and photoreceptor nourishment.
Retinal glia cells
including astrocytes (neurovascular), microglia (immune system) and Mueller (radial glia-like) cells (ionic milieu, guidance during development).
Different subclasses of horizontal, bipolar, amacrine and ganglion cells make distinct contributions to
visual function.
What is the vertical information flow?
Photoreceptors to bipolar cells to ganglion cells
Lateral information flow is mediated by
horizontal cells and amacrine cells
Cells along the vertical path (photoreceptors, bipolar and ganglion cells) release what?
glutamate
Cells mediating lateral information transmission (horizontal and amacrine cells) release what?
mostly GABA or glycineric (some exceptions exist..)
What are the two types of photoreceptors for the retina?
Rods and cones
What types of cells are photoreceptors?
they are ciliated cells.
The outer segment of photoreceptors are connected to what?
the inner segment via a connecting cilium.
Outer segment of the photoreceptor
houses the phototransduction machinery
Inner segment of the photoreceptor
housekeeping machinery (nucleus, mitochondria, Golgi apparatus, ER, etc).
Synaptic terminal of the photoreceptor
contacts bipolar and horizontal cells
Neurotransmitter for the photoreceptor
glutamate
How do photoreceptors respond to light?
With graded hyperpolarizations
In darkness, rods and cones are depolarized near what?
-40mV
When do photoreceptors fire action potentials?
They don’t, they respond with graded hyperpolarizations
Describe neurotransmitter release when rods and cones are depolarized.
Neurotransmitter is released continuously
How do rods and cones compare to most other neurons?
rods and cones maintain a relatively depolarized membrane potential at rest.
In darkness rods and cones are
depolarized near -40 mV and neurotransmitter (glutamate) is continuously released. When stimulated by light, photoreceptors do not fire action potentials but instead respond with graded hyperpolarizations.
What happens to the photoreceptor hyperpolarization?
it then spreads passively to the synapse where it reduces the release of the neurotransmitter glutamate.
Photoreceptors in the dark
they have a circulating current in the dark