Visual pathways: anatomy and lesions week 6 Flashcards
(42 cards)
What is the function of the cornea? Is it vascular or avascular? Where is it located?
Where is the sclera? What is it composed of? Is it vascular or avsacular?
The cornea provides an avascular, transparent protective layer with good optical properties.
The outer white coat, the sclera, is a dense irregular connective tissue structure that is non-distensible and traversed by vessels and nerves of the eye
What does the uvea (middle coat of the eye) consist of?
The uvea is ____ and ____.
Uvea: This middle coat of the eye is pigmented and vascular. It consists of the iris, ciliary body and choroid.
Where is the retina in relationship ot the sclera? To the choroid?
Where does the retina originate from?
Retina and retinal pigment epithelium: The inner coat (deep to sclera and choroid) is the neurosensory component (retina) that originates from the optic vesicle/cup and is part of the central nervous system. The optic cup also gives rise to the retinal pigment epithelium (RPE) that has an intimate functional relationship with the neural retina.
Where is the anterior chamber of the eye?
Where is the posterior chamber of the eye?
Where is aqueous humor produced? What produces aqueous humor?
Explain the flow of aqueous humor.
What is the aqueous humor in contact with posteriorly?
The anterior chamber is the fluid filled space between the cornea and the iris. The posterior chamber is the fluid filled space between the iris and the lens. Aqueous humor is produced in the posterior chamber by the epithelium of the ciliary body, flows over the lens and through the pupil into the anterior chamber. It then flows into the modified veins know as the canals of Schlemm at the outer rim of the anterior chamber at the angle where the iris meets the cornea. Posteriorly, the aqueous humor is in contact with the gelatinous vitreous humor.
What can obstruction of flow of aqueous humor through canals of Schlemm lead to? What are the effects of this?
Obstruction of the outflow of fluid via the canals of Schlemm can lead to glaucoma, a buildup of fluid and pressure in the eyeball, which can damage the retina and optic nerve, with loss of vision proceeding from the retinal periphery to the fovea (the area responsible for finest acuity in the central portion of the visual field) and may lead to blindness. Production and resorption of aqueous humor is normally carefully balanced.
What two muscles is the iris composed of?
How are these 2 muscles innervated?
THE IRIS is composed of two opposing muscles that control the size of the pupil and thus the amount of light reaching the retina. The pupil tends to be larger in the dark and smaller in the light under the influence of the light reflex.
The pupillary constrictor muscle, the iris sphincter, is a small annulus of smooth muscle fibers at the pupillary margin. This muscle is under parasympathetic cholinergic (CN3) control.
The iris dilator muscle consists of radial smooth muscle fibers that are on the outer portion of the iris. They are under sympathetic, adrenergic control.
Where is the ciliary muscle located? What is the function of this muscle?
THE CILIARY BODY in addition to its involvement in aqueous humor production also contains the ciliary muscle component. When this contracts during accommodation, it relaxes the tension on zonular ligaments that suspend the lens. The lens is elastic and assumes a more convex shape – increasing its optical power.
The longitudinal component of the ciliary muscle inserts anteriorly on the scleral spur and its contraction facilitates opening of the angle and aqueous outflow – a property used in medical treatment of glaucoma
What is the function of the lens?
What is presbyopia? When does this typically occur in individuals?
What are cataracts? What do they result from?
The lens serves to focus light on the retina. - The lens loses its elasticity with age and in the 4th decade generally produces an inability to focus during accommodation, called presbyopia.
Opacities of the lens are called cataracts. These are a natural consequence of aging, or may result from trauma, exposure to radiation or ultraviolet light, and metabolic disorders, especially diabetes.
What is the vitreous humor/body?
Explain complications involving the vitreous.
THE VITREOUS BODY is an optically clear gel which is attached at the optic disc and at the perimeter of the retina at the ciliary body.
Complications involving the vitreous include 1) opacities resulting from failure of the primary vitreous to regress completely; 2) cellular infiltration resulting from inflammation or intravitreal hemorrhage, e.g. in diabetic retinopathy; and 2) retinal detachments resulting from vitreous traction at its attachment sites around the optic nerve head and at the peripheral margins of the retina and ciliary body. Traction detachments may occur in trauma, organization of cell infiltrates (scarring) or age-related shrinkage. As the vitreous naturally ages throughout life it shrinks and pulls away from the retina. In some people the vitreous can remain partially stuck to the macula. In these people the vitreous pulls on the surface of the macula and distorts the normal anatomy resulting in vitreomacular traction.
Optic Disc: is frequently used to describe the portion of the optic nerve clinically visible on examination.
Optic cup: the disc has an orange-pink rim with a pale centre. This pale centre is devoid of neuroretinal tissue and is called the cup.
What cells are contained within the retina? What is the function of the retina?
What cell type does the outer nuclear layer of the retina contain?
Where does the outer nuclear layer of the retina receive its blood supply?
The retina contains the light sensitive photoreceptors (rods and cones), bipolar cells, ganglion cells, horizontal cells and amacrine cells organized in a highly laminated fashion to process visual information.
The photoreceptor outer segments are in close proximity to the retinal pigment epithelium. The outer nuclear layer (ONL) of the retina (see attached) containing photoreceptors receives its blood supply from the choroid. The sclera provides structural support.
Where are the retina and retinal pigment epithelium derived from?
What is the specialized basement membrane of the retinal pigment epithleium (RPE)?
What are the functions of the RPE?
The retina and retinal pigment epithelium are derived from the inner and outer layers of the optic cup, respectively. The embryologic origin of part of the diencephalon leaves the retina and optic nerve subject to certain diseases of the central nervous system, e.g. multiple sclerosis.
THE RETINAL PIGMENT EPITHELIUM (RPE) has important functions relative to photoreceptor and retinal maintenance. Its specialized basement membrane is Bruch’s membrane.
- Pigment granules at the cell apex migrate into microvilli between photoreceptor outer segments. This pigmentation provides shielding and prevents light scatter. Albinos, lacking ocular pigmentation, have difficulty with bright light and visual acuity.
- It is involved in vitamin A metabolism important to photoreceptor function. It manufactures the active cis-form of retinal (vitamin A aldehyde) from dietary vitamin A.
- It also recycles the trans form of retinal produced by light activation of photopigment to the active cis form for reutilization by the photoreceptor.
- It phagocytizes rod outer segments which are continually being shed.
- Tight junctions in the junctional complexes between retinal pigment epithelial cells are the basis for the blood-retinal barrier.
- It facilitates transfer of selected metabolites, including vitamin A, from the choroidal blood vessels: the choriocapillaris.
T or F: The retina has a dual blood supply.
True.
What are retinal vessels a branch of? Where do they enter the retina?
What do retinal vessels supply?
Explain what can happen to retinal arteries in retinal strokes.
Explain the involvment of retinal vessels in diabetic retinopathy.
Retinal vessels, which are branches of the central retinal artery, enter the retina at the optic nerve head and s_upply all but the outer nuclear layer_.
- These vessels are end arteries and are subject to occlusion in retinal strokes.
- Occlusions can affect the artery or venous return.
- Retina vessels are also affected in diabetic retinopathy and can proliferate into the vitreous or beneath the retina where they become a source of hemorrhage and scarring.
Where is the choroid (choriocapillaris) located?
What does it supply?
Explain the involvement of the choroid in age-related macular degeneration (AMD).
What does AMD result in?
The choroid (choriocapillaris), between the sclera and the retinal pigment epithelium, supplies the outer retina, i.e. the photoreceptors, and are the exclusive blood supply to the macular region.
- Problems at the choroid/macular interface are important in age-related macular degeneration (AMD), which results in loss of central vision – i.e. color and high resolution cone vision. Abnormal proliferation of capillaries into the vessel-free zone of the macula is the source of these problems - often with leakage of fluid beneath the macula.
What is the function of the optic disc?
What is the lamina cribrosa?
Where is the blind spot in the eye located?
The optic disc is where branches of the central retinal artery enter (and the veins leave) the eye, and nerve fibers (axons of retinal ganglion cells) leave to form the optic nerve.
The optic disc consists of convergent ganglion cell axons from the innermost optic fiber layer of the retina. These axons exit the retina through a fenestrated portion of the sclera called the lamina cribrosa. This is the site for entry of the central retinal artery and egress of the central retinal vein. These vessels provide the blood supply to the inner retina (but not the macula). The optic disc has no photoreceptors and thus provides the blind spot in each eye.
What is the fovea? What is the macula?
What type of photoreceptor is concentrated in the fovea?
Macula/Fovea: The macula represents the center of the visual field. Within the macula is the fovea, the site of best visual acuity. The fovea is the histological “divot” centered on a larger ~1mm region of sharpest central vision called the macula. Most of the cones in the retina are concentrated in the fovea; rods are more peripheral.
Note that the optic disc is more medial (toward the nose)
Explain the pathway that light must travel to be transmitted to the retina.
What is the most important refractive surface and why?
Define the following terms: emmetriopia, myopia, hyperopia, astigmatism
Explain the orientation of images focused on the retina.
Light passing into the eye is transmitted to the retina through the cornea, anterior chamber lens and vitreous. The cornea is the most important refractive surface because of the transition from air to a tissue of much greater optical density. The lens makes fine adjustments in focus: accommodation for near objects results from contraction of the ciliary muscle under parasympathetic control, so that the natural elastic lens assumes a more convex shape as tension is relaxed on its suspensory ligaments (Zonules).
Normal eyes are emmetropic in that the optical system focuses parallel rays at the plane of the retina. In myopic (near sighted) eyes the rays come to focus in front of the retina; in hyperopic (farsighted) eyes they come to focus behind the retina.
Astigmatism results when the refractive index is different along two perpendicular axes. Presbyopia is an age-related effect that occurs when the lens can no longer increase its convexity sufficiently to bring near objects into focus.
Images focused on the retina are reversed and upside down. Point to point relationships in the image as well as congruity of images from the two eyes depend on the organized (visuotopic, retinotopic) projection of central pathways to the cerebral cortex.
Explain the pathway fibers of the retina take to project to the visual cortex.
The Retino-Geniculate-Cortical Pathway:
Retinal Ganglion cell axons exit from the eye as the optic nerve, and traverse the optic chiasm to enter the optic tracts which pass posteriorly around the diencephalon (between the temporal lobe and the midbrain) to synapse in the lateral geniculate nucleus of the thalamus. Principal neurons of this relay nucleus send thalamocortical fibers to the primary visual cortex (striate cortex) of the occipital lobe by means of the optic radiations.
On what side of the visual cortex is the left visual field represented?
What is the key feature for this representation?
This pathway exhibits retinotopic organization so that a point-to-point representation of visual space occurs in the visual cortex. The geometry of the visual pathways insures that this representation of visual space is reversed and therefore congruent with the somatic representation, i.e., left visual field and left body sensation and movement are represented in the right hemisphere. The key feature of this geometry is the partial decussation of optic nerve fibers such that only fibers from the nasal half of each retina cross to the contralateral side, while fibers from the temporal half of each retina proceed uncrossed to the ipsilateral side.
What part of the eye reverses the retinal image?
Explain where in the retina the right and left visual fields are seen.
Explain how/why each point in visual space is doubly represented.
What visual field does the left optic tract represent? The right optic tract?
Retina, Optic Nerves, Chiasm, and Tracts. The lens reverses the retinal image so that the right visual field is seen by the nasal hemi (half) -retina of the right eye and the temporal hemiretina of the left eye. Optic nerve fibers from the nasal retina of the right eye decussate in the optic chiasm to join uncrossed fibers from the temporal retina of the left eye. Ganglion cell axons from the two eyes are mixed at the chiasm and sorted so that fibers representing the same areas of visual space from both eyes run in close proximity in the tracts. This essentially produces a double retinotopic representation in the pathway which is maintained throughout the geniculate and the cortical projection. The left tract, therefore, carries the information about the right visual field- each point in visual space doubly represented by optic axons from the right nasal retina and the left temporal retina.
What layers of the lateral geniculate nucleus (LGN) do crossed fibers (of the optic tract) from the contralateral nasal retina synapse in?
Uncrossed fibers from the ipsilateral temporal retina?
Axons from the two eyes remain segregated to synapse in alternate but retinotopically aligned layers of the lateral geniculate body. Crossed fibers from the contralateral nasal retina synapse in layers 1, 4, & 6 of the geniculate; uncrossed fibers from the ipsilateral temporal retina synapse in layers 2,3, & 5.
What are M-type cells? What are these cells good at sensing?
What are P-type cells? What do these cells sense?
What layers of the LGN do these cell types synapse in?
Axons synapsing with cells in layers 1 and 2 of the geniculate are from large (M-type) ganglion cells of the retina. These constitute the afferent component of the magnocellular subsystem in which convergent wide-field representations of the M cells are particularly good at sensing motion. The smaller ganglion cells (P-type) synapse in layers 3-6 to form a parvocellular subsystem that processes form and color attributes.
What are optic radiations formed by?
What is Meyer’s Loop? What is it formed by? What visual field is represented in it?
Optic radiations: The optic radiations are thalamocortical projections from the geniculate to the primary visual cortex. These are derived from the principal neurons of the geniculate layers, relaying the dual representation of the right visual field to the left primary visual cortex.
Note: the retinal image is also inverted so that the upper visual field seen by the inferior retina remains represented inferiorly along the visual pathway (conversely with the lower visual field). The inferior fibers (upper field) of the optic radiations form Meyer’s loop around the temporal horn of the lateral ventricle where they are widely separated from superior fibers (lower field) that have a more direct course to the occipital lobe.
What is the Brodmann’s number for the primary visual cortex?
What cortical layer do afferents from the LGN synapse in within the primary visual cortex?
Why is the primary visual cortex also known as the striate cortex?
Visual Cortex: Primary visual cortex, also called striate cortex, calcarine cortex, area 17, V1, is where prominent myelinated afferents from the lateral geniculate arriving in cortical layer IV produce a visible stripe known as the line of Gennari. Hence the term “striate” cortex.
