4.1 retina and central visual pathways Flashcards
what are the 3 layers of the eye?
sclera
uvea
retina
what is the sclera?
the outermost layer of the eye.
tough and continuous with dural sheath of the optic nerve
what is the uvea?
pigmented vascular layer. Choroid sitting just deep to sclera, Ciliary body and iris sitting anteriorly
what is the retina?
the innermost neural layer of the eye. Contains a variety of different neural and epithelial cell layers to allow us to see, process information and relay to the optic nerve
what are the 5 layers of the retina?
• Retinal pigment epithelium
• Photoreceptor cells
• Bipolar cells (first order neurones receiving input from photoreceptors). Bipolar cells are connected by horizontal cells which assist with enhancing edges through a process called lateral
inhibition
• Ganglion cell layer (receives input from bipolar cells. ) Axons of
ganglion cells form the…
• Nerve fibre layer
how do we routinely examine the retina?
fundoscopy
what diseases can we detect on fundoscopy?
hypertensive retinopathy, diabetic retinopathy and macular degeneration
where on the retina has the point of highest visual acuity?
macula
what is the fundus?
The fundus of the eye is the interior surface of the eye opposite the lens and includes the retina, optic disc, macula, fovea, and posterior pole.
where does the macula sit in relation to the optic disc?
macula sits laterally
what is the optic disc?
the point of exit of the ganglion cell axons
during fundoscopy, branches of which vessels can be seen on the macula?
Branches of
central retinal artery and vein are visible on the macula
what is Amaurosis fugax?
Occlusion of the central retinal artery (a branch of the ophthalmic artery) causes sudden transient visual loss.
what specialist technique can be used to visualise the layers of the retina?
Optical coherence tomography
what detects light from the temporal visual field?
medial retina / nasal retina
what detects light from the medial visual field?
the temporal retina / lateral retina
where do ganglion cells of the eye project to?
a part of the thalamus called the lateral geniculate
nucleus via the optic tract
where does the lateral geniculate nucleus project to?
projects to the visual cortex of the occipital lobe via the optic radiations
how do the pathways of the ganglion cells of the nasal retina differ from the pathway of the ganglion cells of the temporal retina?
Ganglion cells supplying the temporal retina project to the ipsilateral cerebral hemisphere whereas ganglion cells from the nasal retina project to the contralateral hemisphere via the optic chiasm (i.e. they decussate)
which hemisphere does the left binocular visual field project to?
the right hemisphere as it is detected by the left eyes nasal retina and decussates at the optic chiasm
how is information about the inferior visual field conveyed to the primary visual cortex?
Ganglion cells from the superior retina (i.e. inferior field) project through the
superior optic radiation running through the parietal lobe to reach the primary visual cortex
how is information about the superior visual field conveyed to the primary visual cortex?
Ganglion cells from the inferior retina (i.e. superior field) project through the inferior
optic radiation running through the temporal lobe
what is a scotoma?
A localised defect in the retina that causes a small patch of visual loss
damage to the optic nerve will cause what sort of visual defect?
monocular blindness
damage to the medial chiasm causes what sort of visual defect?
bitemporal hemianopia
tunnel vision
damage to the optic tract causes what type of visual defect?
contralateral homonymous hemianopia
damage to the lateral geniculate ganglion causes what type of visual defect?
contralateral homonymous hemianopia
damage to both the optic radiations can cause what sort of visual defect?
contralateral homonymous hemianopia
non vascular damage to the occipital lobe can cause what sort of visual field defect?
contralateral homonymous hemianopia without macula sparing
occlusion of the posterior cerebral artery can cause what sort of visual defect?
contralateral homonymous hemianopia with macula sparing
why is macula vision spared in an occlusion of the posterior cerebral artery?
This is due to the fact that the area of visual cortex that supplies the
macula receives blood from the deep branch of the middle cerebral
artery
damage to the superior optic radiation in the parietal lobe can cause what visual defect ?
contralateral homonymous inferior quadrantanopia
damage to the inferior optic radiation in the temporal lobe of the brain can cause what sort of visual defect?
contralateral superior homonymous quadrantanopia
what is the pupillary light reflex?
Afferent arm: optic nerve
Processing centres:
Pretectal nucleus which projects bilaterally to Edinger Westphal nuclei (which contain parasympathetic preganglionics)
Efferent arm: oculomotor nerve
Effect: illumination of the eye leads to both direct and consensual pupillary constriction. The consensual reflex is mediated by the bilateral projections from the pretectal nucleus
what is the accommodation reflex?
Afferent arm: optic nerve
Processing centres: Visual cortex (via lateral geniculate nucleus), allowing processing of visual image which then project to oculomotor and Edinger Westphal nuclei
Efferent arm: oculomotor nerve
Effect: focusing on a near object leads to pupillary constriction,
convergence of the eyes (contraction of medial recti) and thickening of the
lens
what is the function of the retinal pogment epithelial cells?
Contain melanin to prevent excessive reflections in the eyeball itself.
what occurs in retinal detachment?
photoreceptors detach from the underlying pigment epithelium and fluid builds up between the 2 layers as they separate. Separation of the neural retina from the pigmented layers. Blurring or loss of vision, visual artefacts.
what is the function of the medial longitudinal fasciculus?
to coordinate the movements of the eyes and allowing the eyes to compensate for movements of the head
what brainstem nuclei are involved in the medial longitudinal fasciculus pathway?
oculomotor nuclei
trochlear nuclei
abducens nuclei
vestibular nucleus
what is the role of the vestibular nucleus in the medial longitudinal fasciculus pathway?
sensory nucleus that receives information about the position of the head form the inner ear (from vestibular apparatus)
what occurs within the medial longitudinal fasciculus?
a lot of crisscrossing connections between the different nuclei responsible for the movement of the eyes.
why does the posture of our head being held at a different angle not affect our vision?
as the vestibular nuclei are connected to medial longitudinal fasciculus and can tell the extra ocular muscles to move in a compensatory way so that we maintain a level horizon
also all nuclei send descending connections down into the spinal cord enabling us to create compensatory movements of the neck or trunk in able to keep us in a neutral position
if there is damage to the medial longitudinal fasciculus (e.g. multiple sclerosis plaque) what would happen?
we wouldnt be able to coordinate the movements of the eye, we would lose conjugate eye movements - internuclear ophthalmoplegia
what is internuclear ophthalmoplegia
ophthalmoplegia in lesions of the medial longitudinal fasciculus, with failure of adduction in horizontal gaze but with retention of convergence.
what is the dolls eye reflex?
this is a reflex that tests the vestibular and oculomotor nuclei clinically used to assess brainstem death
what is internuclear ophthalmoplegia?
Paralysis of the eye balls due to loss of connections between the cranial nerve nuclei. Internuclear ophthalmoplegia (INO) is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. When an attempt is made to gaze contralaterally (relative to the affected eye), the affected eye adducts minimally, if at all.
internuclear ophthalmoplegia can be caused by damage to which pathway?
medial longitudinal fasciculus
what is the medial longitudinal fasciculus?
The medial longitudinal fasciculus (MLF) is a myelinated composite fibre tract found in the brainstem. The MLF primarily serves to coordinate the conjugate movement of the eyes and associated head and neck movements.
what structures are connected by the medial longitudinal fasciculus?
oculomotor nucleus abducens nucleus trochlear nucleus vestibular nucleus spinal cord
a patient has a tumour arising from the right cavernous sinus and invading medially towards the optic chasm. which visual field defect might this result in?
right nasal hemianopia
