Session 4 Flashcards
Describe the layers of the eye
Outermost sclera (tough and continuous with dural sheath of the optic nerve)
Uvea (pigmented vascular layer) - main site of absorption of light, too much bright light would cause photophobia sothis layer absorbs some of those rays leaving a managable amount of light. People who lack pigmentation e.g those with albinism are photophobic as tey dont have a working version of thislayer.
- Choroid sitting just deep to sclera
- Ciliary body and iris sitting anteriorly
Retina (neural layer) from superficial to deep:
- Retinal pigment epithelium (prevents light from ‘bouncing around’ in the eyeball, causing glare)
- Photoreceptor cells - two types rods and cones. Rods are responsible for black and white vision and cones are responsible for colour and high acuity vision (high acuity of cones at macula)
- Bipolar cells (first order neurones receiving input from photoreceptors and connecting to axons). 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
- Interestingly, our retina is the ‘wrong way around’, since light has to pass through the nerve fibre layer and other layers before getting to the photoreceptors.
How is the retina examined?
Examination of the retina by fundoscopy can detect signs of many diseases such as hypertensive retinopathy, diabetic retinopathy and macular degeneration. Also vascular occlusions e.g in the branch of central retinal artery or vein (will also presnt with amaurosis fugax which is described as if a curtain has come down over vision) and looking at optic disk to see papilloedema.
o Optical coherence tomography (OCT) is a specialist technique that can be used to visualise the layers of retina
Describe the normal appearance of the fundus (retina)
Label the image
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The normal appearance of the fundus, with the macula (point of highest acuity) sitting lateral to the optic disc (point of exit of ganglion cell axons).
Branches of central retinal artery and vein are visible on the macula. Occlusion of the central retinal artery (a branch of the ophthalmic artery) causes sudden visual loss known as amaurosis fugax
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Describe and explain the visual pathways. Draw it too
o The eye is a pinhole camera - This implies that light from a lateral visual field is detected by the medial retina and that light from an upper visual field is detected by the inferior retina
o The medial retina is referred to as nasal. Light from the temporal field is detected by the medial retina
o The lateral retina is referred to as temporal. Light from the temporal field is detected by the nasal retina
o Ganglion cell axons project to a part of the thalamus called the lateral geniculate nucleus via the optic tract
o The lateral geniculate nucleus projects to the visual cortex through the optic radiations
o 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) This implies that the left binocular visual field projects to the right hemisphere and vice versa
o Ganglion cells from the superior retina (i.e. inferior field) project through the superior optic radiation running through the parietal lobe
o Ganglion cells from the inferior retina (i.e. superior field) project through the inferior optic radiation running through the temporal lobe
o Visual pathway defects (we always refer to the lost part of the field, not the lost part of the retina)
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Describe the possible visual pathway defects (we always refer to the lost part of the field, not the lost part of the retina)
A localised defect in the retina can cause a small patch of visual loss called a scotoma
Damage to the optic nerve can lead to monocular blindness (Temporal and nasal fibres on the ipsilateral side are affected Therefore the nasal and temporal visual fields are lost on the ipsilateral side - blind in one eye)
Damage to the medial chiasm causes bitemporal hemianopia (Nasal fibres on both sides are affected Therefore, both temporal visual fields are lost)
Damage to the optic tract causes a contralateral homonomous hemianopia (Ipsilateral temporal fibres and contralateral nasal fibres are affected Therefore, the ipsilateral nasal visual field and contralateral temporal visual field are lost)
Damage to the lateral geniculate causes a contralateral homonomous hemianopia
Damage to both optic radiations (e.g in a stroke) causes a contralateral homonomous hemianopia
Non vascular damage to the occipital lobe can cause a contralateral homonomous hemianopia without macular sparing
Occlusion of the posterior cerebral artery causes a contralateral homonomous hemianopia with macular sparing - 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 so macular function (central vision) is retained
Damage to the superior optic radiations (in the parietal lobe) causes contralateral homonomous inferior quadrantanopia. Superior temporal fibre on ipsilateral side is affected leading to loss of inferior nasal visual field. Superior nasal fibre on contralateral side is affected leading to loss of inferior temporal visual field
Damage to the inferior optic radiations (in the temporal lobe) causes contralateral homonomous superior quadrantanopia Inferior temporal fibre on ipsilateral side is affected leading to loss of superior nasal visual field. Inferior nasal fibre on contralateral side is affected leading to loss of superior temporal visual field
Describe the pupillary light reflex
Light stimulates 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
Describe the accomodation reflex of the eye
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 (constrictor pupillae) , convergence of the eyes (contraction of medial recti) and convexity of the lens to increase refractive power (ciliary muscle)
What are the sources of arterial blood to the brain?
The brain receives arterial blood from two sources:
o The anterior circulation is fed by the internal carotid arteries and supplies most of the cerebral hemispheres
o The posterior circulation is fed by the vertebral arteries and supplies the brainstem, cerebellum, some of the temporal lobe and the occipital lobe
How is the circulation of the brain divided?
Divided into the anterior circulation and the posterior circulation
Anterior circulation consists of the anterior cerebral artery, the left and right middle cerebral arteries and their branches.
Posterior circulation consists of the basilar artery and its branches
Describe the anterior circulation of the brain
o Middle cerebral artery - The direct continuation of the internal carotid artery - Cortical branches emerge from the lateral fissure to supply the lateral aspect of the cerebral hemisphere (cortex and underlying white matter), including lateral parts of the frontal and parietal lobes as well as the superior temporal lobe - Deep branches (the lenticulostriate arteries) supply deep grey matter structures including the lentiform nucleus and caudate as well as the internal capsule
o Anterior cerebral artery - A branch of the internal carotid artery - The left and right anterior cerebral arteries anastomose in the midline via the anterior communicating artery - The vessels loop over the corpus callosum and send branches to the adjacent cortex - Cortical branches supply the medial aspect of the frontal and parietal lobes (not the occipital lobe) - There are also branches to the corpus callosum itself
Describe the posterior circulation of the brain
o Basilar artery - Midline vessel formed from the confluence of the vertebral arteries - Has a number of major branches:
• Terminal bifurcation gives rise to the posterior cerebral artery
o Supplies occipital lobe, inferior temporal lobe and thalamus (via thalamoperforator and thalamogeniculate branches)
o Also supplies midbrain
o Posterior communicating arteries branch from these to connect with the anterior circulation (internal carotid artery)
- Superior cerebellar artery supplies the superior aspect of the cerebellum and midbrain
- Pontine arteries supply the pons (including descending corticospinal fibres)
- Anterior inferior cerebellar artery the supplies the anteroinferior aspect of the cerebellum and lateral pons
o The vertebral arteries give rise to two important branches to the brain
- Anterior spinal arteries converge in the midline to supply the anterior 2/3 of the spinal cord
- Posterior inferior cerebellar arteries supply the postero-inferior aspect of the cerebellum
What is Optical Coherence Tomography?
Label the layers
Optical Coherence Tomography (OCT) is a non-invasive diagnostic instrument used for imaging the retina.
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