S4: visual system & introduction to cerebral circulation Flashcards
What are the three layers of the eye?
Outermost sclera (continuous with the dural sheath of the optic nerve)
Uvea (pigmented vascular layer)
Retina (neural layer)
Describe the layers of the retina (superficial to deep)
Retinal pigment epithelium – acts as a main site of light absorption (albinism have absence of this) & helps to anchor the photoreceptor cells
Photoreceptor cells – two different types: rods and cones
Bipolar cells – act to connect the photoreceptor cells to the axons (connected by horizontal cells, which assist with lateral inhibition)
Ganglion cell layer
Nerve fibre layer
What is lateral inhibition?
The horizontal cells detect the area from which the image is coming from
Then, they detect the photoreceptor cells that are most in line, and the cells on either side are inhibited
List pathologies which can be viewed by fundoscopy
Retinopathies
Vascular occlusions (amaurosis fugax = transient loss of vision in one or both eyes)
Macula
Optic disc
Describe the normal appearance of the fundus
Macula sits lateral to the optic disc
Fovea is the dark red circle in the middle of the macula
The optic disc is a white circle (point of exit of ganglion cell axons)
Branches of the central retinal artery and vein are visible on the macula
What is optical coherence tomography?
Specialist technique
Can be used to visualise the layers of the retina
Describe where light from each visual field is detected
Medial retina is referred to as nasal – light from the temporal field is detected by the nasal retinal fibres
Lateral retina is referred to as temporal – light from the nasal field is detected by the temporal retinal fibres
Describe the location of the fibres at the optic chiasm
Nasal fibres decussate
Temporal fibres remain ipsilateral
Describe the pathway of the fibres in the optic tract
Optic tract = from the optic chiasm to the lateral geniculate nucleus
Contain temporal fibres from the ipsilateral side
Contain nasal fibres from the contralateral side
Describe the optic radiations
From the lateral geniculate nucleus to the primary visual cortex (occipital lobe)
Ganglion cells from the superior retina (i.e. inferior field) project through the superior optic radiation running through the parietal lobe
Ganglion cells from the inferior retina (i.e. superior field) project through the inferior optic radiation running through the temporal lobe
List examples of visual field defects
Monocular blindness
Bitemporal hemianopia
Homonomous hemianopia
(NB: named based on the area of visual loss rather than the site of the lesion)
Describe a CN II lesion
Temporal and nasal fibres on the ipsilateral side are affected
Nasal and temporal visual fields are lost on the ipsilateral side
Called MONOCULAR BLINDNESS (complete blindness in one eye)
Describe a lesion in the optic chiasm
Nasal fibres on both sides are affected
Therefore, both temporal visual fields are lost
Called BITEMPORAL HEMIANOPIA
-results in tunnel vision, lose their outer peripheral vision
Pituitary adenomas and aneurysm in the anterior communicating artery are causes of this
Describe a lesion in the optic tract
Ipsilateral temporal fibres and contralateral nasal fibres are affected
Therefore, ipsilateral nasal visual field and contralateral temporal visual field are lost
Called HOMONOMOUS HEMIANOPIA
Describe the visual loss in a lesion of the right superior optic radiation (parietal lobe)
Superior temporal fibre on ipsilateral side is affected (loss of inferior nasal visual field)
Superior nasal fibre on contralateral side is affected (loss of inferior temporal visual field)
Called HOMONOMOUS INFERIOR QUADRANTANOPIA
Describe the visual loss in a lesion of the right inferior optic radiation
Inferior temporal fibre on ipsilateral side is affected (loss of superior nasal visual field)
Inferior nasal fibre in contralateral side is affected (loss of superior temporal visual field)
Called HOMONOMOUS SUPERIOR QUANDRANTOPIA
What happens if both superior and inferior nasal/temporal fibres are affected?
Can happen in a stroke (middle cerebral artery supplies both of them)
Superior and inferior temporal fibres – ipsilateral
Superior and inferior nasal fibres – contralateral
Called HOMONOMOUS HEMIANOPIA
What is macular sparing?
Occipital lobe has a dual blood supply – posterior cerebral artery & middle cerebral artery
In a stroke affecting the posterior cerebral artery:
- Most of the occipital lobe will be lost
- BUT the middle cerebral supplies the occipital pole (represents the macula)
- Therefore, macular function will be spared
Describe the light reflex
Afferent arm: optic nerve
Pretectal nucleus projects bilaterally to Edinger Westphal nucleus
Efferent arm: oculomotor nerve
Effect: illumination of the eye leads to both direct and consensual pupillary constriction
Describe the accommodation reflex
Afferent arm: optic nerve
Visual cortex (via LGN), allows processing of visual image which then projects to oculomotor and EDW nuclei
Efferent arm: oculomotor nerve
Effect: focusing on a near object leads to: convergence (medial rectus), pupillary constriction (constrictor pupillae) & convexity of the lens to increase refractive power (ciliary muscle)
Describe the two sources of arterial blood supply to the brain
1) Anterior circulation: fed by the internal carotid arteries and supplies most of the cerebral hemispheres
2) Posterior circulation: fed by the vertebral arteries and supplies the brainstem, cerebellum, some of the temporal lobe and the occipital lobe
Describe the middle cerebral artery
Direct continuation of the ICA
Cortical branches emerge from the lateral fissure to supply the lateral aspect of the cerebral hemisphere, including lateral parts of the frontal and parietal lobes as well as the superior temporal lobe
Deep branches (lenticulostriate arteries) supply deep grey matter structures, including the lentiform nucleus, caudate & the internal capsule
Describe the anterior cerebral artery
Branch of the ICA
Left and right anterior cerebral arteries anastomose in the midline via the anterior communicating artery
Vessels loop over the corpus callosum (also branches to the corpus callosum itself)
Cortical branches supply the medial aspect of the frontal and parietal lobe
Describe the basilar artery
Midline vessel formed from the confluence of the vertebral arteries
Number of major branches:
1) Posterior cerebral artery: supplies occipital lobe, inferior temporal lobe, thalamus & midbrain; posterior communicating arteries branch from these to connect with anterior circulation
2) Superior cerebellar artery: supplies the superior aspect of the cerebellum and midbrain
3) Pontine arteries: supply the pons
4) Anterior inferior cerebellar artery: supplies the antero-inferior aspect of the cerebellum and lateral pons
Describe the vertebral arteries
Give rise to two important branches:
1) Anterior spinal arteries: converge in the midline to supply the anterior 2/3 of the spinal cord
2) Posterior inferior cerebellar arteries: supply the postero-inferior aspect of the cerebellum
