Eye and Vision Flashcards
Describe the path of light through the eye
Enters through the iris
Focused byt the cornea and lens
Traversing the vitreous humour
Reaches the retina
Describe the cells making up the retina
Rods are highly light sensitive and are specialised for night vision (with the loss of rods leading to night blindness and loss of peripheral vision), converging into one single bipolar cell.
Cones are concentrated in the fovea, providing high acuity vision, day vision, and colour vision. There are three types of cone (blue, red, and green) and, unlike the rods, they have a one cone to one interneurone interaction.
What are the functional classes of cells in the retina?
Photoreceptors, interneurones (bipolar, horizontal and amacrine cells) which combine the signals from the photoreceptors, and ganglion cells (magnocellular (M) and parvocellular (P)) that are the output cells of the retina. These nerves will then enter the visual pathway.
What is the visual field?
The visual field of each eye is the region of space that the eye can see looking straight ahead without movement of the head.
Describe how the retina can be divided and where the visual fields are detected.
The retina can be divided up into two halves by a vertical imaginary line through the centre of the fovea, known as the nasal hemiretina and the temporal hemiretina (these can also be divided into superior and inferior with a horizontal imaginary line).
Due to the action of light passing through the lens, image present on the left of the visual field are detected by the nasal hemiretina of the left eye and temporal hemiretina of the right eye, and vice versa; a similar relationship occurs with the superior and inferior halves of the retina.
Describe the visual pathway.
• Each retina is supplied by nasal and temporal fibres
o Nasal fibres are found in the medial retina, they supply our temporal visual fields
o Temporal fibres are found in the lateral retina, they supply our nasal visual fields
• The temporal fibres run ipsilaterally – they don’t cross at the optic chiasm
• The nasal fibres cross at the optic chiasm
• After the optic chiasm the fibers form the optic radiations – parietal and temporal
• The fibres converge at the occipital lobe in the visual cortex
Describe the muscles of the eye, their movements and innervation.
• Oculomotor nerve (CNIII) innervates
o Superior rectus, elevates, adducts, incyclotorsion
o Inferior rectus - depression, excyclotorsion, adduction
o Medial rectus, adduction
o Inferior oblique - excyclotorsion, elevation, abduction
Levator palpebrae superioris - elevation/retraction of eyelid
• Trochlear nerve (CNIV) innervates
o Superior oblique, incyclotorsion, depression, abduction
• Abducens nerve (CNVI) innervates
o Lateral rectus, abduction
Describe what occurs in CNIII palsy. What are causes?
Ptosis
Down and out
Dilated pupil
Causes:
o Posterior communicating artery aneurysm
o Diplopia and dilated pupil is most common presentation
o Vasculopathies – diabetes/hypertension
o Idiopathic
What occurs in CNIV palsy? What are the causes?
Up and in due to unopposed inferior oblique action
Subtle head tilt as superior oblique has role in eye rotation
Causes: o Head trauma – closed head injuries • CNIV is the thinnest cranial nerve • CNIV has a long course inside the cranium o Tumours o Congenital • Unknown mechanism • Possibly aplasia of CNIV nucleus o Vasculopathies e.g. Diabetic
Describe CNVI palsy.
Inability to abduct the affected eye
Causes: o Raised ICP • Tortuous course of CNVI inside cranium o Pontine tumours o Vasculopathies e.g. Diabetic/hypertension o Idiopathic
Describe the pupillary light reflex.
o Light stimulates the afferent nerve (CNII)
o Synapses at the midbrain (in the pretectal area) onto neurons that supply the Edinger-Westphal nucleus bilaterally
o Stimulates CNIII bilaterally, causes direct and consensual pupillary constriction.
Describe the accommodation reflex.
o Required for near vision:
• Pupils converge (medial rectus)
• Pupils constrict (constrictor pupillae)
• Increase convexity of the lens, increasing refractive power (ciliary muscle)
o It involves the cerebral cortex as it is related to image analysis
• It follows the visual pathway via the lateral geniculate nucleus to the visual cortex, then returns via the edinger-westphal nucleus like the light reflex.
Describe the classification of visual field defects
• Before the optic chiasm: o Signs are unilateral and ipsilateral • At or after the optic chiasm o Signs are bilateral • After the optic chiasm o Signs will be contralateral
What is monocular blindness due to? Causes?
Vision in only one eye
o Due to a lesion of the ipsilateral optic nerve
o Causes:
• Optic nerve glioma or retinoblastoma in children
• Optic sheath meningioma (middle age)
What is bitemporal hemianopia. Causes?
Tunnel vision
Lesion at the optic chasm which affects both nasal fibres - so both temporal fields are lost
Causes include pituitary tumour, anterior communicating artery aneurysm
