4.1 Visual System and intro to cerebral circulation Flashcards
What is the role of horizon cells in the eye?.
They enhance edges via lateral inhibition (less spreading of AP to excitatory neurones that lie laterally)
What are the three layers of the eye?
Outermost sclera
(Tough and continuous with dural sheath of optic nerve)
Uvea (pigmented vascular layer)
Retina (neural layer)
What are the components of the uvea?
Has choroid which sits just deep to sclera
Ciliary body and iris sit anteriorly
What are the components of the retina?
From superficial to deep
- retinal pigment epithelium (prevents glare)
- photoreceptors cells
- bipolar cells (first order neurones), connected by horizontal cells
- ganglion cell layer
- nerve fibre layer
What is a fundoscopy and why is it done?
Examination of the retina
Can detect signs of many diseases such as hypertensive retinopathy, diabetic retinopathy and macula degeneration
What is amaurosis fugax?
Occlusion of the central retinal artery (a branch of the opthalmic artery), causing sudden visual loss
What specialist technique can be used to visualise the layers of the retina?
Optical coherence tomography
What is the pupillary light reflex?
Light is shone into a patients eyes and it constricts
What is the accommodation reflex?
Get patient to fixate on finger, then bring it close to their face while asking them to focus on it
Should see
- eyeball convergence (cross eyed)
- pupillary constriction (to focus)
- lense thickenijg (lense can bend light rays from near object)
Why does the pupil constrict and what is the mechanism?
The papillary light reflex
Light ray enters eye hits retina
It is detected by the retina and activity goes down through the optic nerve until it reaches a part of the midbrain called the pretectal nuclear, which is in the tectum (mickey Mouses double chin)
Pretectal nuclear receives afferents from the optic nerve and then sends impulses up to Edinger-Westphal nucleus in midbrain
EW nucleus contains parasympathetic preganglionic fibres. Sends impulses to ciliary ganglion where it synapses
Parasympathetic post ganglionic neurone projects up to the sphincter pupillae muscle = sphincter constricts to regulate light falling on retina
AFFERENT PART IS THROUGH OPTIC NERVE VISUAL PATHWAYS
EFFERENT PART IS THROUGH PARASYMPATHETIC NERVOUS SYSTEM
How does the accommodation reflex work and how do you get the desired responses of
- pupil constriction
- convergence
- lense thickening
Afferent arm is the retina
Impulses go down through the visual pathways, then we get a synapse at the lateral geniculate
Then projects down to the visual cortex where the patient processes the image of the finger getting closer to their face
This is communicated to the midbrain, to the Edinger-Westphal nucleus and occulomotor nucleus, activating them
EW nucleus then stimualtes ciliary ganglion which in turn, stimulates sphincter pupillae, constricting the lense
Also get lens thickening as it causes contraction of the ciliary muscles that control this
Get convergence through activation of the occulomotor nucleus which controls medial rectus (eye muscle), causing it to contract.
What is the blind spot of the eye?
Where the optic nerve leaves the eyeball, there are no photoreceptors = this is your blind spot
The optic disc
What are the two types of photoreceptors?
Rods and cones
Rods - black and white vision, good for low light conditions
Cones - for colour vision, not good in low light conditions
What is the fovea?
A part of the retina where we have the highest density of photoreceptors (cones)
Also lack of ganglion cell axons that you have elsewhere in the retina = easier for light to reach these cones
The area is the macula and the fovea is in the centre
What is retinal detachment?
Mainly types, however I’m most common form:
Photoreceptors separate from underlying pigment epithelium
= can get fluid building up
Presents with sudden blurring and loss of vision/ seeing visual artefacts
