The Eye and Visual Pathways Flashcards
Describe the functions of the iris, cornea and lens.
- Iris - controls how much light enters the eye via the pupil
- Cornea - focusses light rays
- Lens - provides additional, variable “fine” focus
Describe the distribution of rods and cones in the centre and periphery of the retina, and how this influences vision.
Periphery:
- High density of rods
- Cones are large and widely spaced
- Large receptive fields - the signals from many cones converge onto single ganglion cells
Centre - fovea centralis:
- High density of cones and no rods
- Small receptive fields - each cone synapses with a single bipolar cell, therefore each ganglion cell receives info from one cone
- Overlying layers and blood vessels are absent, so the image is well focussed
Why does each cortical hemisphere process visual information from the contralateral half of the world?
- The optic nerves of the left and right eye meet at the optic chiasm
- At the optic chiasm the nerves cross over and travel to the opposite hemisphere
Explain how the direct and consensual light reflexes are used to assess intracranial pressure after head injury.
- Direct - light shining in one eye causes pupil constriction
- Consensual - pupil constriction in both eyes due to bilateral reflex arc
- Preganglionic parasympathetifc fibres in the oculomotor nerve are vulnerable to raised intracranial pressure
- Absent light reflexes indicate raised intracranial pressure
Describe and explain the control of pupil constriction, including the nerve supply.
- Sphincter pupillae muscle constricts the pupil
- Innervated by the short ciliary nerves - these are postganglionic parasympathetic fibres originating in the ciliary ganglion
- These fibres release ACh
- Preganglionic parasympathetic fibres arise from the Edinger-Westphal nucleus and run in the oculomotor nerve to the ciliary ganglion
Describe and explain the accomodation reflex of the eye.
- Ciliary muscles contract to release tension on the suspensory ligaments, allowing the lens to adopt a rounder shape
- Rounder lens refracts light more, allowing the eye to focus on nearby objects
- When gaze reverts to a distant object, the ciliary muscles relax and the lens returns to its original shape
Describe and explain the control of pupil dilation, including the nerve supply.
- The dilator pupillae contracts to dilate the pupil, under the control of the long ciliary nerves
- Sympathetic postganglionic fibres from the long ciliary nerves release NA
- Thus, threatening situations which induce the fight-or-flight response may cause pupil dilation
Explain the problems underlying myopia/short-sightedness and how it may be corrected.
- Objects are focussed in front of the retina
- Myopic eye can focus on nearby objects by flattening the lens, reducing refraction
- Lens cannot flatten enough to focus distant objects, which appear blurred as they are still focussed in front of the retina
- Myopia corrected using a negative lens, which reduces the refraction of light
Explain the problems underlying hypermetropia/long-sightedness and how it may be corrected.
- Objects are focussed behind the retina
- Hypermetropic eye can focus on far away objects by rounding the lens, increasing refraction
- Lens cannot increase refractive power enough to focus distant objects, which appear blurred as they are still focussed behind the retina
- Hypermetropia corrected using a positive (convex) lens, which increases the refraction of light
Describe common problems in the ageing eye.
- Presbyopia - lens proteins degenerate, lens becomes stiff and focus is fixed
- Cataract - lens proteins become opaque
- Vitreous degeneration - proteins in vitreous “jelly” clump together, spoiling visual clarity
- Presbyopia and vitreous degeneration occurs in all ageing eyes; cataracts do not
- Vitreous detachment - shrinking vitreous pulls away from retina, creating risk of retinal tear
