Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Physiological-clinical importance of the eye. Prof James Ker MBChB, MMED, MRCP, FRCP, PhD, FESC, FACC, L.Akad.SA 2012
Physiological purpose of the visual system: • The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to aid in the guidance of behavior.
The act of seeing: • Begins with the capture of images focused by the cornea and lens upon the retina. • Retina—a light sensitive membrane in the back of the eye. Actually a part of the brain, banished to the periphery to serve as a transducer for the conversion of patterns of light energy into neuronal signals.
Light is absorbed by photopigment in 2 types of receptore: Rods and cones. • 100 million rods • 5 million cones • The rods operate in dim conditions and the cones under daylight conditions
The cones is specialized for color perception and high spatial resolution. • The majority of cones are located in the macula, the portion of the retina serving the central 10 degrees of vision.
The fovea: • This is a small pit in the middle of the macula, packed exclusively with cones and provides the best visual acuity. • Photoreceptors hyperpolarize in response to light and activates bipolar, amacrine and horizontal cells in the inner nuclear layer.
The flow of sensory information converges upon a final common pathway: the ganglion cells. • The ganglion cells translate these into action potentials that propagates along the primary optic pathway to visual centers in the brain.
There are a million ganglion cells in each retina and thus a million fibers in each optic nerve. • The majority of fibers synapse upon cells in the lateral geniculate body, a thalamic relay station. Cells from the lateral geniculate body then projects to the visual cortex.
This massive retinogeniculocortical sensory pathway provides the neural pathway for visual perception. • The lateral geniculate body is the main target of the retina, however some gangion cells project to other subcortical visual nuclei , involved in different functions.
These include: • Pupillary reflexes • Circadian rhythms • Visual orientation and eye movements
Clinical importance: • Primary disease processes affecting the eye • Systemic disease assessment: Hypertension and diabetes, vasculitis • Hypertension: hard/soft exudates • Cardiac disease: Infective endocarditis • Roth Spots • Hypercholesterolemia • Arcus cornealis; Hollenhorst plaque • Brainstem disorders • Skew deviation • Drug intoxications • Morphine: miosis • Optic neuritis: Toxins (ethambutol, methanol) • Pallor of disc • Optic neuritis due to MS
Intracranial hypertension • Papilloedema • Without hypertension: Hypercarbia • Primary metabolic disorders, such as Wilson`s disease: Kayser-Fleischer ring • Copper deposition around eyes • Circadian rhythm implications for the blind • Endocrinopathy: hormone disturbances: cortisol & GH