Visual System I- The Eye Flashcards
What are the three layers of the eye?
- Inner (Retina)
- Middle (Vascular)
- Outer (External Fibrous Skeleton)
What are the two parts of the Outer layer?
- Sclera
2. Cornea
What is the structure of the Sclera?
Tough white covering of the posterior 5/6 of the eye
What is the function of the Sclera?
Protection of internal structures and site for muscle insertion
What is the structure of the Cornea?
Transparent covering of the anterior 1/6 of the eye
What is the function of the Cornea?
Major refractive structure, bending light rays to form images on the retina.
What are the components of the Middle (Vascular) Layer?
- Choroid
- Ciliary Body
- Iris
What is the structure of the Choroid?
- Between Sclera and Retina
- Continuous anteriorly with ciliary body
What is the function of the Choroid?
Pigmented and dense vascular bed
What is the structure of the Ciliary Body?
Ring like thickening posterior to the corneoscleral junction
What are the two parts of the Ciliary Body and what is their function?
- Ciliary Muscle (Muscular portion)- controls the thickness, therefore controlling the focus of the lens
- Ciliary Process (Vascular portion)- secretes aqueous humor
What is the structure of the Iris?
Thin pigmented contractile dipahragm lying on the anterior surface of the lens.
-Central aperture= pupil
What structure decreases the diameter of the pupil?
Sphincter Pupillae
What structure increases the diameter of the pupil?
Dilator Pupillae
What are the components of the Inner (Retina) Layer?
- Optic Part
2. Non Optic Part
What are the components of the Optic Part of the Inner (Retina) Layer?
- Pigmented
- Neural
- Fundus
What is the structure of the Pigmented layer of the Retina?
Composed of epithelium
What is the function of the Pigmented layer of the Retina?
Provides nutrition, protection, and maintenance for the neural layer.
-It absorbs the light not captured by the neural layer
What is the structure of the Neural layer of the Retina?
Contains photoreceptors and associated neurons.
What is the function of the Neural layer of the Retina?
Specialized for sensing light and processing the resultant information.
What is the fundus and what are its components?
- The inner surface of the retina
1. Optic Disc
2. Macula
What is the structure and function of the Optic Disc?
S: Distinct white circular area
F: Region where vacular and neural elements are entering/leaving the eye- contains no photoreceptors.
What is the structure and function of the Macula?
S: Yellow oval area lateral to the optic disc. Has the Fovea.
F: Supports high visual acuity.
What is the structure and function of the Fovea?
S: Small depression in the center of the Macula. Area with the proximal vascular and neural elements are shifted to the side, and has the greatest density of photoreceptors.
F: Greatest ability to resolve detail. High visual aquity.
What is the structure and function of the Optic Part?
S: Anterior continuation of the pigmented layer
F: Picks up random light not absorbed by Retina
Describe the structure of the Lens?
Transparent, biconvex encapsulated structure.
Lying posterior to the iris and anterior to the vitreous body
What is the function of the Lens?
Refractive structure, bending light rays to form images on the retina
Why is it important that the capsule of the lens is anchored to the encircling ciliary body?
The ciliary muscle controls lens accommodation.
What are the two segments in the eye?
- Anterior and Posterior
What does the anterior segment contain and where is it?
-Aqueous humor and is located anterior to the lens
What does the posterior segment contain and where is it?
Vitreous body (gelatinous mass) and is located posterior to the lens
Explain the basic steps of Image Formation.
- Decision to examine an object
- The extraoccular muscles move the eye, placing the object at the center of the visual field.
- The eye is directed at a point on the visual target called the fixation point.
- Light from the fixation point is focused on the fovea, and the remainder of the target is focused on the Retina surrounding the fovea.
What are the properties of Light?
- Electromagnetic radiation that is visible to the eyes
- Characterized by wavelength, frequency, and amplitude.
- Travels in a straight line until it interacts with an object.
Describe absorption.
Transfer of light energy to a particle or a surface
Describe Refraction
Bending of light when it travels from one transparent medium to another, occurs because light speed differs in different media.
What parts of the eye are primarily responsible for the refraction of light?
Cornea and Lens
What structures does light go through before reaching the retina?
Cornea–> Anterior segment–>Lens–>Posterior Segment–>Retina
What are the 2 ways to describe the image being produced?
- Retinal Surface
2. Visual Field
What are the components of the Retinal Surface and where are they?
- Nasal Hemi-Retina- medial to the fovea
2. Temporal Hemi-Retina- lateral to the fovea
What is the visual field?
The view seen by two eyes without movement of the head
-Left and right halves are defined when foveas of both eyes are fixed on a single point in space
What are the 2 zones of the Visual Field?
- Binocular Zone
2. Monocular Zone
Describe the Binocular Zone.
Central Region of the visual field, seen by both eyes.
Describe the Monocular Zone.
Peripheral region on the visual field, seen by only one eye (ipsilateral nasal hemi-retina)
What happens to the image that falls on the retina?
Inverted and reversed
Where does the Right visual hemifield project?
- Nasal hemi-retina of the Right Eye
- Temporal hemi- retina of the Left Eye
What is Retinal Detachment?
Separation of the neural and pigmented layers of the optic part of the retina, due to trauma. The neural part cannot survive detached from the pigmented layer.
What is Macular Degeneration?
Idiopathic degeneration of the photoreceptors in the macula.
What are Cataracts?
Opacities in the lens due to congenital defects, persistent exposure to ultraviolet light, diabetes, high doses of medications, radiation therapy, or poorly understood mechanisms that occur in aging.
What is Glaucoma?
High levels of intraocular pressure that can reduce the blood supply to the eye, due to the inadequate drainage of the aqueous humor.
What does Emmertropic mean?
Normal vision
What is Myopia?
Near Sightedness
-The distant objects are focused in front of the retina, creating a blurred image
What is Hyperopia?
Far Sightedness
-The images of near objects are focused behind the retina, creating a blurred image.
What is presbyopia?
Accommodation for near vision is ineffective. When lens is unable to change shape.
What are the 5 basic classes of neurons in the Retina?
- Photoreceptors
- Bipolar cells
- Ganglion Cells
- Horizontal Cells
- Amacrine Cells
How does information flow in the Direct Pathway? Which cells are involved?
- Vertical Flow of information (directly to the optic nerve)
1. Photoreceptors
2. Bipolar cells
3. Ganglion cells
What is the job of the photoreceptors?
Transduce light energy into a neuronal signal.
-Outer segment= responsible for photo transduction
What is the job of the bipolar cells in the retina?
Interneurons that convey information from photoreceptors to the ganglion cells.
What is the job of the ganglion cells in the retina?
Convey information to the rest of the CNS.
- Forms the optic nerve
- only source of retinal output
How does information flow in the Indirect Pathway? Which cells are involved?
-Lateral information flow. Modifies signals in the direct pathway, is the reason behind contrast and inhibition.
- Horizontal Cells
- Amacrine Cells
What direction does light flow?
Passes through the retinal vasculature (Ganglion cells) and all the subsequent layers before reaching the outer segments of the photoreceptors.
What are the 2 types of photoreceptors, which is more abundant?
- Rods- more abundant
2. Cones
Describe the structure of a Rod.
- Outer segment is long and cylindrical
- contains more stacked membranous discs
- Absent in the fovea, most dense along the periphery of retina
- Many rods converge onto 1 single bipolar cell
Describe the Function of a Rod.
- Very sensitive to dim light–> specialized for night vision
- Achromatic
- Poor spatial resolution
What are the 3 different types of cones and their associated photopigment?
- Short wavelength- Blue
- Medium wavelength- Green
- Long wavelength- Red
What is night blindness?
The inability to see in the dark, can be due to lost rod function.
Describe the structure of a cone.
- outer segment is short and tapering
- Fewer membranous discs
- most dense in the fovea
- Limited convergence; 1 or few cones converge onto 1 single bipolar cell
Describe the function of a cone.
- Less sensitive to light
- Specialized for day vision
- Trichromatic
- High spatial resolution
What is color blindness?
The inability to distinguish certain colors due to a genetic defect.
What is legal blindness?
The inability to see can be due to lost cone function
What is the state of the photoreceptors in the dark?
-Depolarized state, releasing neurotransmitter (glutamate)
What is responsible for the flow of Na+ and Ca++ through the channels?
-cGMP concentration is high in the dark, keeping the channels open for ions through flow.
What is the unique function of neurotransmitters in the dark?
Can be inhibitory
Describe what happens in phototransduction when light hits the photoreceptors.
- Light is absorbed by and activates the photopigment in the photoreceptors.
- Breakdown of cGMP, lowering its concentration
- closure of channels, reduction in flow of Na+ and Ca++
- Cell becomes hyperpolarized, rate of neurotransmitter release is reduced.
- Now photoreceptor is not inhibited, and can send the signal.
When are photoreceptors most sensitive?
Most sensitive to light at low levels of illumination.
What is light adaptation?
going from dark to light, results in a decrease in visual sensitivity
What is dark adaptation?
going from light to dark, results in an increase in visual sensitivity.
What factors are involved in processing of visual input?
- Receptive Fields
- Neuronal Signaling
- Retinal Circuitry
- Retinal Ganglion Cells
What is a receptive field?
The sum of areas in which the stimulus affects the activity of a neuron.
What is the shape of the receptive field of Bipolar and Ganglion cells?
Concentric Center-Surround Organization
- Circular shape, specialized to detect changes in luminance
How do the stimuli in this receptive field act?
Center of the circle- evoke one type of response
Doughnut shaped outer rim- evoke the opposite response
How do cells in the Concentric Center-Surround Organization behave and how are they categorized?
Respond to differences in the intensity of light between one part of the receptive field and another.
-on and off center cells
What does the transformation in the visual signal do?
-Assists higher centers in detecting weak contrasts and rapid changes in light intensity
Describe neuronal signaling in processing of visual input.
Most of the processing in the retina is mediated by graded potentials (due to small and tightly packed cells) and changes in rate of neurotransmitter release.
What type of cell uses action potentials to convey information?
Ganglion cells
Describe retinal circuitry in processing of visual input.
- signals from photoreceptors–>ganglion cells are parallel on ad off center pathways
- Interneurons combine signals, so that electrical responses evoked in ganglion cells depend on the precise spatial and temporal patterns of light
What are retinal ganglion cells and what is their function?
- output cells of the retina
- tonically active, light increasing or decreasing the firing rate of AP
What are the 2 classes of retinal ganglion cells?
- Magnocellular
2. Parvocellular
Describe Magnocellular cells
- Large receptive fields
- respond to large objects
- follow rapid changes in the stimulus
- responsible for analysis of motion (location, speed, and direction)
Describe Parvocellular cells
- Small receptive fields
- respond to specific wavelengths
- responsible for analysis of high acuity (shape and texture) and color.
