**_🧠Neurology🧠 - Visual system Flashcards
Name these eye structures
Name these eye structures
What is the antero-posterior diameter of the eye?
24mm in adults
What are the layers of the coat of the eye?
3 layers
Sclera - hard an opaque
Choroid - pigmented and vascular
Retina - neurosensory tissue
Name these structures?
Give more detail about the sclera
Commonly known as the “white of the eye”
Tough, opaque tissue - serves as the eye’s protective outer coat
High water content
Sits superficially to the choroid and retina
What is the uvea?
Vascular coat of the eyeball
Lies between the sclera and retina
Composed of the iris, ciliary body and choroid
Intimately connected - a disease of one part also affects the other portions (though not necessarily to the same degree)
Give more detail about the retina
Very thin layer of tissue lining the inner part of the eye
Responsible for capturing light rays
Light impulses then sent to the brain via the optic nerve
Outline the optic nerve
Transmits electrical impulses from the retina to the brain
Connects to the back of the eye near the macula
Portion visible on the retina is called the optic disk
What is the macula and what purpose does it serve?
Located roughly in the centre of the retina
Small and highly sensitive part of the retina - responsible for detailed central vision
Fovea is the very centre of the macula - allows the appreciation of detail in central vision, crucial for tasks such as reading
How does the optic nerve form a blind spot?
Where the optic nerve meets the retina there are no light sensitive cells - it is a blind spot
Which anatomical landmark corresponds to the physiological blind spot?
The optic disk
What is the purpose of central vision?
Detail day vision, colour vision, reading, facial recognition
Fovea has the highest concentration of cone photoreceptors
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity
What is the purpose of peripheral vision?
Shape, movement, night vision
Navigation vision
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity
What are the 3 layers of the retina, from superficial to deep?
Outer layer
Middle layer
Inner layer
What does the outer layer of the retina contain?
Photoreceptors (rods/cones - 1st order neurons)
Responsible for the detection of light
What does the middle layer of the retina contain?
Bipolar cells (2nd order neuron)
Local signal processing - improves contrast sensitivity
What does the inner layer of the retina contain?
Retinal ganglion cells (3rd order neuron)
Transmission of signal from the eye to the brain
What is meant by the order of a neuron?
The sequence or hierarchy of neurons in a neural pathway
i.e. 1st order neurons create impulses from a stimulus (e.g. photoreceptors), 2nd order neurons receive impulses from 1st order neurons and so on…
Outline the cells responsible for visual processing
Two main classes - rods & cones
Rods - 100x more sensitive to light than cones
Slow response to light
Responsible for night vision
Cones - Less sensitive to light, but faster response
Responsible for daylight fine vision and colour
120million rods vs 6million cones
Outline the 2 types of lenses
What is emmetropia?
What is ametropia?
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina
Near-sightedness - myopia
Far-sightedness - hyperopia
Presbyopia
Outline myopia
Presents with blurred distance vision
Squinting when looking into the distance
Headache
Outline hyperopia
Patient’s visual acuity at near will blur relatively early
Nature of the blur can vary - tends to be more noticeable when tired, printing is weak or light is inadequate
Asthenopic symptoms: eye pain, headache in frontal region, burning sensation in eyes
What is the near response triad?
Triad of mechanisms that help the eye adapt for near vision
Pupillary miosis (sphincter pupillae)
Convergence of eyes (performed by medial recti muscles)
Accommodation (circular ciliary muscle)
What is the role of the sphincter pupillae in the near response triad?
Causes pupillary miosis - increases depth of field
What is the purpose of convergence in the near response triad?
Performed by the medial recti from both eyes
Aligns both eyes towards a nearby object