Anatomy of the Eye and Visual Pathways Flashcards
Describe the shape of the bony orbit and the bones that make it up
Frontal, zygomatic, maxilla, lacrimal, palatine, sphenoid, ethmoid, pyramidal structure
What are the angles of the medial and lateral walls?
The medial wall is parallel to the midsagittal plane while the lateral wall is at 45 degrees to it.
What is the difference between the anatomical axis and the visual axis?
The visual axis is the line of sight while the anatomical axis is an imaginary plane that goes through the centre of the bony orbit, cutting it into a symmetrical left and right
What angle is the anatomical axis at to the midsagittal plane?
It is at 22.5
When are the visual and anatomical axes in the same plane?
When the eye is abducted by 22.5 degrees, the axes coincide.
Describe the outer layer of the eye
It is known as the fibrous tunic/coating and is a tough and collagen rich tissue. It is composed of the sclera (white) and the cornea (transparent). It is continuous with the dural sheath posteriorly.
Describe the middle layer of the eye
It is known as the vascular coating/tunic and is composed of the choroid, ciliary body and iris.
Describe the choroid
It is found in the vascular layer and is pigmented as it contains melanin. The melanin allows it to absorb light and so limits reflection.
Describe the inner layer of the eye
This is known as the neural layer and is composed of the retina. The retina is made of photoreceptors and multiple other cells arranged in layers. At the centre of the retina, there is the macula lutea where you have the sharp clear vision. There is also the macula lutea which is at the centre of the retina and where you get the sharp and detailed clear vision. At the centre of this, there is a depression called the fovea centralis which is where you get the highest visual acuity.
What is the optic disc? Where is it in relation to the macula lutea.
It is the physiological blind spot. It is a pale area and is where the optic nerve exits. It is also where the central retinal artery and vein branch out. It is medial to the macula lutea.
How do the features of the eye layers appear on a fundus photograph scan of the eye incl arteries, veins, optic disc, macula lutea, fovea centralis,
The veins are darker and wider while the arteries are lighter. The optic disc is brighter and more medial while the macula lutea and fovea centralis are lateral to the optic disc and darker.
Describe how the lens is adapted to change for far/near vision
The lens is connected to suspensory ligaments known as zonule fibres, these are attached to the ciliary muscle which is a ring like muscle. In far vision, the ciliary muscle is relaxed and the ligaments are taut meaning the lens is flat. In close vision, the ciliary muscle is contracted so the suspensory ligaments are relaxed making the lens thick.
What are the two segments of the eye?
There is an anterior segment and posterior. They are divided by the iris.
Describe the structure of the anterior segment of the eye and how fluid flows in it
It is sub-divided into two further chambers - anterior and posterior. The posterior chamber contains the ciliary body which produces aqueous humour. This fluid flows into the anterior chamber -> canal of schlemm.
Name the muscles of the eye and their CN supply
Extraocular muscles - sup, inf, med, lat rectus muscles and inf, sup oblique muscles. CN III (OM) - sup, inf, med rectus and inferior oblique. CN IV (Trochlear) - superior oblique. CN VI (Abducens) - lateral rectus.
Remember LR6 SO4
What muscle moves the upper eyelid and where does it sit?
The lateral palpebrae superioris controls the upper eyelid. It is superior to the superior oblique
What is the origin and insertion of the rectus muscles?
Origin - common tendinous ring of zinn. Insertion - sclera of the eyeball behind the limbus.
What are the origins and insertions of the oblique muscles?
Inferior oblique is O= anterior aspect of the orbital floor (maxilla mainly), I= sclera of the eye posterior to the lateral rectus.
Superior oblique is O= body of the sphenoid bone, I = sclera of the eye, posterior to the superior rectus.
What is the primary function of the medial rectus muscle?
Adduction
What is the primary function of the lateral rectus muscle?
Abduction
What are the primary, secondary and tertiary functions of the inferior rectus?
Depression, extorsion, adduction
What are the functions of the superior rectus?
P= elevation, S= intorsion, T= adduction
What are the functions of the inferior oblique?
P=extorsion, S = elevation, T = abduction
What are the functions of the superior oblique?
P = intorsion, S = depression, T = abduction
What muscles do adduction?
Medial rectus (main muscle), inferior and superior rectus (Tertiary function)
What muscles do abduction?
Lateral rectus (main), inferior and superior oblique (T function)
What muscles do depression?
Inferior rectus (main), superior oblique
What muscles do elevation?
Superior rectus (main), inferior oblique
What muscles do excyclotorsion?
Inferior oblique (main), inferior rectus
What muscles do incyclotorsion?
Superior oblique (main), superior rectus
What is intorsion and extorsion?
Intorsion - inwards rotation or towards midline
Extorsion - outward rotation or towards lateral
Describe the visual fields and the visual pathway and/or draw a diagram. Incl the optic chiasma, retinal axons, nuclei and cortex.
One side of the retina processes the opposite side of the visual field. So temporal retina captures vision from the nasal part of VF and vice versa.The visual fields overlap in middle.
Temporal retinal axons travel on lateral side of optic nerve + vice versa. At optic chiasma, the nasal axons cross over but not temporal. Optic tracts contain temp fibres of same side and nasal of opposite.
Optic tracts reach lateral genicular nucleus -> optic radiations in parietal or temporal lobe -> occipital lobe/primary visual cortex
What are optic radiations and its parts?
These are loops of fibres which go from LGN -> primary visual cortex. Some pass superiorly so through parietal lobe while some go inferiorly so through temporal. Meyer’s loop processes the upper VF while the superior part does the lower VF.
Draw circles of the visual loss for 5 visual field defects and their causes
1 = total right eye visual loss, right circle dark and left circle white. The optic nerve is damaged. 2 = Bitemporal hemianopia, two circles with lateral sides dark. Lesion in midline of optic chiasma. 3 = left nasal hemianopia, left circle medial side dark. Lesion pressing on lateral bit of optic chiasm so same eye affected. 4 = right homonymous hemianopia, both circles right sides affected. Lesion of left optic tract. 5 = left homo hemian with macular sparing. Right optic radiations damaged so left VF affected, however macula lutea still works
