NS 7: The eye and ear special sense organs Flashcards
embryological origin of external ear?
1st pharyngeal cleft= ectoderm, forms EAM
proliferation in 1st and 2nd pharyngeal arches= auricle
embryological origin of middle ear?
1st pharyngeal pouch= endoderm, forms tympanic cavity and eustachian tube
1st and 2nd pharygeal arch cartilages- meckel’s = malleus and incus, reichert’s= stapes.
embryological origin of inner ear?
otic placodes= ectodermal thickening which invaginate to form auditory vesicles, detaching from surface ectoderm which then closes over the top.
how are fibres in optic nerve that are involved in pupillary reflexes routed?
by way of superior colliculi to the Edinger Westphal nucleus= parasympathetic part of oculomotor nerve nucleus- nerve responsible for motor limb of pupillary response to light with innervation of sphincter pupillae muscle.
which fibres cross over in the optic chiasm?
fibres from the nasal 1/2 of the optic disc= origin of optic nerve within the retina. Optic disc= no photoreceptors= blind spot.
which fibres are carried by the right optic tract?
fibres from right 1/2 of each retina (nasal retina of left eye, temporal retina of right eye) = left hemifield
which fibres are carried by the left optic tract?
fibres from left 1/2 of each retina (temporal retina of left eye and nasal retina of right eye)= right hemifield
where do 90% of retinal axons terminate?
the lateral geniculate nucleus in the thalamus
provides orderly representation of contralateral 1/2 of visual field
where is the left 1/2 of the visual field represented?
in the right primary visual cortex
how do ears move from their original position in the neck?
growth of mandible
2 different parts of the auditory (otic) vesicle formed from invagination of otic placodes and subsequent covering by surface ectoderm, and their location?
saccule= ventrally, cochlear duct is formed from this as an outpocketing which pentrates the surrounding mesenchyme is a spiral fashion and allows development of the cochlea. utricle= dorsally, gives rise to the semi-lunar (circular) canals.
what does expansion of the 1st pharyngeal pouch distally create?
tympanic cavity
pouch expands in a lateral direction to contact floor of 1st pharyngeal cleft- gives rise to EAM.
structure of the EAM?
lined by skin throughout, but skin lining outer 1/3 has hairs, sebaceous glands, and ceruminous glands- brown wax, bactericidal.
outer 1/3= elastic cartilage
inner 2/3= tympanic plate of temporal bone
structure of tympanic membrane?
outer 1/3= skin- stratified squamous K epithelium
middle= fibrous layer, CT, forms membrane skeleton
inner= respiratory epithelium= low columnar.
describe how the auricle of the ear develops
mesenchymal proliferation in 1st and 2nd pharyngeal arches surrounding EAM- 3 on each side= auricular hillocks. These fuse to form the auricle.
what happens if the choroid fissure fails to close?
coloboma= cleft in the iris
how is the choroid fissure formed and what is its function?
invagination of inferior surface of optic cup. Allows hyaloid artery to reach inner eye chamber.
describe the formation of the retina
derived from optic vesicle= outpocketing of the diencephalon. Vesicle invaginates forming a double walled optic cup, outer layer= pigmented layer of retina, inner=neural layer, the 2 initally separated by an intra-retinal space but lumen disappears with apposition of the layers.
what does the optic vesicle give rise to?
the retina, lens and ciliary body
how is the lens formed?
derived from lens placode= thickening of surface ectoderm. Optic vesicle developing as outpocketing of diencephalon makes contact with the surface ectoderm helping to induce changes necessary for lens formation.
Lens placode invaginates forming lens vesicle which comes to lie inside mouth of optic cup when loses contact with surface ectoderm, and forms the lens.
when the choroid fissure closes, what happens to the hyaloid artery which ran in it?
degenerates distally
proximal portion becomes central artery of the retina
what is the ciliary body derived from and what does it do?
from optic vesicle, connects choroid with lens.
how is the optic cup, formed from the invagination of the optic vesicle, connected to the brain?
by the optic stalk= outgrowth of diencephalon
what is the groove on the ventral surface of the optic stalk?
choroid fissure- contains hyaloid vessels
what does the optic nerve form from?
the optic stalk following closure of choroid fissure and increasing numbers of nerve fibres and nerve fibres of retina returning to brain lie among cells of inner wall of stalk, and as inner wall then grows, inside and outside walls of stalk fuse.
Contains central artery of retina
what are extraocular muscle of eye derived from?
preotic myotomes
how are eyes moved from side of head to front of face?
growth of facial prominences
what does the optic radiation carry visual information to and from?
from the lateral genicualte nucleus to the primary visual cortex in the occipital lobe of the brain.
what is the primary visual cortex (V1) known as histologically?
Broadman’s Area 17
most likely cause of neural lesions affecting optic nerve?
trauma
optic neuritis- causes pain on movements of eye
optic atrophy e.g. MS
name of visual field defect occuring with lesion of optic chiasm e.g. due to a pituitary adenoma as optic chiasm in an anterior relation of the pituitary gland.
bitemporal hemianopia= as lesion of fibres carrying visual information from the nasal hemi-retinae of both eyes as thses fibres decussate in optic chiasm, and these correspond to the temporal hemi-fields of both eyes due to the pathway of light entry into the eyes- nasal fibres view temporal visual field of eye.
in which part of the visual pathway due fibres belong that travel through the internal capsule?
optic radiation
origin of sympathetic fibres supplying smooth muscle portion of levator palpebrae superioris?
T1/T2
along which artery do S.fibres supplying smooth muscle portion of LPS ascend>
ICA
visual field defect produced by a stroke involving most posterior part of internal capsule, damaging the optic radiation?
homonymous hemianopia
what is THE clinical sign of raised intracranial pressure?
papilloedema= optic disc swelling
how are visual field qudrants projected onto each retina?
in a reversed and inverted pattern due to action of lens- develops from lens placode= ectodermal thickening.
define anopsia
loss of vision
describe how impulses from the upper and lower halves of the visual field are located in different parts of the optic radiation
impulses from contralateral upper quadrant travel ventrally, go into white matter of temporal lobe before moving posteriorly into occipital love where end in lower wall of calcarine sulcus= lingual gyrus
impulses from contralateral lower quadrant move dorsally, sweep posteriorly through white matter of parietal lobe to occipital lobe, where end in upper wall of calcarine sulcus= cuneus.
define homonymous in terms of visual defects?
visual field defect confined to same part of visual field in each eye, so left part of visual field lost in both eyes, or right part.
where do homonymous visual defects occur?
with lesions in visual pathway distal to optic chiasm and so after decussation of fibres from nasal hemi-retinae, so both right and left visual pathways are carrying all the impulses from the contralateral halves of the visual field.
what may cause a right nasal hemianopia?
so visual field lost is from the temporal hemi-retina of the right eye following compression of the right angle of the optic chiasm. May be due to pressure by aneurysm of ICA.
what may cause a left homonymous hemianopia?
this can result from right optic tract compression e.g. abscess or tumour of temporal lobe that compresses optic tract against crus cerebri.
or destruction of internal capsule of right optic radiation e.g. stroke- anterior choroidal artery dysfunction, tumour.
what visual field defect occurs with lesion of right meyer loop or lower part of geniculocalcarine tract?
left homonymous superior quadrantic anopsia/ quadrantanopia e.g. with temporal or occipital lobe tumour
what visual field defect occurs with lesion of upper part of right geniculocalcarine tract?
left homonymous inferior quadrantic anopsia e.g. with parietal or occipital lobe tumour.
what are the first order neurones of the visual system?
bipolar cells- receive transmission of impulses from rods and cones in retina. Axons synapse with dendrites of retinal ganglion cells= second order neurones.
what are the second neurones in the visual pathway and what do they do?
retinal ganglion cells
optic nerve axons from these cells radiate toward optic disc, where they become myelinated and emerge to form the optic nerve.
how is the optic chiasm formed
by the optic nerve fibres from each eye proceeding posteriorly and medially, entering the cranial cavity through the optic canal, and uniting.
where in the visual pathway is the 3rd order neurone?
lateral geniculate nucleus
what are the 2 different types of photoreceptor in the retina connected to bipolar cells?
rods and cones
rods= not present in central retina, photosensitive, dark adapt, many rods converge into one single bipolar cell. No colour vision or fine vision.
cones= concentrated in fovea, high acuity, day vision, colour vision. Three types of cones (blue, red, green)
why are rods not good for high visual acuity?
many converge into 1 single bipolar cell so bipolar cell doesn’t know where visual stimulus has come from.
area of highest visual acuity?
fovea centralis in macula region of retina. high concentration of cones here.
3 major functional classes of neurones in retina?
photoreceptors: rods and cones
interneurones: bipolar, horizontal and amacrine cells combining signals from photoreceptors
ganglion cells: magnocellular and parvocellular
what carries the right hemifeld of vision?
the left optic tract
what might cause a left homoymous hemianopia with macula sparing, and why is the macula spared?
dysfunction of posterior cerebral artery in right visual cortex, or tumour, or trauma.
Macula may be spared if posterior part of the visual cortex is not involved.
Occipital lobe= dual blood supply, so if the posterior cerebral artery is occluded in an occipital stroke, the macula may be spared as occipital pole represents the macula topographically and is the most posterior part of cerbral hemispheres, which also receives blood from a branch of the middle cerebral artery, in addition to blood from posterior cerebral artery supplying the posterior parietal cortex, occipital lobe, and inferior parts of temporal lobe.
how might uncal herniation with raised IC pressure cause a homonymous hemianopia?
results in compression of the ipsilateral posterior cerebral artery, causing ischemia of the ipsilateral primary visual cortex and contralateral visual field deficits in both eyes, so if right posterior cerebral artery, then would produced a left homonymous hemianopia.
where is the edinger-westphal nucleus located?
midbrain
Light is shone into the right eye. Describe how the direct and consensual light reflexes occur.
Activation of photoreceptor cells in retina of right eye, stimulate retinal ganglion cells which send nervous signal along right optic nerve to the pretectal nucleus in the midbrain. The output from here is then to the edinger-westphal nucleus on both sides of the brainstem via crossing at the optic chiasm or in posterior comissure. Then impulse sent along oculomotor nerve on both sides to the ciliary ganglion, and then to the sphincter pupillae muscle which contracts, causing constriction of both pupils.
=4 neurone reflex
how would the pupillary light reflex be affected with a lesion distal to the optic tract?
both direct and consensual would be normal as afferent components of light reflexes in optic tracts intact.
how would the pupillary light reflex be affected with a right optic nerve lesion?
loss of both direct and consensual reflex when light shone in the right eye.
BUT both pupils would constrict when light shone in the left eye as left optic nerve fibres intact and send innervation to edinger-westphal nucleus on both sides of brainstem.
how would the pupillary light reflex be affected with damage to the parasympathtic part of the left oculomotor nerve e.g. with a tumour compressing the fibres?
loss of both direct and consensual responses in the left eye, so left pupil would be dilated (mydriasis) when light shone into this eye, and when light shone into the right eye.
why is the fovea over represented in the lateral geniculate nucleus in the thalamus?
area of highest retinal, ganglion cells, area of best vision
what is the loop of Meyer?
fibres of the optic radiation from the inferior 1/2 of the retina formed by looping around the temporal horn of the lateral ventricle, pass through temporal lobe (below parietal lobe) travel ventrally, damage would produce a superior visual field defect as inferior 1/2 of retina correponds to superior 1/2 of visual field.
why is the primary visual cortex also known as the striate cortex?
prominent stripes of white matter corresponding to myelinated axons contained
segregation of magno and parvocellular channels maintained at V1**
how are neurones with similar receptive fields organised in the visual cortex?
orientations columns
disruption causes no depth perception
what is strabismus?
eye turn
*amblyopia
name given to an eye which is turned out?
exotropia
name given to eye turned in?
esotropia
name given to 1 eye higher than other?
hypertropia
name given to 1 eye lower than other?
hypotropia
where do the magno and parvocellular pathways travel to?*
P pathway ventral stream V2 & V4 Inferior temporal
cortex (object recognition and representation- contours, colour and form)= related to P cells of the lateral geniculate nucleus in the thalamus, concerned with fine detail and colour.
M pathway dorsal stream V5 & V3 Posterior
parietal cortex (object location and motion)- M cells of the lateral geniculate nucleus, concerned with motion.
how do fibres from the superior retina travel?
travel dorsally, through parietal lobe to occipital lobe, shorter path so less susceptible to damage, correspond to inferior part of visual field.
what is accomodation?
this allows images to remain in focus as gaze shifts from far to near objects.
It includes: contraction of ciliary muscles which allow lens to bulge, innervated by PNS part of oculomotor nerve
constriction of pupil
convergence of eyes
centre for accomodation in midbrain
input from visual cortex and output passes to edinger-westphal and oculomotor nuclei. from EW nucleus= pre-ganglionic PNS fibres of oculomotor nerve which synapse in ciliary ganglion, where post-gangl fibres then travel via short ciliary nerves to ciliary and sphincter pupillae muscles.
those from oculomotor nuclei travel directly to medial rectus muscle of each eye.
