Session 7: Special Senses Flashcards
Describe the beginning of the development of the eye
The eye is (broadly) composed of the retina, ciliary body, lens and the iris, each of which must develop during the embryonic period. The development of the eye begins in the 4th week.
Its development begins initially with outpocketings of the forebrain (known as optic vesicles) growing out to make contact with the overlying ectoderm => optic placodes => lens placode.
The ectoderm where the optic vesicle comes in contact with the lens placode (which will become the lens) begins to thicken and the lens placode can then invaginate and begin to pinch off. It then sinks down into the optic vesicle
NB: placodes are thickened (beginning differentiation) ectodermal patches on the developing head.
What happens next in the development of the eye? What os the hyoid artery?
The optic vesicle stretches out to “grasp” the lens placode for invagination and there is a long slit, running down the stalk, called the choroid fissure, which closes around the hyloid artery (failure to do so can result in a coloboma).
The optic vesicle eventually surrounds the lens placode completely.
- The hyloid artery which ran in the choroid degenerates distally yet remains proximally and in turn becomes the central artery of the retina.
What does the optic rim differentiate to form?
The rim of the optic vesicle differentiates to form the ciliary body musculature whilst the extraocular muscles develop from preoptic myotomes developing in the region of the eye.
The preoptic muscles also give rise to some facial muscles.
What is the optic cup? (hint two layers of the retina)
The optic cup that subsequently forms from the optic vesicles will give rise to the retina. The retina is comprised of neural (inner) and pigmented (outer) layers, the iris and the ciliary body. Both the iris (a contractile diaphragm with a central aperture) and the ciliary body (muscular and vascular structure connecting choroid to lens) come from the rim of the optic cup.
- The inner layer of the optic cup is the primordium of the neural layer of retina.
- The outer layer of the optic cup is the primordium of the retinal pigment epithelium.
What does the optic stalk degenerate to form and what is the intraretinal space?
The optic stalk will then degenerate to form the optic nerve (optic stalk was initially an outgrowing from the diencephalon), which becomes responsible for the sensory movements of the eye. Movements of the eye controlled by CN III, IV and VI.
The intraretinal space develops as the retina itself develops from two separate layers; the space is obliterated as the layers fuse to form the retina yet ‘retinal detachment’ can occur as the two layers becomes separated as the intraretinal space is opened up again.
Generally a degenerative change
How do the eye primordia move to the front of the face? And what are congenital cataracts? Describe Congenital Rubella Syndrome
The eye primordia starts positioned at the side of the head, yet as the facial prominences grow (particularly the maxillary prominence), the eyes move to the front of the face and binocular vision is achieved.
Congenital cataracts are opacities of the lens as a result of a genetic defect of exposure to a teratogen such as rubella.
Congenital Rubella Syndrome occurs when a pregnant woman contracts rubella during her first trimester. The classic triad of symptoms is:
- Sensorineural deafness (58% of cases)
- Cataracts or retinopathy (43% of cases)
- Congenital heart disease, especially PDA (50% of cases)
Other symptoms include microcephaly and patent ductus arteriosus
Congenital rubella syndrome has been prevented due to mass immunisation against the rubella virus
What are the 3 parts of the ear?
The ear is comprised of 3 distinct structures, internal, middle and external and each has a discrete embryological origin. The pharyngeal apparatus of the developing head and neck region of the embryo make important contributions to the development of all parts of the ear.
Describe the development of the inner ear
The inner ear comprises of semicircular canals and the cochlea, which forms as a membranous labyrinth/duct system encased in bone. It acts for both hearing (cochlea) and balance (semicircular canals), innervated by CN VIII
The inner ear starts as otic placodes (thickened ectodermal patches) that appear on the back (posterior surface) of the embryonic head. Once they have thickened, they begin to invaginate and sink below the surface. They then pinch off to form the otic vesicle (a completely new structure within the substance of the head, derived from the surface) and the surface ectoderm then closes over.
The otic vesicle then undergoes large morphological changes. Part of it, known as the saccule, elongates and curls up to form the cochlea (responsible for hearing) whereas the other part, known as the utricle, forms the semicircular canals (responsible for balance).
Describe the development of the middle ear
The middle ear is involved in conducting sounds from the external meatus to the inner ear via the auditory ossicles (malleus, incus and stapes).
The tympanic cavity and auditory tube are derived from the 1st pharyngeal pouch whereas the ossicles form from cartilage bar derivatives that undergo remodelling
- The malleus and incus form from Meckel’s cartilage
- The stapes form from Reichert’s cartilage
The 1st pharyngeal pouch expands distally to create the tympanic cavity, yet its proximal end remains narrow to create the Eustachian tube (remains continuous with the nasopharynx). The middle ear ossicles develop within the cartilage bars of 1st and 2nd arches. 3 ossicles then become suspended in the tympanic cavity to complete the formation of the middle ear.
Describe the development of the external ear
The External Ear is composed of the external auditory meatus and the auricles.
- The external auditory meatus forms from the 1st pharyngeal cleft yet the auricles develop from the auricular hillocks (from proliferations within 1st and 2nd pharyngeal arches surrounding the meatus).
The external ears develop initially in the “foetal neck”, yet as the mandible grows, the ears ascend to the side of the head to lie in line with the eyes as the mandible pushes them into position.
Many chromosomal abnormalities are associated with external ear anomalies, as the development of the auricle is very complicated.
Describe the innervation of the ear
Vestibulocochlear (CN VIII)
Innervation of the muscles acting on middle ear ossicles reflex their pharyngeal arch derivation
- Tensor tympani – mandibular branch CN V
- Stapedius – CN VII
Sensory innervation of the external ear provided primarily by CN V and cervical spinal nerves
What are the two main mechanisms congenital deafness can arise from?
Congenital deafness can result from two main mechanisms:
- Middle ear deafness can result from 1st and 2nd pharyngeal arch problems, such as problems with ossicles formation.
- Inner ear deafness can result from maldevelopment of the Organ of Corti (auditory, sensory cells in the cochlear) commonly from a variety of teratogenic agents (normally rubella).
Describe how the eye is effectively a direct part of the brain and thus the optic nerve is a brain tract
The eye is an embryological outpost of the brain deployed as such to sub-serve the brain’s visual information gathering by firstly transducing electromagnetic radiation into electrical energy and then in turn, conducting this energy and feeding it appropriately to various centres of the brain.
This information gathering is efficient and impressive but it is largely possible because the eye is effectively a direct part of the brain. So the optic nerve is thus part of the CNS, not the PNS. It is a brain tract but is called a nerve for reasons of convenience.
The optic nerve is encased in a tube of connective tissues that are continuations of the meninges of the brain. Within the meninges of the optic nerve, cerebrospinal fluid is found that is continuous with that of the ventricular system of the brain. Furthermore, the blood vessels found within the optic nerve are direct continuations of vessels of the brain.
What is the point of origin of the optic disc? where is the visual cortex located?
The point of origin of the optic nerve within the retina is known as the optic disc or blind spot, and is further attended by an organised branching order of these blood vessels. Therefore examining the retina by fundoscopy is in actual fact a direct examination of the vasculature of the brain without opening it.
Understanding the anatomy of the eye, the optic nerve and fundamental issues attendant upon retinal and visual fields are critical; these skills are used routinely to diagnose life-threatening conditions such as meningitis and raised intracranial pressure before they cause irreparable damage to the brain.
The optic nerve carries visual information from the retina to the occipital (visual) cortex. This is a long pathway that can be interrupted at various sites.
Fibres in the optic nerve that are involved in pupillary reflexes are routed by way of the superior colliculi to the parasympathetic part of the third nerve nucleus (the Edinger Westphal nucleus).
What are the two types of photoreceptors in the retina? How are they different?
Light enters through the iris and is focused by the cornea and lens, traversing the vitreous humour, travels through layers of retinal neurones before reaching photoreceptors. The retina forms as the inner layer of the eye and consists of two types of photoreceptors, the rods and the cones.
- Rods are highly light sensitive and are specialised for night vision (with the loss of rods leading to night blindness and loss of peripheral vision), converging into one single bipolar cell. They are not present in the central retina.
- Cones are concentrated in the fovea, providing high acuity vision, day vision and colour vision. There are 3 types of cone (blue, red and green) and unlike the rods, they have a one cone to one interneurone interaction.
The retina has three functional classes in the photoreceptors, interneurons (bipolar, horizontal and amacrine cells) which combine the signals from the photoreceptors, and ganglion cells (magnocellular (M) and parvocellular (P)) that are the output cells of the retina. These nerves will then enter the visual pathway.
Describe the visual and retinal fields
The visual field of each eye is the region of space that the eye can see looking straight ahead without movement of the head.
The retina can be divided into two halves by a vertical imaginary line through the centre of the fovea, known as the nasal hemiretina and the temporal hemiretina. These can also be divided into superior and inferior with a horizontal imaginary line.
Due to the action of light passing through the lens (inversion), image present on the left side of the visual field are detected by the nasal hemiretina of the left eye and temporal hemiretina of the right eye and vice versa.
A similar relationship occurs with the superior and inferior halves of the retina.
Describe the visual pathway up to the LGN
As the neurones leave the optic disc, the axons become myelinated and exit in the optic nerve, which is also covered by in meningeal coverings.
As the optic nerves reach the brain, they form the optic chiasm; fibres from the nasal hemiretina cross to the opposite of the brain whereas those fibres from the temporal hemiretina stay on the same side.
After leaving the optic chiasm, the crossed and uncrossed fibres become the optic tracts.
So the right optic tract contains fibres from right half of each retina; fibres of left nasal hemiretina of left eye and temporal hemiretinal fibres of the right eye, thus containing visual information for the left hemifield.
The left optic tract contains fibres from left half of each retina; fibres of nasal hemiretina of right eye and temporal hemiretinal fibres of the left eye, thus containing visual information for the right hemifield.
The optic tracts on each side project to the corresponding lateral geniculate nucleus.
Describe the Lateral Geniculate Nucleus
- Found in the thalamus and receives projections from the optic tract.
- The LGN consists of 6 layers, arranged in a retinotrophic orderly representation of the contralateral half of the visual field. The layers were divided into magnocellular and parvocellular layers; the fovea having the largest representation within the LGN than any peripheral areas.
90% of retinal axons terminate in lateral geniculate nucleus (part of thalamus). There is major input into the LGN from other centres as well (e.g. reticular formation, brainstem and cortex) => feedback connections.
- The magnocellular pathway appears to act as the motion sensitive pathway (similar sensitivity to rods, sensitive to motion and luminance) whereas the parvocellular (similar sensitivity to cones) pathway appears to be a fine detail and colour vision pathway. The segregation is maintained in the LGN and projections out of the LGN too.
Describe the Optic Radiations
Nerve fibres leaving the magnocellular and parvocellular layers of the LGN project through the optic radiation to the primary visual cortex.
- Fibres from the inferior retina pass through the temporal lobe by looping around the inferior horn (forming Meyers loop) of the lateral ventricle. They carry information from the superior part of the visual field.
- Fibres from the superior retina pass travel straight through the parietal lobe to the occipital lobe in the retrolenticular limb of the internal capsule to the visual cortex. They carry information from the inferior part of the visual field.
Describe the visual cortex
The Primary Visual Cortex (V1) is located in the medial aspect of the occipital lobe.
- Each half of the visual field is represented in the contralateral visual cortex with over-representation of the fovea (because there are so many cone cells) seen as well in the primary visual cortex.
- Information from the primary visual cortex can also be relayed to the secondary visual cortex (V2) and then to the tertiary visual cortex (V3).
- The visual cortex neurones are arranged onto highly-organised columns.
Describe the ventral and dorsal pathways
P pathway => ventral stream => V2 & V4 => inferior temporal cortex (object recognition and colour)
M pathway => dorsal stream => V3 & V5 => posterior parietal cortex (object location and motion).
NB: Ebbinghaus Illusion: optical illusion of relative perception – shows how we don’t see exactly what’s there. The higher visual cortex compares and interprets visual impressions.
Describe key stages in vision development, what may be used to assess visual development etc
Infant Vision
- Newborn; cones are immature
- 4 weeks: able to contrast
Optical Coherence Tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section picture of the retina. It can detect things such as age-related macular degeneration.
- It can also be used in investigation of postnatal retinal development - shows how at birth, the outer nuclear layer is quite thin as the photoreceptors are immature and at 3-5 years, still developing in the fovea.
- Abnormal retinal development can include fovea hypoplasia and retinal dystrophy which can both lead to nystagmus.
What is ambylopia?
Amblyopia: lazy eye caused by abnormal binocular input early in life (input in both eyes is not equal).
The most common problem is poor focusing due to strabismus (abnormal alignment of the eyes; squint), Anisometropia (refractive difference between the two eyes – e.g. myopia (nearsighted), hyperopia (farsighted) or a combination of both, known as Anisometropia)) or deprivation (congenital cataract, ptosis, media opacities).
Strabismus:
- Esotropia: eye turns in
- Exotropia: eye turns out
- Left Hypertropia: eye turns up
- Left Hypotropia: eye turns down
Strabismus typically involves a lack of coordination between the extraocular muscles, which prevents directing the gaze of both eyes at once to the same point in space.
Early treatment of strabisumus, through wearing an eye patch on the dominant eye and/or vision therapy, can correct amblyopia
What is Anisometropia/
Anisometropia: uncorrected difference in refractive power between the two eyes.
Emmetropia (normal): focus on retina
Myopia: focus in front of retina
Hyperopia: focus behind retina
Can be corrected by glasses
Describe Slit Lamp Examination
Slit Lamp Examination: The purpose of the slit lamp is to allow the examiner to examine the media of the eye in a magnified way. The depth of focus on the slit lamp can be adjusted to bring the tissue of interest clearly into view.
The narrow beam projected by the slit lamp provides an optical section of the transparent structures. The different structures can be distinguished by their differences in optical density.
The tissues seen using the slit lamp are the
· Eyelids and conjunctiva
· Cornea
· Anterior chamber
· Iris and pupil
· Lens
· Anterior part of the vitreous body
Describe Fundoscopy
Allows examination of the retina
Fovea: region of highest density of photoreceptors in retina
Optic disc: ganglion cell axons exit, has no photoreceptors = blind spot
Optic disc is on the nasal side - towards the nose (so image shows fundus of the left eye) - image is seen from the front so that left in each image is to the patient’s right.
Describe the sympathetic and parasympathetic innervation to the eye (in the context of the pupillary light reflex). What goes wrong’s in Horner’s?
Sympathetic innervation to the pupil comes via the superior cervical ganglion, which act to innervate the dilator pupillae muscles of the pupil. Dilatation of pupil (dilator pupillae), if disrupted causes Horner’s Syndrome
- Horner’s Syndrome: miosis, partial ptosis 2-3mm (Muller’s muscle paretic) and anhydrosis (lack of sweating of same side of face sudomotor fibres).
However, the main part of the pupillary light reflex comes form the parasympathetic innervation to the eye.
- Parasympathetic: constriction of pupil in response to light (sphincter pupillae)
- Axons from the retinal ganglion project along the optic nerve to the pretectal area of the midbrain. These cells synapse bilaterally with interneurons, passing to the Edinger-Westphal nucleus where they synapse with pre-ganglionic parasympathetic neurones.
- These pre-ganglionic neurones run with CN III and synapse with post-ganglionic parasympathetic neurones in the ciliary ganglion and these then go on innervate the sphincter pupillae muscle.
- Consequently, when the retinal ganglion cells are excited by light, there is an increase in parasympathetic activity to produce miosis and reduce the light entering the eye.
What is the direct reflex and consensual response reflex?
The response in the eye on which the light was directed is called the direct pupillary light reflex and the response in the contralateral eye is called the consensual pupillary light reflex (occurring due to the bilateral projections to the Edinger-Westphal nucleus – due to the crossing over).
Relative afferent pupillary defect (RAPD aka Marcus Gunn pupil): the patient’s pupils constrict less (therefore appearing to dilate) when a bright light is swung from the unaffected eye to the affected eye (compared to just shining a bright light on the unaffected eye); moving a bright light from the unaffected eye to the affected eye would cause both eyes to dilate, because the ability to perceive the bright light is diminished- indicates optic nerve lesion or retinal disease
What are the possible visual field deficits that may occur along the visual field pathway?
depending on where the damage occurs along the visual pathway determines the visual field deficit produced.
- Damage to the right optic nerve will cause complete loss of vision to the right eye
- Damage to the optic chiasm will cause compression on the nasal hemiretinae of both eyes, causing loss to right side on right eye and left side on left eye; this is called non-homonymous bitemporal hemianopia. This is commonly caused by a pituitary adenoma compressing on the optic chiasm, which also means there may be endocrine changes (such as reduced LH and FSH or increase in prolactin or growth hormone).
- Damage to the right optic tract will damage axons from the right temporal hemiretinae and left nasal hemiretinae resulting in loss of the left half of the visual fields in both eyes; this is called a contralateral (left) homonymous hemianopia.
- Damage to the right temporal lobe (including Meyer’s loop) will cause vision to be lost in the superior quadrant of the left visual hemifield; this is called a superior left homonymous quadrantanopia. It commonly occurs through vascular occlusions in the middle cerebral artery causing lesions in the inferior half of the temporal lobe.
- Any damage to regions of the visual cortex will result in homonymous quadrantanopias with associated macular sparing. Macular sparing occurs due to the areas of the visual cortex receiving blood from calcarine artery and collateral branches of the middle cerebral artery, allowing for the integrity of the caudal parts of the visual cortex to be preserved despite any occlusion of the calcarine artery.
what is necessary to maintain diplopia?
n the normal state with the eyes focussed on a distant object, the visual (optical) axes of the eyes are parallel. In order to maintain the focus of each eye on the same point of each eye on the same point of the retina, and so avoid diplopia (double vision), as an object approaches the face it is necessary for each eye to accommodate (change contour of lens) and for the visual axes to converge.
The reflex arcs responsible involve the optic nerves, visual cortex, frontal cortex, brainstem and nerves III, IV and VI.
Any acute disturbance of the visual axes, whether due to an abnormality of nerves III, IV or VI or to a displacement of the globe of the eye, will result in diplopia. However, if the disturbance subsequently remains stable or if a disturbance is of very gradual onset, the brain will compensate for the problem and eliminate the diplopia, at the expense, however of the loss of perception of depth.
Parallel movements of the two eyes to maintain focus on an object, which is moving relative to the eyes are called conjugate movements. Saccades are fast movements of the eyes allowing rapid refixation of gaze from one object to another. Cortical, vestibular and brainstem pathways are involved in these movements.
What is open-angle glaucoma?
Open-angle glaucoma, the most common form of glaucoma, accounting for at least 90% of all glaucoma cases:
- Is caused by the slow clogging of the drainage canals, resulting in increased eye pressure
- Has a wide and open angle between the iris and cornea
- Develops slowly and is a lifelong condition
- Has symptoms and damage that are not noticed.
“Open-angle” means that the angle where the iris meets the cornea is as wide and open as it should be.
What is angle-closure glaucoma?
Angle-closure glaucoma, a less common form of glaucoma:
- Is caused by blocked drainage canals, resulting in a sudden rise in intraocular pressure
- Has a closed or narrow angle between the iris and cornea
- Develops very quickly
- Has symptoms and damage that are usually very noticeable
- Demands immediate medical attention.
It is also called acute glaucoma or narrow-angle glaucoma.
