Structure and function of the eye Flashcards
What are the inner and outer corners of the eye referred to as?
Medial and lateral canthus
What name is given to the border between the cornea and the sclera?
Corneal limbus
What is the clinical significance of the corneal limbus?
Common site for corneal epithelial neoplasm
Describe the nervous innervation of the cornea
Highly sensitive
Ophthalmic branch of trigeminal nerve
Describe the nerve supply to the lacrimal gland
PNS-mediated
CNV
Recall the drainage of tear film
Tear film: lacrimal gland, movement accross eye = superolateral to inferomedial, drainage through puncta to superior/ inferior canaliculus –> lacrimal sac –> nasolacrimal duct
Recall 3 functions of tear film
- Bactericide
- O2 and nutrient supply to ant. seg
- Smooth surface mainenance
Recall the layers of tear film from deep to superficial
Mucous, thick aqueous, oily
Describe the fx of each layer of tear film
mucous: protect eye surface, adhere tear film to eye
Thick aqueous: lubricant, bactericide, nutrition
Oily: prevention of evaporation
Where is the mucous layer of tear film produced?
Goblet cells in conjunctiva
Where is the oily layer of tear film produced?
Meibomian glands within eyelids
What sign in the eyes is indicative of intermittent hypertension?
Rupture of tiny vessels in conjunctiva
What surrounds the retina?
Uvea
What is the uvea made up of?
Predominantly vascular
- Choroid
- Ciliary body
- Iris
What does the choroid lie between?
Retina and sclera
What is the choroid made up of?
Vasculature supplying posterior segment of eye
What is the main component of the sclera?
Collagen
Recall and describe the most significant osmotic gradient that is maintained in the eye
Sclera = high H2O, cornea = low H2O
Sclera has protective fx
Cornea must remain transparent
How is the transparency of the cornea maintained?
Corneal endothelium actively removes water
In emmetropia, what provides the refractive power of the eye?
Cornea = 2/3 of power, lens = 1/3
Recall one advantage and one disadvantage of the barrier function of the cornea?
Ad: prevents infection
Disad: prevents drug entry
What are the 5 structures that make up the cornea, working superficial to deep?
Epithelium Bowman's membrane Stroma Descemet's membrane Endothelium
Which element of the cornea cannot regenerate?
Endothelium
What characteristic of the lens makes it transparent?
Regular structure
Recall the pathophysiology of age-related cateracts
Hydration of lens –> loss of transparency
Describe the shape of the lens
Aspheric: anterior and posterior surfaces have differing curvature
Recall the attachments of the lens
Attached all around by zonules/ lens zonulae, anchored to ciliary body
Recall what happens when the ciliary muscles contract?
Pressure on zonules decreased, lens gets smaller and thicker
Recall what happens when the ciliary muscles relax?
Pressure on zonules increases, lens gets wider and thinner
How can the optic nerve be identified with an ophthalmoscope?
Optic nerve head appears as bright optic disc
Where is the macula?
Roughly in centre of retina
Where is the aqueous humour found?
Anterior segment of eye (in front of lens)
Where is the vitreous humour found?
Posterior segment of eye (behind lens)
What is the fx of aqueous humour?
Nutrient and oxygen delivery
Recall the flow of aqueous humour
Ciiary body –> anterior chamber –> trabecular meshowrk (lies in limbus) –> Schlemm’s canal/ uveal-scleral flow
Describe the 2 methods of aqueous humour drainage
- Schlemm’s canal = modified vein
2. Uveal-scleral flow = passive, PG analogues act here
Describe the composition of vitreous humour
99% water, collagen, GAGs
What is the fx of vitreous?
Mechanical support
What is a normal IOP?
12-21mmHg
Recall the names of the layers and sublayers of the retina
Layers: retinal pigment epithelium and neuroretina
Sublayers of neuroretina: outer/ middle/ inner
Recall the composition of each of the layers of neuroretina
Outer: photoreceptors
Middle: bipolar cells
Inner: ganglion cells
Recall 2 functions of the retinal pigment epithelium
- Waste removal
2. Transport nutrients from choroid to photoreceptor cells
Recall the differences between rod and cone photoreceptors
Cones: colour, central vision, detail, fast response to light; n = 6 mill
Rods: B&W, peripheral vision, sense movement; N = 120 mill
What is the fx of bipolar cells?
Fine-tuning of afferent info
Where do retinal ganglion cells synapse with 4th order neurons?
Lateral genticulate nucleus
What are the 2 oritentations of retinal ganglion cell, and what is the relevance of this
- On-centre: stimulated by light falling on centre of its receptove field and inhibited by light falling on edge
- off-centre: opposite
What is the receptive field of a neuron?
Retinal space within which incoming light can alter the firing pattern of a neuron
Which type of photoreceptors have lower convergence, and what is the clinical relevance of this?
Cones = finer visual acuity as smaller receptive field
What is the main test for colour-blindness?
Ischiara test
What are the 2 possible causes for colour-blindness?
- Shift in the peak of photo-pigment sensitivity
2. Absence of 1 or more of the 3 cone photpigment subtypes
Describe the mediation of light dark adaptation
Photoreceptor bleaching –> inhibition of rods/ cones accordingly (deending on whether it is light –> dark or dark –> light)
Which type of photoreceptor adapts to changes in light level more quickly?
Cones
What name is given to perfect vision?
Emmetropia
What is the mechanical problem in visual impairment?
Parallel lens converges to a point anterior/ posterior to the retina, or to more than one point
What term is used to describe convergence of the parallel lens anterior to the retina?
Myopia (short-sightedness)
What term is used to describe convergence of the parallel lens posterior to the retina?
Hypermetropia (long-sightedness)
What sort of glasses are used to correct myopia?
Concave
What general term is given to eyes that have a refractive error?
Ametropia
What is astigmatism?
Parralel lens focuses on not one point but two
What causes astigmatism?
Elliptical cornea
What term is given to a naturally-occuring loss of accommodation?
Presbyopia
What causes presbyopia?
Stiffening of lens
