Anatomy and Physiology Flashcards
Lacrimal system: Tear film
3 layers of tear film:
- Outer lipid layer
- secreted: Tarsal glands
- function: Retards evaporation of the aqueous layer
Prevent tears from spilling over the edge - Middle Aqueous layer
- secreted: Lacrimal glands - Inner Mucin layer
- secreted: Conjunctival goblet cells
- function: Hydrophilic surface on which aqueous layer can be spread
Components of the visual system (8)
- Eye: External ocular layers and intraocular structures
- Ocular adnexa: Eyelids and related structures
- Orbit: Bony cavity and extraocular structures
- Visual pathways from retina to occipital cortex
- Cranial nerve 2, 3 , 4, 5, 6, 7
- Sympathetic and parasympathetic supply
- Visual association areas
- Areas of recognition and interpretation of visual environment
3 external layers of the globe
OUTER: fibrous layer 1. Conjunctiva 2. Sclera 3. Cornea MIDDLE LAYER: Vascular-muscular and pigmented layer 1. Iris 2. Ciliary body 3. Choroid INNER LAYER: Neural layer 1. External: Retinal pigment epithelium 2. Internal: Neuroretina internally (photoreceptors)
Light pass through all layers of neuroretina to reach photoreceptors
3 Internal zones
- Aqueous
- Lens
- Vitreous
Two segments of the globe
ANTERIOR SEGMENT:
- Cornea
- Anterior chambers
- Iris
- posterior chamber
- Cililary body: Pars plana and Pars Plicata
- Lens
- Zonules
POSTERIOR SEGMENT:
- Sclera
- Choroid
- Retina
- Optic disc
- Vitreous humour
Orbital contents
- Eyeball
- Fat
- Muscle
Orbital blood supply
Arterial supply: Ophthalmic artery
Venous supply: Orbit → Superior and inferior ophthalmic veins→ cavernous sinus.
(Cavernous sinus thrombosis)
Veins draining skin–> Superior ophthalmic vv–> cavernous sinus
Orbital apex
Entry and exit of:
- Nerves to and from the eye
- Vessels to and from the eye
- Site of origin of all the extraocular muscles except IO, which arises inferonasally just behind the orbital margin.
Conjunctiva
Characteristics - Thin transparent mucus membrane - Contains: bv Distribution - Bulbar - Forniceal - Palpebral Continuous - Skin lid margin - Corneal epithelium- limbus
Sclera
Characteristics:
- Collagen Fibres
- Relatively Avascular: Pierced by bv and nn entering and leaving the interior of the eye,
- White of the eye
Distribution
- 4/5 tough, outer protective coating
Continuous
- Anteriorly: Cornea at the limbus
- Posteriorly: Dural sheath of the optic nerve
Cornea
Characteristics
- Avascular
- > Nerve supply
- Optically transparent.
- 5 Layers
Function:
- Refracting element
Uvea
- Iris
- Where: Anterior extension
- What: Pigmented
- Function: pupil size - Ciliary body: Pars plana and pars plicata
- Function: Ciliary muscle–> accommodation
Aqueous secretion - Choroid
- Where: Located posteriorly
- Function: Blood supply
Retina
Characteristics:
- Inner aspects of globe
- Thin multi-layered neural tissue
Layers:
- Ganglion cell layers and bipolar cells
- Interneuron
- Photoreceptors: cones and rods
- Retinal Pigment epithelium
Blood supply (choroidal and retinal):
Arterial: through optic canal→ Internal carotid aa→ Ophthalmic branch→ Central retinal aa that supply retina & Long and short Ciliary aa that supply uvea
Venous: Vortex vv, Anterior Ciliary vv, Central Retinal vv→ Superior Ophthalmic vv and Inferior Ophthalmic vv→ Cavernous sinus
Nerve: through optic canal→optic nn
Retinal structures visible with direct ophthalmoscopy 1. Optic nerve and cup 2. Central retinal aa + branches 3. Central retinal vv + branches 4. Neural retina Macular Lutea with foveal light reflex
Vitreous humour
Characteristic: - Transparent - Avascular - 98 % water Insertion and attachments: - Ora Serrata - Optic nn head - Macula - Arcade vessels
Lens: Anatomy of the lens
The lens consists of three transparent parts:
- Capsule: a thin outer membrane.
- Cortex: a soft layer under the capsule.
- Nucleus: a harder central core that only develops later and is not present in children and young adults.
Lens: Blood and nerve supply of the lens
Does not have a nerve or blood supply. Nutrients diffuse into the lens from the aqueous and the vitreous humour.
Lacrimal drainage system: anatomy
- Lacrimal sac
- Inferior punctum
- Inferior canaliculus
- Inferior turbinate
- Nasalacrimal duct
Virteous humour apearance and consistance
Transparent gel which fills the posterior segment of the globe.–> Water 99% to weight and a network of collagen fibres is responsible for its gel-like character.
Vitreous humour in contact with
Encircled by the hyaloid membrane–>in contact with:
- posterior lens capsule
- zonules
- pars plana epithelium
- retina ( adhesion between posterior hyaloid membrane and retina is loose, bond stronger @ larger retinal blood vessels.)
- optic disc.
Retina: Optic disc
Consists: Nerve fibres from sclera that exit at this point Blind spot (insensitive to light): No retina and choroid
Muscles of the eye
- Rectus: superior, inferior, lateral, medial
- Oblique: Superior, Inferior
- Trochlear
Eyelid
Grey line:
- divides lid margin into anterior and posterior parts.
- marginal subcutaneous fibres of orbicularis oculi.
- Anterior: eyelashes (cilia)
- Posterior: orifices of the tarsal (Meibomian) glands
- mucocutaneous junction: behind the orifices of the tarsal glands
Eyelid: closure
- Orbicularis oculi: facial nerve (VII)
2. elliptical sphincter muscle: facial nerve (VII).
Cornea: Layers of the cornea
From the exterior inwards, the cornea consists of 5 layers:
1. Epithelium: 5-6 layers cells and bm and is continuous with the epithelium of the bulbar conjunctiva.
2. Bowman’s layer: Dense layer of modified stroma.
3. Stroma: 90% of the corneal thickness, and consists of lamellar fibres that run parallel to the corneal surface.
The size and periodicity of these fibres play an important rôle in the optical clarity of the cornea.
4. Descemet’s membrane: clear, elastic, basement membrane of the endothelium.
5. Endothelium: single layer of cells covering the posterior surface of the cornea.
Aqueous humour: secretion
Active secretion by the epithelium of the ciliary processes into the posterior chamber.
Aqueous humour: Circulation
Flow is through the posterior chamber and pupil into the anterior chamber.
Aqueous humour:Drainage
Exit from the anterior chamber is via the trabecular meshwork in the iridocorneal (drainage) angle. The fluid then enters the sinus venosus sclerae (canal of Schlemm), from which it drains into the episcleral veins.
Anatomy: Orbit
- Pear shaped bony cavity with the optic canal as the stalk of the pear.
- Paranasal sinuses lie inferior+ medial; lateral is the temporal fossa and superior the cranial cavity.
- Orbit lies behind orbital septum and has a roof, a floor, medial and lateral walls and an apex.
- Orbital conditions: adjacent structures in
addition to the structures that lie within the orbit.
Anatomy: SPHINCTER PUPILLAE LIGHT REFLEX & ACCOMMODATION
The light reflex pathway for direct and consensual light reflexes, consists of 4 neurons:
- Afferent neurons from retinal ganglion cells synapse in the pretectal nucleus in the midbrain.
- Nasal fibres decussate in the optic chiasm to reach the contralateral pretectal nucleus.
- Temporal fibres do not decussate and terminate in the ipsilateral pretectal nucleus. In this way impulses from each eye reach both sides of the brain. - Intercalated neurons from each pretectal nucleus synapse in the Edinger-Westphal nucleus on both sides–> second decussation of pathway: light stimulus from each eye stimulates both pupils equally.
- Edinger-Westphal parasympathetic outflow runs in III to synapse in the ciliary ganglion.
- Ciliary ganglion efferent fibres run to the iris sphincter.
The stimulus for the iris sphincter response to accommodation originates in the neocortex, flows to the Edinger-Westphal nucleus, and then follows the same path as the light reflex.
Anatomy: NEAR REFLEX
Triad of:
- Accommodation
- Convergence
- Miosis
Anatomy: DILATOR PUPILLAE
Path Sympathetic fibres:
- posterior hypothalamus
- through brainstem
- spinal cord
- exit @ C8-T2
- enter the sympathetic chain
- ascend to the superior cervical ganglion
- join carotid plexus to run with internal carotid artery to cavernous sinus–> to the orbit and eye
Lacrimal system: tear film physiology
Structure unstable–> stabilized by methods:
- renewed by blinking- need normal 3 layers of tear film
- Corneal epithelium surface must be smooth and normal
- Good eyelid apposition
- Good contact of eyelids and anterior surface of eye
Irregular areas–> unstable tear film–> dry spots
- tear film turbulence–> mixing of layers
- mucin adhesion
Functions of the visual systems
- High resolution:
- Dependent upon an accurate focal system
- > Photoreceptor density in the macular and foveolar areas. - Wide field:
- Made possible by the optical configuration of the eye
- > In field which results from binocularity. - Ocular movement:
- Binocular single vision: Coordinated movement needed for a fluid visual experience and maintaining alignment of the eyes to enable the brain to fuse similar images into a single image. - Stereopsis:
- Sense of depth dependent upon binocular single vision and complex processing by higher centres. - Colour representation:
- Dependent on photoreceptors responding to different wavelengths of light. - Integration:
- Accomplished by wide variety of coordinated and complex neural mechanisms to provide a seemingly continuous and cohesive visual experience. - Interpretation:
- Recognition and association
Functions of the retina
- Fovea
- Central visual acuity
- Colour vision - Optic nerve
- Physiological blind spot - Photoreceptors
- Rods: Rhodopsin for night vision
- Cones: Colour photopigments - RPE
- Regenerate photoreceptors - Peripheral retina
- Motion
- Night vision
Refraction
Definition: Bending of light as it cross interface between 2 media of different optical densities
Refraction of the eye:
Light entering eye→ reflected by cornea (> refractive power)→ refracted by lens (< refractive power
Emmetropia
Definition: Perfect refraction condition of the eye→ Refractive power of eye and length of eyeball matched→ parallel rays of light from distant object are focused on retina when ciliary mm relaxed→ Clear image of object on retina
Accommodation
Definition: Refractive power of lens > in order to focus image of nearby object on retina
Parasympathetic stimulation→ Contraction of circular ciliary mm→ Relaxation of zonules→ Lens become convex
Sympathetic stimulation→ Relaxation of circular ciliary mm→ Contraction of zonules→ Lens become concave
Refraction errors
Ametropia:
Myopia
Hypermetropia
Astigmatism
Function of the lens
Cornea and lens: main refractive surfaces of the eye–> focusing a clear image on the retina.
Lacrimal drainage system physiology
Blinking–> tears to flow to the medial canthus –> siphoned into the canaliculi–> squeezed into the lacrimal sac–> They flow down the nasolacrimal duct and into the nose.
Function of lid
The primary purpose of the eyelids is to keep the cornea moist and so maintain its transparency.
The oily secretion of the tarsal (Meibomian) sebaceous glands
- forms a lipid layer on the eyelid margin–> preventing tears from spilling over the edge of the eyelid.
- forms outer lipid layer of the precorneal tear film which retards the evaporation of tears.
Pathology of the Cornea
malfunction of the endothelium –> corneal oedema–> swelling and loss of transparency of the stroma–> > intraocular pressure rises–> difficult for the endothelial pump to pump water out of the stroma and into the anterior chamber against the increased pressure gradient–> > IOP –> corneal oedema–> acute angle closure glaucoma
Cornea function:
- Transparent–> uniform structure, avascularity and state of relative dehydration of the stroma: Allows light to enter the eye and fall on the retina.
- Its anterior surface–> covered by the tear film, is a powerful convex lens.
- Nutrition: Limbal blood vessels, aqueous humour and tears.
- Receive oxygen from atmospheric oxygen dissolved in the tear film.
- Active pump mechanism in the endothelium: transports water from the stroma –> anterior chamber
- -> Dehydration
Aqueous humour functions
- Nutrition, transport of waste products, buffering and thermoregulation of the surrounding avascular ocular structures.
- Optical medium for the conduction of light.
- Maintenance of IOP
The intraocular pressure is determined by the equilibrium between the rates of secretion and drainage of aqueous. The mechanisms that control the intraocular pressure are not completely understood, although they are known to be influenced by a number of physiological factors. The rôle of the vitreous humour is passive, serving only to transmit the prevailing intraocular pressure to all parts of the eye.
Physiology: Retina
- The fibres of the superficial nerve fibre layer lie parallel to the retinal surface.
- The elements of the deeper retinal layers lie parallel to the incident light.
- The photoreceptors are in the wrong position. For the best image they should lie on the retinal
surface. This anomaly can be explained by the evolution of the retina. - The retinal blood vessels supply only the inner retina.
The choriocapillaris supplies the outer retina, including the photoreceptors.
ANATOMY: VISUAL PATHWAY
Visual pathway:
- Retinal photoreceptors synapse indirectly with retinal ganglion cells.
- Axons of retinal ganglion cells form the optic nerve.
- Nasal fibres decussate in the optic chiasm to reach the contralateral lateral geniculate nucleus via the contralateral optic tract.
- Temporal fibres do not decussate but run in the ipsilateral optic tract to terminate in the ipsilateral lateral geniculate nucleus.
- Optic radiation runs from the lateral geniculate nucleus to the ipsilateral visual cortex.
Anatomy: Parts of the Uvea
The uvea is the pigmented vascular layer of the eye. It consists of 3 parts: 1. Iris: Anterior uvea 2. Ciliary body: Anterior uvea 3. Choroid: Posterior uvea
Anatomy: Layers of the conjunctiva
The conjunctiva is a mucous membrane which consists of two layers:
- Epithelium: 2-5 layers of nonkeratinising cells.
- Stroma: richly vascularised connective tissue under the epithelium.
Anatomy: Lymphatic drainage of the conjunctiva
To the preauricular and submandibular lymph nodes.
Anatomy: Clinical subdivisions of the conjunctiva
3 Parts:
- Palpebral conjunctiva: Extends from mucocutaneous jx –>on lid margin –>fornix. Lines the ocular surface of the eyelid.
- Forniceal conjunctiva/fornix: The fold where the conjunctiva is reflected from the eyelid onto the eyeball.
- Bulbar conjunctiva: from fornices–> corneal limbus. Covers the anterior part of the sclera.
NERVE SUPPLY OF THE EXTRAOCULAR MUSCLES
Superior oblique: IV Trochlear
Lateral rectus: VI Abducens
All others: III Oculomotor
Remember that III is responsible for 3 other important muscles
- Levator palpebrae superioris
- Sphincter pupillae via contained parasympathetic fibres which form the efferent pathway for the pupillary reflex.
- Ciliary muscle via contained parasympathetic fibres for accommodation.
Anatomy: Optic nerve Origin
Retinal ganglion cells –> give rise to axons that lie on the internal retinal surface–> form nerve fibre layer–> axons converge on the optic disc, the first or intra-ocular part of the optic nerve, which is situated nasal to the macula.
Anatomy: Optic nerve course
axons leave the eyeball–> through lamina cribrosa sclerae, an area of sclera with small perforations–> posteromedial within the extraocular muscle cone–> leaves orbit–> optic canal–> enters middle cranial fossa
Anatomy: Optic nerve termination
After a short intracranial course, the two optic nerves join to form the optic chiasm.
Anatomy: Optic nerve Misnomer
The optic nerve differs from all other cranial and spinal nerves in that it is not a peripheral nerve, but a central nervous system tract. It has no ability to regenerate if it is damaged.
Anatomy: Optic nerve parts
4 Parts: Ocular bulbar part: 1. Optic Disc Retrobulbar part 2. Intrabulbar part 3. Intracanucular part 4. Inter cranial part
Anatomy: Optic nerve blood supply
Retrobulbar part:
Supplied by a free and complex anastomosis of various branches of the internal carotid artery, rendering it relatively resistant to vascular occlusions.
Optic disc:
Supplied nearly exclusively from the short posterior ciliary branches of the internal carotid artery, rendering it more susceptible to vascular occlusions.
Anatomy:Aqueous humour
Secretion
Circulation
Drainage
Cornea
SECRETION
Active secretion by the epithelium of the ciliary processes into the posterior chamber.
CIRCULATION
Flow is through the posterior chamber and pupil into the anterior chamber.
DRAINAGE
Exit from the anterior chamber is via the trabecular meshwork in the iridocorneal (drainage) angle. The fluid then enters the sinus venosus sclerae (canal of Schlemm), from which it drains into the episcleral veins.
THE CORNEA
The transparency of the cornea is dependent, dehydration. The latter is maintained mainly by an active pump mechanism in the endothelium, which pumps fluid from the corneal stroma into the anterior chamber.
Physiology: Vitreous functions
- Nutrition, transport of waste products, buffering and thermoregulation of the surrounding avascular ocular structures.
- Optical medium for the conduction of light.
- Maintenance of IOP
equilibrium between the rates of secretion and drainage of aqueous.
Mm of the eye Origin insertion innervation Function
Levator Palpabrae Superioris Origin: Lesser wing of the spenoid insertion: Superior tarsus and skin of the superior eye innervation: CN III Function: Elevates eyelid
Superior oblique
Origin: Sphenoid body
insertion: Trochlea to sclera deep to superior rectus
innervation: CN IV
Function: Abducts. depress, Medial rotation
Inferior oblique Origin: Anterior orbital floor insertion: Sclera deep to lateral rectus innervation: CN III Function: Adducts, Elevate, Lateral rotate
Superior rectus
Origin: Common tendinous ring
insertion: Sclera posterior to cornealsclearl jx
innervation: CN III
Function: Elevates, Adducts, Medial rotates
Inferior rectus
Origin: Common tendinous ring
insertion: Sclera posterior to cornealsclearl jx
innervation: CN III
Function: Depress, adduction, Lateral rotation
Medial rectus Origin: Common tendinous ring insertion: Sclera posterior to cornealsclearl jx innervation: CN III Function: Adducts eyeball
Later rectus Origin: Common tendinous ring insertion: Sclera posterior to cornealsclearl jx innervation: CN VI Function: Abducts eyeball
Ophthalmic aa branches
- Central retinal aa
- Short posterior cilliary aa
- Long posterior cilliary aa
- Lacrimal aa
- Supra-orbital aa
- Anterior and posterior Ethmoidal aa
- Dorsal nasal aa
Physiology: Retina layers
- Ganglion cells
- Amacrine cells
- Bipolar cells
- Horizontal cells
- Photoreceptors: Rods and cones
- RPE
Physiology: Transduction in rods–> no light
- Rhodopsin inactive
- Channels open: CGMP, CNG, K pump
- Na and ca comes in
- Cells hyperpolarize–> +
- Ca channels activated
- NT release glutamate
Physiology: Transduction in rods–> Light
- Rhodopsin bleached–> Retinal part jump to RPE
- opsin activated
- CGMP and CNG channels close
- K channel opened
- Na and Ca does not come in
- Cell hyperpolarizes–> -
- NT release < Glutamate
Physiology: Colour vision
Visual pigment of cones stimulated differently by different light wavelengths Colours: Blue: short Green: Medium Red: Long
> Intensity light: Stimulate cones
< Intensity light: stimulate rods
Physiology: Pupil constriction
Sympathetic: Dilate–> Radial pupil mm
Parasympathetic: Constrict–> Circular pupil mm
Physiology: Neural pathway
Eye–> retina–> optic isc–> optic chism–> optic tract–> lateral genucular body–> visual cx
Physiology: Processing of light
Bipolar cells: Light on: Glutamate inhibited Active in light on Inhibit in lights off Light off: Glutamate stimulated Active in lights off Inhibited in lights on
eyelid opening
- Levator palpebrae superioris lifts the upper lid: oculomotor nerve (III).
- Superior tarsal muscle of Müller also plays a rôle in lifting the upper lid: sympathetic system.
