Introduction to the nervous system Flashcards
What are the layers of the eye?
- The fibrous layer (outside) composed of:
- The sclera
- The cornea - The vascular layer
- The iris
- The ciliary body
- The choroid - The sensory coat
- The retina
Explain the Fibrous layer of the eye
- Muscle attach to the fibrous layer
- It is made of collagen which is tidily arranged so that it is transparent.
- This layers doesn’t have any blood vessels in it as they could burst and impede seeing
Explain the vascular layer of the eye
- The Iris: controls the diameter of the pupil and controls the amount of light rays let into the eye
- The ciliary body: suspends the lens and produces aqueous humor
- The choroid: supplies blood to the outer layers of the retina
- The vascular layer is incomplete as it doesn’t cover the pupil
- This layer has a lot of blood vessels
Explain the sensory coat of the eye
- This is the part with rods and cones
- It is where the image forms
What is the structure and function of the eyelids?
• The eyelids protect they eye and are composed of several layers:
- Skin (thin hair and sebaceous glands)
○ Eyelashes with sebaceous glands
- Mucous membrane (conjunctiva)
- Subcutaneous tissue
- 2 layers of muscle: laviatorpalpatorsuperiorus (LPS) and the orbicularis oculi
- Hard palate (tarsal plate): maintains shape
What is the structure and function of the conjunctiva?
- Fornix: angle
- Does not continue over cornea
What is the structure and function of the Lacrimal apparatus?
- Supratemporal part of eye
- Produces fluid to keep the eye moist
- Drains into the lacrimal sac when blinking
- It is then carried through the nasolacrimal duct
Describe the development of the eye
• The eye is an extension of the brain and so is developed from the neurotube
• The specific part of the neurotube it develops from is the diencephalic vesical of the neurotube
- First they grow outwards latterly
- Then there is a lens placode (embryological thickening)
- There is then an invagination of the lens placode
- This forms the optic cup which is 2 layered
- The choroid fissure forms, it will eventually close up
Describe the chambers of the eye
• Posterior chamber: Chamber behind lens
• Anterior chamber: chamber in front of lens
- Posterior segment: posterior part of the anterior chamber (behind the iris)
- Anterior segment: anterior part of the anterior chamber (in front of iris)
• The anterior chamber contains the aqueous humor
• The posterior chamber contains the vitreous humor
Explain the functions of the aqueous and vitreous humor
- The aqueous humour is a watery substance secreted by the ciliary body and helps maintain the pressure of the eyeball (intraocular pressure) at ~21mmHg by being constantly produced, circulated and absorbed
- The Vitreous humor is a gel that pushes the retina back as well as cushions it
Describe the bony orbit
- Frontal bone
- Sphenoid bone (greater and lesser wing)
- Zygomatic bone
- Maxillary bone
- Palatine bone (a tiny bit)
- Lacrimal bone
- Ethmoid bone
- Optic foramen
- Supraorbital fissure
- Inferior orbital fissure
- Inferior orbital groove
• The nasal and superior walls of the orbit are particularly week
• There is a large amount of fat behind the eyeball that cushions the eye
What are the intrinsic muscles of the eye?
- Constrictor pupilae (parasympathetic) (in the iris)
- Dilator pupilae (sympathetic) (in the iris)
- Ciliaris muscle (in the ciliary body) (CN III, parasympathetic)
What are the extrinsic muscles of the eye?
- Medial rectus (MR)
- Lateral rectus (LR)
- Superior rectus (SR)
- Inferior rectus (IR)
- Superior oblique (SO)
- Inferior oblique (IO)
What are the nerves of the eye?
• SO 4, LR 6 and everything else by 3
- CN IV: Trochlea nerve - CN VI: abducent nerve - CN III: Oculomotor - CN II: Optic nerve
Describe the histological features of the cornea
- Non-keratinised stratified squamous epithelium
- Bowman’s membrane
- Stroma (regularly arranged collagen, no blood vessels)
- Descemet’s layer
- single layered endothelium
What are the layers of the tear film?
- Mucinous layer overlying the corneal epithelium
- Aqueous layer lies over the mucinous layer
- Oily layer is the most superficial
What are the functions of the tear film?
- Keeps cornea moist, prevents dying
- Washes away foreign bodies
- Has antibodies and lysosomes to kill microbes
- Smooths out surface of cornea providing a smooth surface for refraction
What are the dynamic of the tear film?
- During a blink, the sharp lower boarder of the upper eyelid distributes the tear film evenly
- When the eyelids are open the aqueous component of tears begin to evaporate, the oily layer comes close to the mucin layer
- When these 2 layers touch, the tear film breaks up and stimulates further blinking
Explain the maintenance of corneal transparency
- Histologically: the regular arrangement of the collagen fibres
- No blood vessels
- The endothelium cell layer has a pump that actively keeps the aqueous humor out as it is constantly trying to get into the stoma which would disrupt the regular arrangement of the collagen fibres
Explain lens transparency with a note on aging
- Avascularity keeps the lens transparent
* The avasuclarity is one of the factors that predisposes it to becoming opaque later in life (cataracts)
Explain the blood aqueous barrier of the eye
- Selectively permeable
- Formed by the nonpigmented layer of the epithelium of the ciliary body and the endothelium of the blood vessels of the iris
Explain the blood retinal barrier of the eye
- The physiological barrier that regulates ion, protein and water flux into and out of the retina
- The inner BRB is formed by tight junctions between the retinal capillary endothelial cells
- The outer BRB is formed by tight junctions between the rental pigment endothelial cells
