Layers of Eye : NEET PG Ophthalmology

 The three layers, namely the sclera, the choroid (the posteriorly extending choroid; extending anteriorly as ciliary body and iris), and the innermost layer which is the retina, can also be seen. Volume of eyeball /globe: 6 ml.  Volume of each orbit: 30 ml  It is Encased in Tenon’s capsule, suspended by orbit and supported by Lockwood’s ligament

Read this blog further to get a quick overview of this important topic for ophthalmology and ace your NEET PG exam preparation.

Three Layers of Eye are

•   Outer fibrous
•   Middle vascular  
•   Inner neural layer 

Outermost Fibrous Layer

It consists of :

Anterior 1/6th: Cornea Clear & transparent

Posterior 5/6th: Sclera Opaque & White

Junction b/w cornea and sclera is known as Limbus

Sclera

It Maintains shape of the eye. It is Site for attachment of Extra ocular muscles (EOM)

Thinnest part of sclera lies behind the attachment of rectus muscles, thickest part of sclera is at posterior pole.

3 layers of Sclera :

1. Episclera 
2. Stroma 
3. Lamina fusca 

Sclera is White and opaque due to irregular arrangement of stromal collagen. Glistening and shiny, tough structure. It appears yellow (icteric) in jaundice.

Cornea

Shape of cornea is like Prolate spheroid. It is More curved in centre than periphery.  Cornea is Transparent , Due to regular stromal collagen and presence of Na+-K+ ATPase pump in endothelium layer.90% light transmission occurs through cornea. Keratometer measures the cornea curvature and the technique is known as keratometry. Instruments used Placido’s disc-In normal cornea, there is no distortion of reflected pattern . In Keratoconus, distortion of reflected pattern is present

6 layers of cornea  are:

1. Epithelium 
2. Bowman’s membrane -Injury to this layer leads to corneal opacity or scar
3. Stroma - it is the Thickest layer
4. Predescemet’s layer (Dua’s layer)
5. Descemet’s membrane -Strongest layer
6. Endothelium -Single layer of hexagonal layer and it Maintain corneal opacity

Thickness of Cornea 

It is Thinnest in centre & Thickest at periphery, Ranging from 500 -600 μ, It is the Most powerful refracting surface eye: 43D (70 %) Cornea is avascular. IOP depends upon central corneal thickness (CCT). It is Measured by Pachymetry

Limbus              

It is Junction of cornea and sclera Surgical limbus is zone of 2 mm . Corneal stem cells are present at limbus. It consists of Longitudinal parallel cells . it is Yellowish brown in colour . It is so known as Palisades of Vogt

In case of deficiency of stem cells due to chemical injury (mc), prolonged contact lens wearing etc. there is decreased regeneration capacity of eye.

Signs of Limbal stem cell deficiency :

• Cells breakdown
• Inflammation
• Persistent epithelial defects
• Ulceration
• Neovascularization
• Conjunctivalisation 

On Impression Cytology: Presence of Goblet cells on cornea (normally goblets cells are present in conjunctiva not in cornea)

MIDDLE LAYER OF EYEBALL 

Uvea                    

Consists of 

1. Iris 
2. Ciliary Body                                                                   
3. Choroid 

Iris 

 It is the Most anterior part of the uveal tract. It is Divided by collarette into central pupillary zone and peripheral ciliary zone two muscles of iris are Sphincter pupillae and Dilator pupillae.

Function - It Controls entry of light by dilating / constricting pupil . There are Two arterial circles :

Major arterial circle - It Lies at root of iris where it attaches to the ciliary body and If it bleeds leads to Hyphema

 Minor arterial circle - It Lies at collarette 

CILIARY BODY       

It has 3 parts :

• Pars Plicata: Anterior half with folds 
• Pars Plana: Posterior half without folds 
• Ciliary Muscle: Enclosed by Pars Plicate & Pars Plana 

Root of the iris is attached to the ciliary body. It is the Thinnest part of iris.its Rupture leads to iridodialysis. Characteristic D-shaped pupils are seen in Iridodialysis. Its function is 

Aqueous humour production, Accommodation. Pars plana only way to access vitreous humour Maximum risk of sympathetic ophthalmitis occurs on injury to ciliary body.

CHOROID    

It is the Posterior most structure of uvea. It is Most vascular structure of eye. Choroid circulation gives 85% of ocular blood flow. Vortex Veins Drains entire uvea but particularly choroid)

Function - It Supplies nutrients to outer retina, and helps in Thermoregulation

4 layers  of choroid are:

• Chorio-capillaries layer
• Bruch’s membrane
• Haller’s layer 
• Sattler’s layer

Ophthalmology Related articles:

Mechanical Trauma: Chemical Injuries of EYE	Uveitis - Symptoms and Causes - NEET PG Ophthalmology	Abnormal Pupils - NEET PG Ophthamology
NEET PG : High-Yield Topics for Ophthalmology	How to Prepare Ophthalmology for PGMEE	Allergic Conjunctivitis (Treatment, causes)
Cornea: Structure, Conditions and Types

INNER MOST LAYER OF EYEBALL

Retina     

It is the Innermost layer stretches from central fovea to peripheral ora serrata which is Present only posteriorly. Central retina consists of  Macula, fovea, optic disc Centre of retina occupied by Macula Centre of Macula Occupied by Fovea Fovea is most sensitive structure to light so brightest and sharpest image forms here.

Microscopic Structure of Retina 

 It has Neurons of  5 Types .Neuroglia [supporting cells] of 3 Types 

Neuron Types 

1. Photo Receptors (Rods/Cones) (1st Order neuron)
2. Bipolar Cells (2nd Order neuron)
3. Ganglion Cells (3rd Order neuron) 
4. Amacrine Cells 
5. Horizontal Cells

Neuroglia cells 

1. M - Muller’s  
2. A – Astroglia
3. M – Microglia

Photoreceptors

Rods	Cones
Peripherally located 120 million in number 1 type Responsible for Peripheral vision  Black & white vision  Night Vision	Centrally located 6 million in number Red, Green & Blue types of cones + Responsible for Central Vision Colour Vision Daylight vision

This is everything that you need to know about layers of eye for your ophthalmology preparation.. For more interesting and informative blog posts like this download the PrepLadder App and keep reading our blog!