General overview
EYEBALL
Ciliary body and some Choroid
Choroid and lens
and pathway
fovea macula and Retina
Optic disk and nerve
100

How many functional parts does every analyser consist of and describe each of them?

3 parts:
-Peripheral(Receptor) Conducting(Intermediate) and Central(Cortical) parts

Peripheral (Receptor) Part

  • Located at the level of sense organs.

  • Detects a specific type of environmental stimulus.

  • Converts physical/chemical energy into electrical signals (nerve impulses).

  • Can amplify, filter, or modify information before sending it to the CNS.

  • Conducting (Intermediate) Part

    • Formed by afferent and efferent neural pathways.

    • Carries impulses from receptors → CNS (cortex) and sometimes from the CNS to effectors.

    • Includes several relay stations:

      • Spinal cord nuclei

      • Brainstem nuclei

      • Subcortical centers (thalamus)

    • Afferent signals undergo processing, modulation, or inhibition before reaching the cortex.

    • Central (Cortical) Part

      • Located in specific cortical regions of the cerebral hemispheres.

      • Responsible for final perception, interpretation, comparison with memory, and forming a behavioral response.

100

Describe SCLERA

  • Dense connective tissue

  • White, opaque

  • Thickness:

    • thickest: near optic nerve (1.2–1.5 mm)

    • thinnest: near muscle insertions (0.3–0.4 mm)

Composition:

  • dense collagen fiber bundles

  • fibroblasts

  • elastic fibers

  • very few blood vessels

Functions:

  • maintains eye shape

  • protects inner structures

  • provides muscle attachment

  • contributes to intraocular pressure stability

100

What is the ciliary body

The ciliary body is a thickened anterior portion of the vascular tunic, located between the iris and the choroid.
It plays three critical roles:

  1. Accommodation (focusing of the lens)

  2. Production of aqueous humor

  3. Anchoring and tensioning the lens via the zonular fibers

100

Lens

  • Transparent, biconvex, enclosed in capsule

  • Subcapsular epithelium → lens fibers

  • Lens fibers elongate, lose nuclei/organelles, filled with crystallins

  • Anchored by zonula fibers to ciliary body

  • Shape changes allow accommodation:

    • Convex → near vision

    • Flattened → distant vision

Age-related changes: Reduced elasticity → presbyopia; accumulation of pigments → cataract

100

Optic disk and optic nerve

Optic Disk (“Blind Spot”)

  • area where ganglion cell axons leave the eye

  • no photoreceptors → insensitive to light

  • entrance point for central retinal vessels

7.2. Optic Nerve

  • formed by 1 million+ ganglion cell axons

  • surrounded by meningeal sheaths (dura, arachnoid, pia)

  • transmits visual information to the brain

Clinical relevance:

  • increased intracranial pressure → papilledema (swelling of optic disk)

200

Types of receptors and describe each receptors

Mechanoreceptors

Respond to mechanical forces:

  • Hearing receptors (cochlea)

  • Balance receptors (vestibular apparatus)

  • Touch receptors in skin

  • Joint + muscle proprioceptors

  • Baroreceptors (blood pressure)

2.2. Chemoreceptors

React to chemical molecules:

  • Taste receptors

  • Olfactory receptors

  • Carotid body chemoreceptors

  • Blood chemistry sensors

2.3. Photoreceptors

  • Located in the retina

  • Detect light waves


    • Detect light waves

    • Rods → low light, shape

    • Cones → color vision

  • 2.4. Thermoreceptors

    • In skin and internal organs

    • Detect temperature changes

  • 2.5. Nociceptors (Pain receptors)

    • Free nerve endings

    • Sense potentially damaging stimuli

200

Describe the CORNEA

A perfectly transparent, avascular structure responsible for two-thirds of total refractive power (~40 diopters).

Properties

  • Uniformly arranged collagen → clarity

  • Rich sensory nerves → high sensitivity

  • Nourished by aqueous humor and tear film

Corneal Layers (from outer to inner)

  1. Epithelium (non-keratinized stratified squamous)

    • 4–5 layers

    • renews rapidly

    • barrier + smooth optical surface

  2. Bowman’s Membrane

    • tough, acellular

    • provides structural support

  3. Stroma / Substantia propria

    • 90% of corneal thickness

    • regularly arranged collagen fibrils



        • keratocytes

        • ground substance rich in keratan sulfate


      • Descemet’s Membrane

        • thick, strong basement membrane

        • resistant to trauma and enzymatic digestion

        • elastic properties


      • Endothelium (simple squamous)

        • maintains hydration

        • critical for transparency

200

what is the choroid

The choroid is the posterior, largest part of the vascular tunic.
It lies between the retina (internally) and the sclera (externally).

Main Functions

  • nourishes outer retinal layers (photoreceptors)

  • absorbs stray light → improves image clarity

  • contributes to temperature regulation of the retina

  • supports intraocular pressure

200

Layers of the choroid and subarachnoidal layer

From outer to inner:

1. Suprachoroid Layer (Lamina suprachoroidea)

  • thin transitional zone

  • loose connective tissue

  • melanocytes

  • elastic fibers

  • carries nerves and vessels

  • connects choroid to the sclera


  • Vascular Layer (Lamina vasculosa)

  • Choriocapillaris Layer (Lamina choriocapillaris)

  • Bruch’s Membrane (Basal complex)

200

Fotoreceptors

The human retina contains ~120 million rods and ~6–7 million cones.

3.1. Rods

  • more numerous

  • responsible for night vision (scotopic vision)

  • extremely sensitive to low light

  • contain photopigment rhodopsin

  • low spatial resolution

  • absent in fovea

Structure of Rods

  • Outer segment: stacked membranous discs containing rhodopsin

  • Inner segment: rich in mitochondria

  • Cell body

  • Synaptic terminal

3.2. Cones

  • responsible for daylight vision (photopic vision)

  • high visual acuity

  • concentrated in the fovea

300

Classification of sense organs

Primary Sensory Organs (Neurosensory)

  • Receptor is a modified neuron itself.

  • Converts stimulus directly into action potentials.

  • Examples:

    • Retina (rods and cones)

    • Olfactory epithelium

3.2. Secondary Sensory Organs (Epithelio-sensory)

  • Specialized epithelial cells act as receptors.

  • These cells synapse with sensory neurons.

  • Examples:

    • Taste buds

    • Hearing receptors (hair cells)

    • Balance receptors (vestibular hair cells)

3.3. Simple Receptors (General sensation receptors)

  • Not forming an organ; located throughout body tissues.

  • Include:

    • free nerve endings

    • encapsulated receptors

    • proprioceptors

    • visceral sensory receptors

300

The vascular tunic and IRIS

The vascular tunic, also known as the uveal tract, is the middle coat of the eyeball. It is rich in blood vessels, melanocytes, and smooth muscle fibers. This layer is essential for:

  • nourishing the retina

  • regulating the amount of light entering the eye

  • producing aqueous humor

  • controlling accommodation (focusing)

  • absorbing stray light to improve visual accuracy

  • The vascular tunic consists of three major parts:

    1. Iris (Iridial part)

    2. Ciliary Body (Corpus ciliare)

    3. Choroid (Choroidea)

      The iris is a thin, circular, pigmented diaphragm that lies between the cornea and the lens.
      At its center is an adjustable opening called the pupil, which controls the amount of light entering the eye.

      Main Functions of the Iris

      • regulates the diameter of the pupil

      • controls the intensity of light reaching the retina

      • absorbs stray light due to abundant melanocytes

      • participates in accommodation via its continuity with the ciliary body

300

Aqueous Humor Formation and Drainage

Formation

  • secreted by non-pigmented ciliary epithelium

  • composition similar to plasma but with very low protein content

  • fills:

    • posterior chamber → flows through pupil → anterior chamber

Drainage Pathway

  1. Anterior chamber angle

  2. Trabecular meshwork

  3. Schlemm’s canal

  4. Episcleral veins

Obstruction of this pathway → glaucoma, increased intraocular pressure, possible optic nerve damage.

300

Vascular layer and Choriocapillaris Layer (Lamina choriocapillaris

Vascular Layer (Lamina vasculosa)

  • contains large arteries and veins

  • numerous melanocytes

  • important for blood supply

  • houses short and long ciliary arteries

3. Choriocapillaris Layer (Lamina choriocapillaris)

  • dense network of fenestrated capillaries

  • closest to the retina

  • supplies:

    • photoreceptors

    • retinal pigment epithelium

  • has high blood flow → important for retinal metabolism

300

The Fovea and Macula

Macula Lutea (“yellow spot”)

  • region responsible for central, high-acuity vision

  • contains a high density of cones

  • minimal blood vessels → less scattering

6.2. Fovea Centralis

  • the center of the macula

  • highest visual resolution in the entire eye

  • most light-sensitive region

  • contains only cones (no rods)

  • inner retinal layers are displaced laterally → photoreceptors directly exposed to light

This design minimizes light scattering and maximizes visual clarity.

400

Structure of the eyeball

The eyeball wall has three major layers:

  1. Fibrous tunic

    • sclera

    • cornea

  2. Vascular tunic (uveal tract)

    • iris

    • ciliary body

    • choroid

  3. Neural tunic

    • retina (sensory layer)

400

The histological layers of the IRIS

The iris has five main layers:

1.1. Anterior Border Layer

  • thin connective tissue layer

  • contains abundant melanocytes

  • determines the color of the eye

    • more pigment → brown eyes

    • less pigment → blue or green eyes

  • albinos lack melanin → iris appears reddish (visible blood vessels)

1.2. Stromal Layer

  • loose connective tissue

  • contains:

    • fibroblasts

    • collagen fibers

    • melanocytes

    • numerous blood vessels

  • houses the two intrinsic muscles of the iris:

    1. Sphincter pupillae muscle (circular smooth muscle)
      → constricts pupil (parasympathetic control)



        1. Dilator pupillae muscle (radial myoepithelial layer)
          → dilates pupil (sympathetic control)

    3. 1.3. Inner Limiting Layer

      • thin connective tissue zone

      • similar to anterior border layer

      • forms a boundary between stroma and epithelium

    4. 1.4. Posterior Pigmented Epithelium

      • two layers of heavily pigmented cuboidal epithelial cells

      • continuation of the retinal pigmented epithelium

    5. 1.5. Iris Pigment Epithelium (double-layered)

      • inner layer: highly pigmented

      • outer layer: partly pigmented + contains myoepithelial components (forms dilator muscle)

400

Accommodation Mechanism (Very Important)

For Distant Vision

  • ciliary muscle relaxed

  • zonular fibers tight

  • lens flat, thin

  • low refractive power

For Near Vision

  • ciliary muscle contracts

  • zonular fibers loosen

  • lens becomes rounder

  • increased refractive power

With age, the lens loses elasticity → presbyopia (difficulty reading close objects).

400

Bruch’s Membrane (Basal complex)

A multilayered structure between the choriocapillaris and retinal pigment epithelium.

Composed of:

  1. Basement membrane of pigment epithelium

  2. Inner collagen layer

  3. Elastic layer

  4. Outer collagen layer

  5. Basement membrane of choriocapillaris

Functions:

  • selective diffusion

  • mechanical support

  • waste exchange between RPE and choroid

Age-related thickening of Bruch’s membrane contributes to:

  • age-related macular degeneration (AMD)

400

Blood supply of the retina and phototransduction

The retina is supplied by two sources:

8.1. Choroidal Circulation

  • supplies the outer layers (photoreceptors, RPE)

  • derived from short posterior ciliary arteries

  • extremely high blood flow

8.2. Central Retinal Artery

  • supplies inner retinal layers

  • runs through optic nerve and branches on the inner retina

Occlusion → sudden, painless vision loss.

Phototransduction is the process by which photoreceptors convert light → neural impulses.

Mechanism Overview (Rods as Example)

  1. Light hits rhodopsin → bleaching

  2. Rhodopsin → activates transducin (G-protein)

  3. Transducin activates phosphodiesterase (PDE)

  4. PDE decreases cGMP levels

  5. cGMP-gated Na⁺ channels close

  6. Photoreceptor hyperpolarizes

  7. Hyperpolarization → reduces glutamate release

  8. Bipolar cells detect the change → activate ganglion cells

  9. Ganglion cells fire action potentials → optic nerve → brain

Key point:

  • Photoreceptors do not fire action potentials; they use graded potentials.

  • Ganglion cells are the only retinal cells that generate action potentials.


500

Development of the eye

The eye develops from three embryonic sources:

5.1. Neural Tube

Forms:

  • optic vesicles → optic cup

  • retina (neurosensory layers)

  • optic nerve

  • iris muscles (sphincter and dilator)

5.2. Surface Ectoderm

Forms:

  • lens

  • corneal epithelium

  • conjunctival epithelium

  • eyelid epithelium

5.3. Mesenchyme (mainly neural crest)

Forms:

  • sclera

  • corneal stroma + endothelium

500

The Pupil and its control

Pupillary Constriction (Miosis)

  • controlled by sphincter pupillae

  • under parasympathetic control (CN III → Edinger–Westphal nucleus)

  • activated by bright light, near vision

Pupillary Dilation (Mydriasis)

  • controlled by dilator pupillae

  • under sympathetic control

  • activated by dim light, fear, excitement

500

Structure of the ciliary body

It has two main regions:

A) Ciliary Muscle

Located beneath the stroma; made of smooth muscle bundles oriented in three directions:

1. Longitudinal (Meridional) Fibers

  • lie closest to the sclera

  • pull the choroid forward during contraction

  • assist in opening drainage channels (useful in lowering intraocular pressure)

2. Radial Fibers

  • intermediate group

  • reduce tension on zonular fibers when contracted

3. Circular (Sphincteric) Fibers

  • innermost group

  • most responsible for lens rounding during accommodation

  • contraction → lens becomes more convex → near vision

    These three sets work together to modify lens curvature.

    B) Ciliary Processes (Corona ciliaris)

    Numerous finger-like folds projecting inward.
    Their functions:

    • major site of aqueous humor production

    • anchor point for zonular fibers (suspensory ligament of the lens)

  • Ciliary Epithelium

    The ciliary processes are lined by a double epithelium:

    1. Outer pigmented epithelium

      • continuous with the retinal pigment epithelium

    2. Inner non-pigmented epithelium

      • continuous with neurosensory retina

      • responsible for producing aqueous humor

      • contains ion pumps (Na⁺, Cl⁻, HCO₃⁻)

500

Visual pathway

Steps of visual signal transmission:

  1. Photoreceptors (rods and cones)

  2. Bipolar cells

  3. Ganglion cells

  4. Optic nerve

  5. Optic chiasm

    • nasal fibers cross

  6. Optic tracts

  7. Lateral geniculate nucleus (LGN) of thalamus

  8. Optic radiations

  9. Primary visual cortex (V1, area 17) in occipital lobe

This pathway results in:

  • depth perception

  • visual field integration

  • color and motion detection

  • conscious visual awareness

500

All RETINA

The retina is the inner, neural layer of the eyeball and the functional core of the entire visual analyser.
It converts light into electrical impulses and sends them to the brain via the optic nerve.
The retina is one of the most complex tissues of the body, containing multiple layers of neurons and supporting cells arranged in a highly organized manner.

1. GENERAL ORGANIZATION OF THE RETINA

The retina consists of two major parts:

1.1. Non-visual (Anterior) Retina

  • covers the ciliary body and iris

  • composed of simple double epithelium

  • does not participate in vision

  • continuation of the pigmented and neural retina but without photoreceptors

1.2. Optic (Visual) Retina

  • lines the posterior 2/3 of the eyeball

  • responsible for light reception and phototransduction

  • LAYERS OF THE RETINA (10 CLASSICAL LAYERS)

    From outer (scleral side) to inner (vitreal side):

    1. Retinal Pigment Epithelium (RPE)

    2. Photoreceptor Layer

      • rods and cones outer & inner segments

    3. External Limiting Membrane

    4. Outer Nuclear Layer

      • photoreceptor cell bodies

    5. Outer Plexiform Layer

      • synapses: photoreceptors → bipolar & horizontal cells

    6. Inner Nuclear Layer

      • bipolar, horizontal, amacrine, Müller cells

    7. Inner Plexiform Layer

      • synapses: bipolar → ganglion ± amacrine cells

    8. Ganglion Cell Layer

      • cell bodies of ganglion cells

    9. Nerve Fiber Layer

      • axons of ganglion cells → optic nerve

    10. Internal Limiting Membrane

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