1_TSH The Eye Structure Function

Learning Objectives

  • Discuss the evolutionary development of the eye.

  • Describe the structure and function of the three layers of the eye:

    • Fibrous tunic

    • Choroid tunic

    • Sensory tunic (retina)

  • Discuss the histology of the sensory tunic (retina).

  • Explain the role of rod cells in eye physiology and vision.

  • Explain the role of cone cells in color vision (trichromatic theory).

  • Discuss the roles of muscles for eye movement and changing the shape of the lens.

The Eye Evolution

  • Light-sensitive cells are present in nearly all organisms.

  • The development of light detection began in the Cambrian period (541 million years ago).

  • Image formation requires a lens, which is a defining feature of the eye.

  • Eyes have evolved independently in different groups such as arthropods, molluscs, and vertebrates, showcasing convergent evolution.

Overview of Eye Functions

Converts light energy into electrical signals using photoreceptor cells.

  • Eye placement aids in binocular vision:

    • Provides wider field of view.

    • Enhances distance and depth perception.

Eye Movement

  • Eyes typically move together, focusing on the same area.

    • Eyes can move in the same direction or opposite directions depending on focus

  • Eye movements:

    • Horizontal movements: medial and lateral rectus muscles.

    • Vertical movements: superior and inferior rectus muscles, & oblique muscles.

  • Eye movement can be conscious (deliberate focus/voluntary) or unconscious (maintaining focus while the head moves/involuntary).

Anatomy of the Eye

  • Key components visible include:

    • Cornea

    • Iris

    • Pupil

    • Sclera (1/6th of the eye is visible).

Tunic Layers of the Eye

  • Three layers (tunics) surrounding a hollow sphere filled with fluids (humors).

    • Fibrous tunic

      • Sclera & cornea

    • Vascular tunic (Uvea)

      • Iris, cilliary body, & choroid

    • Nervous/Sensory tunic (Retina)

      • Pigmented layer & neural layer

Fibrous Layer

  • Outermost layer of the eye made of dense avascular connective tissue.

    • Sclera:

      • White opaque layer providing strength, protection, and muscle attachment.

    • Cornea:

      • Transparent region allowing light entry, providing 2/3 of focusing power.

      • Can be reshaped via laser eye surgery and is transplantable.

Uvea – Vascular Tunic

  • Components:

    • Choroid

    • Ciliary body (and ligaments)

    • Lens

    • Iris

  • Choroid:

    • Contains blood vessels supplying nutrients and oxygen to the retina

    • Contains melanin to reduce light reflection.

  • Ciliary body:

    • Ring-shaped structure with circular and radial ciliary muscles, suspensory ligaments/ zonule filaments.

  • Lens:

    • Transparent, consisting of:

      • Lens capsule (basement membrane)

      • Inner epithelial layer

      • Lens fiber cells

    • Separates humors: aqueous (superficial) and vitreous (deep).

  • Iris:

    • Circular ring of tissue with an opening (pupil).

    • Contains circular pupillary sphincter and radial pupillary dilator muscles.

    • Function - To control the amount of light that enters the eye

Nervous/Sensory Tunic - Retina

  • The retina consists of:

    • Outer pigmented epithelial layer (melanin)

    • Inner light-sensitive neural layer

  • The neural layer includes:

    • Photoreceptor cells (modified nerve cells that detect light)

    • Bipolar neurons

    • Retinal ganglion cells

Rod and Cone Cells in Retina

I. Overview:

  • The retina contains two main types of photoreceptor cells: rods and cones.

  • These cells are responsible for converting light into electrical signals that the brain interprets as vision.  

  • They infoldings of plasma membrane (discs) which increase light absorption.


II. Rod Cells:

  • Number: Approximately 120 million per retina.

  • Light Sensitivity: High sensitivity to light, enabling vision in dim light conditions (night vision).

  • Color Discrimination: Poor color discrimination; primarily responsible for black and white vision.

  • Spatial Distribution: More numerous and prevalent in the peripheral retina, making them important for peripheral vision.

  • Resolution: Provide low-resolution vision (less sharp details).

  • Shape: Rod-shaped.


III. Cone Cells:

  • Number: Approximately 6 million per retina.

  • Light Sensitivity: Function best in bright light conditions.

  • Color Vision: Responsible for color vision.

    • There are three kinds of cones, each maximally sensitive to different wavelengths of light (typically referred to as red, green, and blue).

  • Spatial Distribution: Primarily concentrated in the central retina, particularly the macula, providing sharp, detailed central vision.

  • Resolution: Provide high-resolution vision (fine details).

  • Shape: Cone-shaped.


IV. Key Retinal Areas and Photoreceptor Concentration:

  • Macula:

    • A central area of the retina.

    • Characterized by a high concentration of cone cells.

    • Responsible for sharp, central vision and color perception.

  • Fovea:

    • A small pit located at the center of the macula.

    • Contains the highest density of cone cells in the entire retina.

    • Responsible for the most detailed and acute central vision.

Feature

Rod Cells

Cone Cells

Number

~120 million

~6 million

Light Sensitivity

High (dim light)

Low (bright light)

Color Vision

Poor (black & white)

Good (color)

Spatial Vision

Peripheral vision

Central vision

Resolution

Low

High

Shape

Rod-shaped

Cone-shaped

Concentration

Higher in peripheral retina

Higher in central retina (macula)

Peak Density


Fovea (highest)

Pigmented epithelial layer

The retinal pigment epithelium (RPE) is a single layer of pigmented cells adjacent to the photoreceptors (rods and cones) and the choroid (the vascular layer of the eye).


Key Functions:

  • Light Absorption: The pigment (melanin) within the RPE cells absorb stray light that is not captured by the photoreceptors. This reduces light scatter within the retina, improving visual acuity and preventing glare.

  • Transport of Nutrients and Waste Products: The RPE forms a selective barrier between the choroid (rich in blood vessels) and the neural retina. It actively transports nutrients (like glucose and oxygen) from the choroid to the photoreceptors and removes metabolic waste products from the photoreceptors to the choroid.

  • Provides Retinal to Rods:

    • When light strikes rhodopsin in the rods (or cone pigments in cones), it causes a conformational change in retinal. Specifically, it changes from the cis form (11-cis-retinal) to the trans form (all-trans-retinal).

    • For the photoreceptor to be able to detect more light, the all-trans-retinal needs to be converted back to 11-cis-retinal. This crucial conversion process occurs within the RPE cells.

    • The RPE contains the necessary enzymes and binding proteins to take up the all-trans-retinal released by the photoreceptors, convert it through a series of enzymatic steps back to 11-cis-retinal, and then shuttle this regenerated 11-cis-retinal back to the photoreceptors.


Macula Function

The macula is a small, highly sensitive area located in the central part of the retina, the light-sensitive tissue at the back of the eye. It's responsible for our sharp, central vision and our ability to see fine details and colors clearly.


Location: Situated in the centre of the retina containing a central pit called the fovea centralis that contains the highest [ ] of cones.


Macular Degeneration: Occurs when the macula becomes damaged over time.

  • Blurry or reduced central vision, making it difficult to see fine details, read, drive, and recognize faces.  

  • Correlated with ageing

  • area affected is mainly macula and fovea so only central vision is affected


Optic disc: The point where the optic nerve leaves the retina at the back of the eye.

  • This area contains no photoreceptors which is why it is called the Blind Spot

  • The optic disc serves as the exit point for the axons of retinal ganglion cells

  • It also contains the central retinal artery and vein, which enter and exit the eye at this point, supplying blood to the retina

Photoreceptor Cell Arrangement

  • The retina has a unique "back-to-front" structure where photoreceptors face away from the incoming light.

  • Layers of Neurons:

    • The retina is organized into distinct layers containing different types of neurons.


Types of Neurons in the Retina

  • The retina contains five main types of neurons:

    • Photoreceptor cells (Rods and Cones)

    • Bipolar cells

    • Ganglion cells

    • Horizontal cells

    • Amacrine cells

Signalling in the Retina

  • Neurotransmitters:

    • Excitatory Cells: Rods, cones, bipolar cells, and ganglion cells release the excitatory neurotransmitter glutamate.

    • Inhibitory Cells: Horizontal and amacrine interneurons release the inhibitory neurotransmitter GABA.

  • Ganglion Cell Function:

    • Ganglion cells process specific aspects of visual information, including color, brightness, and motion.

    • They also play a role in controlling light sensitivity and circadian rhythms.


Role of Interneurons

  • Horizontal Cells (HC):

    • Connect photoreceptors and bipolar cells.

    • Help process visual inputs within the outer retina.

  • Amacrine Cells (AC):

    • Connect bipolar cells and ganglion cells.

    • Help process visual inputs within the inner retina.

Photoreceptor Cells: Rods and Cones

  • Rods:

    • Responsible for scotopic vision (high sensitivity in dim light).

    • High sensitivity but poor detail vision.

    • Non-functional in bright light.

    • Require 30-60 minutes for dark adaptation.

    • Rhodopsin:

      • Photopigment composed of opsin protein and 11-cis-retinal (a derivative of vitamin A).

      • Maximum sensitivity to blue-green light (500 nm).

      • Night blindness may result from vitamin A deficiency, affecting rhodopsin production.

    • Visual Cycle of Rhodopsin:

      • Light causes isomerization of 11-cis-retinal to 11-trans-retinal.

      • This induces a conformational change in opsin, forming activated rhodopsin.

      • Activated rhodopsin triggers an electrical signal that is sent to the brain.

  • Cones:

    • Responsible for photopic vision (color vision in bright light).

    • Require good illumination to function.

    • Provide sharp, detailed central vision (high visual acuity).

    • Mainly located in the fovea, the central part of the macula.

    • High visual acuity is supported by a near 1:1 ratio of cones to ganglion cells in the fovea.

    • Trichromatic Theory (Young-Helmholtz):

      • Three types of cones, each maximally sensitive to a different range of wavelengths:

        • Blue cones: Maximum sensitivity around 430 nm.

        • Green cones: Maximum sensitivity around 530 nm.

        • Red cones: Maximum sensitivity around 562 nm.

      • The brain compares the responses of these three cone types to perceive the full spectrum of intermediate colors.

      • Most primates are trichromatic, while many other mammals have dichromatic vision (two types of cones).

Color Blindness

  • Overview:

    • A congenital condition characterized by a reduced ability to distinguish between certain colors.

    • More prevalent in males (1:12) compared to females (1:200).

    • Exists in various forms, with the inability to distinguish red and green being the most common.

Color Blindness Testing

Ishihara Colour Cards: These are specifically designed plates or cards used by eye care professionals to screen individuals for color blindness, particularly red-green color vision deficiencies.

  • Testing Colour Discrimination: The cards feature a background of numerous small, coloured dots arranged randomly. Within this background, dots of a slightly different color are arranged to form a number or a winding line.

  • Red-Green Colour Blindness Detection:

    • People with normal colour vision will be able to easily distinguish the pattern (number or line) formed by the differently colored dots against the background.

    • People who are red-green color blind (either partially or completely) will have difficulty seeing the intended pattern. They might see a different number, no number at all, or be unable to trace the line. This is because they cannot differentiate between the specific shades of red and green used in the design.