eye

THE NERVOUS SYSTEM - SPECIAL SENSE ORGANS

THE EYE - SPECIAL SENSE ORGANS

VISION

  • Vision is defined as the special sense of sight, primarily based on the transduction of light stimuli received through the eyes.

THE LOCATION OF THE EYES

  • The eyes are anatomically located within either orbital cavity of the skull.

  • Bony orbits surround the eyeballs to provide protection and anchor the soft tissues associated with the eye.

ACCESSORY STRUCTURES OF THE EYE

Accessory Structures

  • Accessory structures are critical for the protection, lubrication, and movement of the eye.

  • They include:

    • Eyebrows

    • Eyelids

    • Conjunctiva

    • Lacrimal Apparatus

    • Extrinsic Eye Muscles

Eyebrows

  • Function:

    • Protect the eyes by preventing perspiration from entering and causing irritation.

    • Help shade the eyes from direct sunlight.

Eyelids

  • Function:

    • Protect the eyes from foreign objects.

    • The blink reflex occurs in response to a sudden approach of an object, prompting eyelids to close rapidly.

    • Blinking, which averages about 20 times per minute, spreads tears across the surface of the eyes for lubrication.

The Conjunctiva

  • Definition:

    • A thin, transparent mucous membrane that covers the inner surface of the eyelids and the anterior surface of the eye.

  • Functions:

    • Provides lubrication through its secretions.

    • Inflammation of the conjunctiva is called conjunctivitis.

Lacrimal Apparatus

  • Components:

    • The lacrimal apparatus includes the lacrimal gland, situated in the superior lateral corner of the orbit, and the nasolacrimal duct with associated structures in the inferior medial corner.

  • Function:

    • The lacrimal gland produces tears that pass over the eye's anterior surface.

    • Most tear fluid evaporates from the eye surface, with excess tears collected in small ducts called lacrimal canaliculi at the medial angle of the eyes.

    • These canaliculi lead into a lacrimal sac, which is an enlargement of the nasolacrimal duct that opens into the nasal cavity.

    • Tears serve to lubricate and cleanse the eye and contain enzymes that combat infections.

Extrinsic Eye Muscles

  • There are six skeletal muscles responsible for the movement of each eyeball, referred to as extrinsic eye muscles.

  • They include:

    • Superior rectus muscle

    • Inferior rectus muscle

    • Medial rectus muscle

    • Lateral rectus muscle

    • Superior oblique muscle

    • Inferior oblique muscle

Muscles that Move the Eye

  • The extrinsic eye muscles originate outside of the eye and insert onto the surface of the eye's sclera.

  • Located within the eye socket, they cannot be observed on the visible eyeball.

Movement of Muscles

  • These muscles control the movement of the eye in various directions.

  • They are involved in different functions; for example, the levator palpebrae superioris muscle elevates the upper eyelid.

Basic Muscle Functions

  • Levator

    • Action: Opens the eye (elevates eyelids).

    • Origin: Roof of orbit (sphenoid bone).

    • Insertion: Skin of upper eyelids.

  • Superior Oblique

    • Action: Moves eyes up and toward the nose, rotates eyes.

    • Origin: Sphenoid bone.

    • Insertion: Surface of eyeball between superior rectus and lateral rectus.

  • Superior Rectus

    • Action: Moves eyes up and medially.

    • Origin: Common tendinous ring.

    • Insertion: Superior surface of the eyeball.

  • Inferior Rectus

    • Action: Moves eyes down and medially.

    • Origin: Common tendinous ring.

    • Insertion: Inferior surface of the eyeball.

  • Lateral Rectus

    • Action: Moves eyes laterally (abduction).

    • Origin: Common tendinous ring.

    • Insertion: Lateral surface of the eyeball.

  • Medial Rectus

    • Action: Moves eyes medially (adduction).

    • Origin: Common tendinous ring.

    • Insertion: Medial surface of the eyeball.

  • Inferior Oblique

    • Action: Moves eyes up and laterally; rotates eyeball.

    • Origin: Maxilla (floor of orbit).

    • Insertion: Surface of eyeball between inferior rectus and lateral rectus.

PARTS OF THE EYE

Structural Overview

  • The eye contains several key anatomical parts vital for its function, including:

    • Iris

    • Sclera

    • Pupil

    • Lens

    • Vitreous body (filled with vitreous humor)

    • Cornea

    • Anterior chamber (filled with aqueous humor)

    • Posterior chamber

    • Suspensory ligaments

    • Ciliary body and muscle

    • Optic disc (blind spot)

    • Retina

    • Choroid

    • Macula lutea

    • Fovea centralis (central depression)

    • Optic nerve and retinal blood vessels.

Sclera

  • Definition: The sclera is the white, tough outer covering of the eye, constructed from connective tissue.

Cornea

  • Definition: The cornea is the transparent section of the sclera that permits light entry into the eye.

Choroid

  • Function: The choroid contains many blood vessels and pigment cells to absorb light and prevent reflection within the eye.

Retina

  • Description: The innermost layer of the eye comprising light-sensitive cells known as rods and cones.

Ciliary Body

  • Structure: The ciliary body consists of smooth muscle that holds the lens in position and alters its shape upon contraction and relaxation.

Lens

  • Characteristics: The lens is biconvex, transparent, and flexible, consisting of numerous layers of cells.

  • Function: It changes shape to facilitate focusing light on the retina, but mature cells lack the ability to repair or regenerate.

Iris

  • Description: The iris is the colored diaphragm of the eye made of smooth muscle that regulates the amount of light entering through the pupil.

  • Pupil: The pupil is the round opening in the iris that serves as a gateway for light entry.

Autonomic Control of the Iris

  • The size of the pupil is regulated by the autonomic nervous system:

    • Parasympathetic stimulation leads to pupil constriction.

    • Sympathetic stimulation causes pupil dilation.

Compartmentalization of the Eye

  • The interior of the eye is divided into two compartments:

    • Anterior Compartment: Filled with aqueous humor.

    • Posterior Compartment: Houses the vitreous humor.

Aqueous Humor and Vitreous Humor

  • Aqueous Humor: Produced by the ciliary body, this fluid fills the anterior compartment and is necessary for nutrient supply and maintaining ocular pressure.

  • Vitreous Humor: Occupies the posterior compartment, assisting in maintaining ocular pressure and refracting light.

Detailed Anatomy of the Retina

  • The retina is equipped with photosensitive cells and features a pigmented epithelial layer that prevents light from reflecting back within the eye.

  • Photosensitive cells consist of rods and cones, functioning in dim lighting (rods) or bright light and color (cones).

Rods and Cones

  • Rods:

    • Sensitive to low light levels, allowing for vision in dim environments but lacking color perception.

    • Contains rhodopsin, which degrades to opsin and retinal in light. - Vitamin A is essential to synthesize retinal, and its deficiency leads to night blindness.

  • Cones:

    • Enable color vision and require brighter light levels to function effectively. - There are three types of cones sensitive to red, green, and blue wavelengths, which their combinations produce the complete spectrum of colors.

Synapses and Signal Transmission

  • Both rods and cones synapse with bipolar cells, which in turn synapse with ganglion cells. The axons of these ganglion cells form the optic nerve.

Structure of Rods and Cones

  • Both cell types comprise:

    • Outer Segment: Rod-shaped in rods and cone-shaped in cones.

    • Inner Segment: Intermediate layer of the photoreceptor cells.

    • Synaptic Terminal: Closest to the interior of the eye, facing bipolar cells.

Photoreceptor Functions

  • The outer segment contains membranous discs with photopigments responsible for light detection.

Neuronal Pathway for Visual Signals

  • Visual pathways from the eye progress from retinal photoreceptors through bipolar cells, leading to the ganglion cells and eventually the brain's visual cortex located in the occipital lobe.

Macula Lutea and Fovea Centralis

  • Macula Lutea: A yellowish region in the retina critical for sharp vision.

  • Fovea Centralis: A depression in the macula lutea that provides the sharpest vision for directly observed objects.

  • Optic Disc: Located medial to the fovea, this blind spot lacks receptor cells making it the exit point for the optic nerve.

REFRACTION OF LIGHT BY THE EYE

Sensitivity of Photoreceptors

  • Photoreceptors in the eye respond to visible light wavelengths ranging from 400 to 700 nm.

Refraction Process

  • Light rays diverge in various directions from a light source. To form an accurate image on the retina, these rays need to be focused inward through the process called refraction, defined as the bending of light rays as they transition through different mediums.

Role of the Eye's Refractive Structures

  • The cornea and lens are primary contributors to the eye's refractive capabilities.

  • The cornea is responsible for the most significant refraction due to its fixed structure and the transition of light from air to cornea involving a significant density difference.

  • The lens' refractive power is adaptable and modifies with its shape, essential for near and distant vision.

Accommodation

  • The lens's strength can be adjusted based on its shape, moderated by the ciliary muscles, allowing for the focusing on objects at different distances.

Distant Object Vision

  • Light rays from objects over 20 ft away are parallel, requiring minimal lens curvature to focus on the retina.

  • During this state, ciliary muscles relax, tensions apply to suspensory ligaments, flattening the lens for weaker refraction.

Near Object Vision

  • Light rays diverging from nearby objects require a stronger lens power to focus correctly on the retina.

  • Ciliary muscles contract, the ligaments slacken, allowing the lens to adopt a more spherical shape for enhanced refraction.

Eye Condition Classifications

  • Emmetropia (Normal Eye): The lens requires no adjustment for objects further than 20 feet, but accommodates for nearer items.

  • Myopia (Nearsightedness): Occurs when the eyeball is excessively long or the lens is overly strong, causing distant objects to focus before reaching the retina, resulting in blurriness.

  • Hyperopia (Farsightedness): This condition arises when the eyeball is too short or the lens too weak, resulting in near objects being out of focus behind the retina.

Image Reversal and Perception

  • The optical system inverts and reverses images on the retina; the brain compensates for this, perceiving objects in their correct orientation.

PHOTOTRANSDUCTION

Overview

  • Phototransduction is the process where light stimuli transform into electrical signals via chemical reactions in photopigments.

Light Activation in Rods

  • Retinal in rods exists as 11-cis retinal that, upon exposure to light, converts to all-trans retinal, activating rhodopsin.

Phototransduction in Darkness

  • The outer segment's plasma membrane of rods has chemically gated Na+ channels kept open in the presence of cyclic-GMP (cGMP).

  • The resting transmembrane potential is about -40 mV, causing continuous neurotransmitter (glutamate) release across the synapse. This outflow of sodium ions is termed DARK CURRENT.

Exposure to Light

  • Activation of rhodopsin and subsequent Na+ channel closure leads to hyperpolarization in the outer segment of the photoreceptors.

Relay of Signals

  • The transition potentials manifest towards the synaptic terminal, altering neurotransmitter release, consequently affecting bipolar and ganglion cells synapses.

Rods vs. Cones - Types of Phototransduction

  • On-Center Bipolar Cells respond to light increases at their center pathways, while Off-Center Bipolar Cells respond inversely to light input.

Rod and Cone Properties

  • Rods are higher in number (120 million) and more sensitive but yield low acuity vision (night vision).

  • Cones are less abundant (6 million) yet are concentrated in regions responsible for color vision and sharp detail detection.

Photopigments in Vision

  • Four types of photopigments exist:

    • One specific to rods.

    • Three in cones (blue, green, red).

  • Each pigment is tailored to absorb distinct light wavelengths.

Night and Color Vision

  • Rods detect light intensity, facilitating night vision in grayscale.

  • Color differentiation relies on the simultaneous stimulation of cones by various wavelengths, enabling perception of vibrant colors influenced by comparative operation among the three cone types.

ADAPTATION TO LIGHT

Dark Adaptation

  • The capability to adjust vision upon transitioning from a bright environment to a darker one, leading to heightened sensitivity in rods over time.

Light Adaptation

  • The process when adapting from low light to brighter conditions, in which the transition creates initial hypersensitivity, leading to color distinction as cones regain responsiveness.

VISUAL FIELDS

Visual Field Definition

  • The visual field represents the peripheral view available without head movement. It is divided such that the left cortex interprets inputs from the right visual field and vice versa.

VISUAL PATHWAY

Processing Information from the Retina

  • Each optic nerve transmits signals from dual halves of the retina to the optic chiasm, where visual pathways rearrange.

  • Fibers from medial retinae cross to the opposite sides while lateral fibers remain on their originating side.

Optic Tracts Characteristics

  • From the optic chiasma, optic tracts relay information from one retina's lateral half and another's medial half to corresponding brain hemispheres.

DEPTH PERCEPTION AND BINOCULAR VISION

Definition of Depth Perception

  • Depth perception engages the ability to view the world in three dimensions and assess distances, critical for spatial navigation.

Binocular Field of Vision

  • The binocular visual field is the area of overlapping perception from both eyes, enhancing depth and spatial awareness.

Overview

Phototransduction is the process where light stimuli transform into electrical signals via chemical reactions in photopigments.

Light Activation in Rods

Retinal in rods exists as 11-cis retinal that, upon exposure to light, converts to all-trans retinal, activating rhodopsin. This activation begins a series of biochemical events, leading to the closing of sodium channels.

Phototransduction in Darkness

The outer segment's plasma membrane of rods has chemically gated Na+ channels kept open in the presence of cyclic-GMP (cGMP).

  • The resting transmembrane potential is about -40 mV, causing continuous neurotransmitter (glutamate) release across the synapse. This outflow of sodium ions is termed DARK CURRENT.

  • In the dark, rods maintain high levels of cGMP, keeping the sodium channels open and allowing sodium ions to flow in, contributing to their depolarized state.

Exposure to Light

  1. Activation of Rhodopsin: When light hits the rhodopsin molecule, it causes a conformational change that initiates the phototransduction cascade.

  2. Closure of Sodium Channels: The activation of rhodopsin activates a G-protein called transducin, which then activates phosphodiesterase (PDE). This enzyme reduces cGMP levels, causing the closure of the sodium channels.

  3. Hyperpolarization: As sodium channels close, sodium influx decreases, leading to hyperpolarization of the outer segment.

  4. Reduced Neurotransmitter Release: The reduced depolarization leads to a decrease in the release of glutamate at the synaptic terminal, altering the signaling to bipolar cells.

Relay of Signals

  • The transition potentials manifest towards the synaptic terminal, altering neurotransmitter release, consequently affecting bipolar and ganglion cells synapses.

  • Noteworthy is the response differentiation wherein On-Center Bipolar Cells respond to light increases at their center, while Off-Center Bipolar Cells react inversely to light input.

Rods vs. Cones

  • Rods are higher in number (120 million) and more sensitive but yield low acuity vision (night vision).

  • Cones are less abundant (6 million) and less sensitive, concentrated in regions responsible for color vision and sharp detail detection.

Photopigments in Vision

Four types of photopigments exist:

  • Specific to rods.

  • Three in cones (blue, green, red).

  • Each pigment is tailored to absorb distinct light wavelengths, detailing their functionality under varying light conditions.

Night and Color Vision

  • Rods detect light intensity, facilitating night vision in grayscale.

  • Color differentiation relies on the simultaneous stimulation of cones by various wavelengths, enabling perception of vibrant colors influenced by comparative operation among the three cone types.

Phototransduction is the process where light stimuli transform into electrical signals via chemical reactions in photopigments. It begins with light activation in rods, where the retinal exists as 11-cis retinal, that converts to all-trans retinal upon exposure to light, activating rhodopsin. In darkness, rods maintain high levels of cyclic-GMP (cGMP), keeping sodium channels open and allowing sodium ions to flow in, resulting in a depolarized state known as DARK CURRENT. When light hits rhodopsin, it activates a G-protein called transducin, which activates phosphodiesterase (PDE), reducing cGMP levels and causing the closure of sodium channels. This closure leads to hyperpolarization of the outer segment and a decrease in neurotransmitter release at the synaptic terminal, ultimately changing the signaling to bipolar and ganglion cells. On-Center Bipolar Cells respond to light increases, while Off-Center Bipolar Cells act inversely. Rods are more numerous and sensitive but offer low acuity, while cones are less abundant and specialized for color vision and detail detection.

Dark current is the flow of sodium ions into photoreceptor cells, particularly rods, during darkness. It occurs because in the absence of light, the outer segment's plasma membrane of rods retains high levels of cyclic-GMP (cGMP), keeping chemically gated sodium channels open. This results in a depolarized state with a resting transmembrane potential of about -40 mV, leading to continuous release of the neurotransmitter glutamate across the synapse.