Vision and Hearing Notes

Vision and Hearing

Introduction

• Balance is also related to hearing, sharing the same sensory structures.
• Vision is the primary focus due to its complexity, followed by sound and balance.
• The lecture covers the anatomy of the eye (external and internal features), light processing, and the auditory system.

Importance of Vision

• 70% of sensory receptors are dedicated to visual information.
• The visible portion of the eye is about one-sixth of the eyeball's overall size.

Accessory Structures of the Eye

Eyebrows

• Function: Protection from particles, sweat, and excessive sunlight.
• Also contribute to aesthetics.

Eyelashes and Eyelids

• Eyelids: Thin skin structures with tarsal plates for support.
• Commissures: Lateral and medial points where eyelids meet.

Lacrimal Apparatus

• Lacrimal gland: Produces tears, located in the superior-lateral orbit.
• Tears flow medially across the eye and drain through:
  • Lacrimal canaliculi: Small tunnels at the superior and inferior ends of the medial commissure.
  • Lacrimal sac: Collects tears.
  • Nasolacrimal duct: Drains into the nasal cavity (explains runny nose when crying).

Lubrication of the Eyes

• Eyes lack a physical barrier, so lubrication is crucial for protection.
• Tears: Lubricate and cleanse the eye; contain enzymes to break down bacteria.
• Reflexive blinking: Occurs every few seconds to spread secretions and prevent buildup (eye boogers).

Eyelashes (Function)

• Protection from particles; highly sensitive to touch, triggering a blink reflex.
• Richly innervated to sense touch, allowing the eyes to close when something gets too close.

Glands of the Eyelids

• Tarsal glands: Located inside the tarsal plates; secrete oil to lubricate the eyes.
• Ciliary glands: Modified sweat glands, also producing lubricant.
• Conjunctiva: Transparent mucous membrane covering the white of the eye; secretes mucus.

Conjunctivitis (Pink Eye)

• Inflammation of the conjunctiva, often due to bacterial infection (contagious).
• Treated with antibiotics (e.g., eye cream).

Eye Muscles

• Six skeletal muscles control eye movement; innervated by cranial nerves.

Eye Muscles (Specifics)

• Medial and lateral rectus muscles.
• Superior and inferior rectus muscles.
• Inferior oblique and superior oblique muscles (twisted muscles).

Strabismus (Cross-Eyed)

• Caused by weakened or elongated eye muscles, leading to an imbalance.
• Treatment: Eye patch to strengthen the weaker muscle; surgery may be required.

External and Internal Structures of the Eye

Layers of the Eye

• Conjunctiva
• Fibrous layer
  • Sclera: White of the eye; thick connective tissue for physical protection.
  • Cornea: Transparent modified region of the fibrous layer.
    • Avascular to maintain clarity.
    • Bulges outward for bending and focusing light.
    • Densely packed with pain receptors.
    • Can be replaced via corneal transplants.

Vascular Layer (Choroid)

• Contains blood vessels and melanin for color.
  • Choroid: Highly vascular, pigmented layer.
  • Ciliary body: Thickened region of the choroid with smooth muscles.
  • Suspensory ligaments: Connect the ciliary body to the lens.

Lens Accommodation

• Ciliary muscles contract or relax to adjust tension on ligaments and change the lens shape.
  • Relaxed muscles: Increased tension on ligaments, flattening the lens (for distant vision).
  • Contracted muscles: Reduced tension on ligaments, causing the lens to bulge (for close vision).

Autonomic Nervous System Involvement

• Sympathetic nervous system: Activates for distant vision (fight or flight); flattens the lens.
• Parasympathetic nervous system: Activates for close vision (relaxed state); lens becomes rounded.

Iris and Pupil

• Iris: Colored portion of the eye; contains two layers of smooth muscle.
• Pupil: Opening in the center of the iris; controls light entry.

Muscles of the Iris

• Sphincter pupillae: Circular muscle that constricts the pupil (parasympathetic).
• Dilator pupillae: Muscle that dilates the pupil (sympathetic).

Function of Melanin in the Choroid

• Absorbs excess light, preventing interference with the image on the retina.

Retina

• Contains photoreceptors (rods and cones) for light detection.

Photoreceptors

• Rods: Sensitive to low light; provide black and white vision; located in the periphery.
• Cones: Enable color vision and high visual acuity; concentrated in the fovea centralis.

Fovea Centralis and Macula Flutiae

• Fovea centralis: Area with the highest density of cones, providing the sharpest vision.
• Macula flutiae: Outer region of the fovea centralis.
• Not perfectly centered due to the presence of blood vessels and the optic nerve exit.

Light Processing in the Retina

• Light travels through several layers of cells before reaching the photoreceptors.
• Photoreceptors (rods and cones) are activated by photons, triggering an action potential.

Neural Pathway

• Signal pathway:
  • Rods and cones → bipolar cells → ganglion cells → optic nerve
• Axons of ganglion cells form the optic nerve, which sends signals to the brain.

Visible Light Spectrum

• Different frequencies of radiation from the sun.
• Some animals can see UV light.

Optic Nerve and Visual Field

• Neurons from each eye cross over at the optic chiasm.
• Combined input from both eyes provides a full field of view.
• Optic nerve → optic chiasm → optic tracts → visual cortex of the occipital lobe.

Interior Chambers of the Eye

Segments and Chambers

• Anterior segment:
  • Cornea to the lens; contains the anterior and posterior chambers.
  • Anterior chamber is from the cornea to the pupil.
  • Posterior chamber is from the pupil to the back of the lens.
  • Filled with aqueous humor: A filtrate of blood that maintains eye shape and keeps layers attached.
• Posterior segment:
  • Lens to the back of the eye; contains vitreous humor.

Aqueous Humor

• Produced by blood vessels in the choroid layer, flows from the posterior to the anterior chamber, and drains into the scleral venous sinus.
• Constantly produced and recycled.

Pathway of Light

• Cornea → anterior chamber → pupil → posterior chamber → lens → posterior segment/retina.

Pathway of Nerve Impulses

• Photoreceptors (rods and cones) → bipolar cells → ganglion cells → optic nerve → optic chiasm → optic tracts → visual cortex of the occipital lobe.

Eye Disorders

Cataracts

• Clouding of the lens due to damaged protein fibers.
• Can be treated with lens transplantation.

Astigmatism

• Irregularities in the shape of the lens cause light to focus on different parts of the retina.
• Can be corrected with glasses or LASIK surgery.

Farsightedness and Nearsightedness

• Farsightedness:
  • Can see far objects well; trouble seeing close objects.
  • Requires glasses for correction.
• Nearsightedness:
  • Can see close objects well; trouble seeing far objects.
  • Requires glasses for correction.

Introduction to the Auditory System

• The ear is responsible for hearing and balance.