Lec-vision-hearing2025

Special Senses

Sensation of Vision

• Reduced Eye Model: All refractive surfaces of the eye can be combined as one single lens.

  • Refractive Power: Measured in diopters (D), which is the reciprocal of focal length in meters.

  • Cornea: Focal length of 24 mm, refractive power of 42 D.

  • Lens: Focal length of 44 mm, refractive power of 23 D.

• Human Eyeball (Bulbus Oculi):

  • Optic Axis: Line connecting anterior and posterior poles.

  • Visual Axis: Line from cornea to fovea centralis (lateral to posterior pole).

  • The eyeball is primarily located within the bony orbital cavity and is linked to it by ocular muscles.

• Conjunctiva:

  • Bulbar Portion: Covers the eyeball.

  • Palpebral Portion: Covers the eyelid.

Formation of an Image on the Retina

• Lens System: Focuses images on the retina; images are inverted and reversed.

  • Brain's perception corrects the orientation back to upright.

• Accommodation: Ability to change the lens shape to focus on nearby objects.

  • In Children: Refractive power can increase voluntarily from 20 D to about 34 D.

  • Controlled by Parasympathetic Nerves: Ciliary muscles are regulated by the third cranial nerve.

Presbyopia

• Definition: Loss of accommodation with age as the lens becomes larger and less elastic.

  • Decreases from 14 D to 2 D between ages 45-50 and to essentially 0 D by age 70.

• Causes:

  1. Decreased elasticity of lens due to physical changes.

  2. Decreased convergence of eyeballs due to weakened ocular muscles.

• Correction: Biconvex lenses for bifocal glasses to aid both near and distant vision.

Retina

• Light-sensitive Membrane: Forms innermost layer of the eye and houses visual receptors (rods and cones).

• Structure: 10 layers including bipolar cells, ganglion cells, horizontal cells, amacrine cells.

• Fluid System:

  • Vitreous Humor: Viscous fluid behind the lens.

  • Aqueous Humor: Thin fluid in front of retina, produced by ciliary processes.

  • Functions: Maintain shape and pressure of eyeball, provide nutrients and remove metabolic waste.

Intraocular Pressure and Glaucoma

• Definition: Fluid pressure in the eye due to aqueous humor. Normal range: 12-20 mm Hg.

  • Glaucoma occurs when pressure exceeds 25 mm Hg, causing potential vision loss.

  • As pressure increases, optic nerve axons compress, leading to neuronal damage and vision loss.

Cataract

• Definition: Opacity in the lens; major cause of blindness globally.

  • Develops due to cell accumulation and fluid buildup leading to cloudiness.

• Neural Basis of Visual Process:

  • Rods and cones are visual receptors with varying distribution. Fovea contains only cones; peripheral retina contains more rods.

  • Rods: Sensitive to light, responsible for night vision, cannot resolve details.

  • Cones: Responsible for bright light vision and color discrimination.

Night Blindness

• Definition: Impairment in low-light vision; linked to vitamin A deficiency.

  • Can result from poor diet or decreased absorption.

  • Initial deficiencies affect rod function; prolonged leads to retinal degeneration.

Visual Acuity

• Definition: Ability to detect details, measured by distance using Snellen charts.

• Field of Vision: Extended visual field of 160° horizontally and 135° vertically, divided into four parts: Temporal, Nasal, Upper, Lower fields.

Visual Pathway

• Components: Pathway follows from retina through optic nerve, optic chiasma, optic tract to lateral geniculate body and finally to the visual cortex.

• Neurons:

  • First Order: Bipolar cells in the retina.

  • Second Order: Ganglionic cells, forming the optic nerve.

  • Third Order: Lateral geniculate neurons reaching the cortex.

Repair and Reflexes - Visual Pathway Lesions

• Various visual defects arise from specific lesions, categorized as:

  1. Anopia: Vision loss in one eye.

  2. Hemianopia: Loss of half of the visual field, classified as homonymous or heteronymous.

• Lesions at different pathways create distinct visual field defects.

Color Vision

• Discrimination: Human eyes perceive approximately 150 colors; dependent on cone function.

• Visible Spectrum: Range represented by red, orange, yellow, green, blue, indigo, violet (ROYGBIV) determined by wavelength.

• Notable rays outside this spectrum (IR and UV) do not invoke visual sensations.

Color Blindness

• Definition: Inability to perceive certain colors; inherited condition, prevalent in males.

• Testing: Utilizes Ishihara charts, colored wool, and lantern tests.

• Classification: Based on Young-Helmholtz theory into monochromatism (total color blindness) and others.

Errors of Refraction

• Emmetropia: Normal vision with proper refractive power; ametropia refers to refractive errors.

• Myopia (Nearsightedness): Shortened focal distance on distant objects; corrected using biconcave lenses.

• Hypermetropia (Farsightedness): Difficulty focusing on close objects; corrected with biconvex lenses.

• Astigmatism: Irregular curvature of lens causing multiple focus points; corrected with cylindrical glasses.

Hearing and Auditory System

• External Ear: Contains auricle and external auditory meatus.

• Middle Ear: Holds auditory ossicles (Malleus, Incus, Stapes) and Eustachian tube for pressure equalization.

• Internal Ear: Composed of cochlea (for hearing) and vestibular apparatus (for equilibrium).

Organ of Corti and Hair Cell Function

• Description: Receptor organ composed of hair cells and tectorial membrane.

• Function: Hair cell stimulation leads to auditory impulses through auditory pathways to the brain.

• Impedance Matching: Converts sound energy into mechanical vibrations with minimal loss.

Sound Characteristics

• Frequency: Measured in hertz; humans hear between 20 - 20,000 Hz, most sensitive around 2000-3000 Hz.

• Loudness: Amplitude of sound waves measured in decibels (dB).

Reflex Activity and Reflex Arcs

• Definition: Unconscious response to stimulation, serving as protective mechanisms.

• Components of Reflex Arc: Receptor, afferent nerve, central processing unit, efferent nerve, and effector organ.

Classification of Reflexes

• Types: Based on origin (inborn vs. acquired), center location (cerebellar, cortical, midbrain, etc.), purpose (protective, antigravity), and structures involved (superficial, deep, visceral, pathological).

• Examples: Includes simple and complex reflexes depending on the number of synapses involved (monosynaptic vs. polysynaptic).