Eyes and Ears

👁 Anatomy of the Eye

Cross-Sectional Anatomy

The eye has three layers when viewed in cross-section:

• Sclera: The outer, white, tough, fibrous protein layer that provides protection.

• Choroid Layer: Consists of capillary beds that supply blood to the photoreceptors.

• Retina: The inner lining where photoreceptors are located.

Longitudinal Anatomy

As light passes through the eye, it encounters the following structures:

• Cornea: The clear protein layer at the anterior part of the eye.

• Aqueous Humor: A watery fluid in the anterior chamber.

• Pupil: The opening into the interior of the eye, located in the middle of the iris.

• Iris: The colored part of the eye made of smooth muscle that controls the diameter of the pupil.

• Lens: Suspended behind the pupil by suspensory ligaments attached to the ciliary body.

• Vitreous Humor: A gel-like material (colloid) in the posterior chamber.

• Fovea: The focal point at the back of the posterior chamber.

Summary of Light Passage

Light passes through:

1. Cornea

2. Aqueous humor (anterior chamber)

3. Pupil (opening in iris)

4. Lens

5. Vitreous humor

6. Fovea

Optic Nerve and Blind Spot

• Photoreceptors: Rods and cones in the retina convert light into nerve impulses.

• Optic Nerve: Carries visual information to the visual interpretation centers in the brain.

• Blind Spot: An area on the retina where the optic nerve forms, lacking photoreceptors. The brain filters out this area, so we are typically unaware of it.

📸 Photoreceptors: Rods and Cones

Photoreceptors are cells in the retina that detect different wavelengths of light and translate them into nerve impulses.

• Rods: Sensitive to dim light, enabling black and white vision.

• Cones: Provide color vision and are most sensitive to red, green, and blue light. The brain interprets varying degrees of stimulation from these cones as different colors.

💔 Pathology of the Eye

Colorblindness

• Inability to see certain colors properly.

• Most common form is red-green colorblindness, where the red and green photoreceptors don't function correctly.

• Sex-linked trait, more common in males due to the mutation being on the X chromosome.

Normal Vision

• Light bends or refracts as it passes through the eye.

• Ideally, light rays converge on the fovea for clear vision.

• The lens is primarily responsible for refracting and bending light.

Myopia (Nearsightedness)

• Difficulty seeing objects clearly at a distance. The farther away an object is, the blurrier it appears.

• Occurs because the focal point is not correctly aligned on the fovea.

👁 Vision Problems & Corrections

Nearsightedness (Myopia)

Nearsightedness occurs when the focal point of light entering the eye falls in front of the fovea (the central focal point on the retina).

• Results in blurry distance vision.

• Corrected with:

  • Glasses

  • Contact lenses

  • Surgical procedures to adjust the focal point

Farsightedness (Hyperopia)

Farsightedness is the opposite of nearsightedness; the focal point falls behind the retina.

• Results in difficulty reading fine print or seeing close objects.

• Corrected with:

  • Lenses

  • Surgical procedures

Astigmatism

Astigmatism is related to an irregular curvature of the cornea.

• Results in blurry vision.

Glaucoma

Glaucoma is characterized by high intraocular pressure.

• Elevated pressure can damage the retina and the optic nerve.

• Historically a leading cause of blindness.

• Can be treated with drugs to relieve intraocular pressure.

👂 Anatomy and Function of the Ear

The ear is divided into three main regions: the outer ear, the middle ear, and the inner ear.

Outer Ear

• Pinna: Channels sound waves down the ear canal.

• Auditory Canal: Conducts sound waves to the tympanic membrane.

• Tympanic Membrane: The eardrum, which vibrates in response to sound waves.

Middle Ear

The middle ear contains the ossicles (three small bones):

• Oval Window: Area of the middle ear wall that separates the middle ear from the inner ear. The stapes is attached to it.

• Eustachian Tube: Connects the middle ear to the pharynx (back of the throat) to equalize pressure.

• Round Window: Another thin area of the wall separating the middle and inner ear, which helps regulate pressure.

Inner Ear

• Cochlea: A snail-like structure containing the organ of Corti, the organ of hearing that translates fluid waves into nerve impulses.

• Semicircular Canals: Three canals involved in balance and spatial orientation.

Function

1. Sound waves enter the outer ear and are channeled down the ear canal.

2. The tympanic membrane vibrates at the same frequency as the sound waves.

3. The ossicles (malleus, incus, and stapes) also vibrate at the same frequency.

4. The stapes (stirrup) pushes back and forth on the oval window, creating a fluid wave in the cochlea.

5. The round window bulges outward into the middle ear to regulate pressure.

6. The fluid wave in the cochlea stimulates the organ of Corti, which translates the wave into nerve impulses.

👂 How the Cochlea Works

Anatomy and Function of the Cochlea

The eardrum (tympanic membrane) connects to the ossicles (hammer, anvil, and stirrup). The stirrupconnects to the oval window. When the stirrup pushes in, it creates a fluid wave in the perilymph inside the cochlea. This wave moves around the cochlea, causing the round window to bulge out, alleviating pressure.

Organ of Corti

The organ of Corti is located in the middle of the cochlea and is responsible for hearing. It converts fluid waves into nerve impulses that the brain interprets as sound.

• Lined with ciliated cells tuned to different sound frequencies.

• When a fluid wave moves around the cochlea, the cilia bend.

• This bending creates a nerve impulse.

Differentiating Sound: Pitch and Volume

The cochlea differentiates pitch and volume.

Pitch:

• Determined by the frequency of the sound wave.

• Low-frequency sound waves stimulate ciliated cells on the front side of the cochlea.

• High-frequency sound waves stimulate cells on the opposite side of the cochlea.

Volume:

• Determined by the number of hair cells stimulated.

Equilibrium: Semicircular Canals, Saccule, and Utricle

Aside from the cochlear apparatus, the inner ear also includes the semicircular canals, saccule, and utricle, which are all essential for equilibrium.

Dynamic Equilibrium:

• Controlled by the semicircular canals.

• Gives us a sense of balance when we're moving.

• Fluid inside the canals moves and bends hair cells.

Static Equilibrium:

• Controlled by the saccule and utricle.

• Maintains posture and equilibrium when we're not moving.

Summary Table