Special Senses: Eye Structure Quiz

Special Senses

• Special senses include smell, taste, sight, hearing, and equilibrium.

The Senses

• Smell
• Taste
• Sight
• Hearing
• Equilibrium

Structure of the Eye (Fibrous Layer)

• Sclera: White connective tissue layer, seen as the "white of the eye".
• Cornea: Transparent, central anterior portion, allows light to pass through, repairs easily, the only human tissue that can be transplanted without fear of rejection.

Structure of the Eye (Vascular Layer)

• Choroid: Blood-rich nutritive layer in the posterior of the eye. Pigment prevents light scattering.
• Ciliary body: Smooth muscle attached to the lens.
• Iris: Pigmented layer that gives the eye color.
• Pupil: Rounded opening in the iris.

Structure of the Eye (Sensory Layer)

• Retina: Contains two layers – outer pigmented layer and inner neural layer.
• Contains receptor cells (photoreceptors):
  • Rods: Found towards the edges of the retina, allow dim light and peripheral vision, perception is in gray tones.
  • Cones: Allow detailed color vision, densest in the center of the retina. Fovea centralis - area of the retina with only cones.
• Signals leave the retina toward the brain through the optic nerve.
• Optic disc (blind spot): Where the optic nerve leaves the eyeball; no photoreceptor cells are present.

Structure of the Eye (Lens)

• Biconvex crystal-like structure.
• Held in place by a suspensory ligament attached to the ciliary body.
• Cataracts: Result when the lens becomes hard and opaque with age, vision becomes hazy and distorted, eventually causing blindness in the affected eye.

Two Segments of the Eye

• Anterior/Aqueous segment: Anterior to the lens, contains aqueous humor.
• Posterior/ Vitreous segment: Posterior to the lens, contains vitreous humor.

Anterior Segment (Aqueous Humor)

• Watery fluid between the lens and cornea.
• Similar to blood plasma.
• Helps maintain intraocular pressure.
• Provides nutrients for lens and cornea.
• Reabsorbed into venous blood through the scleral venous sinus (canal of Schlemm).

Posterior Segment (Vitreous Humor)

• Gel-like substance posterior to the lens.
• Prevents the eye from collapsing.
• Helps maintain intraocular pressure.

Pathway of Light Through the Eye

• Light must be focused to a point on the retina for optimal vision.
• The eye is set for distance vision (over 20 feet).
• Accommodation: The lens must change shape to focus on closer objects (less than 20 feet).
• Images formed on the retina are real images. Reversed from left to right and upside down. Smaller than the object.

Defects of Vision

• Emmetropia: Eye focuses images correctly on the retina.
• Myopia (nearsightedness): Distant objects are blurry, light from those objects fails to reach the retina and focused in front of it. Results from an eyeball that is too long.
• Hyperopia (farsightedness): Near objects are blurry, distant objects are in focus. Results from an eyeball that is too short or from a "lazy lens".
• Astigmatism: Images are blurry, results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens.

Homeostatic Imbalances of the Eyes

• Night blindness: Inhibited rod function, hinders night vision.
• Color blindness: Genetic condition, inability to see certain colors. Due to the lack of one type of cone (partial color blindness).
• Glaucoma: Can cause blindness due to increase in pressure within the eye.
• Hemianopia: Loss of the same side of the visual field of both eyes; results from damage to the visual cortex on one side only.

The Ear

• Houses two senses: hearing and equilibrium.
• Receptors are mechanoreceptors.
• Different organs house receptors for each sense.

Anatomy of the Ear

• Divided into three areas:
  • External (outer) ear
  • Middle ear (tympanic cavity)
  • Inner ear (bony labyrinth)

The External Ear

• Involved in hearing only.
• Structures of the external ear include:
  • Auricle (pinna)
  • External acoustic meatus (auditory canal): Narrow chamber in the temporal bone, lined with skin and ceruminous (wax) glands, ends at tympanic membrane.

The Middle Ear (Tympanic Cavity)

• Air-filled cavity within the temporal bone.
• Only involved in the sense of hearing.
  • Two tubes associated with the inner ear:
    • Opening from the auditory canal covered by the tympanic membrane.
    • Auditory tube connecting the middle ear with the throat, equalizes pressure during yawning or swallowing. This tube is otherwise collapsed.

Bones of the Middle Ear

• Three bones (ossicles) span the cavity:
  • Malleus (hammer)
  • Incus (anvil)
  • Stapes (stirrup)
• Vibrations from the eardrum move the malleus → incus → stapes → inner ear.

Inner Ear or Bony Labyrinth

• Includes sense organs for hearing and balance.
• A maze of bony chambers within the temporal bone.
• Cochlea: Involved in hearing.
• Vestibule (static equilibrium): Position of the head.
• Semicircular canals (dynamic equilibrium): Movement of the head.

Organs of Equilibrium

• Semicircular canals
• Vestibule
• Vestibular nerve

Mechanism of Hearing

• Sound waves are channeled through the external ear to the middle ear. Vibrations in the eardrum cause vibrations to the three tiny bones (ossicles). The vibrations are amplified and passed to the inner ear.
• Fluids in the cochlear canals stimulate the spiral organ of Corti.
• Sensory signals travel to the auditory cortex via the vestibulocochlear nerve.

Olfaction (Smell)

• Olfactory receptors are in the roof of the nasal cavity.
• Neurons with long cilia.
• Chemicals must be dissolved in mucus for detection.
• Impulses are transmitted via the olfactory nerve.
• Interpretation of smells made in the cortex.

The Sense of Taste

• Taste buds house the receptor organs.
• Most are on the tongue.
• Soft palate
• Cheeks

Taste Sensations

• Sweet receptors (sugars)
• Sour receptors (acids)
• Salty receptors (metal ions)
• Bitter receptors (alkaloids)

Chemical Senses (Taste and Smell)

• Both use chemoreceptors, stimulated by chemicals in solution.
• Taste has four types of receptors, smell differentiates a range of chemicals.
• Senses complement each other and respond to similar stimuli.

Developmental Aspects of the Special Senses

• Formed early in embryonic development.
• Eyes are outgrowths of the brain.
• All special senses are functional at birth.

Test your knowledge on the special senses, with a focus on the structure of the eye. This quiz covers key components such as the sclera, cornea, choroid, and retina. Strengthen your understanding of how these parts contribute to vision and sensory perception.