Anatomy and Function of the Eye

Special Senses

• Special senses include smell, taste, sight, hearing, and equilibrium.
• Special sense receptors are large, complex sensory organs or localized receptor clusters.

Nervous System Functions

• The nervous system has three basic functions.
• Each special sense gathers unique sensory info, which, when integrated, influences motor output.
• For example, seeing a ball coming towards your head may cause a motor output to move your body out of the way.
• Each type of sensory info is processed in a specific area of the cerebrum.

The Eye

• 70% of all sensory receptors are in the eyes.
• Each eye has over 1 million nerve fibers carrying information to the brain.

Accessory Structures

• Accessory structures of the eye include extrinsic eye muscles, eyelids, conjunctiva, and the lacrimal apparatus.
  • Extrinsic eye muscles produce gross eye movements.
  • Eyelids meet at the medial and lateral commissures (canthus).
  • Tarsal glands lubricate the eye.
  • Ciliary glands are located between the eyelashes.
  • Conjunctiva is a membrane lining the eyelids and eyeball.
  • It connects with the transparent cornea.
    • It secretes mucus to keep the eye moist.
  • Lacrimal apparatus is the lacrimal gland and ducts.
    • Lacrimal gland produces lacrimal fluid (tears).
    • Located at the lateral end of each eye.
    • Tears drain across the eye, into the lacrimal canaliculi, then the lacrimal sac, and into the nasolacrimal duct, which empties into the nasal cavity.
    • Tears contain dilute salt solution, mucus, antibodies, and lysozyme (bacteria-destroying enzyme).
    • Tears cleanse, protect, moisten, and lubricate the eye.

Anatomy of the Eye

• Three layers (tunics) form the wall of the eyeball: fibrous, vascular, and sensory.
  • Fibrous layer: sclera + cornea
    • Sclera is the white connective tissue layer, seen as the "white of the eye."
    • Cornea is the transparent, central anterior portion that allows light to pass through. It can repair itself easily and is the only human tissue that can be transplanted without fear of rejection.
  • Vascular layer: choroid, ciliary body, and iris
    • Choroid is a blood-rich nutritive layer that prevents light scattering.
    • Ciliary body is modified anteriorly into two smooth muscle structures.
      • Iris regulates the amount of light entering the eye.
      • Pupil is the rounded opening in the iris.
  • Sensory layer: retina; contains rods and cones, specialized receptor cells (photoreceptors)
    • Outer pigmented layer absorbs light and prevents scattering.
    • Inner neural layer contains receptor cells (photoreceptors)
      • Rods: important in dim light and peripheral vision (gray tones only).
      • Cones: important for detailed color vision and concentrated in the fovea centralis.
    • Electrical signals pass from photoreceptors via a two-neuron chain (bipolar neurons, ganglion cells), and leave the retina through the optic nerve.
    • The optic disc is the blind spot where the optic nerve leaves the eyeball.

Internal Structures

• Humors are fluids filling the interior of the eyeball.
  • Lens divides the eye into two chambers
    • Anterior segment: located anterior to the lens, and contains aqueous humor (clear, watery fluid similar to blood plasma).
      • Aqueous humor maintains intraocular pressure and provides nutrients for lens and cornea.
      • Aqueous humor is reabsorbed into venous blood through the scleral venous sinus (canal of Schlemm).
    • Posterior segment: located posterior to the lens, and contains vitreous humor (gel-like substance).
      • Vitreous humor prevents the eye from collapsing, and helps maintain intraocular pressure.
  • Ophthalmoscope: instrument used to illuminate the interior of the eyeball (fundus), allowing doctors to detect problems like diabetes, arteriosclerosis, and optic nerve/retina degeneration.

Physiology of Vision

• Light pathway through the eye and refraction

  • Light is focused onto the retina by the cornea, aqueous humor, lens, and vitreous humor.
  • Eye is set for distant vision (over 20 ft).
  • Accommodation: adjusting lens shape to focus on closer objects (less than 20 ft).
    • Light from distant objects are focused precisely on the retina, while images from closer objects are focused in front of the retina. A concave lens is required to refract light more for myopic (nearsighted) individuals.

• Real images are:

  • Reversed left to right.
  • Upside down
  • Smaller than the object.

• Visual fields and visual pathways to the brain

  • Optic nerve: a bundle of axons that exit the back of the eye carrying impulses from the retina.
  • Optic chiasma: location where optic nerves cross over so fibers from the medial side of each eye cross over to the opposite side of the brain.
  • Optic tracts
    • Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye
    • Synapse with neurons in the thalamus.
  • Optic radiations: axons from the thalamus run to the occipital lobe; synapse with cortical cells; vision interpretation
  • Optic cortex (in occipital lobe): the point where visual interpretation occurs.

• Visual fields: each eye has a slightly different view, but overlapping fields result in binocular vision, which provides depth perception (three-dimensional vision).

• Eye reflexes

  • Convergence: reflexive movement of the eyes medially when focusing on a close object.
  • Photopupillary reflex: bright light causes pupils to constrict.
  • Accommodation pupillary reflex: viewing close objects causes pupils to constrict, which allows focusing on near objects.

The Ear - Hearing and Balance

• Ear houses two senses: hearing and balance.
• Receptors are mechanoreceptors.
• Different organs house receptors for each sense.
  • The ear is divided into external (outer), middle, and internal (inner) ears.

Ear Anatomy

-External (outer) ear
    - Auricle (pinna): collects sound waves.
    - External acoustic meatus (auditory canal): lined with skin and ceruminous glands (produce cerumen, or earwax), containing eardrum
-Middle ear cavity (tympanic cavity)
    - Air-filled, mucosa-lined cavity within temporal bone.
    - Only involved in hearing.
    - Located between tympanic membrane and oval window and round window.
    - Pharyngotympanic tube (auditory tube): links middle ear cavity with the throat, equalizing pressure. Three ear ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
        - Function: transmit vibrations from tympanic membrane to the inner ear fluids via a lever-like system. Vibrations travel from malleus → incus → stapes → oval window
-Internal (inner) ear
    - Includes sense organs for hearing and balance.
    - Bony labyrinth (osseous labyrinth): contains perilymph.
        - Cochlea: functions in hearing; spiral-shaped bony tube.
        - Vestibule: small, oval, central portion.
        - Semicircular canals: functions in equilibrium; three bony tubes.
    - Membranous labyrinth: suspended in perilymph; contains endolymph
        - Crista ampullaris: responds to rotational movements / angular acceleration
        - Maculae: receptors for static equilibrium; report on head position; hair cells embedded in otolithic membrane.

Equilibrium

• Equilibrium receptors of the inner ear are called the vestibular apparatus.
• Vestibular apparatus has two functional parts:
  • Static equilibrium: Maculae –report the position of the head and help one keep their head upright. In the vestibule. Hair cells here are embedded in otolithic membrane, otoliths (tiny stones) float in gel around hair cells. Movement causing otoliths to roll and bend hair cells.
  • Dynamic equilibrium: Crista ampullaris –responds to angular or rotational movement of the head. in the ampulla of each semicircular canal. Tuft of hair cells covered with cupula (gelatinous cap). Movement causes cupula to drag against the endolymph. Hair cells stimulated and impulse to vestibular nerve → cerebellum.

Hearing

• The spiral organ of Corti lies within the cochlear duct.
• Receptors are hair cells found on the basilar membrane.
• Gel-like tectorial membrane can bend hair cells.
• Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe.
• Pathway of vibrations from sound waves: eardrum → ossicles → oval window → inner ear fluids → basilar membrane → hair cells of spiral organ of Corti → cochlear nerve → temporal lobe.
• High-pitched and low-pitched sounds affect different parts of the basilar membrane and cochlea. This ultimately affects different hair receptors.

Hearing and Equilibrium Deficits

• Deafness: any degree of hearing loss.
  • Conduction deafness: sound vibrations transmission through the external/middle ear is hindered.
  • Sensorineural deafness: damage to the nervous system structures involved in hearing.
• Ménière's syndrome: affects the inner ear can cause progressive deafness and/or vertigo.

Chemical Senses: Smell and Taste

• Chemical receptors: stimulated by chemicals in solution.
• Taste has 5 types of receptors.
• Smell can differentiate a wider range of chemicals.
• Both senses complement each other.
  • Olfactory receptors are found in the roof of the nasal cavity.
    • Receptor cells have olfactory hairs, detecting chemicals dissolved in mucus; impulses travel to olfactory nerve → olfactory cortex.
  • Taste buds contain receptor organs (gustatory cells).
    • Many located on the tongue, soft palate, and superior part of pharynx/cheeks.
    • Gustatory cells have gustatory hairs (microvilli) that extend through taste pores; stimulated by chemicals dissolved in saliva.
    • Impulses travel to gustatory complex by facial nerve, glossopharyngeal nerve, and vagus nerve.

Developmental Aspects of Special Senses

• Special sense organs form early in embryonic development.
• Maternal infections (first 5–6 weeks of pregnancy) may cause visual abnormalities/sensorineural deafness.
• The infant has poor visual acuity (farsighted), and lacks color vision and depth perception at birth.
• Vision develops until age 8 or 9.
• Age related issue in the eyes: presbyopia (decreasing lens elasticity, difficulty focusing on close objects), dryness from decreased lacrimal gland secretions etc.
• Age related issue in the ears: presbycusis (type of sensorineural deafness) results from otosclerosis (ear ossicles fuse).
• Taste and smell acuity is most acute at birth; sensitivity decreases with age.

This quiz covers the anatomy and function of the eye, including the cornea, sclera, and lens. It explores the importance of each structure and potential issues arising from compromised function or transparency. It also covers related concepts such as intraocular pressure and nutrient supply.