Eye and Ear Anatomy

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Eye and Ear Overview

• Overview of the special senses which include sight, hearing, and balance.
• Includes the structures associated with the special senses.

Eye Structure

• The eye consists of three tunics: Fibrous, Vascular, and Retina.

Fibrous Tunic

• This is the outermost layer of the eye, consisting of the sclera and the cornea.
• Sclera is the posterior 5/6 of the fibrous tunic.
• The sclera is responsible for protection, maintaining shape, and muscle attachment; "skleros" translates to hard.
• The cornea is the transparent, anterior 1/6 of the fibrous tunic.
• Cornea refracts light.
• The cornea has sensory capabilities from surface irritation via cranial nerve V1.

Fibrous Tunic and Eye Muscles

• There are 4 rectus muscles that primarily pull the eye up (superior), down (inferior), in (medial), and out (lateral).
• There are 2 oblique muscles that primarily rotate the eye; superior and inferior obliques.

Vascular Tunic

• The vascular tunic has three components: choroid, ciliary body, and iris.
• The choroid is a network of capillaries supplying oxygen and nutrients to the retina, and absorbs unnecessary light.
• The ciliary body consists of a ciliary muscle that acts like a sphincter.
• Suspensory ligaments connect the ciliary body muscle to the lens.
• The ciliary body controls the shape of the lens.
• The shape of the lens changes for distant and near vision.
  • For distant vision, ciliary muscles are relaxed and the lens is flattened.
  • For near vision, the ciliary muscles contract, making the lens more spherical.
• The iris is the colored part of the eye.
• Iris has two smooth muscle groups that control the amount of light entering the eye through the pupil.
  • Sphincter pupillae constricts the pupil (parasympathetic innervation from CN 3).
  • Dilator pupillae dilates the pupil (sympathetic innervation from the spinal cord).

Retina

• The retina has two layers: a thin outer pigmented layer and a thin inner neural layer.
• The outer pigmented layer receives light rays.
• The inner neural layer houses photoreceptor cells (rods and cones) and their neurons.
• The inner neural layer is where light is processed.
• The optic disk (blind spot) is where axons of neurons come together to form CN2 (Optic Nerve).
• The macula lutea is a distinct area where light is focused, and sharpest focus occurs at a small pit in the center called the fovea centralis.

Eye Cavities

• The lens separates the eye into two segments/cavities.
• The anterior cavity contains aqueous humor, a transparent, watery fluid continuously produced by ciliary processes and circulating to provide nutrients and oxygen to the cornea and lens.
• The posterior cavity contains vitreous humor, a transparent and gelatinous permanent fluid aiding in maintaining eye shape and supporting the retina.

Accessory Structures of the Eye

• Eyelids (palpebrae) protect the anterior eye and distribute lacrimal fluid during blinking.
• Eyelids are made of skin, part of the orbicularis oculi, tarsal plates & glands, and a layer of conjunctiva.
• Conjunctiva = 1 continuous membrane
  • Surface 1: Inside of Eyelid, called Palpebral Conjunctiva
  • Surface 2: Surface of Sclera, called Bulbar/Ocular Conjuctiva
  • Bulbar/Ocular conjunctiva does not cover the cornea.
• Mucous membrane moistens and protects the eye; blood vessels there supply anterior sclera.
• Lacrimal apparatus is responsible for tear production and drainage
  • Lacrimal fluid (tears) is produced by the lacrimal gland (innervated by CN7).
  • Lacrimal fluid is dispersed across the eye.
  • Lacrimal fluid enters 2 canaliculi through a small opening known as a punctum.
  • Lacrimal fluid drains through the nasolacrimal duct.
  • Lacrimal fluid enters the nasal cavity under the inferior nasal.

Ear Structure

• The ear has three main parts: external, middle, and internal ear.

External Ear

• The external ear includes the auricle, external acoustic meatus, and tympanic membrane.
• The auricle directs sound waves.
• The external acoustic meatus is a canal with skin.
• The tympanic membrane, also known as the eardrum (tympanon = drum), vibrates and transmits sound wave energy to 3 bones of the middle ear.

Middle Ear

• The middle ear consists of an air-filled tympanic cavity, three auditory ossicles (bones), two muscles, and the auditory (Eustachian) tube.
• The three auditory ossicles (bones) are malleus, incus, and stapes.
  • Malleus is attached to the tympanic membrane.
  • Stapes presses on the oval window (beginning of inner ear).
• Two muscles:
  • Tensor tympani m.
  • Stapedius m.
  • These restrict ossicle movement during loud noise to protect the inner ear
• The auditory (Eustachian) tube connects the tympanic cavity to the upper throat.
• The auditory (Eustachian) tube opens during chewing, yawning, and swallowing to equalize pressure.

Inner Ear

• Inner ear consists of membrane-lined cavities in the temporal bone (petrous portion).
• Inner ear consists of three parts: cochlea, vestibule, and semicircular canals.
  • The cochlea is the snail shell.
  • The vestibule is the entrance court (vestibulum).

Overview of Hearing (Cochlea)

• Membranes are fluid filled.
• Stapes bone vibrates and presses on fluid at the oval window, because of sound.
• Receptors of the spiral organ recognize movement of fluid through the cochlea to create nerve impulses.
• The cochlear portion of CN 8 carries nerve impulses.

Overview of Equilibrium

• Membranes are fluid filled.
• Movement of the head (rotation) activates receptors in semicircular canals.
• Change in linear position (start-stop, up-down, side-side) activates receptors in the vestibule.
• The vestibular portion of CN8 (vestibulocochlear nerve) carries these impulses to the brainstem.