Ear and equilibrium

Anatomy of the Ear

The ear is divided into 3 regions: the external (outer) ear, the middle ear, and the internal (inner) ear.

External Ear (Pinna)

Consists of the auricle, external auditory canal, and tympanic membrane, and extends from the auricle to the tympanic membrane.

Auricle

The flexible external structure that is commonly called the ear. Collects sound waves and directs them toward the external auditory canal. The rim of the auricle is called the helix and the fleshy, inferior portion is the lobule.

External Auditory Canal

Conducts sound waves from the auricle to the tympanic membrane.

Tympanic Membrane/Eardrum

Converts sound waves to vibrations that are transferred to middle ear structures.

Middle Ear

Air-filled cavity within the temporal bone that extends from the tympanic membrane to a thin bony partition which contains the oval and round windows. Middle ear structures include auditory ossicles, oval window, round window, and auditory tube.

Auditory Ossicles

Small bones within the cavity that are connected by synovial joints. These bones transfer vibrations from the tympanic membrane to the oval window. The malleus is the outermost bone and is attached to the tympanic membrane. The incus is the middle bone and connects to the stapes. The innermost bone is the stapes, which connects to the incus and oval window.

Oval Window

The oval window is the membrane covered opening that separates the middle and inner ear and transfers vibrations to the inner ear.

Round Window

Membrane covered opening between the middle ear and cochlea of the inner ear.

Auditory Tube

Connects the middle ear to the nasopharynx and the air pressure of the middle ear with atmospheric air.

Sound Conduction

Air to bones to water: middle ear serves as an impedance match.

Internal Ear

Housed within the temporal bone. It consists of cavities within the bone called the bony labyrinth that encloses a series of connected membranous sacs, the membranous labyrinth.

Bony Labyrinth

Contains a fluid called perilymph that surrounds the membranous labyrinth. Endolymph is a fluid within the membranous labyrinth. The bony labyrinth has 3 main regions: the vestibule, the semicircular canals, and the cochlea.

Vestibule

Is in the middle area of the bony labyrinth that contains 2 sections of membranous labyrinth, the utricle and the saccule. These help with static equilibrium.

Utricle

The utricle is the posterior section of the membranous labyrinth within the vestibule, and it houses equilibrium receptors. Maculae are thousands of hair cells, covered by the otolithic membrane (little stones gravity pulls on).

Saccule

Anterior section of the membranous labyrinth within the vestibule. The saccule is continuous with the utricle and also houses equilibrium receptors.

Semicircular Canals

3 bony canals posterior to the vestibule that project posteriorly, laterally, and superiorly from the vestibule. Responsible for dynamic equilibrium. Each canal is at right angles to the other two.

Semicircular Ducts

Sections of membranous labyrinth within the semicircular canals that connect with the utricle and contain equilibrium receptors.

Cochlea

Spiral area of the bony labyrinth anterior to the vestibule. The cochlear duct is the section of membranous labyrinth within the cochlea and contains the hearing receptors.

Hearing and Equilibrium Nerve

Hearing and equilibrium receptors initiate nerve impulses that are carried by the vestibulocochlear nerve to the brain. This nerve has 2 branches: the vestibular branch that carries nerve impulses generated by equilibrium receptors and the cochlear branch that carries nerve impulses generated by the hearing receptors.

Sound Waves Become a Nerve Impulse

Cranial nerve 8 goes to medulla to midbrain to thalamus to temporal lobe (auditory area) of the cerebral cortex.

Hearing Loss

Hearing loss can be described as either conduction deafness or sensorineural deafness.

Conduction Deafness

Occurs when there is a decrease ability to conduct the energy of sound waves through the external and middle ear to hearing receptors in the inner ear. Ear wax buildup, damage to the tympanic membrane, or fusion of auditory ossicles may cause conduction deafness.

Sensorineural Deafness

Caused by damage to hearing receptors, damage to the cochlear branch of the vestibulocochlear nerve, or damage of the neural pathways to the auditory cortex.

Equilibrium Receptors

Provide information that enables the body to maintain balance. There are two types of equilibrium receptors: static and dynamic. Inflammation of or injury to equilibrium receptors results in an inability to maintain body position, vertigo, and/or dizziness.

Static Equilibrium Receptors

Provide information about body positions relative to the force of gravity (standing upright vs being upside down).

Dynamic Equilibrium Receptors

Provide information about body position in response to sudden movement such as rotation, acceleration, and deceleration (spinning, going faster, stopping).

Vertigo

Is the sensation of circular motion either of oneself or external objects, while dizziness is often used to describe faintness, unsteadiness, or lightheadedness. Severe vertigo may be accompanied by nystagmus - rapid, involuntary movement of eyeballs.

Unilateral Deafness Test

Weber test - Tuning fork on forehead.

Conduction Deafness Test

Rinne test - Tuning fork on mastoid process and listening for when it stops being heard.

Static Equilibrium Test

Balance test - Stand still with eyes closed.

Dynamic Equilibrium Test

Barany test - Rotate on a stool, observe eye movements.

Nose and Olfaction

The nose contains the receptors for the sense of smell or olfaction.

Olfactory Epithelium

Specialized area of the epithelium lining the nasal cavity. The olfactory epithelium covers the inferior surface of the cribriform plate and extends over the superior nasal concha.

Olfactory Receptor Cells

Are bipolar neurons whose dendritic end is embedded in the mucus layer covering the surface of the olfactory epithelium and whose axons form the olfactory nerves.

Olfactory Nerve

Pass through olfactory foramina in the cribriform plate and synapse on neurons in the olfactory bulb. Nerve impulses then travel along the olfactory tract to the primary olfactory cortex in the temporal lobe of the cerebrum. Olfactory receptors adapt to odors very quickly.