Anatomy of the Ear

• Outer ear includes the external pinna and auditory canal
  • It collects sound waves and channels them to the tympanic membrane (eardrum)
• Middle ear consists of the malleus (hammer), incus (anvil), and stapes (stirrup)
  • It transmits vibrations to the oval window, which is a membrane beneath the stapes
  • It connects to the Eustachian tube, which connects to the pharynx and equalizes pressure
• Inner ear, also known as the labyrinth, is composed of semicircular canals and the cochlea, both of which are fluid-filled chambers
  • Semicircular canals are responsible for equilibrium
  • The cochlea functions in hearing
  • Vibrations at the oval window cause fluid motion, which excites receptors

Cochlea

• Consists of two fluid-filled canals called perilymph, specifically, vestibular and tympanic canals
• Contains the cochlear duct with endolymph
• Basilar membrane, located on the floor of the cochlear duct, supports the organ of Corti
  • The organ of Corti contains mechanoreceptors and hair cells with hair projections
• The tectorial membrane is positioned over the organ of Corti, allowing many hair cells to make contact with it
• Sound waves induce vibrations in the basilar membrane, leading to the bending of hair cells and activation of mechanoreceptors

Hearing

• Vibrating objects create pressure waves in the air
  • The tympanic membrane vibrates, and the middle ear transmits these vibrations to the oval window -Pressure waves are created in the fluid inside the cochlea
  • These waves exert pressure on the cochlear duct and the basilar membrane of the organ of Corti
• Hair cell projections are deflected.
  • Bending in one direction causes depolarization, increasing neurotransmitter release
  • Increased neurotransmitter release leads to a higher frequency of auditory nerve sensations
  • Bending in the opposite direction causes hyperpolarization, reducing neurotransmitter release
• Healthy young humans can hear frequencies between 20 and 20,000 Hz
  • Frequencies below 20 Hz travel through perilymph around the apex to the round window
  • Frequencies above 20 Hz take a shortcut through the cochlear duct, causing the basilar membrane to vibrate
• Maximum displacement occurs where fibers are tuned to a specific frequency
  • Fibers near the oval window are short and stiff and detect high frequencies
  • Fibers at the apex are longer and more flexible, responding to low frequencies
• The cochlea distinguishes pitch because the basilar membrane is not uniform
  • Different regions of the basilar membrane are tuned to particular frequencies

Equilibrium

• The vestibular apparatus in the inner ear detects body movement, position, and balance
• Fluid-filled sacs, the utricle and saccule, are used to perceive position in relation to linear acceleration in the horizontal plane, as well as head tilting and vertical movements
• Semicircular canals detect angular motion of the head in any direction
• Each saccule and utricle contains a macula
  • Maculae contain flat epithelial patches with hair cells
  • The hairs of these cells are embedded in an overlying otolith membrane, a jelly-like mass containing tiny stones called otoliths
• In the utricle, the maculae are horizontal, and the hairs are vertically oriented when the head is upright
  • Utricles respond best to acceleration in the horizontal plane and head tilting
• In the saccule, the macula is nearly vertical, and the hairs protrude horizontally into the otolith membrane
  • Saccules respond best to movements like the sudden acceleration of an elevator
• Maculae receptor activation occurs when the head starts or stops moving in a linear direction
  • Inertia causes the otolith membrane to slide backward or forward
• When hairs bend toward the kinocilium, hair cells depolarize
  • This increases neurotransmitter release and sends more impulses to the brain
• When hairs bend away from the kinocilium, hair cells/receptors hyperpolarize
  • This decreases neurotransmitter release and sends fewer impulses to the brain
• Maculae respond to changes in the velocity of head movement
• The crista ampullaris detects rotational movements and serves as the receptors for rotational acceleration
• Semicircular canals are located in all three planes of space
  • All rotational movements of the head disturb one or another pair of cristae
• Each crista consists of supporting cells and hair cells similar to those in the cochlea and maculae
• A gelled mass that forms an ampullary cupula (pointed cup)
  • It consists of a loosely organized network of gelatinous strands radiating outward to contact other hair cells
• Inertia causes the endolymph in the semicircular ducts to briefly move in the direction opposite the body’s rotation, deforming the cristae in the duct
• As the hairs bend, the hair cells depolarize, and impulses reach the brain at a faster rate
• Bending in the opposite direction causes hyperpolarization and generates fewer impulses