Ears

Auditory Transduction

• Auditory transduction is the process by which sound waves are converted into electrical impulses.

• Sound waves travel through the external auditory canal to the tympanic membrane (eardrum).

• The tympanic membrane vibrates in response to sound, influenced by frequency and amplitude.

  • Lower frequency sounds create slower vibrations.

  • Higher frequency sounds produce faster vibrations.

Anatomy of the Ear

• Tympanic membrane is cone-shaped, connected to auditory ossicles: malleus, incus, stapes.

• The ossicles amplify the sound vibrations transmitted from the tympanic membrane.

• They pivot on ligaments, including:

  • Anterior malleol ligament

  • Posterior incudal ligament

• The stapes transfers vibrations into the bony labyrinth, filled with perilymph fluid, adjusting due to the flexible round window membrane.

Cochlea Structure

• The cochlea has three main sections:

  • Scala vestibuli (ascending portion)

  • Scala tympani (descending portion)

  • Cochlear duct (between scala vestibuli and tympani, filled with endolymph)

• Membranes: Reissner's membrane and basilar membrane respond to vibrations.

• Organ of Corti resides on the basilar membrane, where hair cells generate nerve impulses to the brain via the cochlear nerve.

Tonotopic Organization

• Vibrations along the basilar membrane vary with frequency:

  • Lower frequencies cause vibrations closer to the cochlear apex.

  • Higher frequencies activate areas closer to the base.

• Each area corresponds with specific frequencies, enabling sound discrimination.

Hearing Range and Age

• Human hearing range is typically up to ~20,000 hertz.

• High frequencies are often the first lost with age due to noise exposure damaging hair cells.

• Hair cells responsible for high frequencies deteriorate first, making it harder to hear these pitches with age.

Sound Processing in the Brain

• Auditory signals travel through the cochlear nerve to the brain:

  • Goes through the medulla, midbrain, and thalamus.

  • Eventually reaches the primary auditory cortex in both temporal lobes.

• Sound localization occurs by comparing the timing of sound arrival at each ear.

Hearing Disorders

• Tinnitus: Permanent ringing in the ears, often related to damage to hair cells or nerves.

• Can occur naturally or be a focus of the brain on internal sounds.

• Meniere's Syndrome: A disorder affecting the inner ear, causing extreme vertigo and uncertainty in duration.

• Frequent exposure to loud sounds can lead to hearing loss by damaging hair cells.

Equilibrium and Balance

• The vestibular system consists of:

  • Semicircular canals (for rotational movements)

  • Utricle and saccule (for linear movements)

• Hair cells in these structures send continuous action potentials for balance assessment.

• Nystagmus: Involuntary eye movements often experienced after spinning, indicating how the vestibular system responds to motion.

Motion Sickness

• Caused by conflicting sensory information; feeling motion when still.

• Can be alleviated by visual cues indicating motion to counteract disorientation.

Summary of Key Concepts

• The ear's anatomy and its functioning are essential for hearing and balance.

• Understanding auditory transduction reveals how sound is perceived.

• Aging affects hearing capacity, especially in high frequencies, emphasizing the need for ear protection.

• Auditory processing in the brain is crucial for sound localization and interpretation.