Lec16 hearing and vestibular handout

Sensory Physiology: The Ear

Goals

• Describe general anatomy of the ear.

• Describe cellular mechanisms of hearing and balance.

Outline

1. Introduction

2. General structure of the ear

   • External, middle, and parts of the inner ear (cochlea)

   • Structure and function

   • Hearing: transduction by hair cells (conversion of sound into electrical signals)

   • Auditory pathways

3. Equilibrium (balance, acceleration)

   • Parts of the inner ear (vestibular apparatus, semicircular canals, otolith organs)

   • Structure and function

   • Vestibular pathways

Introduction to Hearing

• Hearing is the brain’s perception of sound energy.

• Sound transduction: conversion of mechanical energy of sound waves to electrical energy.

• Pitch: Interpretation of frequency.

• Loudness: Perception of intensity or amplitude of sound waves.

• Sound processing provides information about distance, direction, and movement.

Wavelength

• Sound waves consist of alternating peaks of compressed air and valleys of low compressed air.

General Structure of the Ear

External Ear

• Pinna (Auricle): Elastic cartilage covered with skin.

• External Auditory Canal (Meatus): Channel leading to the tympanic membrane.

• Tympanic Membrane: Vibrates in response to sound.

Middle Ear

• Eustachian Tube: Connects the middle ear to the pharynx.

• Ossicles: Three small bones (Malleus, Incus, Stapes) that amplify sound.

  • Malleus: Attached to tympanic membrane.

  • Incus: Between malleus and stapes.

  • Stapes: Connects to oval window.

Inner Ear

• Bony Labyrinth: Protects the membranous labyrinth; tunnels in the temporal bone.

• Membranous Labyrinth: Fluid-filled organs.

  • Cochlear Duct: Contains Organ of Corti.

  • Semicircular Ducts: Each has a crista ampullaris.

  • Utricle/Saccule: Each contains a macula.

Hearing Mechanism

Sound Transmission

• Sound waves hit the tympanic membrane, causing it to vibrate.

• Vibrations are transmitted through the ossicles to the oval window.

• Vibrations create fluid waves in the cochlea, pushing on flexible membranes of the cochlear duct.

• Hair Cells: Bend, opening ion channels, creating electrical signals that release neurotransmitters and generate action potentials in sensory neurons.

Inner Ear Structure

• Cochlea: The cochlear duct has three ducts: vestibular duct and tympanic duct (both with perilymph), cochlear duct (filled with endolymph).

• Tectorial Membrane: Covers hair cells in the Organ of Corti.

Hair Cell Function

• Hair cells have sterocilia connected by tip-links, which open mechanically gated K+ channels.

• At rest, about 10% of the ion channels are open, sending a tonic signal.

• Excitation: Bending towards kinocilium depolarizes the cell, increasing action potential frequency.

• Inhibition: Bending away hyperpolarizes the cell, decreasing action potential frequency.

Basilar Membrane and Frequency Sensitivity

• High-frequency sounds stimulate the stiff region near the round window; low frequencies stimulate the flexible distal end.

• The displacement of the basilar membrane’s active hair cells determines perceived pitch.

Auditory Pathways

• Primary sensory neurons send axons via vestibulocochlear nerve (VIII) to the cochlear nucleus in the medulla.

• From cochlear nucleus, signals transverse several brain areas, including the thalamus and auditory cortex.

Hearing Loss

• Types:

  • Conductive: Damage to the eardrum or middle ear.

  • Sensorineural: Damage to hair cells or cochlear implants can help.

  • Central: Lesions in the auditory cortex can impair processing.

Equilibrium and Vestibular Apparatus

• Otolith Organs and Semicircular Canals: Filled with endolymph.

  • Macula in Utricle and Saccule: Detect linear acceleration and head position.

  • Crista Ampullaris in Semicircular Canals: Detect rotational acceleration.

Equilibrium Pathways

• Hair cells in the vestibular apparatus synapse with the vestibular nerve, which projects to the cerebellum, thalamus, and cerebral cortex, controlling eye movements and balance.