The Nervous System: Sensory Systems

The Nervous System: Sensory Systems — The Ear and Hearing

Chapter Outline

• 10.4 The Ear and Hearing
• 10.5 The Ear and Equilibrium

Learning Outcomes

• Describe the two sensory systems of the ear.

10.4 The Ear and Hearing

Anatomy of the Ear

• Outer Ear
  • Pinna: The external part of the ear that collects sound waves.
  • External Auditory Meatus: The canal that directs sound waves to the tympanic membrane.
• Middle Ear
  • Tympanic Membrane: Also known as the eardrum, it vibrates in response to sound waves.
  • Ossicles: Three small bones that amplify sound vibrations.
  • Malleus: Also known as the hammer.
  • Incus: Also known as the anvil.
  • Stapes: Also known as the stirrup, connects to the oval window.
  • Oval Window: Membrane-covered opening leading from the middle ear to the inner ear.
  • Round Window: Another membrane-covered opening that helps with pressure release.
• Inner Ear
  • Cochlea: A spiral-shaped organ responsible for transducing sound energy into neural signals.
  • Vestibular Apparatus: Involved in balance and spatial orientation.
  • Eustachian Tube: Equalizes pressure between the middle ear and the environment, also known as the auditory tube.

The Nature of Sound Waves

• Definition: Sound waves are mechanical waves caused by air molecules put into motion.
• Wavelength: The distance between successive peaks of a wave.
  • Compressed regions (high pressure) and rarefied regions (low pressure).
• Properties of Sound Waves:
  • Loudness: Related to the amplitude of sound waves, measured in Decibels (dB), which uses a logarithmic scale.
  • Pitch: Determined by the frequency of sound waves, measured in Hertz (Hz), with a typical human hearing range from 20 Hz to 20,000 Hz; optimal hearing occurs between 1000 Hz and 4000 Hz.

Sound Amplification in the Middle Ear

• Mechanism:
  • Tympanic Membrane: Vibrates at the same frequency as incoming sound waves.
  • Movement of Ossicles: Acts as the first level of amplification, where each ossicle increases the vibration.
  • Movement of Oval Window: Acts as the second level of amplification; it is an area of large to small surface area transition—from tympanic membrane to oval window.

Signal Transduction for Sound

• Definition: The conversion of sound energy into action potentials.
• Cochlea of Inner Ear:
  • Structure consists of fluids:
  • Perilymph: Fills the scala vestibuli and scala tympani.
  • Endolymph: Fills the scala media.
• Anatomy of Cochlea:
  • Hair Cells of the Organ of Corti:
  • Inner Hair Cells: Serve as receptor cells for sound.
  • Outer Hair Cells: Modulate the sensitivity of inner hair cells.
  • Stereocilia: Extensions from hair cells that are embedded in the tectorial membrane, arranged from short to tall.
  • Connecting Proteins: Include tip link proteins that play a crucial role in transduction.

Mechanism of Sound Transduction

• Steps:
  1. Mechanical stress from vibrations causes the deflection of the basilar membrane and hair cells.
  2. Bending of stereocilia against the tectorial membrane.
  3. At rest (no sound), there is a baseline tone under which tip link proteins maintain tension.
  4. When bending occurs towards the taller stereocilia: ion channels open, resulting in depolarization (K+ influx and Ca2+ entry).
  5. When bending occurs towards shorter stereocilia: ion channels close, resulting in hyperpolarization.

Coding for the Qualities of Sound

• Intensity Coding: Reflects the amplitude (in dB), which corresponds to the degree of deflection of stereocilia and subsequently the opening of ion channels.
• Frequency Coding: Related to the pitch (in Hz), determined by the specific location of deflection along the basilar membrane.

Neural Pathways for Sound

• Hair cells act as receptor cells that interact with the modified neurons:
  • Afferent neurons of the cochlear nerve (cranial nerve VIII).
  • One hair cell corresponds to one cochlear nerve fiber.
  • Auditory pathways: Cranial nerve VIII enters the medulla and synapses with neurons in the cochlear nuclei.
  • Some pathways remain ipsilateral while others decussate (cross over).
  • Signals travel to the thalamus then to the auditory cortex.
  • Tonotopic Arrangement: Specific arrangement in the auditory cortex related to frequency.

Clinical Defects: Deafness

• Conductive Deafness: Inadequate conduction of sound waves through the external and/or middle ear.
• Sensorineural Deafness: Inadequate transduction of sound waves to electrical signals in the inner ear, affecting the cochlea and cranial nerve VIII.
• Central Deafness: Damage occurring in the Central Nervous System (CNS) neural pathway responsible for sound processing.

10.5 The Ear and Equilibrium

Anatomy of the Vestibular Apparatus

• Components:
  • Semicircular Canals: Detect rotational movements.
  • Anterior Canal: Detects head movements up or down on an axis.
  • Posterior Canal: Detects head movements left to right on an axis.
  • Lateral (Horizontal) Canal: Detects turning of the head side to side on an axis.
• Hair Cells: Located in ampulla, responsible for detecting motion through their stereocilia and kinocilium embedded in a gelatinous mass called cupula.

Detection of Movement

• Dependent on the differential effects of motion on opposite sides of the head during head rotation:
  • Acceleration: Depolarization occurs on one side and hyperpolarization on the other.
  • Post-Rotation: The reverse effect occurs as the body stops moving.

The Utricle and Saccule - Linear Acceleration

• Anatomy: Located between semicircular canals and cochlea, oriented to detect linear acceleration.
  • Utricle: Responsible for detecting forward and backward motion.
  • Saccule: Responds to vertical (up and down) motion.

Receptor Cells

• Receptor cells in these regions are similar to those found in the cochlea (hair cells, stereocilia, kinocilium).
• They are surrounded by gelatinous material and contain otoliths, which assist in motion detection due to inertia.