Hearing and Equilibrium Notes

Special Senses - Hearing

The Nature of Sound

• Sound is an audible vibration of molecules.
• A vibrating object pushes air molecules, creating sound waves.
• Pitch refers to the frequency of the sound wave, which vibrates specific parts of the ear.
• The human hearing range is approximately 20 Hz (low pitch) to 20,000 Hz (high pitch).
• Speech frequencies typically fall within the range of 1500-4000 Hz, where human hearing is most sensitive.
• Loudness is related to the amplitude or intensity of sound energy.

Ear Anatomy

• The ear consists of three main parts: the outer ear, middle ear, and inner ear.
• Outer Ear: Directs sound waves towards the tympanic membrane.
  • Auricle (pinna): The visible part of the ear.
  • Auditory canal: The passage leading to the tympanic membrane.
  • Ends at the tympanic membrane (eardrum)
• Middle Ear
• Inner Ear

Middle Ear

• The middle ear is an air-filled tympanic cavity located within the temporal bone.
• Components:
  • Auditory tube (Eustachian tube): Connects the middle ear to the nasopharynx, equalizing air pressure on both sides of the tympanic membrane.
  • Ear ossicles: Three small bones that transmit vibrations:
    • Malleus ("hammer")
    • Incus ("anvil")
    • Stapes ("stirrup")
  • Stapedius and tensor tympani muscles: Attach to the stapes and malleus, respectively, and help dampen loud sounds.

Inner Ear

• Bony labyrinth: A series of passageways within the temporal bone.
• Membranous labyrinth: Fleshy tubes lining the bony tunnels.
  • Filled with endolymph, a fluid similar to intracellular fluid.
  • The membranous labyrinth floats in perilymph, a fluid similar to cerebrospinal fluid.

How Hearing Works

1. Sound waves travel through the external auditory canal, causing the tympanic membrane to vibrate.
2. Vibrations are propagated by the ossicles (malleus, incus, stapes).
3. The stapes transfers vibrations to the oval window.
4. The oval window transfers vibrations to the perilymph in the scala vestibuli.

Inner Ear Details

• Sound waves represent alternating areas of high and low pressure.
• The tympanic membrane vibrates in response to sound waves.
• Vibrations are amplified across the ossicles.
• Vibrations against the oval window set up a standing wave in the fluid of the scala vestibuli.

Organ of Corti

• The organ of Corti contains:
  • Tectorial membrane
  • Stereocilia
  • Hair cells

Sound Sensation

• Waves in scala vestibuli perilymph lead to waves in scala tympani perilymph, causing vibration of the basilar membrane.
• As the basilar membrane vibrates, hair cells move up and down.
• The tectorial membrane does not move.

From Vibration to Action Potential

• Stereocilia are bathed in high K^+, creating an electrochemical gradient.
• Tips of stereocilia are embedded in the tectorial membrane.
• Bending of stereocilia in response to basilar membrane movement pulls on tip links and opens ion channels.
• K^+ flows into hair cells, causing depolarization and release of neurotransmitter.
• Neurotransmitter stimulates sensory dendrites at the base of the hair cells.

Potassium Gates

• Tip links connect stereocilia, mechanically gating K^+ channels.
• When unstimulated, the K^+ gate is closed.
• When stimulated, the K^+ gate opens, allowing K^+ to flow into the hair cell.

Sensory Coding

• Vigorous vibrations excite more inner hair cells over a larger area, triggering a higher frequency of action potentials.
• The brain interprets this as a louder sound.
• Pitch is determined by which part of the basilar membrane vibrates:
  • At the basal end (narrow and stiff), the brain interprets signals as high-pitched.
  • At the distal end (5 times wider and more flexible), the brain interprets signals as low-pitched.

Loudness vs. Frequency

• The graph illustrates loudness in decibels (dB) versus frequency in hertz (Hz).
• Threshold of pain is around 120 dB.
• Normal conversation (speech) is around 60 dB.
• Threshold of hearing is at 0 dB.

Processing Pathways for Hearing

1. The cochlear nerve joins the vestibular nerve to form the vestibulocochlear nerve (CN VIII).
2. First stop: Pons (both sides).
3. Next stop: Midbrain (inferior colliculus).
4. Next stop: Thalamus.
5. Primary auditory cortex: Located in the temporal lobe within the lateral sulcus.

Sound Localization

• Performed in the brain stem.
• Two methods:
  • Interaural time difference: The difference in time it takes for a sound to reach each ear.
  • Interaural intensity difference: The difference in intensity of a sound between the two ears.

Special Senses - Equilibrium

Equilibrium

• Sensed by the vestibular apparatus.
• Components:
  • Saccule and utricle: Detect static equilibrium (stationary head position) and linear acceleration (change in velocity in a straight line).
  • Semicircular ducts: Detect angular acceleration (change in rate of rotation).

Sensation of Position and Linear Acceleration

• Maculae:
  • Located in the utricle and saccule.
  • Contain hair cells and otoliths.
  • Otoliths are embedded in the otolithic membrane.
• When the head is upright, the otolithic membrane rests on the hair cells.
• When the head is tilted forward, the force of gravity causes the otolithic membrane to shift, bending the stereocilia.
• This bending of stereocilia leads to a shearing force that causes the stereocilia to bend in the direction of linear acceleration.

Sensation of Angular Acceleration

• Crista ampullaris:
  • Located within the ampulla of each semicircular canal.
  • Contains hair cells embedded in a gelatinous cupula.
• When the head is still, the cupula is in its resting position.
• As the head rotates, the endolymph within the semicircular canals moves in the opposite direction, causing the cupula and stereocilia to bend.
• Movement of two canals within a plane provides information about the direction in which the head is moving.
• Activation of all six canals gives a precise indication of head movement in three dimensions.

Vestibular Projection Pathways

• Vestibulocochlear nerve carries signals to the brainstem.
• Vestibular nuclei in the pons and medulla receive input from the vestibular apparatus.
• Projections:
  • Thalamus to vestibular cortex: Awareness of spatial orientation and movement.
  • Nuclei for eye movement: Compensatory eye movements.
  • Cerebellum: Motor coordination.
  • Reticular formation: Postural reflexes.
  • Vestibulospinal tracts: Innervate antigravity muscles.

Equilibrium Projection Pathways

• Hair cells synapse on the vestibular nerve.
• The vestibular nerve joins the cochlear nerve to form the vestibulocochlear nerve.
• First stop: Pons and Medulla (both sides).
• Then:
  • Cerebellum: Control of head, eye movements, and posture.
  • Nuclei of CN III, IV, and VI (vestibulo-ocular reflex).
  • Reticular formation: Control of blood circulation and posture.
  • Vestibulospinal tracts: Innervate antigravity muscles.
  • Thalamic relay to the cerebral cortex for awareness of position and movement.