Vertigo and Motion Sickness Quiz

Hearing & Equilibrium

• Hearing: The process of detecting sound
• Equilibrium: The sense of balance
• External ear: The outer part of the ear; includes the pinna and ear canal
  • Pinna: Collects and funnels sound waves
  • Ear canal: Amplifies sound waves
• Middle ear: The cavity between the eardrum and inner ear
  • Tympanic membrane (eardrum): Vibrates in response to sound waves
  • Auditory ossicles (malleus, incus, stapes): Amplify and transmit vibrations
• Inner ear: Fluid-filled chambers containing the structures for hearing and balance
  • Cochlea: Organ of hearing; contains receptor cells (hair cells)
    • Basilar membrane: Vibrates in response to fluid waves
  • Vestibular system: Responsible for maintaining balance
    • Semicircular canals: Detect head rotation
    • Otolith organs (utricle and saccule): Detect head position and linear movement

Learning Outcomes for Hearing

• Explain factors affecting sound pitch and loudness
• Explain the transmission of sound through the auditory system
• Describe the transmission of impulses in the auditory pathway
• Define air and bone conduction of sound
• Differentiate between conduction and nerve deafness

Impedance Matching

• Auditory ossicles (malleus, incus, stapes): Act as a lever system to transfer sound waves to the oval window
• The ossicular lever system increases the force transmitted substantially
• The tympanic membrane surface area is larger than the oval window surface area
• Combining these two factors increases the pressure exerted on the cochlear fluid, exceeding the increase in pressure across the middle ear approximately 20 times.

Other Structures in the Middle Ear

• Skeletal muscles (tensor tympani and stapedius): Involved in the tympanic reflex to protect auditory receptors
• Round window: Dissipates the sound waves
• Eustachian tube: Equalizes pressure inside the middle ear with atmospheric pressure

Tympanic Reflex

• Trigger: Very loud sound
• Action: Tensor tympani muscles contract, pulling the malleus inward
• Action: Stapedius muscles contract, pulling the stapes outward and away from the oval window
• Purpose: Protects auditory receptors from excessive stimulation

Inner Ear: Cochlea

• Space between bony and membranous labyrinths Contains perilymph (rich in Na+ ions)
• Scala media: Contains endolymph (rich in K+ ions)
• Endolymph has a higher K+ concentration compared to perilymph or fluids outside of the inner ear

Endolymph

• High K+ plays a role in generating auditory signals
• Low Na+
• Secreted by stria vascularis
• Endolymph is high in K+

Auditory Receptors: Hair Cells

• Bending of stereocilia towards kinocilia depolarizes the hair cells; toward away hyperpolarizes the hair cells
• Receptor potential leads to neurotransmitter release (glutamate)
• Depolarization leads to neurotransmitter release (glutamate); leads to depolarization of afferent neurons

Sound Transmission

• Sound waves strike the tympanic membrane, which vibrates
• Vibration transfers energy to the auditory ossicles
• Stapes vibrations against oval window creates fluid waves in the cochlea
• Fluid waves cause hair cells to bend and generate electrical signals
• Signals transferred to the brain via the vestibulocochlear nerve

Auditory Pathway

• Bipolar cells in the spiral ganglion
• Cochlear nuclei in the medulla; then cross to the contralateral side of the brainstem
• Third order neurons
• Inferior colliculus
• Fourth order neurons
• Medial geniculate nucleus.
• Fifth order neurons
• Auditory cortex in the temporal lobe.

Conduction of Sound

• Ossicular/air conduction: Sound waves conducted to the inner ear via the tympanic membrane and auditory ossicles. Main pathway for normal hearing
• Bone conduction: Transmission of vibrations from the skull to the fluid of the inner ear

Sound Waves

• Loudness: Related to amplitude; higher amplitude, louder sound
• Pitch: Related to frequency; higher frequency, higher pitch
  • Frequency range: 20-20,000 Hz audible to humans
  • Best pitch discrimination: 1000-4000 Hz

Anatomy of the Cochlea

• Spiral-shaped organ
• Basilar membrane: Narrower, thicker at the base, wider and thinner at the apex, tailored to different frequencies
• High frequencies stimulate hair cells at the base; low frequencies stimulate hair cells at the apex

Different Frequencies Stimulate Hair Cells

• 20,000 Hz (High frequency)
• 1,500 Hz (Medium frequency)
• 20 Hz (Low frequency)
  • Different frequencies stimulate hair cells at different locations along the basilar membrane

Determination of Loudness

• Loudness: Determined by amplitude
• Frequency of action potentials in single auditory nerve fibers is proportional to loudness
• Vibration of basilar membrane and recruitment of hair cells determines loudness
• High frequency of action potentials in auditory nerves correspond to loudness

Determination of Pitch

• Pitch: Determined by frequency
• Higher frequency, higher pitch
• Frequency affects loudness
• Audible frequency range for humans: 20-20,000 Hz
• Best pitch discrimination: 1000-4000 Hz

The Auditory Pathway

• Anterior auditory pathway to prefrontal cortex
• Posterior auditory pathway to posterior parietal cortex
• Primary auditory cortex
• Secondary auditory cortex
  • Auditory association cortex

Thresholds of hearing

• Threshold of hearing: 0 dB (Sound intensity = Standard sound)
• Threshold of pain: ~120 dB

Hearing Loss (Deafness)

• Conduction deafness: Impaired sound transmission in external or middle ear (obstructions or damage to ossicles)
• Nerve deafness: Degeneration of hair cells (chronic exposure to loud noise, aging, damage to the auditory pathway, or tumors)

Audiometry

• Pure tone audiometer: Provides standard tones of different pitches and variable intensity
• Used to determine hearing threshold at various frequencies

Weber & Rinne Tests

• Weber: Tuning fork placed on head, used to diagnose issues in bone conduction
• Rinne: Tuning fork placed on mastoid process and then near ear; used to differentiate between conduction and sensorineural loss

Cochlear Implants

• Used in sensorineural hearing loss
• Bypasses damaged hair cells and directly stimulates auditory nerve fibers
  • Sounds are converted to electrical signals for stimulation through the auditory nerve fibers

Equilibrium (Balance)

• Learning Outcomes: Functions of utricle and saccule; Function of semicircular canals; Effects of labyrinthectomy
• Utricle: Detects horizontal movement
• Saccule: Detects vertical movement
• Semicircular canals: Detect angular acceleration/deceleration
• Labyrinthectomy: Removal of inner ear; can impair balance

Otolith Organs

• Detect position of head to gravity (static equilibrium) - Pulling of gravity on otoliths triggers tonic discharge in the macula
• Detect linear movement of the head (dynamic equilibrium)
  • Utricle (horizontal movement); Saccule (vertical movement)

Semicircular Canals

• Detect angular/rotational acceleration and deceleration
• Endolymph contributes inertial mass to the canal's response to movement

Vestibulo-ocular Reflex (VOR)

• Initiated by impulses from semicircular canals
• Compensates for head movements to allow eyes to remain fixed on a stationary point
• At beginning of rotation, eyes drift in the opposite direction then rapidly move back toward movement

Vestibular Nucleus

• Vestibulospinal tract: Regulates muscle tone in response to head movement
• Impulses to the neck muscles and extraocular muscles that provide eye fixation and equilibrium
• Impulses to the cerebellum to monitor information, make corrective adjustments, and maintain equilibrium

Input & Output of the Vestibular System

• Vestibular receptors, visual receptors, and somatic receptors (skin, muscles, joints) transmit information to the cerebellum
• Cerebellum processes information, makes corrective adjustments, and sends impulses to the vestibular nuclear complex
• Vestibular nuclear complex relays impulses to cranial nerves III, IV, VI, and XI to control eye movements, neck, and skeletal muscles in limbs, trunk, and body to maintain equilibrium

Motion Sickness

• Warning signals (nausea, vomiting, excessive salivation) precede from sensory mismatch
• Simultaneous multiplanar angular accelerations, a mismatch between visual and vestibular stimuli

Labyrinth Dysfunction

• Vertigo: Sensation of movement of the external environment or the head; occurs from lesions to vestibular pathways like cerebellum and brain stem
• Ménière's syndrome: Characterized by recurrent vertigo with tinnitus and progressive nerve deafness; usually comes from swelling in inner ear canals

Test your knowledge on vertigo and motion sickness with this quiz. Explore the conditions, cranial nerves involved in eye movements, and symptoms of labyrinth dysfunction. Understand what typically accompanies vertigo and what is not associated with motion sickness.