Auditory and Vestibular Systems + Acoustics

Inner Ear

Houses the organs of hearing and balance

Functions:

• Converts acoustic energy into electrical signals which are sent to the brain

• Maintain balance by detecting position and motion

Cochlea

Spiral shaped

Auditory portion of the inner ear

Scala Vestibuli

Located superiorly to the cochlear duct

Contains perilymph

Scala Tympani

Located inferiorly to the cochlear duct

Contains perilymph

Scala Media

Middle region

Houses organ of corti

Contains endolymph

Basilar Membrane

Stiff

Organ of corti rests on this

Narrow at base (sensitive to high frequencies), wide at apex (sensitive to low frequencies

Critical for fluid displacement as a traveling wave

Tectorial Membrane

Connected to outer hair cells

Organ of Corti

Contains four rows of hair cells

Three OHC rows

1 IHC row

Inner Hair Cells

One row

Arranged tonotopically (High frequencies at base, low at apex)

Responsive to 20-20000 Hz

Responsible for frequency analysis

Outer Hair Cells

Three rows

Arranged tonotopically

Provide amplification and sharpen frequency analysis

Sound

• Change in pressure

• Displacement of molecules in a certain medium

• Produces a waveform

• Passes through air and bone

Intensity

Refers to the strength of sound

Unit = dB

Range of hearing: 0-120 dB

Frequency

Refers to the number of cycles of a vibrating body per unit time

Unit: Hz

Range of hearing: 20-20000 Hz

Traveling Wave

Fluid in inner ear is displaced when sound is transmitted across the middle ear

Low frequencies cause deformation of hair cells near apex, high frequencies deform the base

Traveling Wave Process

1. Waves displace basilar membrane

2. Hair cells move

3. Hair cells depolarize and neuron fires

Fast Acting Fibers

Dorsal acoustic stria

Fibers synapse in the dorsal cochlear nucleus

Function as a general warning

Decussate and ascend in the lateral lemniscus to the inferior colliculus

Tonotopic Organization

Cells responsive to different frequencies are found in different places at each level of the auditory system

High frequencies at base, low frequencies at apex

Auditory Reflex

Involuntary and bilateral

Protects hearing structures from loud sound

Neural mechanisms mediate the reflex —> involuntary stapedius contraction stiffens the stapes

Vestibular System

• Monitors orientation of body with respect to gravity

• Engages several pathways that are responsible for making compensatory movements and adjustments in body position

• Engages pathways that protect the cortex to provide perceptions of gravity and movement

Vestibule

Communicates anteriorly with the cochlea and posteriorly with the semicircular canals

Houses the saccule and utricle

Semicircular Canals

Houses the organs for balance + semicircular ducts

Three: Superior, Lateral, Posterior

Open into the vestibule

Lateral Canal

Detects head movements in the transverse plane

Anterior Canal

Detects head movements in the saggital plane

Posterior Canal

Detects movements in the frontal/coronal plane

Otolith Organs/Otoliths

• Utricle and Saccule

• Receptors are called Macula

• Lie against the walls of the inner ear between the semicircular ducts and cochlea

• Detect linear acceleration and whether our heads are in equilibrium

Utricle

Detect horizontal acceleration

Saccule

Detects vertical acceleration

Lateral Vestibulospinal Tract

Arises in the lateral vestibular nucleus

Descends into the anterior white matter down the length of the spinal cord (ventral horn)

Modulates postural adjustments for the trunk and limbs

Medial Vestibulospinal Tract

Starts in the medial vestibular nucleus

Descends no lower than the mid-thoracic cord

Mediates head position by controlling muscles of head, neck, shoulder

Some vestibular afferents go directly to the cerebellum through…

Inferior Cerebellar Peduncle

Most 1st order afferents synapse in the…

Medial and inferior vestibular nuclei and then ascend to the cerebellum

Most of these vestibular afferents innervate the…

Flocculonodular lobe and fastigial nucleus