THE AUDITORY AND VESTIBULAR SYSTEMS

THE NATURE OF SOUNDSound is an audible variations in air pressure, defined by:1) frequency: Number of cycles (distance between successive compressed patches)per second expressed in units called Hertz (Hz). Human Range is btw 20 Hz to 20,000 Hz2) Intensity: Difference in pressure between compressed and rarefied patches of air. It determines the loudness of the sound.

Sounds propagate at a constant speed: 343 m/sec

THE AUDITORY SYSTEM

THE MIDDLE EARSound Force (pressure) is amplified by the Ossicles, producing greater pressure at oval window (smaller surface) than tympanic membrane, in order to move more efficiently the fluid inside the cochela

The Attenuation Reflex: response where onset of loud sound causes tensor tympani and stapedius muscle contraction. It’s used to adapt ear to loud sounds, or understand speech better in noisy environment (more attenuation of low sounds)

THE INNER EAR

Perilymph: Fluid in scala vestibuli and scala tympaniEndolymph: Fluid in scala mediaEndolymph has an electric potential 80 mV more positive than perilymph (Endocochlear potential)

THE INNER EAR

Basilar Membrane is wider at apex, stiffness decreases from base to apex

THE INNER EARPressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out. Endolymph movement bends basilar membrane near base, wave moves towards apex

THE INNER EAR

The Organ of Corti and Associated Structures. Here the mechanical energy of the sound is transformed in electrical signal by the auditory receptor cells (hair cells).Each hair cells has around 100 stereocilia.Rods of corti provide structural support. Hair cells form synapses with bipolar neurons that have their body in the spiral ganglion. Their axons form the auditory nerve

THE INNER EARTransduction by Hair CellsWhen sound arrives, basilar membrane moves. According to the movement, stereociliabends on one or the other direction: i.e. Basilar membrane upward, reticular lamina up and stereocilia bends outward

THE AUDITORY PATHWAY

Auditory nerve

Superiorolive

MGN

Auditorycortex A1

MGN

INFORMATION ABOUT THE SOUNDInformation About Sound Intensity is encoded in 2 ways:

Firing rates of neurons and number of active neuronsStimulus Frequency

Frequency sensitivity: in Basilar membrane is Highest at base, lowest at cochlea apex. This coding is kept separate along the auditory pathways (tonotopy)

Phase Locking is another way to code for frequencyConsistent firing of cell at same sound wave phase. Only for frequency below 4kHz

SOUND LOCALIZATION: HORIZONTAL PLANEInteraural time delay: Time taken for sound to reach from ear to ear

Duplex theory of sound localization:Interaural time delay: 20-2000 Hz

Interaural intensity difference: 2000-20000 Hz

Interaural intensity difference: Sound at high frequency from one side of ear

Sound waves

Sound waves

Sound waves

Sound waves

Sound shadow

Sound shadow

Sound shadow

SOUND LOCALIZATION: VERTICAL PLANE

pinnaPath 2, direct sound

Path 2, reflected sound

Path 2, direct sound

Path 2, reflected sound

Path 3, direct sound

Path 3, reflected sound

Based on reflections from the pinna

THE AUDITORY CORTEX: BA 41

Primary auditory cortex

Secondary auditory cortex

Axons leaving MGN project to auditory cortex via internal capsule in an array called Acoustic Radiation

THE VESTIBULAR SYSTEM

Importance of Vestibular SystemBalance, equilibrium, posture, head position, eye movement

The Vestibular Labyrinth

THE VESTIBULAR SYSTEM

The Otolith Organs (saccule and utricle). Detect force of gravity (linear acceleration) and tilts (change of angle) of the head.Saccule is vertically oriented and utricle horizontally oriented

Crystals of calcium carbonate

Bending of the hairstoward kinocilium: depolarization

THE VESTIBULAR SYSTEM

The Semicircular Canals. Detect rotation of the head and angular acceleration

Crista: Sheet of cells where hair cells of semicircular canals clusteredAmpulla: Bulge along canal, contains cristaCilia: Project into gelatinous cupulaKinocili oriented in same direction so all excited or inhibited together

Filled with endolymph

endolymph

Three semicircular canals on one side helps sense all possible head-rotation anglesEach Canal paired with another on opposite side of headRotation causes excitation on one side, inhibition on the other

CENTRAL VESTIBULAR PATHWAY

S1/M1 Face area

VESTIBULO-OCULAR REFLEX (VOR)

Function: Line of sight fixed on visual target

Mechanism: Senses rotations of head, commands compensatory movement of eyes in opposite direction.

Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei excite extraocular muscles

Motion of the head

Motion of the eyes