Chapter 11 The Auditory and Vestibular Systems

Introduction Sensory Systems Sense of hearing, audition Detect sound Perceive and interpret nuances Sense of balance, vestibular system Head and body location Head and body movements

The Nature of Sound Sound Audible variations in air pressure Sound frequency: Number of cycles per second expressed in units called hertz (Hz) Cycle: Distance between successive compressed patches

The Nature of Sound Sound Range: 20 Hz to 20,000 Hz Pitch: High pitch = high frequency; low frequency = low pitch Intensity: High intensity louder than low intensity

The Structure of the Auditory System

The Structure of the Auditory System Auditory pathway stages Sound waves Tympanic membrane Ossicles Oval window Cochlear fluid Sensory neuron response

The Middle Ear Components of the Middle Ear

5 – Stapedius muscle 9 – Tensor Tympani muscle

The Middle Ear Sound Force Amplification by the Ossicles Pressure: Force by surface area Greater pressure at oval window than tympanic membrane, moves fluids The Attenuation Reflex Response where onset of loud sound causes tensor tympani and stapedius muscle contraction Function: Adapt ear to loud sounds, understand speech better

The Inner Ear Anatomy of the Cochlea Perilymph: Fluid in scala vestibuli and scala tympani Endolymph: Fluid in scala media Endocochlear potential: Endolymph electric potential 80 mV more positive than perilymph

The Inner Ear Physiology of the Cochlea Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out The Response of Basilar Membrane to Sound Structural properties: Wider at apex, stiffness decreases from base to apex Research: Georg von Békésy Endolymph movement bends basilar membrane near base, wave moves towards apex

Georg von Békésy

The Inner Ear Travelling wave in the Basilar Membrane

The Inner Ear The Organ of Corti and Associated Structures

The Inner Ear Transduction by Hair Cells Research: A.J. Hudspeth. Sound: Basilar membrane upward, reticular lamina up and stereocilia bends outward

External ear Middle ear Internal ear Air External acoustic meatus Malleus, incus, stapes (ossicles) Tympanic membrane Oval window Fluids in cochlear canals Pinna Upper and middle Lower Pressure Time Spiral organ (of Corti) stimulated One vibration Amplitude Amplification in middle ear

Central Auditory Processes Auditory Pathway

Central Auditory Processes Response Properties of Neurons in Auditory Pathway Characteristic frequency: Frequency at which neuron is most responsive - from cochlea to cortex Response Properties more complex and diverse beyond the brain stem Binaural neurons are present in the superior olive

Mechanisms of Sound Localization Techniques for Sound Localization Horizontal: Left-right, Vertical: Up-down Localization of Sound in Horizontal Plane Interaural time delay: Time taken for sound to reach from ear to ear Interaural intensity difference: Sound at high frequency from one side of ear Duplex theory of sound localization: Interaural time delay: 20-2000 Hz Interaural intensity difference: 2000-20000 Hz

Mechanisms of Sound Localization Interaural time delay and interaural intensity difference

Mechanisms of Sound Localization The Sensitivity of Binaural Neurons to Sound Location

Mechanisms of Sound Localization Delay Lines and Neuronal Sensitivity to Interaural Delay Sound from left side, activity in left cochlear nucleus, sent to superior olive Sound reaches right ear, activity in right cochlear nucleus, first impulse far Impulses reach olivary neuron at the same time summation action potential

Mechanisms of Sound Localization Localization of Sound in Vertical Plane Vertical sound localization based on reflections from the pinna

Auditory Cortex Primary Auditory Cortex Axons leaving MGN project to auditory cortex via internal capsule in an array Structure of A1 and secondary auditory areas: Similar to corresponding visual cortex areas

The Vestibular System Importance of Vestibular System Balance, equilibrium, posture, head, body, eye movement Vestibular Labyrinth Otolith organs - gravity and tilt Semicircular canals - head rotation Use hair cells, like auditory system, to detect changes

The Vestibular System The Otolith Organs: Detect changes in head angle, linear acceleration Macular hair cells responding to tilt

Otolithic membrane Kinocilium Ster eocilia Depolarization Hyperpolarization Receptor potential (Hairs bent towar kinocilium) d (Hairs bent away from kinocilium) Nerve impulses generated in vestibular fiber Increased impulse frequency Decreased impulse frequency Excitation Inhibition

The Vestibular System The Semicircular Canal Structure

The Vestibular System Push-Pull Activation of Semicircular Canals Three semicircular canals on one side Helps sense all possible head-rotation angles Each paired with another on opposite side of head Push-pull arrangement of vestibular axons:

The Vestibular System The 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

The Vestibular System The Vestibulo-Ocular Reflex (VOR)

Vestibular receptors Visual receptors Somatic receptors Input Vestibular nuclear complex Reticular nuclei Cerebellum Central nervous system processing Oculomotor control (cranial nerve nuclei III, IV, VI) (eye movements) Spinal motor control (cranial nerve nuclei XI and vestibulospinal tracts) (neck movements) Output