Auditory and Vestibular Systems Flashcards
(38 cards)
• Sensory systems: hearing &
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
Audible variations in air pressure • Cycle: distance between successive compressed patches of air • Sound frequency: number of cycles per second expressed in hertz (Hz) • Audible Sound: Range: 20 Hz to 20,000 Hz Pitch: high pitch = high frequency low pitch = low frequency Intensity (amplitude): high intensity louder than low intensity
Pathway: Sound waves →
Sound waves → Tympanic membrane → Ossicles → Oval window → Cochlear fluid → Sensory neuron response
The Middle Ear Role
Sound force amplification by the ossicles – Pressure: force per surface area – Greater pressure at oval window than tympanic membrane, moves fluids. •
The attenuation reflex – Response when onset of loud sound causes tensor tympani and stapedius muscle contraction
Function: adapts ear to loud sounds, protects inner ear, enables us to 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 electrical potential 80 mV more positive than perilymph
Physiology of the cochlea …
Physiology of the cochlea – Motion at oval window pushes perilymph into scala vestibuli, makes round window membrane bulge.
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 toward apex.
The Inner Ear—(cont.) inmnervation
The innervation of hair cells – One spiral ganglion fiber synapses with one inner hair cell, numerous outer hair cells • Amplification by outer hair cells—cochlear amplifier – Function: sound transduction – Motor proteins: change length of outer hair cells – Prestin: protein required for outer hair cell movements
Auditory Pathways
Characteristic frequency: frequency at which a 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.
Encoding Sound Intensity
• Encoding information about stimulus intensity – Firing rates of neurons – Number of active neurons • Membrane potential of activated hair cells more depolarized or hyperpolarized • Loudness perceived is correlated with number of active neurons
• Tonotopic maps
Encoding Sound Frequency
Tonotopic maps on the basilar membrane, spiral ganglion, and cochlear nucleus – From the base to apex, basilar membrane resonates with increasingly lower frequencies. – Tonotopy is preserved in the auditory nerve and cochlear nucleus. • In cochlear nucleus, bands of cells with similar characteristic frequencies increase from anterior to posterior.
Phase Locking
• Low frequencies: phase locking on every cycle or some fraction of cycles • High frequencies: not fixed
• Localization of sound in horizontal plane
• Localization of sound in horizontal plane – Interaural time delay: difference in time for sound to reach each ear – Interaural intensity difference: sound at one ear less intense because of head’s sound shadow
Delay Lines and Neuronal Sensitivity to Interaural Delay
Sound from left side, activity in left cochlear nucleus sent to superior olive • Sound delayed to right ear, activity in right cochlear nucleus • Impulses reach olivary neuron at the same time → summation → action potential
Sensitivity of Binaural Neurons to Sound Location
Localization of Sound in Vertical Plane
Vertical sound localization based on reflections from the pinna
Primary Auditory Cortex
Axons leaving MGN project to auditory cortex via internal capsule in array called acoustic radiation. • Structure of A1 and secondary auditory areas: similar to corresponding visual cortex areas
• Principles of auditory cortex
Tonotopy, columnar organization of cells with similar binaural interaction – Unilateral lesion in auditory cortex: almost normal auditory function (unlike lesion in striate cortex: complete blindness in one visual hemifield) – Different frequency bands processed in parallel
The 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 Otolith Organs
• Detect changes in head angle, linear acceleration • Macular hair cells responding to tilt
Push–pull Activation of Semicircular Canals
• Three semicircular canals on each side – Help sense all possible head rotation angles • Each paired on opposite side of head. • Push–pull activation of vestibular axons
Central Vestibular Pathway
The Vestibulo-Ocular Reflex (VOR)
• Function: fixate line of sight on visual target during head movement • 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
What differentiates the auditory system from the vestibular system?
