Chapter 11: the Auditory and Vestibular Systems Chapter 1: Applying Research to Everyday Exercise and Sport

Neuroscience: Exploring the Brain, 4e

Chapter 11: The Auditory and Vestibular Systems Chapter 1: Applying Research to Everyday Exercise and Sport

• 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

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Nature of Sound • Audible variations in air pressure • Vibration of the air : Rhythmic movement • Cycle: distance between successive compressed patches of air • Frequency: Distance between successive compressed patches • Sound frequency: number of cycles per second expressed in hertz (Hz)

Copyright © 2016 Wolters Kluwer • All Rights Reserved Audible Sound • Range: 20 Hz to 20,000 Hz • Pitch (tone): determined by the frequency High pitch = high frequency; low pitch = low frequency • Intensity (amplitude): difference in pressure High intensity louder than low intensity • Sounds in real world: combination of different frequency waves at different intensities

• Pinna: collect sounds • Auditory canal: Entrance ~ internal year (2.5cm) • Tympanic membrane: eardrum • Ossicles: transfer movements of the tympanic membrane • Oval window: Second membrane receives movements of ossicles • Cochlea: transform physical motion into a neuronal response

• Outer ear: pinna ~ tympanic membrane • Middle ear: tympanic membrane ~ ossicles • Inner ear: apparatus medial ~ oval window

Copyright © 2016 Wolters Kluwer • All Rights Reserved Auditory Pathway • Neural response generated in the ear • Signal reaches to the thalamus medial geniculate nucleus (MGN) • MGN projects to primary auditory cortex, A1 (@temporal lobe)

• Middle ear: air-filled cavity containing the first elements in response to sound • 1) Tympanic membrane, 2) three ossicles: malleus, incus, stapes and footplate • Continuous to the nasal cavities via the Eustachian tube

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Middle Ear • Sound force amplification by the ossicles – Sound – Tympanic membrane – Ossicles – Oval window – Cochlea: filled with fluid, cannot be moved by sound – Ossicle provides necessary amplification in pressure – (1) Greater pressure at oval window than tympanic membrane, (2) small oval window moves fluids (20 times greater).

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Middle Ear • The attenuation reflex – Tensor Tympani muscle: bone in the cavity of middle ear attaches to the malleus – Stapedius muscle: fixed anchor bone ~ stapes – 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 – Reflex has a delay of 50-100msec  not useful to a very sudden sound – Attenuation reflex suppresses low frequencies  loud explosion can still damage ears – Suppresses our own voice while speak

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Middle and Inner Ear • Inner ear = Cochlea (auditory) + labyrinth (vestibular) • Cochlea anatomy – Spiral shape (similar to a straw wrapping a pencil) – Hollow tube + Central pillar, 32mm long/2mm diameter – Two windows: oval window + round window

• Reissner’s membrane: separates scala vestibuli and media • Basilar membrane: separates scala tympani and media • Organ of Corti: contains auditory receptor neurons • Tectorial membrane: hang over the organ of Corti

• Helicotrema: a hole in the membrane where scala tympani and vestibuli meet • Perilymph: fluid in scala vestibuli and scala tympani, similar to cerebrospinal fluid (low K+, high Na) • Endolymph: fluid in scala media, similar to intracellular fluid (high K+, low Na+) • Stria vascularis: endothelium lining that generates ionic concentration • Endocochlear potential: endolymph electrical potential 80 mV more positive than perilymph

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Inner Ear—(cont.) • Physiology of the cochlea – Motion at oval window pushes perilymph into scala vestibuli, makes round window membrane bulge – If the cochlea were completely rigid – However, some structures in the cochlea are not rigid • The response of basilar membrane to sound – Structural properties: (1) Basilar membrane is wider at apex, (2) its stiffness decreases from base to apex

Copyright © 2016 Wolters Kluwer • All Rights Reserved Traveling Wave in the Basilar Membrane • Research: Georg von Békésy – Endolymph movement bends basilar membrane near base, wave moves toward apex. – The distance the wave travels depends on the frequency of the sound – High frequency: shorter propagation; low frequency: travel to the apex – Tonopony: systematic organization of sound frequency within an auditory structure

• Organ of Corti – Hair cells + Rods of Corti + Supporting cells • Hair cells : Auditory receptors, contain 100 stereocilia – Bending of cilia  formation of neural signal • Rods of Corti – Span the basilar membrane & reticular lamina, provide structural support.

• Inner hair cells – Hair cells between modiolus and the rod s of Corti • Spiral ganglion - Bipolar cells receiving synaptic input and entering auditory nerve • Auditory-vestibular nerve (Cranial nerve VIII) - projects to the cochlear nuclei in the Medulla

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Bending of Stereocilia • Transduction by Hair cells – Basilar membrane, rods of Corti, reticular lamina, hair cells move as a unit • All the cilia move as a unit

• Sound: basilar membrane upward, reticular lamina up, and stereocilia bend outward • Stereocilia bend in one direction  hair cell depolarized • Stereocilia bend in other direction  hair cell hyperpolarized • Resting potential = -70 mV • Movement of sterocilia from 0.3~20nm transduces signals • Ion channel mediating the mechanosensitive transduction is unknown

• Tip link: a stiff filament that connects each channel  When cilia pointing up: open a part of channels, allowing a small amount of K+  Displacement of clila in one direction: increase tension, rate of opening, K+ influx • K+ entry  depolarization  activates voltage-gated Ca2+ channel  Ca2+↑  Glutamate release  spiral ganglion activation • In other neurons, K+ induces hyperpolarization; here, depolarization

• Hair cells and the Axons of the Auditory nerve – Spiral ganglion neurons: the first auditory pathway to fire AP – Inner hair cell : Outer hair cell = 1: 3 – 95% Inner hair cell input, 5% Outer hair cell input – Vast majority of information comes from inner hair cells • Amplification by outer hair cells—Cochlear amplifier – Function: sound transduction, act like a tiny motor amplifying the movement – (1) Mechanisms involves motor proteins: change length of outer hair cells (change in length + potential) – Prestin: Hair cells primary motor protein, protein required for outer hair cell movements

• Amplification by outer hair cells—Cochlear amplifier – (2) Myosin is attached to the upper end of the tip links, enhances the movement of hair cells

– Treatment of Furosemide significantly reduces the movement of the basilar membrane  inactivation of outer hair cell motor proteins, loss of cochlear amplifier

• Efferent fibers from the brain stem – Efferent fibers release acetylchline, changes shape of the outer hair cells  Change inner hair cell response

• Antibiotics damage hair cells – Excessive exposure to certain antibiotics leads to outer hair cell damage – This damage is thought to be a consequence of damage to the cochlear amplifier

Copyright © 2016 Wolters Kluwer • All Rights Reserved Central Auditory Pathways • Anatomy of Auditory Pathways – Spiral ganglion  Dorsal cochlear nucleus and ventral cochlear nucleus (@Medulla) – Ventral cochlear nucleus  Superior olive (@brain stem)  Inferior colliculus (@midbrain) – Dorsal cochlear nucleus bypass the superior olive – All ascending auditory pathways converge onto the inferior colliculus – Inferior colliculus  Medial geniculate nucleus (MGN, @thalamus)  Auditory cortex

• Anatomy of Auditory Pathways 1. Projections and brain stem nuclei other than ones described contribute to the auditory pathways 2. There is extensive feedback in the auditory pathways 3. Each cochlear nucleus receives input from just the one ear on the ipsilateral side

• 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 – Layer I contains few cell bodies; layer II and III contains pyramidal cells; MGN axons terminate on layer IV

• 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 • Neuronal response properties – Frequency tuning in neurons: similar characteristic frequency – Isofrequency bands running mediolaterally across A1 cortex

Copyright © 2016 Wolters Kluwer • All Rights Reserved The Vestibular System • Balance, equilibrium, posture; head, body, eye movement • Vestibular labyrinth – Otolith organs— gravity and tilt (Saccule+Utricle) – Semicircular canals— head rotation, three canals lie in 90’ – Use hair cells, like auditory system, to detect changes – Each hair cell makes an excitatory synapse  vestibular nerve (a branch of auditory-vestibular nerve) – Cell bodies of vestibular nerve lie in Scarpa’s ganglion

• Saccule and utricle detect changes in head angle, linear acceleration • Tilting changes angle between otolith organs, direction of the force of gravity; Linear acceleration (riding a car, an elevator-sudden stop) generates force • Macula: A sensory epithelium in the otolith organ; vertically oriented within saccule, horizontally the utricle • Macular hair cells responding to tilt

• Otoconia: tiny crystals of calcium carbonate, 1-5um diameter, locate at the surface of gelatinous cap, key to the tilt sensitivity • Kinocilium: tall cilium of which bend results in depolarization • Saccular maculae: vertically oriented, utricular maculae: horizontal • Any tilt or acceleration of the head will excites some hair cells, inhibits other

• Semicircular canals: detect turning movement of the head, sense angular acceleration • Hair cells are clustered within the crista, located within the ampulla • Bending  emdolymph tends to stay behind  endolymph exerts a force on cupula  moves the cilia  excites/inhibits the release of NT

• Cranial nerve VIII (vestibular axon)  medial and lateral vestibular nuclei @ brain stem, and cerebellum • Cerebellum, visual and somatic sensory system  vestibular nuclei (combining the information) • Otolith organ  lateral vestibular nucleus  vestibulospinal tract  maintain posture (balancing) • Semicircular canals  medial vestibular nucleus  Medial longitudinal fasciculus  orient head

• Vestibular nuclei  ventral posterior (VP) nucleus @thalamus • @Cortex; Integration of body movement, eye, visual scene information  maintains body position and orientation  maintaining equilibrium, planning movement

• Hearing and balance – Nearly identical sensory receptors (hair cells) – Movement detectors: periodic waves of air pressure, rotational and linear forces – Auditory system: senses external environment – Vestibular system: senses movements of itself

• Auditory system parallels visual system – Tonotopy (auditory) or retinotopy (visual) preserved from sensory cells to cortex – Convergence of inputs from lower levels  neurons at higher levels have more complex responses. • Higher level visual neurons binocular • Higher level auditory neurons binaural