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Hearing & Balance MHD – Neuroscience Module January 28, 2016 Gregory Gruener, MD, MBA Vice Dean for Education, SSOM Professor & Associate Chair, Department of Neurology LUHS/Trinity Health and Catholic Health East

Objectives Describe the structure and function of sensory receptors, otolithic (otoconial) organs and semicircular canals Diagram the Auditory pathway within the CNS Diagram the Vestibular pathway within the CNS Describe nystagmus, vestibuloocular reflex and the rationale for why we experience dizziness

Bony and membranous labyrinth

Netter. Atlas of human anatomy, 3 rd ed. 2003

Nature Rev Neurosci 2006;7:19-29 Nolte; Essentials of the human brain 2009 Cochlea

Schwander M et al. JCB 2010;190:9-20 Nolte; Essentials of the human brain 2009 Hair cells in the Organ of Corti Signals from each inner hair cell are relayed to the brain via 10 to 20 afferent fibers (through a ribbon synapse) of the spiral ganglion of the 8 th cranial nerve. These hair cells transmit frequency, intensity and timing information to the spiral ganglion because of the unique morphology of the ribbon synapse. Through the central process of the spiral ganglia this information is transmitted to the brainstem.

Nature Rev Neurosci 2006;7: Nolte; Essentials of the human brain 2009 Organ of Corti (about 33mm long) Signals from each inner hair cell are relayed to the brain via 10 to 20 afferent fibers (through a ribbon synapse) of the spiral ganglion of the 8 th cranial nerve. Outer hair cells have both sensory and motor capabilities and possess electromotility that underlies their active process. They have sparse afferent innervation (5-10% of spiral ganglia neurons make contact with them) and mainly contacted by efferent nerves, which regulate their electromotility and influence inner hair cell sensitivity. Outer hair cells enhance responsiveness of the inner hair cells (Active process) and their sterocilliary attach to the tectorial membrane

Hearing encompasses frequencies from 20 Hz to 20 kHz, but resolution extends to one-thirtieth of the interval between successive keys on a piano Evolved to and optimized to process behaviorally relevant natural sounds Not passive, but enhanced by the active process of cochlear hair cells. Active process arises from the motility of the mechanoreceptive outer hair cells (about 12,000 with 3,500 inner hair cells) and results in: –Amplification of acoustic signals several hundred-fold. –Sharpens frequency selectivity (why a hearing aid can augment signal, but not loss of frequency resolution, key to discrimination of speech sounds) –Compressive nonlinearity (telescoping a million-fold variation in the amplitude of sound into only a hundred-fold range - enables the ear to encode an enormously broad range of sound intensities). –Otoacoustic emissions - in a very quiet environment, a normal human cochlea can produce spontaneous otoacoustic emissions, which are tones, and an epiphenomena (like feedback from a public-address system) “Summary” of the Cochlea's role

Castro, Merchut, Neafsey, Wurster; Neuroscience: An Outline Approach, 2002 Spiral ganglion projections to the brainstem

Nolte; Essentials of the human brain 2009 CNS auditory pathway

Vestibular system and the eyes Stabilize our gaze during a head movement or when we shift gaze to a new target The effector system is the extraocular muscles The vestibular system, cerebral cortex, cerebellum and brainstem are all involved These types of eye movements include: –Vestibuloocular reflex –Optokinetic response –Smooth pursuit –Saccadic eye movements –Vergence eye movements

Integration within vestibular pathways Trends in Neurosci 2012;35:

Schuenke M, Schulte E, Schumacher U. Thieme Head and Neuroanatomy, 2007 Nolte; Essentials of the human brain 2009

The “Otoliths” function to… The otoliths are structures within the utricle and the saccule Serve to detect linear motion of the head Provides our ability to “sense” which way is up or position! (i.e. gravity) The Canals function to … Serves to detect angular motion of the head

Semicircular canals 1.Three canals are located on each side (horizontal, anterior and posterior) of the head 2.Fluid-filled and opening at one end into the utricle 3.Each canal has a swelling (ampulla), sealed by a membrane the cupula and hair cells are embedded in the cupula (this sensory epithelium is called the crista ampullaris) 4.When the head turns, the fluid “lags” because of inertia and the cupula is bent 5.As the cupula bends, so does the kinocilium causing the hair cells to either increase or decrease their firing rate Nolte J. The Human Brain, 6 th ed., 2009

Castro, Merchut, Neafsey, Wurster; Neuroscience: An Outline Approach, 2002

Vestibular pathways

Vestibulospinal tracts: Review Medial Vestibulospinal Tract (or “descending MLF”) From the Medial Vestibular nucleus Bilateral projection (?) Cervical spinal cord only – medial ventral funiculus Neck muscles Maintains head erect Lateral Vestibulospinal Tract From the Lateral Vestibular nucleus Uncrossed projection Entire length of the cord - ventral funiculus Proximal limb muscles Maintains balance by acting on the limbs

Vestibulo-ocular reflex (VOR) Stabilizes the image on the retina during a rotation of the head and faster than visual tracking As the head rotates the VOR rotates the eyes with the same speed, but in the opposite direction Without this reflex, the image would appear “smeared” upon the retina Once the head stops moving the eyes remain in that same direction of gaze –“stabilization” occurs through the nucleus prepositus hypoglossi –tonic activation maintains the activation/activity of the involved cranial nerve nuclei (the 3 rd and 6 th )

Head is rotating to the right The right horizontal canal is activated Right vestibular nucleus is “activated’ The left 6 th nucleus (via PPRF) is activated and the left lateral rectus muscle contracts The left PPRF “activates neurons in the right 3 rd nucleus and the right medial rectus contracts …both eyes begin to move to the left Vestibuloocular reflex Nolte; Essentials of the human brain 2009

What is nystagmus ? 1.Rhythmic back and forth movement of the eyes 2.Usually the movement is slow in one direction (“smooth”) and fast (“saccadic”) in the other 3.When you induce it by spinning yourself around…. 1.The VOR is generating the slow phase which helps to keep an eye on a target 2.Once the eye approaches the maximum that it can turn, a saccade will then occur moving the eyes in an opposite direction and onto a new target (Optokinetic nystagmus or OKN)

How do we get dizzy? Vestibular input without vision –While spinning in a chair with your eyes closed (the constant motion eventually results in the cupula membrane returning to its baseline) you suddenly open your eyes Sense of motion via the visual system, but without vestibular “confirmation” –Looking out a car window when an adjacent car moves away (false sense of motion) Sense of motion via the vestibular system, but without visual confirmation (“a disconnect”) –In the cabin of a boat during a storm (motion sickness)