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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

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Bony and membranous labyrinth

http://education.yahoo.com/reference/gray/illustrations/figure?id=924

Bony and membranous labyrinth

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Bony and membranous labyrinth

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

Cochlea

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

Hair cells in the

Signals from each inner are relayed to the brain via 10 to 20 afferent fibers (through a ribbon ) of the spiral of the 8th 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 .

Schwander M et al. JCB 2010;190:9-20 Nolte; Essentials of the human brain 2009

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Organ of Corti (about 33mm long) Outer hair cells enhance responsiveness of the inner hair cells (Active process) and their Signals from each inner hair cell are relayed to the sterocilliary attach to the tectorial membrane brain via 10 to 20 afferent fibers (through a ribbon synapse) of the spiral ganglion of the 8th 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 make contact with them) and mainly contacted by efferent nerves, which regulate their electromotility and influence inner hair cell sensitivity.

Nature Rev Neurosci 2006;7:19-29 Nolte; Essentials of the human brain 2009 http://universe-review.ca/I10-85-cochlea.jpg

“Summary” of the Cochlea's role

• 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 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)

Spiral ganglion projections to the brainstem

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

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CNS auditory pathway

Nolte; Essentials of the human brain 2009

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 , cerebral cortex, 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;35:185-196

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

The “Otoliths” function to…

• The otoliths are structures within the utricle and the • 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

www.med.uwo.ca/physiology/courses/medsweb

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

http://thalamus.wustl.edu/course/audvest.html Nolte J. The Human Brain, 6th ed., 2009

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

Vestibular pathways

Vestibulospinal tracts: Review

• Lateral • Medial Vestibulospinal Tract (or “descending MLF”)

•From the Lateral Vestibular •From the Medial Vestibular nucleus nucleus • Uncrossed projection • Bilateral projection (?) • Entire length of the cord - • Cervical only – ventral funiculus medial ventral funiculus

• Proximal limb muscles • Neck muscles • Maintains balance by acting • Maintains head erect on the limbs

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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 3rd and 6th)

Vestibuloocular reflex

Head is rotating to the right The right horizontal canal is activated Right vestibular nucleus is “activated’ The left 6th nucleus (via PPRF) is activated and the left lateral rectus muscle contracts The left PPRF “activates neurons in the right 3rd nucleus and the right medial rectus contracts

…both eyes begin to move to the left

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)

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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)

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