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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
http://education.yahoo.com/reference/gray/illustrations/figure?id=924
Bony and membranous labyrinth
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 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 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 brainstem.
Schwander M et al. JCB 2010;190:9-20
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 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
neurons 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
Outer hair cells enhance responsiveness of the
inner hair cells (Active process) and their
sterocilliary attach to the tectorial membrane
Nolte; Essentials of the human brain 2009
http://universe-review.ca/I10-85-cochlea.jpg
“Summary” of the Cochlea's role
• 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)
Spiral ganglion projections to the brainstem
Castro, Merchut, Neafsey, Wurster; Neuroscience: An Outline Approach, 2002
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 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;35:185-196
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 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
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
Castro, Merchut, Neafsey, Wurster; Neuroscience: An Outline Approach, 2002
Vestibular pathways
Vestibulospinal tracts: Review
• Lateral Vestibulospinal Tract
• Medial Vestibulospinal Tract
(or “descending MLF”)
• From the Lateral Vestibular
nucleus
• Uncrossed projection
• Entire length of the cord ventral funiculus
• From the Medial Vestibular
nucleus
• Bilateral projection (?)
• Cervical spinal cord only –
medial ventral funiculus
• Proximal limb muscles
• Maintains balance by acting
on the limbs
• Neck muscles
• Maintains head erect
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)
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)