1. Vestibular system Part of the membranous labyrinth of the inner ear
Involved in a form of proprioception
The vestibular apparatus detects head movements and the position of the head in space
- requires two sets of sensory epithelia to transduce angular and linear acceleration of the head
- together they from five receptor organs (3 semicircular canals; as well as utricle and saccule)
Receptive organs are ensheathed by connective tissue
3/04/09
Subconscious motor

2. Vestibular system Vestibular portions of inner ear:
Membrane-lined fluid filled cavities in temporal bone (contains endolymph. connected with cochelar duct through ductus reuniens)
3 semicircular canals (anterior, posterior and horizontal)
Respond to angular acceleration (yes, no, tipping of head)
Utricle & Saccule
Respond to linear acceleration & gravity
3/04/09
Subconscious motor

3. Vestibular Components
A and P canals are oriented in vertical planes perpendicular to each other
H canal is oriented horizontally
sense events in 3 dimensions of space
Utricle & Saccule respond to linear acceleration & gravity
vestibular component of 8th cranial nerve; 20,000 myelinated axons
3/04/09
Subconscious motor

4. Receptors = Hair Cells
- Depolarize when stereocilia are bent towards kinocilium (non-motile cilium; results in functional polarity of hair bundle)
Endolymph is high in K+; girdle of tight junctions separates endolymph from perilymph (like extracellular fluid; high in Na+)
afferents fire both tonically and phasically – firing can persist or adapt – resulting in mechanisms to signal sustained stimulation (acceleration from gravity) and abrupt changes in acceleration
3/04/09
Subconscious motor

5. Semicircular canals Respond to angular acceleration 3 on each side
Filled with fluid
Perpendicular to each other
Pairs of canals in same plane
3/04/09
Subconscious motor

6. Semicircular canals Mechanism of stimulation:
Hair cells (7,000) located in ampulla - Gelatinous Cupula covers stereocilia
(each “canal” is a closed tube of ~8 mm in diameter filled with endolymph)
During rotation of head in the plane of a canal:
Fluid moves around canal (acceleration detected by inertia)
Fluid flow interrupted by cupula (Tilts the cupula; Stereocilia bent)
Afferents excited on one side & inhibited on the other
3/04/09
Subconscious motor

7. Semicircular canals Mechanism of stimulation:
Fluid presses against one side of cupula
Cupula bows, displacing the haircells
All hairbundels share common orientation
Angular acceleration in preferred direction (towards kinocilium) depolarizes haircells and stimulates afferents, acceleration in opposite direction hyperpolarizes receptors
three canals are almost precisely perpendicular to one another
representing 3 mutually orthogonal axes
3/04/09
Subconscious motor

8. Utricle and Saccule
Ovoidal sac of membranous laby-rinth about 3mm long
- Utricle: 30,000 HC, saccule: 20,000 HC
Respond to linear acceleration & gravity
One of each on each side
Utricle - macular surface horizontal
Saccule - macular surface vertical
Proportional activity in 2 channels for info on acceleration along all axes
Mechanism of stimulation:
hair cells in macular surface
Stereocilia covered by gelatinous matrix
Otoliths embedded in gelatin
Otoliths more dense than water (fine, dense particles, “ear dust”)
Mass lags behind movement of head
gelatinous layer shifts with respect to underlying epithelium
deflects haircell bundles
elicits electrical response
3/04/09
Subconscious motor

9. Utricle and Saccule
- Linear acceleration or gravity forces otoliths to move gelatin and bend stereocilia
Utrical signals horizontal forces (utricle has variations in axes in populations of hair cells; tilt in any direction will depolarize some cells and hyperpolarize others)
Saccule signals vertical forces
3/04/09
Subconscious motor

10. Utricle and Saccule
Otoliths
3/04/09
Subconscious motor

11. Vestibulo-cochlear Nerve
nerve along which the sensory cells (hair cells) of the inner ear transmit information
consists of the cochlear nerve (hearing), and the vestibular nerve (balance)
emerges from the medulla oblongata and enters the inner skull via the internal auditory meatus in the temporal bone, along with the facial nerve.
3/04/09
Subconscious motor

12. Vestibular Information and pathways
Vestibular information is used in 3 ways
Control eye muscles so that in spite of changes in head position, the eyes can remain fixed on same point
Reflex mechanisms for maintaining upright posture
Conscious awareness of the potion and acceleration of body, perception of space surrounding the body and memory of spatial information
Pathways
Information is relayed from vestibular apparatus to nuclei in brainstem via vestibular branch of cranial nerve VIII
Transmitted through multineuronal pathway through the thalamus to vestibular centers in parietal lobe and cerebellum
descending projections sent to spinal chord to affect postural reflexes
vestibular information integrated with info from joints, tendons and skin
3/04/09
Subconscious motor

13. Vestibulo-Spinal Tracts; vestibulo-spinal reflexes
(medial and lateral)
Medial:
Provides basic postural control
receives much input from semicircular canals
Causes movement of head and shoulders to coordinate head and eye movements (ends at cervical cord)
Descend in the ipsilateral column of spinal cord; terminate in ventro-medial spinal gray matter; innervate axial and proximal muscles
3/04/09
Subconscious motor

14. Vestibulo-Spinal Tracts; vestibulo-spinal reflexes
Lateral:
Concerned with goal-directed limb movement such as reaching and manipulating
receives much input from utricle and saccule
Changes muscle tone in response to gravity
Descending pathway descend to dorsal part of lateral column of spinal cord
3/04/09
Subconscious motor

15. Vestibulo-Spinal Tract: vestibulo-ocular reflexes
Other vestibular pathways ascend to oculomotor nuclei: CN III (oculomotor nerve; controls most of the eye's movements, constriction of the pupil, and maintains an open eyelid), CN IV (trochlear nerve; innervates a single muscle: the superior oblique muscle of the eye), CN VI (abducens nerve; controls the movement of a single muscle, the lateral rectus muscle of the eye)
Cause eye movement in response to head rotation: Nystagmus
3/04/09
Subconscious motor

16. Projections from Vestibular Nuclei; vestibulo-spinal reflexes
Vestibular connections (postural control [medial] and limb movement [lateral]) to the cerebellum
Thalamic information relayed to cortex - allow for conscious perception of head position and movement
3/04/09
Subconscious motor

17. Central vestibular connections
Afferent fibers relay through 4 vestibular nuclei (superior, lateral, medial and inferior)
2 vestibulospinal tracts
Lateral:
receives much input from utricle and saccule
Changes muscle tone in response to gravity
Medial:
receives much input from semicircular canals
Causes movement of head and shoulders to coordinate head and eye movements
Strong input to cerebellum
3/04/09
Subconscious motor

18. Central vestibular connections
Together, vestibular reflexes stabilize eyes and body when head moves
Vestibulospinal reflexes enable skeletomotor system to compensate for head movement
Vestibuloocular reflexes keep eyes still when head moves
3/04/09
Subconscious motor

19. Nystagmus; vestibulo-ocular reflexes
Stabilize eyes when head moves
3/04/09
Subconscious motor

20. Nystagmus; vestibulo-ocular reflexes
Stabilize eyes when head moves
you can read a book while shaking your head if the book is still (visual processing slower than vestibular processing for image stabilization)
vestibular apparatus signals how fast head is moving, ocular motorsystem uses info to stabilize eyes (visual image motionless on retina)
slow eye movement in opposite direction of head movement (driven by vestibular system; otholith reflex)
nystagmus to reset to center of gaze (driven by brain stem circuits)
3/04/09
Subconscious motor

21. Vestibulo-occular control
Subject seated on stool and rotated to left
Initial response (hard to visualize)
Slow tracking eye movements to right
Fast eye movements back to left
Nystagmus: alternate slow (otolith reflex) and fast eye movement (brain stem)
Semicircular canals habituate, eyes begin to move in space
Response to stopping turning (post-rotatory)
Head stops but fluid continues moving left
Eyes track slowly left, quick movement to right
Nystagmus normal for head rotation and repetitive moving object (optokinetic)
Nystagmus without movement = sign of lesion
Post-rotatory nystagmus
3/04/09
Subconscious motor

22. Vestibulo-occular control
Coffee cup example:
gently twist your coffee; watch a bubble at fluid boundary
at beginning, coffee tends to maintain its original orientation and thus counter rotates the cup
at conclusion of turning, when cup decelerates, coffee moves in opposite direction (post rotatory nystagmus)
Post-rotatory nystagmus
3/04/09
Subconscious motor