Dr Dini Sri Damayanti,MKes

The vestibular system conveys sensory information about head orientation and motion from receptors in the inner ear. Its three main functions are :

• Coordination of head and eye movement
• Helping to maintain upright posture and balance (equilibrium)
• Conscious perception of spatial orientation and motion

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The Balance System depends on a complex set of interactions between a variety of systems:

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• Brainstem and cerebellum
• Eyes
• Spinal cord
• Postural Muscles�Cortices

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• 45% from AICA
• 24% superior cerebellar artery
• 16% basilar
• Two divisions: anterior vestibular and common cochlear artery

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• Membranous labyrinth is surrounded by perilymph
• Endolymph fills the vestibular end organs along with the cochlea

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• Perilymph
• Similar to extracellular fluid
• K+=10mEQ, Na+=140mEq/L
• Unclear whether this is ultrafiltrate of CSF or blood
• Drains via venules and middle ear mucosa

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•  The inner (internal) ear contains the receptors for both the auditory system and the vestibular system.
• Bony labyrinth. Within the petrous part of the temporal bone lies the inner ear . It contains the bony labyrinth, a series of communicating bony cavities: the cochlea (part of the auditory system), the vestibule and the semicircular canals.
• The three semicircular canals are perpendicular to one another and correspond to the three planes of space . .They are the lateral (horizontal) canal, and the superior (anterior) and posterior canals in the vertical plane.

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• Membranous labyrinth. The bony labyrinth encloses the membranous labyrinth, a closed system of sacs and ducts filled with endolymph and bathed in perilymph. 
• The membranous labyrinth includes two sacs, the utricle and saccule within the vestibule; and the semicircular ducts within the semicircular canals. The semicircular ducts, utricle and saccule contain the receptors of the vestibular system. 

• Each of these canals has one swollen or ampulated end, which contains the endorgans. The posterior and superior canals share their non-ampulated ends in what is called the common crus.

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• Each semicircular duct has a dilated end, the ampulla, within a corresponding dilation of the semicircular canal . Each ampulla contains an ampullary crest (crista ampullaris)  with epithelium that contains mechanoreceptive hair cells. 
• The hair cells are receptors whose stereocilia and kinocilium are embedded in a gelatinous mass called the cupula.

• Angular acceleration sets up currents in the endolymph that cause the cupula to away. This deflects the cilia and excites the hair cells . Consequently, the semicircular canals are sensitive to angular acceleration of the head and function in dynamic (or kinetic) equilibrium

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• Gravitational pull and linear acceleration, which represent static equilibrium, are detected by the utricle and saccule.
• These structures also have collections of hair cells in the macula of the utricle and the macula of the saccule. The hair cells have a gelatinous covering that contains rounded crystals of calcium carbonate called otoliths.
• Gravitational pull on the otoliths causes deflection of the stereocilia and kinocilium of the hair cells in the maculae. The utricular macula lies horizontally when the head is upright and detects horizontal linear acceleration. The saccular macula is oriented vertically and is the major gravitational sensor.

• Sensory cells are either Type I or Type II
• Type I cells are flask shaped and have chalice shaped calyx ending
• One chalice may synapse with 2-4 Type I cells
• Type II cells – cylinder shaped, multiple efferent and afferent boutons

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• Each afferent neuron has a baseline firing rate
• Deflection of stereocilia toward kinocilium results in an increase in the firing rate of the afferent neuron
• Deflection away causes a decrease in the firing rate

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• Calcium carbonate or calcite
• 0.5-30um
• Specific gravity of otolithic membrane is 2.71-2.94
• Central region of otolithic membrane is called the striola

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• The primary sensory axons of the vestibular system comprise the vestibular nerve, a division of the vestibulocochlear nerve (VIII).
• The cell bodies of these bipolar neurons are located in the vestibular (Scarpa's) ganglion in the lateral end of the internal auditory canal (internal acoustic meatus).
• The peripheral axonal processes of these cell bodies innervate the hair cells, and the central axonal processes enter the brain stem at the cerebellopontine angle.

• Superior vestibular nerve: superior canal, lateral canal, utricle
• Inferior vestibular nerve: posterior canal and saccule

• Instead of delineating each vestibular nucleus (there are four), understand where they are located in the brain stem. They extend from caudal medulla to mid-pons in the lateral floor of ventricle IV . Note their relationships with the inferior cerebellar peduncles, the solitary nucleus and tract, and the somatic and visceral motor nuclei.

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The vestibular nuclei have pathways to the

• A.  Extraocular muscles - to coordinate eye movements with head movements
• B.  Spinal cord - to maintain upright posture and balance
• C.  Cerebral cortex - to perceive spatial orientation and motion

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Medial longitudinal fasciculus (mlf).

• The central connections of the vestibular system are important in understanding conjugate eye movements.  The medial longitudinal fasciculus (mlf) is a fairly complex fascicle that contains axons from the vestibular nuclei of both sides.
• The mlf runs longitudinally near the midline beneath ventricle IV and the periaqueductal gray matter of the midbrain .  Secondary vestibular neurons project rostrally via the mlf to the motor neurons in the abducens nucleus ,trochlear nucleus and oculomotor nucleus

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• Membranous labyrinth moves with head motion
• Endolymph does not causing relative motion
• Cupula on right canal deflected towards utricle causing increase in firing rate, left deflects away causing a decrease in firing rate.
• Reflex causes movement of eyes to the left with saccades to right
• Stabilizes visual image

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• Patient is lying down with horizontal canals oriented vertically (ampulla up)
• Cold water irrigation causes endolymph in lateral portion to become dense and fall causing deflection of cupula away from utricle with a decrease in the firing rate
• This causes nystagmus with fast phase (beat) away from the stimulus

• With warm water irrigation column of endolymph becomes less dense, rises and causes deflection of cupula toward the utricle
• Results in increase firing rate and nystagmus which beats towards the stimulation
• COWS (cold opposite, warm same)

• The vestibular nuclei give rise to two vestibulospinal tracts.
• The medial vestibulospinal tract projects bilaterally within the descending part of the mlf to end in the ventral horns mainly in the upper cervical cord. It promotes stabilization of head position.
• The lateral vestibulospinal tract projects ipsilaterally in the anterolateral white matter) to end in the ventral horns throughout the cord. It functions in reflex postural mechanisms to activate motor neurons of extensor (antigravity) muscles and promote upright posture and balance.

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• Senses head movement and head relative to gravity
• Projects to antigravity muscles via 3 major pathways:
• Lateral vestibulospinal tract
• Medial vestibulospinal tract
• Reticulospinal tract

Thalamus: VLp (Vim)

• The vestibular nuclei project bilaterally to the thalamus, particularly its cerebellar territory: the posterior part of the ventral lateral nucleus (VLp) also called the ventral intermediate nucleus (Vim) in humans.

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• Thalamic vestibular neurons project to cerebral cortex. Unilateral stimulation of the vestibular labyrinth results in mainly ipsilateral activation of vestibular cortex.
• In contrast to the other sensory systems, there is no unimodal primary vestibular cortex.  That is, there is no cortical area that receives only vestibular input. Instead, there are multiple multisensory vestibular cortical areas:  their neurons also receive somatosensory and/or visual motion input.

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• The dominant vestibular area in humans is in the posterior insula.  Infarctions of this region cause subjective visual vertical tilt.

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• Sensory cranial nerves: contain only afferent (sensory) fibers
• ⅠOlfactory nerve
• ⅡOptic nerve
• VIII Vestibulocochlear nerve
• Motor cranial nerves: contain only efferent (motor) fibers
• Ⅲ Oculomotor nerve
• Ⅳ Trochlear nerve
• ⅥAbducent nerve
• Ⅺ Accessory nerve
• Ⅻ Hypoglossal nerve
• Mixed nerves: contain both sensory and motor fibers---
• ⅤTrigeminal nerve,
• Ⅶ Facial nerve,
• ⅨGlossopharyngeal nerve
• ⅩVagus nerve

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C: Mixed

O: sensory from taste receptors of anterior 2/3 of tongue / motor from pons

D: Sensory to sensory nuclei of pons / motor muscles of facial expression, visceral motor to tear gland.

• Facial muscles (five branches fan out over face from stylomastoid foramen)
• Temporal
• Zygomatic
• Buccal
• Mandibular
• Cervical
• “chorda tympani” (crosses interior ear drum to join V₃ )
• Taste to anterior 2/3 of tongue
• Submandibular, sublingual salivary glands
• Lacrimal glands

Human Anatomy, Frolich, Head/Neck IV: Cranial Nerves

Thankyou