Lec 7: Vestibulocochlear Nerve Vestib Div

Vestibular Nerve

Vestibulo-ocular

Vestibulospinal

Vestibular Circuitry

function parts of the cerebellum:

• vestibulocerebellum

• spinocerebellum

• cerebrocerebellum

what is vestibulocerebellum

– Flocculonodular lobe
– Vestibular control of eye movement, posture, balance

what is spinocerebellum

– Vermis & Paravermis
– Receives postural sensory information from spinocerebellar tracts
– Sends outgoing signals to rubro, vesibulo and reticulospinal tracts
– Postural stability

what is cerebrocerebellum

– Cerebellar hemispheres
– Connects to Dentate nucleus
– Sends outgoing signals to motor cortex via thalamus
– Motor planning and control

what is Vestibulospinal Tract (VST)

• Maintain balance in the body
• responsible for antigravity muscle tone (and neck movements) in response to the head being tilted in any direction

• Starts at the Lateral Vestibular Nucleus (LVN) in Pons/Medulla
• Descends the full length of spinal cord through the anterior column
• Communicates with every segment on the way down
• No Decussation – totally ipsilateral

lateral VST

• Starts at the Medial Vestibular Nucleus (MVN) in Pons/Medulla

• Axons descend Bilaterally through the medial portion of the anterior column

• Stops at the thoracic spinal cord

• Reflexive movements of the head and neck

  • Control neck flexors and extensors during falls

medial VST

Information relevant to maintaining balance:

• CN VIII – Semicircular canals and maculae of the inner ear
• CN II
• Ascending cutaneous and proprioceptive information from the spinal cord
• Cerebellum

• Vestibular Nerve
• Ends at 4 nuclei in the Dorsolateral Medulla and Pons

Equilibrium and Orientation

what is Vestibular apparatus?

– Equilibrium receptors in semicircular canals and vestibule
– Vestibular receptors monitor static equilibrium
– Semicircular canal receptors monitor dynamic equilibrium

• Sensory receptors for static equilibrium

• One in each saccule wall and one in each utricle wall

• Monitor the position of head in space
  – necessary for control of posture

• Respond to linear acceleration forces, but not rotation

• Otolith membrane studded with otoliths (Calcium Carbonate Crystals)

Vestibular Receptors: Maculae

There are 2 Maculae in each ear:

• utricle

• saccule

• Both respond to linear acceleration or tilt of the head in a forwards & backwards direction
  – Tilting head changes the effect of gravity on the Otolith membranes

• Macula in utricle responds to Lateral Acceleration & lateral tilt of the head (side to side)

• Macula in saccule responds to vertical acceleration

• Hair cells synapse with vestibular nerve fibers

Maculae – Utricle & Saccule

• Anterior
• Posterior
• Horizontal (lateral)

Semicircular Canals

• Sensory receptor for rotational acceleration
  – One in each semicircular canal
  – Major stimuli are rotational movements

• Crista hair cells extend into gel-like mass called ampullary cupula

The Crista Ampullares (Crista)

• Equilibrium information goes to reflex centers in brain stem
  – Allows fast, reflexive responses to imbalance

Equilibrium Pathway to the Brain

Three modes of input for balance and orientation:

– Vestibular receptors (maculae ofutricle and saccule, and cristae of semicircular canals)
– Visual receptors
– Somatic receptors

what are the 3 semicircular canal planes

• horizontal canals

• anterior canals

• posterior canals

what is horizontal canals?

Activated by:
– Cranial/Cervical Rotation
– Spinning on ice

what is anterior canals ?

Activated by:
– Neck flexion
– Front flip

what is posterior canals

Activated by:
– Neck extension
– Back flip

The three semicircular canals ararranged in

orthogonal planes
– at right angles to each other – like the corner of a box

Each semicircular canal is paired with a

complementary canal in the opposite labyrinth
– each pair responds to specific angular motion

• The medial and inferior vestibular nuclei are located close to one another in the rostral medulla

• The smaller lateral and superior nuclei lie within the pons

Vestibular nuclei and pathways

The four vestibular nuclei give rise to both ascending and descending tracts, sending projections to the:

– Cerebellum
– Brainstem nuclei
• especially the extraocular nuclei
– Cerebral Cortex
– Spinal cord

Information from the Semicircular canals goes to the __

Medial Vestibular Nucleus

– The medial nucleus activates the Medial Vestibulospinal Tract
• Reflexive movements of the head and neck

Information from the Utricle and Saccule goes to the

Lateral Vestibular Nucleus

The Lateral nucleus activates the__
• Activates extensors in limb and trunk muscles

Lateral Vestibulospinal Tract

Lateral Nuclei to the:

– Spinal Cord
• Lat. Vestibulospinal Tract

Medial Nuclei to the:

– Spinal Cord
• Medial Vestibulospinal Tract
• AKA Medial Longitudinal Fasciculus (MLF)
– Descending part

Other Destinations for Vestibular Nuclei signals:

– Ipsilateral Cerebellum
• Via Inferior Cerebellar Peduncle
– Bilateral CN 3,4&6 Nuclei
• Via MLF, Ascending part
– Contralateral cerebral cortex

Maintain head, trunk, and eye alignment during movements

1. Vestibulo-ocular Reflex
2. Vestibulospinal Reflex
3. Vestibulocollic reflex

• Movement in any direction excites the vestibular neurons on the side to which the head turns
  – inhibiting contralateral neurons

• Central projections excite the ipsilateral vestibular nuclei

• Vestibular nuclei excite the ipsilateral abducens nucleus and contralateral oculomotor nucleus

• Controls and coordinates binocular gaze opposite in direction to the head movement

Vestibulo-ocular Reflex

• For example, with bilateral stimulation of the posterior canals:

• resulting eye movement is oblique and downward, as it excites the inferior recti and superior obliques
  – and inhibits the superior recti and inferior oblique

Vestibulo-ocular Reflex example

what is Vestibulospinal Reflex

• regulates full-body balance and postural responses
• Excite limb Extensors, Antigravity muscles
• Via Lateral Vestibulospinal Tract

what is Vestibulocollic Reflex

• coordinates vestibular activity, with input from proprioceptors in the neck to precisely coordinate and align head position during postural adjustments

• Via Medial Vestibulospinal Tract