Lec 2: Vestibular System

85 slides on lecture deck

Patent information: 6 month history of dizziness, since starting new job as manager 9 months ago. Symptoms occur when turning head to right, occasionally feels like she is going to fall. Now afraid to drive. Neck is often stiff and sore in the evening. Uses heat pads while watching TV and working at night. Can you think of underlying causes of this person’s dizziness?

• Dysequilibrium

• Otoconia

• Trauma

• Stress & Anxiety

#1 cause of emergency department visits

Otologic/vestibular pathology

>70% of pts with initial complaints of dizziness will NOT have…

resolution of symptoms at a 2-week follow up

Components of Vestibular rehabilitation

• differential diagnosis

• skilled particle repositioning

• gaze stabilization

• customized habituation (decreased sensitivity)

• Balance Re-training

• progressive fitness and gait re-education

• visual motion desensitization

• cervicokinesthetic re-education

• extensive education and treatment of co-morbidities

• identification and treatment of migraines and anxiety factors

What are the two major functions of the vestibular system in normal function?

1. Postural control — helps maintain balance and upright posture.

2. Visual control — stabilizes gaze during head movement and helps stabilize the head relative to vertical.

What information does the vestibular system provide for postural control?

Sensory information about head position in space relative to gravity and head acceleration/movement

• this input helps indicate appropriate motor response (muscle activation)

Describe the overall pathway shown in the anatomical overview of the vestibular system.

INPUT:
Vestibular + visual + somatosensory
↓
PROCESSING:
Vestibular nuclear complex
↓
OUTPUT:
VOR + VSR
↓
Eye stability + balance/postural stability

The cerebellum participates in adaptive processing and fine-tuning.

What are the two major motor outputs/reflexes of vestibular processing?

• Vestibulo-ocular reflex (VOR) → controls/stabilizes the eyes

• Vestibulospinal reflex (VSR) → contributes to balance and postural stability

What is the vestibulo-ocular reflex (VOR)?

produces eye movement opposite to head movement to maintain a stable visual target during head motion.

• purpose is gaze stabilization with head motion

  • Head right → eyes left

  • Head left → eyes right

What might happen visually if the VOR cannot adequately stabilize gaze during head movement?

The visual environment may appear blurred or unstable during head movement because the eyes cannot adequately maintain fixation.

What is the vestibulospinal reflex (VSR)?

A vestibular-mediated motor response that activates appropriate postural muscles in response to changes in head/body position to help maintain balance and upright posture.

• purpose: head stabilization with respect to vertical

What is the role of the cerebellum in normal vestibular function?

provides adaptive processing. It helps fine-tune, calibrate, and modify vestibular responses based on sensory information, experience, and errors.

What is sensory conflict in postural control?

this occurs when vestibular, visual, and/or somatosensory systems provide different information about movement or position.

Example: On a moving bus, vision may suggest that you are stationary relative to the inside of the bus while the vestibular system detects acceleration.

peripheral vestibular system anatomy

• this is in the inner ear and is made up of

  • bony labyrinth = outer container

  • membranous labyrinth = inner container

Bony labyrinth

• outer, hard structure carved into the temporal bone

• includes

  • semicircular canal (SCC)

  • cochlea

  • vestibule = utricle + saccule

  • filled with PERILYMPH fluid = similar to CSF (low K, high Na)

Membranous Labyrinth

• this sits inside (it’s suspended in) the bony labyrinth by supportive connective tissue

• contains membranous portions of semicircular canal (SCC) and Otoliths (aka saccule + utricle)

• filled with ENDOLYMPH fluid (High K, Low Na)

Saccule and utricle = vestibule: Function?

senses movement and position in addition to the semicircular canal (SCC)

• these structures are AKA otolith organs

What are the 3 semicircular canals?

• horizontal (lateral)

• anterior (superior)

• posterior (inferior)

  • these are a part of membranous labyrinth located with the petrous portion of temporal bone

when testing the horizontal (lateral) semicircular canal, you would need to…

tilt head into 30 degrees of flexion before providing horizontal movement (rotation)

• anterior = flexion/extension

• posterior = side bending

Hair cells are contained in each…

ampulla (SCC) and macular of otolith organs (utricle and saccule)

• inside ampulla is cupula → inside cupula is hair cells which are sensitive to endolymph fluid (fluid moves and bends these cells)

kinocilia

big hair cells

• big hairs bend TOWARD the big hair → 🔴 OFF / less firing signals to the brain

• K → S = hyper-polarization = inhibition

Stereocilia

small hair cells

• small hairs bend TOWARD the big hair → 🟢 ON / more firing signals to the brain

• S → K = depolarization = excitation

hair cells in the SCC detect…

• angular acceleration

• sensitive to higher-frequency motion

hair cells in utricle (an otolith organ) detect

• linear acceleration in horizontal planes → like driving

• sensitive to static head tilts and low-frequency motion

hair cells in saccule (an otolith organ) detect

• linear acceleration in vertical planes → like elevator

• sensitive to static head tilts and low-frequency motion

What is the main function of vestibular hair cells?

Convert head movement/displacement into neural signals that can be sent to the brain.

What does it mean that hair cells “transduce mechanical force into electrical nerve signals”?

Physical bending of the hair cells from head/fluid movement is converted into an electrical signal.

What can happen when vestibular hair cells are damaged?

The CNS can learn to compensate/adapt for the altered vestibular

what are the central processors of the vestibular system?

vestibular nuclei and cerebellum

Where does cortical processing of vestibular information in the central vestibular system occur?

The parieto-insular vestibular cortex (PIVC)

What major brain areas does the central vestibular system connect with for higher-level processing such as (arousal, conscious of the body, and discriminating between movement of self and the environment)?

The vestibular cortex, thalamus, and reticular formation

What initiates the vestibular afferent pathway?

Hair cells in the peripheral vestibular system detect movement and generate electrical signals/action potentials in vestibular afferents.

What stimulates hair cells in the semicircular canals (SCC)?

Movement of the cupula

What stimulates hair cells in the otolith organs (utricle and saccule)?

Movement of the otoconia

What is the vestibular (Scarpa’s) ganglion?

The sensory ganglion containing the cell bodies of vestibular afferent neurons.

Which cranial nerve carries vestibular information toward the CNS?

CN VIII — the vestibulocochlear nerve, specifically its vestibular division

Where are the four vestibular nuclei located?

In the brainstem, specifically the pons and medulla

Which part of the cerebellum is especially involved in vestibular processing?

The flocculonodular lobe

What structure relays vestibular information toward the cerebral cortex?

The thalamus

Where does vestibular information reach the cerebral cortex for conscious processing?

The parieto-insular vestibular cortex

What are the key principles of the vestibular system?

• Vestibulo-ocular Reflex (VOR) gain and phase  

• Tonic Firing Rate 

• Push & Pull Mechanism  

• Inhibitory Cutoff  

• Velocity Storage System 

What is the normal resting firing rate of regular vestibular afferents?

About 70–100 spikes/second

How do regular vestibular afferents respond to head movement?

Head movement causes:

• Increased firing when stimulated on the same side & Decreased firing on the opposite side

regular vestibular afferents function primarily in…

VOR (vestibulo-ocular reflex) → gaze/eye stabilization

irregular vestibular afferents function primarily in…

the VSR (vestibulospinal reflex) → balance/postural control

• these nerves typically have no resting firing rate

How do irregular vestibular afferents respond to stimulation?

They can produce highly fluctuating/variable spikes

What is the push-pull mechanism of the vestibular system?

Each SCC has a partner on the opposite side of the head that lies in the same plane. 

• When the head moves in the plane of that canal pair: 

  • One canal is excited (increased firing). 

  • The partner canal is inhibited (decreased firing). 

• This coding is important bc it allows the brain to sense head rotation more precisely. 

Which semicircular canals are paired in the push-pull mechanism?

• Right horizontal ↔ Left horizontal

• Right anterior ↔ Left posterior

• Left anterior ↔ Right posterior

The pairs lie approximately in the same plane

If the right anterior canal is excited, what happens to the left posterior canal?

inhibited

If the left anterior canal is excited, what happens to the right posterior canal?

inhibited

How does damage to one semicircular canal affect the push-pull mechanism?

It creates an imbalance between the paired canals, so the brain receives unequal vestibular information and may falsely perceive movement

If BPPV affects the right anterior canal, why can symptoms occur during movement in that canal's plane?

The displaced otoconia (ear crystals) cause abnormal stimulation of the right anterior canal, disrupting its normal push-pull relationship with the left posterior canal.

• If otoconia fall into the R canal, symptoms may mimic activation of that canal (dizziness/nystagmus when tested). 

• But the L posterior canal is always working in tandem (push–pull balance disrupted).

What is the vestibulocollic reflex (VCR) and what is its function?

activates the neck muscles in response to vestibular input/head movement to help stabilize the head.

• neck/head stability

  • “Torticollis = neck’’

Spectrum of Dizziness

• Vertigo = spinning

• Imbalance = loss of VSR, proprioception, visual/motor loss, or psychogenic

• Vertical diplopia = skew deviation

• Oscillopsia = loss of VOR, nystagmus, or BVL

• Floating/swimming/internal spine = anxiety, depression, somatoform

• Lightheadedness = decreased blood flow CNS

• Motion sickness/Visual vertigo = visual-vestibular mismatch

• Nausea/Vomiting = stimulation of dorsal medulla

• Rocking = Maladaptation

Bilateral Vestibular Hypofunction (BVH)

type of vestibular disorder typically caused by toxicity via meds or physical trauma

Persistent postural perceptual dizziness (3PD)

• type of vestibular disorder

• very specific to inner ear

• Sx include ringing in the ears

Peripheral vestibular disorders

• pathology of inner ear vestibular structures as well as the vestibular portion of CN 8

• such pathology diminishes available sensory information regarding head position and movement

Peripheral vestibular disorders include…

• Neuritis

• Labyrinthitis

• Bilateral vestibular loss

• Meniere’s Disease

• BPPV

• Vestibulopathy after surgical procedures (e.g. labryinthectomy and acoustic neuroma)

Central vestibular disorders

• primarily involve the vestibular nuclear complex and the cerebellum, and structures of the reticular activating system, midbrain, and higher centers of cortical function

• Pathology of these structures affects the integration and processing of sensory input from the vestibular, visual, and somatosensory systems

Most common central vestibular disorders include…

• brainstem strokes

• head trauma

• migraine-related vestibulopathy

• MS

• cerebellar degeneration

Pt examination: vestibular system slide