Clinical Neuroscience Week 11 – Vestibular System

Introduction to Vestibular and Auditory Systems

• The vestibular system detects head movement.
• The auditory system interprets sound.
• The vestibular system processes multimodal sensory information from feet sensations and vision.
• It acts as a conflict resolution system for discrepancies between vision and head movement .
• The vestibular system drives motor output through feedback in relation to gravity that informs balance.
• It controls gaze stabilization, which is the ability to keep eyes on a target during head movements.
• The vestibular organ is located behind the tympanic membrane and is embedded within the temporal bone.
• The organ detects linear movements, such as acceleration in a car or riding an elevator.

Components of the Vestibular System

• The vestibular system comprises the vestibular apparatus/end organ, the central processor/vestibular nuclei, and the motor output/vestibulospinal tract.
• Information travels through the 8th cranial nerve to the cochlear nerve, then to the vestibular nuclei.
• Located in temporal bone

Anatomy of the Ear

• The ear is divided into three parts: the external ear, middle ear, and inner ear.
• The middle ear contains three small bones: malleus, incus, and stapes which transmit information from the tympanic membrane to the vestibular organ.
• Inner ear problems are related to the vestibular or cochlear portion.

Organization of the Vestibular System

• Sensory input includes visual, vestibular, and proprioceptive (somatosensory) sensations.
• The adaptive processor is the cerebellum.
• Hallmarks of the vestibular system include detecting head movements and how the head movement impacts your balance.
• Primary processor is the vestibular nuclei.
• Motor output goes to:
  • motor neurons.
  • Eyes and posture muscles.

Anatomy of the Peripheral Vestibular System

• The labyrinth contains:
  • Bony (outside): 3 semicircular canals (anterior, posterior, horizontal/lateral) and 2 otolith organs (utricle and saccule).
  • Cochlea.
  • Membranous (inside).
• The 8th Cranial Nerve transmits information to the central nervous system.
• The labyrinthine artery provides nourishment to the organ.

Bony Labyrinth

• A series of cavities within the temporal bone contains:
  • Cochlea: Responsible for hearing transmission.
  • Vestibule: Contains otolith organs (utricle and saccule) and 3 semicircular canals.
• Contains perilymph.
• Fluid that fills the space between the bony and membranous labyrinth.
• High sodium-to-potassium ration that helps with nerve conduction.
• Cushions and supports inner membranous labyrinth located within bony labyrinth.

Membranous Labyrinth

• Suspended within the bony labyrinth.
• Filled with endolymph.
  • Closed system.
  • Sensory receptors, or hair cells, are bathed in endolymph.
• Cross section of labyrinth layers:
  • Bony labyrinth (outside), for protection.
  • Perilymph.
  • Membranous labyrinth.
  • Endolymph. Important for integrating the semi-circular canals within vestibular organ.
• Ampullae: Bulbous ending at the end of each canal that contains hair cells that transmit information to vestibular cochlear nerve and then to the brainstem.

Semicircular Canals

• Used to detect angular motion (up/down, left/right, diagonal).
• Three canals per ear, sitting at a specific angle.
• Anterior canal sits at 45 degrees with respect to the nose.
• Posterior canal sits at 45 degrees with respect to the nose.
• Horizontal canal sits at 30 degrees tilted upwards towards the forehead.
• Each canal has a dilated end called the ampulla.

Orientation of the Semicircular Canals

• Horizontal canal is approximately 30 degrees tilted up or superior from the horizontal plane.
• Posterior canal sits at 45 degrees with respect to the individual’s nose.

Ampulla; Cupula

• Contains two types of hair cells: kinocilia (tallest) and stereocilia (shorter).
• Bending hair cells towards the kinocilia excites the nerve.
• Bending hair cells away from the kinocilia inhibits the nerve.
• Cupula and endolymph have the same specific gravitational density.
• Canals are insensitive to gravity forces.
• Cupula is a flower-like, gelatinous structure that extends from one end of the ampulla to the other, supporting hair cells and detecting head movements.
• The endolymph within cannot pass though.

Bony Labyrinth: Two Otolith Organs

• Utricle responds to horizontal movement; Saccule responds to vertical movement.
• Examples of movements:
  • Horizontal: Riding in a car, running down a street.
  • Vertical: Riding an elevator.
• Othoconia "rocks" Informal term = “rocks”
• Millions that lie on top of the utricle and saccule.
• Register tilt.

Utricle & Saccule – Deflecting Hair Cells

• In a neutral head position, otoconia rests on top of the hair cells.
• Tilting the head causes gravity to pull or bend the otolithic membrane down, bending hair cells and causing excitation or inhibition.

Push-Pull Relationship

• Head rotation in one direction excites the vestibular organ on that side and inhibits the opposite side.
• Example: Excitation on the left; inhibition on the right during a left head turn.
• Benefit: Helps provide sensory redundancy.
  • Deficits in one ear can be compensated through vestibular information from the other ear.
• Allows the brain to ignore certain changes if both sides are stimulated.
• May help in recovery process.
• Injury to the vestibular organ disrupts the push-pull relationship, potentially causing the brain to perceive head movement when none is present, or delaying/absenting perception.
• Issues will result in:
  • Dizziness.
  • Vertigo.
  • Imbalance.
  • Visual deficits.

Vascular Supply of PVS

• Blood supply comes from basilar artery through the anterior inferior cerebellar artery (AICA).
• Branches off into the labyrinth artery and contribute to anterior portion of the vestibular artery as well as the posterior portion.
• Clots can cause damage to these organs.
• Vascular disease: Can cause temporary deficits within the vestibular organs.

Vestibular System Malfunctioning

• Subjective symptoms include dizziness, vertigo, oscillopsia (bouncing vision), lightheadedness, and imbalance.
• Objective symptom is nystagmus, which is the involuntary beating of the eyes with a fast and slow phase.
• Nystagmus beats toward the more neurally active or "good" ear, helping localize lesions and identify pathology. Basic Nystagmus
• Characterized by fast and slow phase.
• Nystagmus is named for the direction from the person’s point of view.
• Slow phase = quick jerk to the left and slow recoil to the right.

Clinical Application: Benign Paroxysmal Positional Vertigo (BPPV)

• Spinning sensation or perceived sensation of movement when there is no physiological movement.
• When an individual lies down or rolls over in bed, they get a brief burst of dizziness or spinning when physiologically they’re not moving.
• Caused by otoconia displacing into one of the semicircular canals, causing turbulence and atypical firing of hair cells.
• Has a fairly good prognosis
• Can be treated with physical therapy techniques to relocate the crystals back to the utricle and saccule.
• Example: individual lies down or rolls over in bed, and they get a brief burst of dizziness or spinning when physiologically they’re not moving.

Central Vestibular System (CVS)

• Vestibular nuclei: Located within the pons or pontomedullary junction. The nuclei synthesize vestibular organ information to determine appropriate motor output.
• Cerebellum: the hallmark features of the central vestibular system.

Getting to the Central Nervous System

• Cranial Nerve 8 transmits afferent signals project from scarpas ganglion through the Internal Auditory Canal with cochlear and facial nerve, sensory branch, and labyrinthine artery.
• Enters brainstem at ponto-medullary junction.

Anatomy of CVS

• Four primary vestibular nuclei on each side (total of 8).
• Nuclei process input from CN VIII, visual, auditory, and somatosensory systems concurrently.
• Other inputs to the vestibular nuclei come from the cerebellum, ocular motor nuclei, and brainstem.

Output from Vestibular Nuclei

• Effector organs of the vestibulo-ocular reflex (VOR) or vestibulospinal reflex (VSR).
• VOR is the integration of the vestibular system in the eyes or the ocular motor system.
• VSR is the integration of how the vestibular system and certain spinal muscles work together in a reflexive pattern.
• Medial and lateral vestibulospinal tracts:
  • Extraocular/neck muscles (MVST).
  • Postural muscles (LVST), mainly trunk and proximal muscles.
• Damage to the vestibular organ will present as postural weakness and eye movement difficulties.

Vestibular Pathway

• Sensory Input: Hair cells (within SCC and otolith organs) and Vestibular peripheral neurons
• Central:
  • Vestibular nuclei (4 nuclei).
  • Medial longitudinal fasciculus.
  • Cerebellum (flocculonodular lobe).
• Output:
  • Extra ocular motor neurons (CN III, IV, VI).
  • Neck motor neurons.
  • Limb motor neurons.
  • Thalamic nuclei: VPI (ventral posterior inferior) and VPL (ventral posterior lateral).
  • Vestibular cortex in parietal lobe.

Vestibular Reflexes

• Vestibulo-ocular reflex (VOR) maintains stable visual fixation during head motion, reflexively keeping eyes on the target when the head turns.
  • Eye movement (degrees) = head movement (degrees)=< 0.8 Hz – when the reflex kick in.
• Vestibulospinal reflex (VSR) stabilizes the head and body.
• Vestibulocollic reflex (VCR) stabilizes the head in space.

Gaze Stabilization

• Reduces retinal slip and maintains image focus; mainly utilized through VOR.
• Example: As an individual turns their head ten degrees to the left, their eyes will turn ten degrees to the right.
• Dysfunction can cause of dizziness or imbalance.

Auditory Systems

• External ear focuses sound into the external auditory meatus.
• Air pressure vibrates the tympanum.
• Three ting bones (malleus, incus, and the stapes) convey vibrations across a fluid-filled middle ear cavity.
• Vibration of the stapes will stimulate the round window and cochlea.

Cochlea

• Cochlea transmits sounds heard to be interpreted.
• Parts of the Cochlea:
  • Cochlear duct.
  • Basilar membrane, extending the full length and dividing the cochlea into upper and lower chambers. Organ of Corti or Organ of hearing-comprised of receptor or hair cells that will eventually terminate in the cochlear portion of the 8th cranial nerve.
• Converts mechanical energy from the upper chamber into a neuronal signal conveyed by the 8th cranial nerve.
• The tympanic membrane vibrations move the ossicles, which vibrate the membrane at the opening of the upper chamber, moving fluid, stimulating the cochlear duct, and vibrating the basilar membrane.

Auditory Pathway

• Organ of Corti -> Cochlear nuclei -> Medial geniculate body -> Primary auditory cortex (conscious hearing).
• The information will travel down the 8th cranial nerve and will synapse on the cochlear nuclei within the ponto medullary junction.
• Information goes to one of three locations:
  • Reticular formation.
  • Inferior colliculus; helps orient and rotate the head and eyes to wherever the sound stimulus is coming from.
  • Primary auditory cortex within the temporal lobe; that helps with conscious sound or hearing.
• When the cochlea is stimulated and sound is being transmitted through the cochlea and the hair cells are being bent or activated, the information will travel down the 8th cranial nerve and will synapse on the cochlear nuclei within the ponto medullary junction.
• Information on one side will eventually get processed and interpreted through both sides of the central nervous system.

Clinical Application: Sensorineural Hearing Loss/Deafness

• Disease of the cochlea or neuronal pathway from the cochlea to the brain.
• Caused by damage to the inner ear portion through congenital issues, viral illness, or infection, deteriorating cochlear hair cells or the organ of Corti.
• Signs and symptoms include hearing discrepancies or disruption, lack of hearing, or ringing in the ears (tinnitus).
• Treatment:
  • Hearing aid (to amplify the sound).
  • Physical therapy for balance.
  • Cochlear implant.