The Vestibular System

• The Purpose of the Vestibular System:

  • to monitor static position and movement of the body/head and maintain positional equilibrium in the body

  *Static position – how the body is positioned relative to gravity

  *Monitor movement - acceleration (in a straight line or rotational)

  • Key in maintaining and controlling - Stable posture, Detection of body and head movements , Control of body movements , Control of eye movements for maintenance of gaze
  • Works closely with the proprioception system and visual system - mismatches b/w them causes confusion, imbalance, nausea, travel sickness
• Vestibular and Other Systems:

  • impulses synapse at vestibular nuclei in brainstem and motor responses carried out through cerebellum

  Propriocetpion System: detecting muscle movement and stretch through somatic receptors in skin, muscle and joints

  • muscle spindles - embedded in skeletal muscle fibres
  • Golgi tendon organs - at the interface of muscles and tendons
  • joint receptors - mechanoreceptors embedded in joint capsules

  Visual System:

  • eye movement control - extraocular muscles controlled by CN III,IV, VI)
• Symptoms of Balance Problems:

  • Damage to either side of the vestibular system can result in disorientation and vertigo as a result of conflicting messages being sent to the brain
  • Sense of motion or spinning (vertigo)
  • Feeling of faintness or lightheadedness
  • Loss of balance or unsteadiness
  • Falling or feeling like you might fall
  • Feeling a floating sensation or dizziness
  • Vision changes, such as blurriness
  • Confusion
  • Nausea
• Components of the Vestibular System:

  • Vestibular labyrinth - continuous with cochlea and consists of 3 semi-circular canals

  *each canal situated in plane which head can rotate

  *each canal filled with fluid called endolymph - movement of fluid corresponds to plane of movement

  • Ampulla - expansion of canals, contains hair cells (sensory receptors of vestibular system)

  *movement of stereocilia here releases neurotransmitters which are sent back to brain on the plane of movement

  • 2 otolith organs (maculae) = utricle and saccule

  
• The 3 Semi-Circular Canals:

  • orthogonal (at 90o to each other) which are sensitive to rotational movement/acceleration of the head and body in the three dimensions - nodding up/down, shaking side/side, tilting left/right
  • Each fluid canal filled with endolymph has a widened region called an ampulla, where sensory haircells sit and top hairs (stereocilia) project into a gelatinous mass called the cupula
  • ears are mirror images - rotation in 1 direction causes excitation (neuron firing) in one ear, but reduces firing in the other ear
• 2 Maculae/Otolith Organs:

  • detect forwards/backwards movements and gravity

  *utricle = detects movements in horizontal plane

  *saccule = detects movements in vertical plane

  • striola (midline) = either side, haircells have opposite orientation, so that one side are stimulated whilst other side are inhibited
  • calcium carbonate crystals (otoliths) respond to gravity - leads to movement/displacement of hair cell layers

  *causing either stimulation or inhibition of haircell activity depending on direction of the head tilt
• Sensory Cells of Vestibular System:

  • cells that respond to the mechanical stimulation are similar to those within the cochlea except those in the vestibular system have a tall "kinocilium".

  *Tilt of the stereocilia towards the kinocilium =  depolarization of the cell , release of neurotransmitter , increase neuron firing rate

  *Tilt away from kinocilium =  hyperpolarization , reduction in neuron firing rate
• Neural Pathways and Reflexes:

  • Ascending pathway = neural signal from vestibular system travels along vestibular branch of CN VIII + ascends to vestibular nuclei which sit in the medulla oblongata of the brainstem
  • Descending pathways - most of vestibular's function occur via reflex (involuntary actions)

  *Vestibulocervical Reflex = from medial vestibular nuclei to cervical spinal cord (detects the movement of head and control of neck muscles)

  *Vestibulospinal Reflex = from lateral vestibular nuclei down the spinal cord (controls muscles of trunk&limbs in to maintain stable posture and gait)
• The Vestibule-occular Reflex:

  • vestibular system detects movement of the head and controls the action of the eye muscles to maintain a stable image on the retina as the head moves. 
  • As head moves left, eye movement moves right...

  *Left horizontal canal excited , right horizontal canal inhibited

  *Left extraocular muscles (of each eye) relax, right extraocular muscles (of each eye) contract

  • As head moves right, eye movement moves left...

  *Left horizontal canal inhibited, right horizontal canal excited

  *Left extraocular muscles contract, right extraocular muscles relax