Vestibular System

• what is Acoustic neuroma?

  benign tumour of the myelin-forming cells of the vestibulocochlear nerve (CN VIII) (also called vestibular schwannoma) - located in the cerebellopontine angle.
• what is Benign Positional Vertigo?

  - otoconia dislodged from utricle and migrate into semicircular ducts (often posterior one).

  - When head moves, gravity-dependent movement of otoconia causes abnormal fluid displacement in the affected semicircular duct and resultant vertigo
• what is Ménière's Disease?

  too much endolymph and distention of membranous labyrinth => attacks of severe vertigo, nausea, nystagmus, hearing loss and tinnitus, plus eventual permanent progressive hearing loss.
• Describe some disorders of the vestibular system.

  
• What is the vestibulo-ocular reflex?

  "Small rotation of the head is accompanied by movement of the eyes through the same angle but in opposite direction"
• What do ascending & descending tracts from the vestibular nuclei do?

  - Project to nuclei of extra-oculomotor nerves and to the cervical spinal cord to coordinate head movements with eye movements: vestibulo-ocular reflex

  
  

  - Project to cerebellum & lower spinal levels to coordinate extensor & flexor muscles to maintain balance & posture

  
• Where are the vestibular nuclei located?

  In the medulla and the pons

  
• The vestibular nerve separates from the cochlear nerve before reaching the vestibular nuclear complex.

  how many nuclei does the vestibular nuclear complex consist of?

  four nuclei: medial, lateral, superior, and inferior

  
• what does the vestibular nerve join forces with?

  the cochlear nerve and together they form the 8th cranial nerve

  
• after the primary affernet fibres bundle together, what do they beocme?


  the vestibular nerve
• What does the vestibular ganglion contain?

  Cell bodies of bipolar neurones (primary afferent fibres) whose peripheral processes form synaptic contact with hair cells of the vestibular sensory end organs

  
• how is information about head movement in space sent to the brain?

  from the bipolar neurones (primary afferent fibres) and then via the vestibular nerve and ganglion
• How are the stereocilia oriented in the saccule?

  It's also oriented in the same direction relative to the striola, but unlike the utricle, they point away from the striola (therefore, effectively point in different directions)

  
• So what does the saccule respond to?

  It responds to vertically directed linear force & detects position of head in space
• Compare the saccule with the utricle.


  It's essentially like the utricle: also has a macula, but this is oriented vertically when person is in upright position
• What does the position of the stereocilia mean for the utricle?


  It means the utricle can respond to tilt or linear acceleration in many directions. Tilt in any direction will depolarise some cells & hyperpolarise others- leads to complex signal to brain with accurate measure of head position
• How are the stereocilia in the utricle arranged?

  The stereocilia of hair cells in utricle don't all face the same way; they all point towards a curving landmark- the striola

  
• What does gravity do to the otolithic mass when the head is tilted?


  Gravity acts on heavy otolithic mass so it sags in direction of tilt & bends hair cells so movement of hair cells is NOT required. This means static head position can be detected.

  
  

  -> acceleration is not required

  
• What can the macula also detect?

  Linear acceleration (frequent fluctuations lead to motion sickness)
• What is the position of the macula when upright?

  Macula is roughly horizontal so otoliths rest directly on it

  
• What are otoconia?

  Calcium carbonate crystals

  
• What is really distinctive about the utricle?

  The stereocilia of the hair cells are also embedded in a gelatinous material, which is called the otolithic membrane, but has structures called otoconia sitting on top of it

  
• Where are hair cells located in the utricle?

  macula
• Where is the utricle located?

  Within the vestibule
• how do the otolithic organs detect movement?

  within the organs, hair cells detect movement when crystals of calcium carbonate called otoconia shift in response to it. Leading to movement to cells below the otoconia and displacement of hair cells.
• what plane of movements does the saccule detect?

  vertical plane
• what plane of movements does the utricle detect?

  horizontal plane
• what movements does the otolithic organs detect?

  - forward and backward movements

  - gravitational forces
• what are the two otolithic organs?

  utricle and saccule
• how do the hair cells respond to movement in semicircular canals?

  If an individual hears something so turns head to the left (anticlockwise). So the fluid inside the ducts moves in the opposite direction (clockwise); due to inertion.

  
  

  - ON THE LEFT HAND SIDE:

  the stereocilia will be bent towards their axis of polarity - resulting in depolarisaton of hair cells and increased firing on the left-hand side of the afferent fibres

  
  

  - ON THE RIGHT HAND SIDE:

  the stereocilia move in the opposite direction of axis of polarity so decreased firing of afferent fibres

  
  

  ----------> increased firing on left so the brain knows you've rotated the head towards the left

  
  

  NOTE: this is a DYNAMIC system, so it needs acceleration

  
• how do the semicircular canal detect head movements?


  The semicircular canals are filled with endolymph.

  When the head is moved, it causes the movement of endolymph through the canal that corresponds to the plane of the movement.
• what head movements do the semicircular canals detect?

  Each of the canals can detect one of the following head movements:

  - nodding up and down

  - shaking side to side

  - tilting left and right.
• How many semicircular canals are there?

  three

  
• What is the effect of the hair cells being arranged in this way?

  Hair cells only respond along their axis, so each ampulla responds with depolarisation in 1 direction, and hyperpolarisation in the other.

  -> all hair cells do responses together so the system is VERY sensitive
• How are the hair cells in the ampulla arranged?

  They're arranged so that the axis of polarity always points in 1 direction e.g. in horizontal semicircular ducts, they go towards the utricle

  
• What is the cupula?

  The cupula is a structure composed of the hair cells of the inner ear embedded in a gel

  
• What is the ampullary crest?

  A region of thickened epithelium within the ampulla where the hair cells are located

  
• What is inside the ampulla?

  ampullary crest

  
• What is the ampulla in the ear?


  Slightly more dilated region of semicircular canal

  
• what happens to the mechanotransducer channels at rest?


  the channels are a little bit open

  - there is always a steady low level release of neurotransmitter and firing rate