NEUROBIOLOGY B7

• Vestibular system

  Drives very rapid, reflexive movements of the head and eyes to try and maintain horizontal view
• Rotational eye movements are used by many animals (including humans) to keep the world horizontal
• Statocyst

  Structure in crabs and molluscs similar in function to the otoliths of mammals
• Vestibular system

  Also known as the labyrinth of the inner ear, has 2 primary components: semicircular canals and utricle/saccule (otolith organs)
• Semicircular canals detect rotational movements, filled with endolymph
• Utricle and saccule (otolith organs) detect tilt and linear translations
• Vestibular system contains hair cells similar to those of the cochlea, synapsing with afferent neurons that project to vestibular nuclei
• Vestibular system is best developed in agile animals, helps control sense of 'self-motion' and balance, and stabilises eye movements
• Displacement of endolymph in ampulla of semicircular canals

  1. Endolymph flows within the canals during rotational movement

  2. This flow displaces against the hair cells, deflecting their hair bundles

  3. The deflection of hair bundles activates sensory fibres

  4. The firing rate of sensory fibres corresponds to the amount of angular acceleration detected by the hair cells
• Graded potential changes and hair cells

  • Hair cells within the semicircular canals respond to movement of the fluid as the head turns
  • Graded potential or spike firing rate then encodes the direction of fluid flow and its speed, providing the sensory cue required to turn the eyes to compensate for head movement
  • Excitation: depolarisation of receptor potential --> increased impulse frequency
  • Inhibition: hyperpolarisation of receptor potential --> decreased impulse frequency
• Semicircular canals

  • Left and right semi-circular canals work together to detect rotational movements in yaw, pitch and roll
• Otolith organs (saccule, utricle)

  • Driven by otoconia composed of calcium carbonate in a protein matrix, which move due to inertia and push against the stereocilia
  • Utricle is sensitive to tilt and horizontal movement, saccule is sensitive to vertical acceleration
• Otoliths are highly developed in fish, critical for balance underwater
• Hair cells in utricle and saccule

  Respond to movement of the fluid and the inertia of the otoconia, encoding linear acceleration
• Orientation of hair cells permits variation in mechanosensitivity, allowing discrimination in sensing direction
• Vestibular inputs signalling body posture and motion can be ambiguous (can't tell the difference when tilting head and moving entire body)
• Fixing gaze

  1. Generate a perfectly calibrated divergence of output to neck muscles and eye muscles so they move in concert

  2. Could be driven by the magnocellular pathways of the visual system, but vision is too slow due to 2nd messenger transduction and motion detection delays

  3. Hair cells of the vestibular system provide a solution for more rapid sensing of motion
• Vestibulo-ocular reflex (VOR)

  An example of 'closed loop' feedback, compensating for head movements to keep the eyes still in space
• VOR

  • Does not depend on visual input, works even in total darkness or with eyes closed
  • The semicircular canals provide a head velocity signal
  • The VOR provides an equal and opposite eye velocity signal to keep the eyes still
  • Eye movements lag head movements by less than 10ms
• VOR

  1. Hair cells in the ampullae respond to fluid movement as the head turns, stimulating afferent pathways that drive the rectus muscles of the eyes

  2. The two halves of the head respond in opposite fashion
• Excitatory and inhibitory connections in VOR

  Work together to drive the VOR, with inputs from the contralateral organ playing a crucial role
• Disruption between left and right vestibular nuclei

  Perceived as head rotation, may produce nausea or vomiting
• Alcohol intoxication

  Changes in fluid volume can affect the VOR
• Nystagmus

  Involuntary eye movement with alternating smooth pursuit and quick saccade in opposite direction, caused by stimulation of the semicircular canals while the head is not in motion
• Adaptability of the VOR

  The vestibular system (not vision) drives the reflex, but a 'feed-forward gain control' from a visual error signal (via cerebellum) can adjust the gain of the VOR over time