Reflex Eye Movements

• Reflex Eye Movements


  Eye movements that are automatic responses to stimuli
• Reflex eye movements?.??.?.?


  • Repeatable: the same stimulus always evokes the same response (not influenced by internal states)
  • Mediated by spinal circuits (circuits located in the spinal cord)
  • Mediated by reflex arcs: mechanisms are basically chains of neurons (pathways) between sensory receptors and muscles
• Reflex eye movements will be used to address the following three misconceptions regarding the nature of reflexes
• Why move the eyes?


  • To keep the images on the retinas relatively still
  • To position the images of things of interest onto the foveas (the most sensitive regions of the retinas)

  
• If retinal images move on the retina, it is impossible to see clearly => it is vital to keep the images steady
• When we stand or sit still, our heads are typically fairly still as well
• When we walk or run or drive on a bumpy road, our heads move up and down (bob), side to side (roll and weave) and sometimes forwards and backwards (nod): a lot of this movement cannot be prevented
• If the eyes jiggle around with an angular motion of 1° amplitude it is impossible to see clearly
• Vestibular-ocular reflex (VOR)


  A reflex that involves the vestibular organs, keeping the eyes fixated on an object of interest as the head moves
• Without the VOR, every time the head jiggles, the eyes would jiggle and clear vision would be impossible
• Just living jiggles your head – breathing jiggles it, blood pulsing through the arteries jiggles it
• If the vestibular organs are lost, balance is quickly recovered (not as good as before, but plenty good enough for everyday living)
• The VOR keeps your eyes fixated on an object of interest as your head moves
• The vestibulo-ocular reflex


  • Eyes counter-rotate so that they remain pointing in the same direction
  • Operates in all three dimensions: horizontal, vertical and torsional components
• The VOR is mediated by circuits which do not involve the spinal cord at all
• We also have reflexes that keep our heads stable – meaning that the VOR doesn't have to do all the work – the vestibulocollic and cervicocollic reflexes
• Most invertebrates and some amphibians cannot make eye movements, birds have only a very limited ability to do so
• Types of eye movement


  • Saccadic eye movements (very rapid shifts or jumps from one position in the orbit to another)
  • Slow, smooth movements that gradually shift an eye's position in its orbit

  
• Saccadic movements


  Shifts in the direction of gaze from one thing to another, made using rapid eye movements
• Conjugate eye movements


  • Eyes move together in the same direction, through the same angle
  • Disconjugate eye movements: eyes move together in opposite directions

  
• Horizontal saccades


  Simultaneous contraction of left eye lateral rectus + right eye medial rectus
• Horizontal saccades


  To make a leftward saccade, activating signals are sent to the left abducens nucleus, which excites the motor neurons and also interneurons that carry the signal to the right oculomotor nucleus
• Muscles involved in horizontal saccades

  • Medial and lateral recti
• Left saccade: simultaneous contraction of left eye lateral rectus + right eye medial rectus
• Horizontal saccades


  1. Excitatory signals sent to left abducens

  2. Excite motor neurons and interneurons that carry signal to right oculomotor nucleus
• Contraction of muscles on left sides is not sufficient, muscles on right sides need to relax
• How right side muscles relax


  By shutting off (inhibiting) excitation of these muscles
• Paramedian pontine reticular formation (PPRF)


  Generates a burst of excitation that causes the eyes to move rapidly to a new position (the saccadic movement itself)
• Nucleus prepositus hypoglossi (NPH)


  • Contains loops that feedback the output back into the nucleus, so it cannot be described in terms of chains (arcs)
• Pathway for reflex horizontal saccades


  1. Retina(s)

  2. Superior colliculi

  3. PPRF
• Pathway for voluntary horizontal saccades


  1. Frontal cortex

  2. Superior colliculi

  3. PPRF
• When making a voluntary saccade

  You often also turn the head
• Turning the head


  Elicits the VOR response that tries to keep the eyes looking in the same direction
• Vestibular-ocular reflex (VOR)


  Reflex response that tries to keep the eyes looking in the same direction when the head turns
• To look in a new direction, you need to turn the VOR off when making a voluntary head turn
• Whether a VOR response occurs depends on your intent
• 6 Common Misconceptions about reflex behaviour


  • Simple responses
  • Stereotyped responses
  • Mediated by spinal circuits
  • Mediated by reflex arcs
  • Repeatable
  • Not acquired or modified by learning and experience
• Questions on saccade circuits should focus on what the circuit does and what the different parts do, not on the names of nuclei
• If the pathway transmitting signals to the left lateral rectus is severed


  The eye cannot be pulled to the left
• If the nucleus paragiganto-cellularis is removed


  The inhibition on the muscles moving the eye to the right is removed, opposing the leftward movement