Gaze shifting & gaze holding ocular motor functions


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Gaze shifting & gaze holding ocular motor functions

  1. 1. Gaze shifting & Gaze holding Ocular motor functionsGauri S. Shrestha, M.optom, FIACLE
  2. 2. What do Eye Movements Do? Keep visual images relatively stable on the retina Change the angle of gaze Prevent the fading of retinal images Gauri S. Shrestha, M.optom
  3. 3. Keeping Visual Images Stable Prevents retinal slip which leads to motion smear  5 deg/sec can cause motion smear Example: head movement Motion Smear causes blur and the inability to accurately judge object location in space Eye movements used to prevent motion smear  Vestibular Ocular Reflex (VOR)  Optokinetic Nystagmus (OKN) Gauri S. Shrestha, M.optom
  4. 4. Change the Angle of Gaze Align fovea with the object of interest Eye movements used to change gaze angle  Saccades  Pursuits  Vergence Gauri S. Shrestha, M.optom
  5. 5. Prevent Image Fading Perfectly stabilized images fade  Troxler’s Effect Eye Movements used to prevent image fade  Drifts  Tremors  Microsaccades Miniature ocular movements that constantly occur Gauri S. Shrestha, M.optom
  6. 6. Eye Movement Development From birth and continues to develop  VOR, OKN From birth and continues to develop  Saccadic movements From 6-8 weeks and continues to develop  Pursuit movements From 3 months and continues to develop  Vergence, accommodation  BINOCULAR VISION Gauri S. Shrestha, M.optom
  7. 7. How does the eye move? The eye is tightly packed in the orbit 2 forces must be overcome  Viscous drag of the orbit  Elastic restoring forces of the orbital tissues (primary resting position) 2 types of neural activity required to overcome these forces Gauri S. Shrestha, M.optom
  8. 8. Neural Activity Required Velocity (pulse) Signal  Phasic increase in neural activity allowing the EOM to move quickly  Overcomes the viscous drag of the orbit Position (step) Signal  Sustained increase in neural activity allowing the eye to maintain the eye at the new position  Overcomes the elastic restoring forces of the orbital tissues Gauri S. Shrestha, M.optom
  9. 9. Velocity and Position Signals Without pulse (velocity), eye movement is slow Without step (position), eccentric eye position cannot be maintained Pulse and step signals must match for accurate eye movements and steady fixation How do we achieve this? Gauri S. Shrestha, M.optom
  10. 10. Neural Integration The brain converts the pulse (velocity) signal into the step (position) signal What happens if the system fails?  Eye is carried to new position, but drifts back to the central position  Causes nystagmus (jerky eye movements) Gauri S. Shrestha, M.optom
  11. 11. Motor Plasticity Eye movements can undergo adaptation changes  When a patient receives new glasses  Larger movements needed for plus lenses Cerebellum has important role in plasticity  Flocculus and nodulus  Dorsal vermis Gauri S. Shrestha, M.optom
  12. 12. Eye Movement ControlAnatomical and Neural Factors Gauri S. Shrestha, M.optom
  13. 13. What is Needed for Accurate Eye Movements The brain needs to know where the eye is located with respect to the head and orbit  THE AFFERENT SYSTEM The brain needs to know where and by how much to move the eye  THE EFFERENT SYSTEM Clinical Application: Disorders can cause the inability to accurately judge visual space  Inaccurate ‘positional sense’ in targeting task Gauri S. Shrestha, M.optom
  14. 14. The Afferent System Provides information about eye position to the brain How is this achieved?  Two sources (subsystems)  Proprioception  Efference copy/Corollary discharge Gauri S. Shrestha, M.optom
  15. 15. Afferent System-Proprioception Includes Muscle Spindles in the human EOM that respond to stretch Includes the Palisade Tendon Organ in the human EOM that responds to tension These signals are sent to the brain through the trigeminal nerve Gauri S. Shrestha, M.optom
  16. 16. The Afferent System-Efference Copy AKA: Corollary Discharge This is a ‘copy’ of the motor signal to move the eye that is sent back to the brain Anatomical origin: unknown Also involved in distinguishing between real motion or self motion Gauri S. Shrestha, M.optom
  17. 17. The Efferent System Provides brain with information about how much and where to move the eye Neural information from this system provides innervation to the ocular motor nuclei to move the EOM Gauri S. Shrestha, M.optom
  18. 18. Innervation of the EOMs Oculomotor (Cranial Nerve 3)  MR, SR, IR, IO Trochlear (Cranial Nerve 4)  SO Abducens (Cranial Nerve 6)  LR Gauri S. Shrestha, M.optom
  19. 19. Review of these systems Afferent System  Proprioception  Corollary Discharge or Efference Copy Efferent System  Sends pulse and step neural signals to the EOM Gauri S. Shrestha, M.optom
  20. 20. The EOM Fibers Two Major Function  Move the eyes (quickly or slowly)  Change eye position  Keep the eyes relatively stationary  Maintain new eye position Gauri S. Shrestha, M.optom
  21. 21. Physiological Types of Fibers Twitch Fibers (burst)  All or none action potential Non-Twitch Fibers (tonic)  Graded contractions Few fibers are a combination Gauri S. Shrestha, M.optom
  22. 22. Twitch Fibers (Burst) All or None response Fast-fatiguing fibers Known as global fibers  Fibers that are closer to the eyeball Good for rapidly moving the eye to a new position Gauri S. Shrestha, M.optom
  23. 23. Non-Twitch Fibers (step) Receive the step signal Graded contraction High oxidative capacity Known as orbital fibers  Fibers that are closer to the orbit Good for maintaining the new eye position Gauri S. Shrestha, M.optom
  24. 24. Fiber Positions Gauri S. Shrestha, M.optom
  25. 25. Actions of EOMsCardinal Positions Gauri S. Shrestha, M.optom
  26. 26. Actions of EOMsFrom Primary Position Primary Secondary TertiaryMR AdductionLR AbductionIR Depression Extorsion AdductionSR Elevation Intorsion AdductionIO Extortion Elevation AbductionSO Intorsion Depression Abduction Gauri S. Shrestha, M.optom
  27. 27.  Five movement subsystems: 1. Saccadic systems 2. Pursuit systems 3. Vestibulo-ocular reflex 4. Optokinetic reflex 5. Vergence Gauri S. Shrestha, M.optom
  28. 28. Saccadic systems Saccade named after the French word describing the rapid turning of a horses head Saccades are very fast yoked eye movements that have a variety of function Speed of saccade – 400-700/s Saccades produce the quick phase of vestibular & OKN to avoid turning the eyes to their mechanical limitations Saccades also occur withS.head M.optom Gauri Shrestha, movements.
  29. 29. Saccadic systems∀ Undershoot or overshoot during saccades is corrected by micro saccades or glissade∀ Saccadic waltz (pulse-slide-step) called glissade (pulse) & tonic (step) innervations of the saccade∀ Saccade in neonates is inaccurate . Developed by 1 yr.∀ Saccade respond very quickly because of their burst or pulse innervations∀ Saccadic system involves pulse step mechanism.∀ Burst of electrical activity is required to move the eye to the desired position-pulse Gauri S. Shrestha, M.optom
  30. 30. Saccadic systems∀ After pulse, further energy required to maintain the eye in desired position & counter elastic recoil –step∀ Speed of saccade is greatest midway between 1/3 & halfway of saccade movement -max. Velocity peak (MVP)∀ Larger the saccade greater the MVP Gauri S. Shrestha, M.optom
  31. 31. Pulse –Step theory∀ Pulse –step theory is due to 3 groups of neuron.∀ Burst neuron ∀ Cause rapid electrical discharge with rapid acceleration medium ∀ At the end stage of the saccade inhibitory burst neuron stimulate antagonist muscle to stop the movement. Pause neuron ∀ Inhibit firing of burst neuron until initiation of the saccade ∀ Its activity is suspended immediately after the start of the saccade Gauri S. Shrestha, M.optom
  32. 32. Pulse –Step theory Gauri S. Shrestha, M.optom
  33. 33. Pulse –Step theory Tonic neuron ∀ Responsible for maintaining muscle tone ∀ Its activity increase after saccade to maintain gaze position represents the step Gauri S. Shrestha, M.optom
  34. 34. Pulse –Step theory∀ Saccade are regulated by Neural integraters (NI). NI converts velocity command into appropriate position command – step (pulse step mechanism) i.e. pulse is integrated to produce steps.∀ The pulse innervation produced by the burst cell, controls the velocity of the saccade and the step of innervation produced by tonic cell, controls the final position of the eye upon completion of the saccade. Gauri S. Shrestha, M.optom
  35. 35. Anatomical pathways Frontal eye field (frontal cortex, area 8)--> ant. Limb of internal capsule-- > decussate in lower midbrain-- > synapse at horizontal gaze center (PPRF-Paramedian Pontine Reticular Formation) -- > CN VI nucleus--> motor neurons to LR and Interneurons to MR subnucleus(CNIII) Gauri S. Shrestha, M.optom
  36. 36. Anatomical pathways Motor neurons of MR i.e. right frontal cortex initiates saccades to left Also superior colliculus initiate the contralateral saccade in response to novel visual stimuli Cerebellum also plays a major role in controlling saccadic pulse size and so aids in co-ordinated eye movements. Gauri S. Shrestha, M.optom
  37. 37. Smooth pursuit Tracking or following movement∀ Much slower than saccadic with maximum speed at 400/s, if speed higher than that??∀ Latency- 100- 125msec∀ Slow eye movements are also generated by vestibules In neonates tracking usually accompanied by series of saccades. Full development by 3-4 months of age Gauri S. Shrestha, M.optom
  38. 38. Smooth pursuit Functions∀ Cancellation of VOR during head tracking foveas∀ Cancellation of OKN during fixation and tracking∀ Foveate moving isolate targets (stabilizes moving objects on retina when background. Gauri S. Shrestha, M.optom
  39. 39. Smooth pursuit Control of pursuit∀ Parietal cortex (medial, temporal..)∀ MT, MST∀ Horizontal pursuit is initiated by ipsilateral occipito-paritetal cortex--> PPRF--> motor neurons of LR & MR i.e. left occipital cortex is responsible for left pursuit∀ Vertical pursuit originate in occipito parietal region, interstitial nucleus of cajal (INC)--> CN IV, III Gauri S. Shrestha, M.optom
  40. 40. Vestibulo-ocular reflex (VOR)∀ The first class of stabilizing eye movement that components for brief head and body rotation∀ Generates slow eye movements in response to head movement maintaining steady eye position∀ Semicircular canals of the vestibulo labyrinth signals how fast the head is moving and oculomotor system responds to this signal by rotating the eyes in an equal and opposite velocity. Gauri S. Shrestha, M.optom
  41. 41. Vestibulo-ocular reflex (VOR)∀ Stabilizes the eyes relative to the external world and keep visual images fixed on the retina∀ Works in total darkness responds to acceleration and deceleration but not to constant velocity∀ Control initial image stabilization∀ Otoliths (saccula) compensates for head tilt movement that cause torsional eye movement∀ Proprio-receptors in neck muscles also contribute towards VOR Gauri S. Shrestha, M.optom
  42. 42. Vestibulo-ocular reflex (VOR)∀ Can be tested with Doll head movement and inducing vestibular nystagmus using swinging baby test or caloric test∀ Caloric test --> Pt head inclined at 600 so that Horizontal Semicircular canals lies vertically-->∀ COWS (Cold water – fast phase of nystagmus towards opposite labyrinth, warm water – fast phase towards same labyrinths) Gauri S. Shrestha, M.optom
  43. 43. Anatomical pathways Labyrinth--> CN VIII nucleus – PPRF & CN III, IV &VI nucleus Gauri S. Shrestha, M.optom
  44. 44. Optokinetic Reflex (OKN)∀ 2nd ocular stabilization system that responds to currents of image motion∀ Also referred as railway / parade nystagmus∀ OKN supplements VOR in several ways∀ OKN responds to constant velocity∀ Both OKN & VOR exhibit jerk nystagmus (following movement and then saccade) Gauri S. Shrestha, M.optom
  45. 45. Optokinetic Reflex (OKN)∀ Active OKN or pursuing, the phase follows target towards the periphery away from primary gaze∀ Passive OKN or starring, the fast phase (saccade) to where the target is emerging from and then has a slow phase back to primary gaze.∀ OKN is responsible to large fields 20- 500∀ Max. Velocity rarely exceeds 500/sec usually in close to the stimulus velocity below 300/sec Gauri S. Shrestha, M.optom
  46. 46. Optokinetic Reflex (OKN)∀ Can be demonstrated with OKN drum, look at the stripes, pursuit and saccades occurs --> refers OKN∀ Slow target velocities --> good correspondence∀ >30-1000/s velocities- eye movement lags behind target movement∀ Beyond 1000/s – OKN cant be demonstrated Gauri S. Shrestha, M.optom
  47. 47. Optokinetic Reflex (OKN)∀ OKN development in an infant from the sub- cortical crossed input—stimulates a nasal ward slow phase of OKN (Both eyes move smoothly towards covered eye)∀ After 3 months, infants cortical projection predominate and horizontal OKN responds to both temporal ward and nasal ward image motion.∀ Vertical OKN can also tested Gauri S. Shrestha, M.optom
  48. 48. Vergence∀ Disjunctive or Vergence eye movements are movements of the eyes in opposite direction∀ They can be horizontal , vertical and cyclo∀ Vergence doesnt appear in animals with laterally placed eyes∀ Units of measurement- degrees, prism Diopters, meter angle∀ Maddox classification of Vergence- tonic, proximal, disparity & accommodative∀ Stimuli- retinal disparity, diplopia, accommodation, convergence Gauri S. Shrestha, M.optom
  49. 49. Gauri S. Shrestha, M.optom
  50. 50. Vergence∀ Development – in full term from neonates but un- coordinated- accurate convergence developed by 2-3 months of age∀ Slower than saccades∀ Supra nuclear control of Vergence- unclear- frontal eye fields and occipital regions produces convergence.∀ Middle temporal region and parietal cortex discharge in response to retinal disparity and objects moving in depth∀ Neurons near oculomotor nucleus act as immediate pre- motor neurons for Vergence Gauri S. Shrestha, M.optom
  51. 51. Vergence The supraoculomotor nucleus contains burst and tonic cells as well as phasic-tonic cells that are characteristic of the saccadic pathways. Gauri S. Shrestha, M.optom
  52. 52. Vergence It is thought that velocity signals related to disparity stimlui innervate the bursters and that velocity information is integrated to form the position signal that is processed by the tonic cells. Accommodative-vergence is already represented by these cells so that the wiring for cross-coupling between accommodation and convergence must occur more centrally in the pathways. Gauri S. Shrestha, M.optom
  53. 53. Supranuclear disorders∀ Gaze palsies – disorders of saccade amplitudes and appropriateness∀ Saccadic dysmetria∀ Vertical gaze palsy∀ Internuclear ophthalmoplegia Gauri S. Shrestha, M.optom