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THE TOP TEN THINGS YOU SHOULD KNOW ABOUT THE OCULOMOTOR SYSTEM 10. Movements of the eyes are produced by six extra-ocular muscles. If they, or the neural pathways controlling them, are not functioning normally, eye movements are abnormal. Video OF Duane’s Video of Opsoclonus • Additionally, accommodation and pupillary responses are produced by intraocular muscles Meet the muscles Muscle Primary action Example Medial rectus Adduction Towards the midline/nose Lateral rectus Abduction Away from the midline/nose Superior rectus Elevation Inferior rectus Depression Superior oblique Intorsion Inferior oblique Extorsion Meet the muscles (Cont.) Muscle Primary action Ciliary muscle Positive accommodation: acts against suspensory ligaments Sphincter pupillae iris muscle Pupilloconstriction Dilator pupillae iris muscle Pupillodilation 9. The stretch reflex is absent. Gently press on your eye and you’ll see the world move. • Proprioceptive feedback from the extraocular muscles is not used to keep track of eye position. • The brain keeps track of eye position by keeping track of the signals sent to the motoneurons that innervate the extraocular muscles. This is known as efference copy or corollary discharge. 8. Except for changes in viewing distance, normal eye movements are yoked. • Yoking: the eyes move the same amount in the same direction. • Vertical eye movements are normally always yoked. • Projections from the abducens nucleus to medial rectus motoneurons by way of the medial longitudinal fasciculus provides the basis for horizontal yoking. During convergence, the eyes move equal amounts in opposite directions. MVN - Medial vestibular nucleus NPH - Nucleus prepositus hypoglossi EBN - Excitatory burst neuron IBN - Inhibitory burst neuron Excitatory Inhibitory VIDEO SHOWING INTERNUCLEAR OPHTHALMOPLEGIA 7. Eye movements are controlled by distinct neurological subsystems. • Eye movements stabilize the image of the external world on the retina • Eye movements bring images of objects of interest onto the fovea FUNCTIONAL CLASSES OF EYE MOVEMENTS Extra-ocular muscles Class of Eye Movement Main Function Vestibular Holds images of the seen world steady on the retina during brief head rotations Optokinetic Holds images of the seen world steady on the retina during sustained head rotation Visual fixation Holds the image of a stationary object on the fovea Smooth pursuit Holds the image of a small moving target on the fovea; with optokinetic responses, aids gaze stabilization during sustained head rotation Nystagmus quick phases Reset the eyes during prolonged rotation and direct gaze toward the oncoming visual scene Saccades Bring images of objects of interest onto the fovea Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on both foveas FUNCTIONAL CLASSES OF EYE MOVEMENTS Intra-ocular muscles Class of Eye Movement Main Function Accommodation Focuses images on fovea Pupillary Light Reflex Controls illumination levels of retina 6. Vestibular responses. You can’t read without them. Your head turns in one direction with a certain velocity, and because of the vestibular ocular reflex (VOR), your eyes turn with an equal velocity (if the VOR gain is 1.0) in the opposite direction. This reflex has a latency of less than 10 millseconds. Once the transient head rotation ceases, your eyes have turned to a new position. They need to remain at that position and not drift back to primary position. To achieve this, a tonic signal proportional to the integral of the eye velocity signal is generated and sent to the extraocular motoneurons to maintain the new eye position. VOR gain is low at low frequencies Vestibulo-ocular reflex MVN - Medial vestibular nucleus NPH - Nucleus prepositus hypoglossi EBN - Excitatory burst neuron IBN - Inhibitory burst neuron Excitatory Inhibitory Increased firing rate with rightward head turns FUNCTIONAL CLASSES OF EYE MOVEMENTS Extra-ocular muscles Class of Eye Movement Main Function Vestibular Holds images of the seen world steady on the retina during brief head rotations Optokinetic Holds images of the seen world steady on the retina during sustained head rotation Visual fixation Holds the image of a stationary object on the fovea Smooth pursuit Holds the image of a small moving target on the fovea; with optokinetic responses, aids gaze stabilization during sustained head rotation Nystagmus quick phases Reset the eyes during prolonged rotation and direct gaze toward the oncoming visual scene Saccades Bring images of objects of interest onto the fovea Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on both foveas 5. Optokinetic responses. The world drifts without them. When large-field stimuli move, your eyes tend to track the overall movement. This is an adaptive response to the slip of the image of the outside world on the retina that occurs when VOR gain is not 1.0. This response is mediated by neurons in the pretectum and the medial superior temporal (MST) region of cortex. These neurons indirectly modulate vestibular neurons. VESTIBULAR NUCLEUS NEURON A. ROTATION IN DARKNESS (Vestibular but no Optokinetic) B. ROTATION IN LIGHT (Vestibular and Optokinetic) C. NO ROTATION. OPTIC FLOW. (Optokinetic but no Vestibular) Vestibular-optokinetic interactions Schematic summary of vestibular-optokinetic interaction occurring in response to velocity-step rotations. Graphs on the left show characteristics of the stimulus (head velocity during rotation or drum velocity during optokinetic stimulation); graphs on the right show the responses (slow-phase eye velocity, quick phases having been removed). R, right; L, left; t, time. In the top panel, constant-velocity rotation to the left in the dark produces slowphase movements to the right (per-rotatory nystagmus, RN) with initial eye velocities equal to head velocity (VOR gain = 1.0). When rotation stops, nystagmus starts in the opposite direction (postrotatory nystagmus, PRN). In the middle panel, an optokinetic stimulus (drum rotation to the right) causes a sustained optokinetic nystagmus (OKN), with slow phases to the right during the entire period of stimulation. When the lights are turned off during stimulation, eye movements do not stop immediately but persist as optokinetic afternystagmus (OKAN). In the lower panel, the subject is rotated in the light (natural situation of self-rotation). This gives a combined vestibular and optokinetic stimulus. The response is a sustained nystagmus. When the chair stops rotating, eye movements stop nearly completely: postrotatory nystagmus is suppressed by the opposite-directed optokinetic after-nystagmus and by visual fixation of the stationary world. FUNCTIONAL CLASSES OF EYE MOVEMENTS Extra-ocular muscles Class of Eye Movement Main Function Vestibular Holds images of the seen world steady on the retina during brief head rotations Optokinetic Holds images of the seen world steady on the retina during sustained head rotation Visual fixation Holds the image of a stationary object on the fovea Smooth pursuit Holds the image of a small moving target on the fovea; with optokinetic responses, aids gaze stabilization during sustained head rotation VOLUNTARY Saccades Bring images of objects of interest onto the fovea VOLUNTARY Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on both foveas VOLUNTARY 4. Saccadic eye movements. You can’t look at anything interesting without them. • FAST - 40-90 MS IN TOTAL DURATION •BALLISTIC Pulse of firing rate is required to produce the transient force needed to move the eye rapidly despite viscous drag Sustained firing rate is required to hold the eye in a new postion despite the elastic forces that try to return it to primary position Right Medial Rectus Motoneuron - Saccades Horizontal saccades are generated in the paramedian pontine reticular formation (PPRF) VERTICAL SACCADES ARE GENERATED HERE Vertical saccades are generated in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) MVN - Medial vestibular nucleus NPH - Nucleus prepositus hypoglossi EBN - Excitatory burst neuron IBN - Inhibitory burst neuron Excitatory Inhibitory Increased firing rate with rightward head turns OMNIPAUSE NEURON (OPN) EBN Burst size proportional to saccade size NI - Neural Integrator Excitatory burst neuron- small saccade Excitatory burst neuron- medium saccade Excitatory burst neuron - large saccade Omnipause neuron - various saccades OMNIPAUSE NEURON (OPN) EBN Burst size proportional to saccade size NI - Neural Integrator THE SUPERIOR COLLICULUS PROJECTS TO THE PPRF 2-D map of contralateral saccades SUPERIOR COLLICULUS MOTOR MAP A block diagram of the major structures that project to the brain stem saccade generator (premotor burst neurons in PPRF and riMLF). Also shown are projections from cortical eye fields to superior colliculus. FEF, frontal eye fields; SEF, supplementary eye fields; DLPC, dorsolateral prefrontal cortex; IML, intramedullary lamina of thalamus; PEF, parietal eye fields (LIP); PPC, posterior parietal cortex; SNpr, substantia nigra, pars reticulata. Not shown are the pulvinar, which has connections with the superior colliculus and both the frontal and parietal lobes, and certain projections, such as that from the superior colliculus to nucleus reticularis tegmenti pontis (NRTP). Disorders of the saccadic pulse and step. Innervation patterns are shown on the left, eye movements on the right. Dashed lines indicate the normal response. (A) Normal saccade. (B) Hypometric saccade: pulse amplitude (width ´ height) is too small but pulse and step are matched appropriately. (C) Slow saccade: decreased pulse height with normal pulse amplitude and normal pulse-step match. (D) Gaze-evoked nystagmus: normal pulse, poorly sustained step. (E) Pulse-step mismatch (glissade): step is relatively smaller than pulse. (F) Pulse-step mismatch due to internuclear ophthalmoplegia (INO): the step is larger than the pulse, and so the eye drifts onward after the initial rapid movement. Experimental cerebellectomy completely abolishes the adaptive capability-for both the pulse size and the pulse-step match.296 Monkeys with lesions restricted to the dorsal cerebellar vermis cannot adapt the size of the saccadic pulse; they have pulse-size dysmetria .416,416a On the other hand, monkeys with floccular lesions cannot match the saccadic step to the pulse to eliminate pulse-step mismatch dysmetria.298 This evidence suggests that the repair of conjugate saccadic dysmetria is mediated by two different cerebellar structures: the dorsal cerebellar vermis and the fastigial nuclei control pulse size, and the flocculus and paraflocculus control the pulse-step match. FUNCTIONAL CLASSES OF EYE MOVEMENTS Extra-ocular muscles Class of Eye Movement Main Function Vestibular Holds images of the seen world steady on the retina during brief head rotations Optokinetic Holds images of the seen world steady on the retina during sustained head rotation Visual fixation Holds the image of a stationary object on the fovea Smooth pursuit Holds the image of a small moving target on the fovea; with optokinetic responses, aids gaze stabilization during sustained head rotation Nystagmus quick phases Reset the eyes during prolonged rotation and direct gaze toward the oncoming visual scene Saccades Bring images of objects of interest onto the fovea Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on both foveas • Smooth pursuit: Tracking eye movements conjugate. Velocity of visual target • Visual cue: retinal slip velocity of visual target. 3. Smooth pursuit eye movements. You can’t track anything interesting without them • Smooth pursuit: Tracking eye movements conjugate. Velocity of visual target. Slow. • Visual cue: retinal slip velocity of visual target. SMOOTH PURSUIT PATHWAYS FUNCTIONAL CLASSES OF EYE MOVEMENTS Extra-ocular muscles Class of Eye Movement Main Function Vestibular Holds images of the seen world steady on the retina during brief head rotations Optokinetic Holds images of the seen world steady on the retina during sustained head rotation Visual fixation Holds the image of a stationary object on the fovea Smooth pursuit Holds the image of a small moving target on the fovea; with optokinetic responses, aids gaze stabilization during sustained head rotation Nystagmus quick phases Reset the eyes during prolonged rotation and direct gaze toward the oncoming visual scene Saccades Bring images of objects of interest onto the fovea Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on both foveas 2. Vergence. Without it, you can’t get a closer look. • Vergence: Eye movements in depth. Disconjugate - left and right eyes move in opposite directions. Far viewing F F Near target F F Blurred images fall on non-corresponding retinal locations blur and disparity signals Accommodation and convergence F F Eyes converge and lenses focus - reduced blur and disparity signals Far viewing F F Near target F F Blurred images fall on retina - blur signal Accommodation and accommodative convergence F F Eyes change focus - reduced blur signal. Also accommodative convergence 2-D map of saccades MVN - Medial vestibular nucleus NPH - Nucleus prepositus hypoglossi EBN - Excitatory burst neuron IBN - Inhibitory burst neuron Excitatory Inhibitory Increased firing rate with rightward head turns Internuclear ophthalmoplegia - adduction during convergence is not reduced FUNCTIONAL CLASSES OF EYE MOVEMENTS Intra-ocular muscles Class of Eye Movement Main Function Accommodation Focuses images on fovea Pupillary Light Reflex Controls illumination levels of retina MODULATE ROOM LIGHTS Pupillary light reflex Pupillary light reflex Direct Consensual Pupillary light reflex Direct Consensual Pupillary light reflex Direct Consensual Pupillary light reflex Direct Consensual 1. Pupillary light reflex. If it’s absent, there’s a problem. AFFERENT DEFECTS: PUPILS APPROX. EQUAL IN SIZE. BUT RESPONSE TO LIGHT IN ONE EYE IS LESS THAN THE RESPONSE TO LIGHT IN THE OTHER EYE. EFFERENT DEFECTS: PUPILS MAY BE OF DIFFERENT SIZES (ANISOCORIA). PUPIL OF ONE EYE REACTS MORE TO LIGHT IN EITHER EYE THAN THE PUPIL OF THE OTHER EYE TO LIGHT IN EITHER EYE. Pupillary light reflex: Afferent deficit Pupillary light reflex: Afferent deficit Neutral Density Filter (0.5 log unit) 0.5 log unit Relative Afferent Pupillary Deficit (RAPD) Pupillary light reflex: Efferent deficit TOP TEN LIST 1. Pupillary light reflex. If it’s absent, there’s a problem. 2. Vergence. Without it, you can’t get a closer look. 3. Smooth pursuit eye movements. You can’t track anything interesting without them 4. Saccadic eye movements. You can’t look at anything interesting without them. 5. Optokinetic responses. The world drifts without them. 6. Vestibular responses. You can’t read without them. 7. Eye movements are controlled by distinct neurological subsystems. 8. Except for changes in viewing distance, normal eye movements are yoked. 9. The stretch reflex is absent. Gently press on your eye and you’ll see the world move. 10. Movements of the eyes are produced by six extra-ocular muscles. If they, or the neural pathways controlling them, are not functioning normally, eye movements are abnormal.
