First you tell them what your gonna The oculomotor system tell them - PDF document

First you tell them what your gonna The oculomotor system tell them • The phenomenology of eye movements. Or • The anatomy and physiology of the Fear and Loathing at the Orbit extraocular muscles and nerves. • The supranuclear control of eye movements: motor control and cognitive Michael E. Goldberg, M.D. plans. The purposes of eye movements The vestibuloocular reflex. • Keep an object on the fovea • Fixation • Smooth pursuit • The semicircular canals provide a head • Keep the eyes still when the head moves velocity signal. • Vestibulocular reflex • The vestibuloocular reflex (VOR) • Optokinetic reflex provides an equal and opposite eye • Change what you are looking at ( move the velocity signal to keep the eyes still in fovea from one object to another) • Saccade space when the head moves. • Change the depth plane of the foveal object • Vergence – eyes move in different directions Smooth pursuit matches eye The vestibular signal habituates, and is supplemented by vision – the optokinetic response velocity to target velocity 1

6 Muscles move the eyes Saccades move the fovea to a new position Levator Palpebrae Superior Rectus Lateral Rectus Medial Rectus Superior Oblique Inferior Oblique Inferior Rectus How the single eye moves The obliques are counterintuitive • Horizontal: • Each oblique inserts behind the equator of • Abduction (away from the nose) the eye. • Adduction (toward the nose). • The superior oblique • Vertical: rotates the eye • Elevation (the pupil moves up) downward and intorts it! • Depression (the pupil moves down) • The inferior oblique rotates the eye upward • Torsional: and extorts it. • Intorsion: the top of the eye moves towards the • Vertical recti tort the nose eye as well as elevate • Extorsion: the top of the eye moves towards the or depress it. ear. 3 Cranial Nerves Control the Eye Oblique action depends on orbital position Levator Palpebrae Superior Rectus • The superior oblique depresses the eye Inferior Rectus Nerve III: when it is adducted Oculomotor (looking at the Medial Rectus nose). • The superior oblique Inferior Oblique intorts the eye when Nerve IV: it is abducted Trochlear Superior Oblique (looking towards the ear) Nerve VI: Lateral Rectus Abducens 2

Left fourth nerve palsy Listing’s Law • Hyperopia in central gaze. • Worse on right gaze. • Torsion must be constrained or else vertical • Better on left gaze. lines would not remain vertical. • Worse looking down to • Listing’s law accomplishes this: the axes of right rotation of the eye from any position to any • Better looking up to other position lie in a single plane, Listing’s right. plane. • Head tilt to right improves gaze. • This is accomplished by moving the axis of • Head tilt to left worsens rotation half the angle of the eye movement gaze. The pulleys: something new in Pulley Anatomy orbital anatomy and physiology. • How is Listing’s law accomplished? • Extraocular muscles have two layers • A global layer that inserts on the sclera • An orbital layer that inserts on a collagen-elastin structure between the orbit and globe. This structure serves as a PULLEY through which the global layer moves the eye. • Moving the pulleys accomplish listings law. (Demer). Horizontal rectus pulleys change The pulleys their position with horizontal gaze. 3

Oculomotor neurons describe eye Eye muscle nuclei position and velocity. Mesencephalic Eye position – the step Thalamus Lateral Medial Reticular Superior Colliculus Formation Inferior Colliculus III Abducens neuron Eye velocity – the pulse IV Medial - Lateral Sp/s Pulse Cerebellum Eye Position Step VI Neuron Pontine Vestibular Nuclei Abducens neuron Nuclei Horizontal saccades are generated in The transformation from muscle the pons and medulla activation to gaze Thalamus Superior Colliculus • The pulse of velocity and the step of position are generated independently. Inferior Colliculus • For horizontal saccades the pulse is Medial III longitudinal generated in the paramedian pontine reticular IV fasciculus formation. Cerebellum • The step is generated in the medial vestibular nucleus and the prepositus hypoglossi by a Paramedian VI Pontine neural network that integrates the velocity Reticular signal to derive the position signal. Formation Vestibular Nuclei and Pontine Nucleus Prepositus Nuclei Hypoglossi Neurons involved in the generation Digression on Neural Integration of a saccade • Intuitively, you move your eyes from position to position (the step). • Higher centers describe a saccadic position error. • The pontine reticular formation changes the position error to a desired velocity (the pulse). • The vestibulo-ocular reflex also provides the desired velocity. • In order to maintain eye position after the velocity signal has ended, this signal must be mathematically integrated. ` 4

Generating the horizontal gaze Left lateral rectus Right medial rectus . signal Abducens nerve Oculomotor nucleus and nerve: motor Paramedian neurons only pontine reticular • The medial rectus of one eye and the formation Medial (saccade velocity) lateral rectus of the other eye must be longitudinal Abducens fasciculus coordinated. nucleus: motor neurons and • This coordination arises from interneurons. interneurons in the abducens nucleus Medial that project to the contralateral medial vestibular nucleus: eye rectus nucleus via the medial position, VOR and smooth longitudinal fasciculus. Nucleus prepositus pursuit velocity hypoglossi (eye position) Vertical movements and vergence are To reiterate organized in the midbrain • Ocular motor neurons describe eye position and velocity. Mesencephalic Posterior commissure Thalamus • For smooth pursuit and the VOR the major signal is the Reticular Superior Colliculus velocity signal, which comes from the contralateral medial Formation vestibular nucleus. • The neural integrator in the medial vestibular nucleus and Inferior Colliculus rIMLF nucleus prepositus hypoglossi converts the velocity signal into a position signal which holds eye position. III • For horizontal saccades the paramedian pontine reticular Medial IV formation converts the position signal from supranuclear Longitudinal centers into a velocity signal. Cerebellum Fasciculus • This signal is also integrated by the medial vestibular nucleus and the nucleus prepositus hypoglossi. Paramedian • Abducens interneurons send the position and velocity signals VI Pontine to the oculomotor nucleus via the medial longitudinal Reticular fasciculus. Formation Pontine Vestibular Nuclei Nuclei Internuclear ophthalmoplegia Supranuclear control of saccades • The medial longitudinal fasciculus is a vulnerable fiber tract. • The brainstem can make a rapid eye • It is often damaged in multiple sclerosis movement all by itself (the quick phase and strokes. of nystagmus). • The resultant deficit is internuclear • The supranuclear control of saccades ophthalmoplegia requires controlling the rapid eye movement for cognitive reasons. • The horizontal version signal cannot • In most cases saccades are driven by reach the medial rectus nucleus, but the attention convergence signal can. 5

Supranuclear control of saccades Humans look at where they attend Supplementary Eye Field Posterior Parietal Cortex Frontal Eye Field Caudate Nucleus Superior Colliculus Substantia Nigra Pars Reticulata Reticular Formation Supranuclear Control of Saccades The effect of lesions • Superior colliculus drives the reticular formation to • Monkeys with collicular or frontal eye field lesions make contralateral saccades. make saccades with a slightly longer reaction time. • The frontal eye fields and the parietal cortex drive the • Monkeys with combined lesions cannot make colliculus. saccades at all. • The parietal cortex provides an attentional signal and • Humans with parietal lesions neglect visual stimuli, the frontal eye fields a motor signal. and make slightly hypometric saccades with longer reaction times. Often their saccades are normal: if • The substantia nigra inhibits the colliculus unless they can see it they can make saccades to it. • It is inhibited by the caudate nucleus • Humans with frontal lesions cannot make • Which is, in turn, excited by the frontal eye field. antisaccades. The Antisaccade Task The Antisaccade Task • Look away from a stimulus. • The parietal cortex has a powerful signal describing the attended stimulus. • The colliculus does not respond to this signal. • The frontal motor signal drives the eyes away from the stimulus. • Patients with frontal lesions cannot ignore the stimulus, but must respond to the parietal signal 6