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Transcript
Part 4 Key consequence of Hering’s law of equal innervation is that the two eyes are seen by the brain as a single, non-separable organ Conjugate commands will rotate both eyes in the same direction of the same amount Brain sees the two eyes as a single “cyclopean eye” Vergence commands will rotate both eyes in the opposite direction of the same amount Symmetrically control the angle between the two eyes Any eye movement is the sum of an appropriate pair of conjugate and vergence commands Cranial III----Oculomotor nerve Innervation of IR, IO, MR, SR and levator palpabrae Parasympathetic innervation of the constrictor pupillae and ciliary muscle Ipsilateral oculomotor nucleus IR, IO, MR Contralateral oculomotor nucleus SR Ipsilateral and contralateral nuclei of levator Ipsilateral Edinger-Westphal nucleus Accommodation with ipsilateral only cortical input Pupillary reflex with bilateral inputs from both pretectal nuclei Lesion of CN III Ipsilateral eye: strabismus (eye abducted and intorted/depressed), ptosis, dilation of the pupil (decreased tone of constrictor) and loss of accommodation reflex Argyll-Robertson pupil: both direct and consensual light reflex lost but accommodation is intact Problem is in the pretectal nuclei Cranial IV----Trochlear nerve SO Receives its input from the contralateral trochlear nucleus Lesion of CN IV Ipsilateral eye: strabismus as extorsion (unopposed action of IO), vertical diplopia due to extorted eye, weakness of downward gaze, and presence of head tilt to compensate for the abnormal tilt Cranial VI----Abducens nerve LR Origin is the ipsilateral abducens nucleus Lesion of CN VI Inability to look laterally beyond midline, horizontal diplopia (tends to look with head sideways) Most common isolated muscle palsy Cardinal positions of gaze Up/right, right, down/right, down/left, left, up/left In each cardinal position of gaze, one muscle is the primary mover and is yoked to the corresponding muscle of the other eye In extreme tertiary positions, the adductive and abductive effects of SR and IO swap Final common pathway Every oculomotor command has to generate the appropriate version and vergence commands and obey Sherrington’s second law Horizontal circuitry: vestibular, optokinetic, smooth pursuit Medial vestibular nucleus Excitatory input to contralateral abducens nucleus Inhibitory input to ipsilateral abducens nucleus Excitatory input to ipsilateral oculomotor nucleus through ascending tract of Deiters Saccadic excitatory burst neurons (EBN) Project to ipsilateral abducens nucleus Saccadic inhibitory burst neurons (IBN) Project to contralateral abducens nucleus Abducens interneurons Interconnect the abducens and the contralateral oculomotor nuclei Abducens motorneurons Connect directly to the ipsilateral lateral rectus Medial longitudinal fasciculus Transmits a copy of the activity of the abducens nucleus to the contralateral oculomotor nucleus Nucleus prepositus hypoglossal Receives a copy of the horizontal velocity commands and integrates it to obtain a positional (tonic) signal Excitatory connection to ipsilateral abducens nucleus Inhibitory connection to contralateral abducens nucleus Vergence: the main output of the vergence system reaches both oculomotor nuclei (identical contractions and relaxations of the MR of both eyes) Has its own neural integrator to obtain the vergence position tonic signal Neural integrator All oculomotor commands are velocity commands Tonic signal is needed to hold the eye in the new position (elastic forces would bring the eyes back) Step change in firing from the tonic firing before the movement and after the movement is obtained bi integration of the velocity command in the neural integrators The tonic firing and the velocity command (pulse) are summed together at the motorneuron level “Step” is a change in the tonic firing of the neural integrator All conjugate systems are believed to share the same integrators Horizontal: vestibular nuclei + NPH Vertical and torsion: vestibular nuclei + interstitial nucleus Vergence system has its own vergence integrator Tonic firing rates of the motorneurons innervating the EOMs control the tonic forces generated by the muscles and are present also in the absence of an actual oculomotor command Their coordinated balance determines the position of the globe An appropriate pulse/step calibration allows the reaching of the movement goal much faster and with highly optimized dynamics An oculomotor command is always a velocity command (pulse), At the motorneuron level, there is also a tonic component, which change associated to the movement is obtained by integration of the pulse at the neural integrator Horizontal RVOR, TVOR, smooth pursuit and OKR velocity signals are also sent to the abducens nuclei by excitatory and inhibitory cells in the vestibular nuclei For vertical saccades, the vertical and torsional EBNs and IBNs are located in the riMLF EBNs and IBNs for upward and downward saccades are not automatically segregated and are present on both the right and left sides Right riMLF codes clockwise and left riMLF encodes counterclockwise Vertical EBNs project directly to the vertical oculomotor neurons in III and IV nuclei Vertical IBNs have connections to the vertical integrator to generate the “negative” step for the antagonist muscle and send inhibitory pulses to the motorneurons Braking mechanism Helps stop the eye rotation at the end of the saccade Obtained by a burst (pulse) in the opposite EBN/IBN near the end of the saccade Brief contraction of the antagonist muscle Brief relaxation of the agonist muscle MLF damage: Internuclear ophthalmoplegia Weakness of adduction during conjugate horizontal eye movements but usually there are minor or no effects on convergence movements Convergence commands are sent directly to the two MR with no crossed connections through the MLF Weakness due to the lack of the command coming from the contralateral side to coordinate the horizontal eye movement Example: lesion to right MLF causes weakness of adduction of right eye on attempted leftward gaze One and a half syndrome Occurs when three is a single unilateral lesion of the paramedian pontine reticular formation involving the abducens nucleus and ipsilateral MLF Example: left lesion- cannot generate any leftward conjugate eye movement (left eye is missing direct activation of LR); can generate rightward movements in right eye Intact convergence feeding directly into the oculomotor nuclei Strabismus: misalignment of the visual axis with respect to an object causes the two images of the object to fall on non-corresponding area of the retina---disparity Problem is when subject, even with actively “fixating” an object, fails to align both foveas on it Ambylopia: reduced vision in an eye that has not received adequate uses during early childhood Visual confusion: false matching of similar objects (now interpreted by the brain as the same object) Particular aspect of diplopia is monocular diplopia, which persists even when the other eye is patched Heterotropia: misalignment of the visual axes when both eyes are viewing a single target (abnormal) Esotropia: one eye turned inward Exotropia: one eye turned outward Phoria: change in eye alignment when binocular vision is briefly interrupted by patching one eye Horizontal phoria is present in normal subjects Phoria is normal in that it is largely invariant with direction of gaze A dependence with the eye position (non-concomitant phoria) is a sign of muscular or neuronal problems Alternating cover test Switch to cover the good eye (bad one looking) we have a certain amount of phoria of the good eye----primary deviation Good eye is receiving the same signal needed to align the bad eye, signal which is stronger than normal, due to the weakness on the bad eye More motorneuron firing is needed to drive the bad eye on the target Cover the bad eye (good eye looking) but it was driven more off when patched Result is a larger movement of the good eye when uncovered---secondary deviation Secondary deviation of the good eye is larger than the secondary deviation of the weak eye