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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