Download Motor Physiology

Eye Motor
Physiology
Dr Besharati MD
Axes of Fick, Center of Rotation
A movement of the eye around a theoretical center of
rotation is described with specific terminology.
Two helpful concepts are the axes of Fick and Listing's
plane. The axes of Fick are designated as x, y, and z.
The x-axis is a transverse axis passing through the center
of the eye at the equator; vertical rotations of the eye
occur about this axis.
The y-axis is a sagittal axis passing through the pupil;
involuntary torsional rotations occur about this axis.
The z-axis is a vertical axis; horizontal rotations occur
about this axis.
listing's Plane
Listing's equatorial
plane contains the
center of rotation and
includes the x and z
axes. The y-axis is
perpendicular to
Listing's plane.
Positions of Gaze
Primary position is the position of
the eyes when fixating straight ahead.
Secondary positions are straight up,
straight down, right gaze, left gaze
Tertiary positions are the 4 oblique
positions of gaze: up and right, up
and left, down and right, down and left.
Cardinal positions are up and right, up
and left, right, left, down and right,
down and left .
Extraocular Muscle Action
The 4 rectus muscles have
traditionally been thought of as fixed
straight strings running directly from
the orbital apex to the muscle
insertions.
The oblique muscles, historically, were
thought to simply attach obliquely to the
globe. In light of ongoing discoveries that
lend support to the active pulley
hypothesis, some of the older concepts
and descriptions of extraocular muscles
and their actions are currently
undergoing revision.
Arc of contact
The point of effective, or physiologic, insertion
is the tangential point where the muscle first
contacts the globe. The action of the eye
muscle may be considered a vector of force
that acts at this tangential point to rotate the
eye. The length of muscle actually in contact
with the globe constitutes the arc of contact.
The traditional concepts of arc of contact and
muscle plane, based on straight-line 2
dimensional models of orbital anatomy, do not
take into account the recently discovered
effective muscle pulleys and their effect on
linearity of muscle paths.
Primary, secondary, and tertiary
action
With the eye in primary position,
the horizontal rectus muscles are
purely horizontal movers around
the z-axis (the vertical axis), and
they have a primary action only.
The vertical rectus muscles have a
direction of pull that is mostly
vertical as their primary action,
but the angle of pull from origin
to insertion is inclined 23° to the
visual axis, giving rise which is defined
as any rotation of the vertical corneal
meridians.
torsion
Intorsion is the secondary action for
the superior rectus;
extorsion is the secondary action for
the inferior rectus; and adduction is
the tertiary action for both muscles.
Because the oblique muscles are
inclined 51 0 to the visual axis,
torsion is their primary action.
Vertical rotation is their secondary
and horizontal rotation their tertiary
action
Field of action
The term field of action has
Been used in 2 ways to describe
Entirely separate and distinct
concepts:
• to indicate the direction of
rotation of the eye from primary
position if the muscle was the
only one to contract.
• to refer to the gaze position (one
of the cardinal positions) in which
the effect of the muscle is most
readily observed.
direction of rotation
For the LR, the direction of rotation and the
gaze position are both abduction; for the
MR, they are both adduction. However, the
direction of rotation and the gaze position
are not the same for all muscles. For
example, the IO, acting alone, is an abductor
and elevator, pulling the eye up and out; but
its elevation action is best observed in
adduction. Similarly, the SO, acting
alone, is an abductor and depressor, pulling
the eye down and out; but its depression
action is best observed in adduction.
fields of action
The clinical significance of fields of action is that a deviation
(strabismus) that increases with gaze in some directions may result
from the weakness of the muscle normally pulling the eye in that
direction. For example, an acute left sixth nerve palsy in an adult can
be diagnosed by asking the patient with diplopia by 3 questions:
1. Is the diplopia horizontal or vertical? Patient's answer: Horizontal
2-Is the diplopia worse at distance or at near? Patient's answer:
distance (implicating the lateral recti )
3. Is the diplopia worse on looking to the left or to the right? Patient's
answer: Looking to the left (the field of action of the left lateral rectus )
Changing muscle action with different
gaze positions
The gaze position determines the effect of EOM contractions on the rotation
of the eye. The different positions are primary gaze and the 6 cardinal
positions. In each of these 6 cardinal positions, each of the 6 EOM has
different effects on the eye rotation based on the relationship between the
visual axis of the eye and the orientation of the muscle plane to the visual
axis. Each cardinal position minimizes the angle between the visual axis and
the muscle plane of the muscle being tested, thus maximizing the horizontal
effect of the medial or lateral rectus or the vertical effect of the SR, IR, SO, or
IO. By having the patient move the eyes to the 6 cardinal positions, the
clinician can isolate and evaluate the ability of each of the 6 EOM to move the
eye
muscle actions
With the eye in primary position, the
horizontal rectus muscles share a common
horizontal plane that contains the visual
axis .The relative strength of the horizontal
rectus muscles can be assessed by
observing the horizontal excursion of the
eye as it moves medially from primary
position to test the MR and laterally to test
the LR.
The muscle actions of the vertical rectus
muscles and the oblique muscles are more
complex because, in primary position, the
muscle axes are not parallel with the visual
axis
muscle actions
In primary position, the superior and inferior rectus muscle planes form an angle of
23° with the visual axis (y-axis) and insert slightly anterior to the z-axis .
Therefore, from primary position, the contraction of the SR has 3 effects:
Primary elevation around the x-axis, secondary intorsion around the y-axis, and
Adduction around the z-axis.
The relative strength of the SR muscle can be most readily observed by aligning the
visual axis parallel to the muscle plane axis-that is, when the eye is rotated 23° in
abduction. In this position, the SR becomes a pure elevator and its elevating action is
maximal. To minimize the elevation action of the SR, the visual axis should be
perpendicular to the muscle axis at a position of 67° of adduction. In this position, the
SR would become a pure intorter. Because the globe cannot be adducted this far,
there is still a SR elevating action in maximal adduction.
muscle actions
The action of the IR is similar to
that of the SR. Because the IR is
attached to the globe inferiorly,
its action from primary position
is primarily depression,
secondarily extorsion, and
tertiarily adduction . Its action
as a depressor is maximally
demonstrated in 23° of
abduction and minimized
in adduction.
muscle actions
The 2 oblique muscle planes course
in a direction from the anteromedial
aspect of the globe to the
posterolateral, forming an angle of
approximately 51 ° with the visual
axis .Because of the large angle
formed in primary position, the
Primary action of the SO is intorsion,
with a secondary depression and
tertiary abduction.
muscle actions
As the muscle plane is aligned
with the visual axis in extreme
adduction, the SO action can
be seen as a depressor. With
Abduction of the eye, the
visual axis becomes
perpendicular to the muscle
plane, and the muscle action is
one of intorsion.
muscle actions
The action of the IO is similar
to that of the SO. In primary
position, the primary action is
extorsion, with secondary
elevation and tertiary
abduction. The IO action as an
elevator is best seen in
adduction and, as an extorter,
In abduction.
Eye Movements
Motor Units
An individual motor nerve fiber and its
several muscle fibers constitute a
motor unit.
EMG records motor unit electrical
activity. An EMG is useful in
investigating normal and abnormal
innervation and can be helpful in
documenting paralysis, recovery from
paralysis, and abnormalities of
innervation in myasthenia gravis
and muscle atrophy. However, this test
is not helpful in ordinary comitant
strabismus.
Recruitment during fixation or
following movement
As the eye moves farther into
abduction, more and more lateral
rectus motor units are activated
and brought into play by the brain
to help pull the eye. This process is
called recruitment. In addition, as
the eye fixates farther into
abduction, the frequency of activity
of each motor unit increases until
it reaches a peak.
Monocular Eye Movements
Ductions are monocular rotations of the
eye. Adduction is movement of the eye
nasally; Abduction is movement of the
eye temporally. Elevation is an upward
rotation of the eye; Depression is a
downward rotation of the eye. Intorsion
is defined as a nasal rotation of the
superior portion of the vertical corneal
meridian. Extorsion is a temporal
rotation of the superior portion
of the vertical corneal meridian
Ductions
The following terms relating to the muscles
used in monocular eye movements are also
important:
Agonist: the primary muscle moving the eye
in a given direction
Synergist: the muscle in the same eye as the
agonist that acts with the agonist to produce a
given movement (eg, the IO is a synergist with
the agonist SR for elevation of the eye)
Antagonist: the muscle in the same eye as
the agonist that acts in the direction opposite
to that of the agonist; the MR and LR are
antagonists
Sherrington's law
Sherrington's law of reciprocal
innervation states that increased
innervation and contraction
of a given EOM are accompanied
by a reciprocal decrease in innervation
and contraction of its antagonist. For
example, as the right eye abducts, the
right LR receives increased innervation
while the right MR receives decreased
innervation.
Binocular Eye Movements
When binocular eye movements are
conjugate and the eyes move in the
same direction, such movements
are called versions.
When the eye movements are
disconjugate and the eyes move in
opposite directions, such
movements are known as vergences
(eg, convergence and divergence).
Versions
Right gaze (Dextroversion) is movement of both eyes to the
patient's right.
Left gaze (Levoversion) is movement of both eyes to the patient's left.
Elevation, (Sursumversion), is an upward rotation of both eyes;
Depression, (Deorsumversion), is a downward rotation of both eyes.
Dextrocycloversion, both eyes rotate so that the superior portion of
the vertical corneal meridian moves to the patient's right.
Levocycloversion is movement of both eyes so that the superior
portion of the vertical corneal meridian rotates to the patient's left.
yoke muscles
The term yoke muscles is used to
describe 2 muscles (1 in each eye)
that are the prime movers of their
respective eyes in a given position
of gaze. For example, when the
eyes move or attempt to move into
right gaze, the right LR and the left
MR are simultaneously innervated
and contracted. These muscles are
said to be "yoked" together.
yoke muscles
Each EOM in 1 eye has a yoke muscle in the other eye.
Because the effect of a muscle is usually best seen in a given
direction of gaze, the concept of yoke muscles is used to
evaluate the contribution of each EOM to eye movement.
Hering's law of motor correspondence states that equal and
simultaneous innervation flows to yoke muscles concerned
with the desired direction of gaze. The most useful application
of this law is in evaluating binocular eye movements and, in
particular, the yoke muscles involved.
Hering's law
Hering's law has important clinical implications, especially when the
practitioner is dealing with a paralytic or restrictive strabismus.
Because the amount of innervation to both eyes is always determined
by the fixating eye, the angle of deviation varies according to which
eye is fixating. When the normal eye is fixating, the amount of
misalignment is called the primary deviation. When the paretic or
restrictive eye is fixating, the amount of misalignment is called the
secondary deviation. The secondary deviation is larger than the
primary deviation because of the increased innervation necessary to
move the paretic or restrictive eye to the position of fixation.
Hering's law
Hering's law is also necessary to explain the following example. If a
Patient has a right SO paresis and uses the right eye to fixate an object
that is located up and to the patient's left, the innervation of the right
IO required to move the eye into this gaze position is reduced because
the right IO does not have to overcome the normal antagonistic effect
of the right SO. Therefore, according to Hering's law, less innervation is
also received by the right IO muscle's yoke muscle, the left SR. This
decreased innervation could lead to the incorrect impression that the
left SR is paretic
Vergences
Convergence is movement of both eyes nasally relative to
a given starting position;
Divergence is movement of both eyes temporally relative
to a given starting position.
Incyclovergence is a rotation of both eyes so that the superior
portion of each vertical corneal meridian rotates nasally;
Excyclovergence is a rotation of both eyes so that the superior
pole of each vertical corneal meridian rotates temporally.
Vertical vergence movement, though less frequently
encountered, can also occur: 1 eye moves upward and the
other downward
Tonic convergence
The constant innervational tone to the EOM
when a person is awake and alert. Because of
the anatomical shape of the bony orbits and
the Position of the rectus muscle origins, the
alignment of the eyes under complete
muscle paralysis is divergent. Therefore,
convergence tone is necessary in the awake
state to maintain straight eyes even in the
absence of strabismus.
Accommodative convergence
Accommodative convergence of the visual axes Part of the synkinetic
near reflex. A fairly consistent increment of accommodative
convergence (AC) occurs for each diopter of accommodation
(A), giving the accommodative convergence/accommodation (AC/A)
ratio.
Abnormalities of this ratio are common, and they are an important
cause of strabismus.
With an abnormally high AC/ A ratio, the excess convergence tends to
produce esotropia during accommodation on near targets. An
abnormally low AC/ A ratio tends to make the eyes exotropic when the
person looks at near targets.
Other Convergence
Voluntary convergence a
conscious application of the near
synkinesis.
Proximal (instrument) convergence
an induced convergence movement
caused by a psychological
awareness of near; this movement
is particularly apparent when a
person looks through an instrument
such as a binocular microscope.
Fusional convergence
A movement to converge and position the eyes
so that similar retinal images project on
corresponding retinal areas. Fusional
convergence is accomplished without changing
the refractive state of the eyes and is prompted
by bitemporal retinal image disparity
Fusional divergence
The only clinically significant form of divergence. It
is an optomotor reflex to diverge and align the eyes
so that similar retinal images project on
Corresponding retinal areas. Fusional divergence is
accomplished without changing the refractive state
Of the eyes and is prompted by binasal retinal
image disparity.
Supra nuclear Control Systems for Eye
Movement
There are several supra nuclear eye movement systems. The saccadic
system generates all fast (up to 400o-500o/sec) eye movements, such
as eye movements of refixation. This system functions to place the
image of an object of interest on the fovea or to move the eyes from
one object to another. Saccadic movements require a sudden strong
pulse of force from the EOM to move the eye rapidly against the
viscosity produced by the fatty tissue and the fascia in which the globe
lies. The study of saccadic velocity is of practical value in determining
paresis of muscles and abnormal innervation.
Supra nuclear Control Systems for
Eye Movement
The smooth pursuit system generates all
following, or pursuit, eye movements.
Pursuit latency is shorter than for
saccades, but the maximum peak
velocity of these slow Pursuit
movements is limited to 30o - 60o/sec.
The vergence system controls
disconjugate eye movement, as in
convergence or divergence.
Supranuclear control of vergence eye
movements is not yet fully understood.
nonoptic reflex systems
The nonoptic reflex systems integrate
eye movements and body movements.
The most clinically important of these
systems is the labyrinthine reflex system,
which involves the semicircular canals of
the inner ears. Other, less important,
systems involve the utricle and saccule
of the inner ears. The cervical, or neck,
receptors also provide input for this
nonoptic reflex control.