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16 Theory and Practice of Squint and Orthoptics
Fig. 1.12 Scheme to show the cranial nerve nuclei as projected onto the posterior surface of the brainstem.
nerve lies in between the two divisions, while the
abducent nerve lies inferolateral to them.
Intraorbital part
TROCHLEAR NERVE
The trochlear (fourth cranial) nerve is entirely
motor in function and supplies only the
superior oblique muscle of the eyeball.
In the orbit (Fig. 1.9), the smaller superior
division ascends on the lateral side of optic
nerve and supplies the superior rectus and the
levator palpebrae superioris. The larger, inferior
division divides into three branches: (1) nerve
to the medial rectus passes inferior to the optic
nerve, (2) nerve to inferior rectus passes
downward and enters the muscle on its upper
Nucleus
The trochlear nucleus is situated in the
ventromedial part of the central grey matter
of the midbrain at the level of inferior
colliculus (Figs 1.12 and 1.13). It is caudal to
and continuous with the third nerve nucleus
complex.
aspect and (3) nerve to inferior oblique (longest
of the three branches) passes in between the
inferior rectus and lateral rectus and supplies
the inferior oblique from its posterior border.
It gives off the motor root to the ciliary
ganglion.
Course and distribution
For the purpose of description, the course of the
trochlear nerve can be divided into fascicular,
precavernous, intracavernous and intraorbital
parts.
Anatomy of Extraocular Muscles and Related Structures
17
Fig. 1.13 Trochlear nerve nucleus, its central connections and course of fascicular and basilar parts of the nerve.
Fascicular part
The fasciculus consists of efferent fibres which
after leaving the nucleus, pass posteriorly
around the aqueduct in the central grey matter
and decussate completely in the anterior
medullary velum (Fig. 1.13).
Precavernous part
The trochlear nerve trunk after emerging from
the dorsal aspect of midbrain winds round
the superior cerebellar peduncle and the
cerebral peduncle just above the pons. It then
runs forwards and enters into the cavernous
sinus.
Intracavernous part
In the cavernous sinus, the nerve runs forwards
in its lateral wall lying below the oculomotor
nerve and above the first division of the fifth
cranial nerve (Fig. 1.10). In the anterior part of
the cavernous sinus, it rises, crosses over the 3rd
nerve and leaves the sinus to pass through the
lateral part of the superior orbital fissure (where
it lies superolateral to annulus of Zinn and
medial to the frontal nerve) (Fig. 1.11).
Intraorbital part
After entering the orbit through the lateral part
of the superior orbital fissure, the nerve passes
medially above the origin of levator palpebrae
superioris (Fig. 1.14) and ends by supplying the
superior oblique muscle through its orbital
surface.
ABDUCENT NERVE
The abducent (sixth cranial) nerve is a small,
entirely motor nerve that supplies the lateral
rectus muscle of the eyeball.
18 Theory and Practice of Squint and Orthoptics
Fig. 1.14 Showing the course of trochlear nerve.
Nucleus
The abducent nucleus is situated in the lower
part of pons, closely related to the fasciculus of
the facial nerve (Fig. 1.15).
Course and distribution
For the purpose of description, the course of
the abducent nerve can be divided into:
Fascicular, basilar, intracavernous and intraorbital parts.
Fascicular part
The fasciculus consists of efferent fibres which
start from the nucleus, pass forward and emerge
by some 7 to 8 rootlets from the junction of pons
and medulla which join to form one nerve
(Fig. 1.15).
Basilar part
The nerve then runs forwards, upwards on the
back of the petrous temporal bone near its apex.
Fig. 1.15 Abducent nerve nucleus and its central connections.
Anatomy of Extraocular Muscles and Related Structures
Fig. 1.16 Course of sixth cranial nerve.
19
At the sharp upper border of the petrous bone,
the nerve bends forward at right angle and
enters the cavernous sinus.
Intracavernous part
In the cavernous sinus, the nerve runs almost
horizontally forward, occupying a position
below and lateral to the internal carotid artery
(Fig. 1.10). The nerve then leaves the cavernous
sinus to enter the orbit through the middle part
of the superior orbital fissure within the annulus
of Zinn (Fig. 1.11). In the superior orbital fissure,
the abducent nerve lies inferolateral to the
oculomotor and nasociliary nerves.
Intraorbital part
In the orbit, the nerve runs forwards and enters
the ocular surface of the lateral rectus muscle
just behind its middle portion after dividing into
three or four branches (Fig. 1.16).
BIBLIOGRAPHY
1. Bisaria KK. Cavernous portion of the trochlear
nerve with special reference to its site of entrance.
J. Anat. 159:29–35, 1988.
2. Duane’s Ophthalmology, Chapter 32
Embryology and Anatomy of the Orbit and
Lacrimal System. (eds Tasman W, Jaeger EA)
Lippincott/Williams & Wilkins, 2007.
3. Hoya K, Kirino T. Traumatic Trochlear Nerve
Palsy Following Minor Occipital Impact. Neurol
Med Chir 40:358–360, 2000.
4. Joseph L. Demer (April 2002). “The Orbital
Pulley System: A Revolution in Concepts of
Orbital Anatomy”. Annals of the New York
Academy of Sciences. Neurobiology of eye
movements: from molecules to behavior. 956:
17–32. doi:10.1111/j.1749-6632.2002.tb02805.x.
5. Mehta MP; Perry JD (2015). “Medial orbital
wall landmarks in three different North
American populations”. Orbit. 34 (2): 72–8. PMID
25804299. doi:10.3109/01676830.2014. 997394.
6. Orbit at the US National Library of Medicine
Medical Subject Headings (MeSH).
7. Tenon JR, Naus J, Blanken R (March 2003).
“Anatomical observations on some parts of the
eye and eyelids. 1805”. Strabismus. 11 (1): 63–
8. PMID 12789585. doi:10.1076/stra.11.1.63.
14089.
8. Vilensky, Joel; Robertson, Wendy; SuarezQuian, Carlos (2015). The Clinical Anatomy of
the Cranial Nerves: The Nerves of “On Olympus
Towering Top”. Ames, Iowa: Wiley-Blackwell.
ISBN 978-1-118-49201-7.
20 Theory and Practice of Squint and Orthoptics
2
Physiology of
Ocular Motility
BASIC KINEMATICS
Positions of gaze
•
Centre of rotation
•
Fick's axes
•
Translatory and rotatory movements
•
Position of rest
•
MECHANICS OF ACTIONS OF
EXTRAOCULAR MUSCLES
Factors involved
•
Classical concept of actions
•
• Kinematics of muscle pulleys
Field of action
•
BASIC KINEMATICS
POSITIONS OF GAZE
To understand the ocular movements and their
mechanics, a frame of reference against which
the movements may be quantitated is
necessary. The primary position of the eye is
that position from which all other ocular
movements are initiated, changing the position
of eyeball from primary to secondary or
tertiary. All the extraocular muscles have a
given tone for every position of gaze. A total
of 9 positions of gaze have been described.
These include one primary, 4 secondary and 4
tertiary positions (Fig. 2.1).
AGONIST, SYNERGISTS,
ANTAGONISTS AND YOKE MUSCLES
FUNDAMENTAL LAWS GOVERNING OCULAR MOTILITY
Donders’ law
•
Listing’s law
•
Hering’s law
•
Sherrington’s law
•
OCULAR MOVEMENTS
Monocular movements (Ductions)
•
Binocular movements
•
– Versions
– Vergences
Secondary positions of gaze
These are the positions assumed by the eyes
while looking straight up (supraversion),
straight down (infraversion), to the right
(dextroversion) and to the left (levoversion)
(Fig. 2.1B, D, F, H).
Tertiary positions of gaze
These describe the positions assumed by the
eyes, when combination of vertical and
horizontal movements occurs. These include
position of eyes in dextroelevation, dextrodepression, levoelevation and levodepression
(Fig. 2.1A, C, G, I).
Primary position of gaze
The primary position has been defined by
Scobee as that position of the eyes in binocular
vision when, with the head erect, the object of
regard is at infinity and lies at the intersection
of the sagittal plane of the head and a horizontal
plane passing through the centres of rotation of
the two eyeballs (Fig. 2.1E).
Cardinal positions of gaze
These are the positions which allow examination
of each of the 12 extraocular muscles, of the two
eyes, in their main field of action. There are six
cardinal positions of gaze, viz., dextroversion,
levoversion, dextroelevation, levoelevation,
dextrodepression and levodepression (Fig. 2.1A,
C, D, F, G, I).
21Physiology of Ocular Motility
Fig. 2.1 Diagnostic positions of gaze. Primary position (E); secondary positions (B, D, F, H); tertiary positions (A, C, G,
I); cardinal positions (A, C, D, F, G, I).
CENTRE OF ROTATION
It refers to a hypothetical point around which the
eyeball performs rotatory movements. Earlier this
has been assumed that the centre of rotation is a
fixed point. However, newer experiments
indicate that translatory movements do take place
to some extent (i.e. laterally, vertically, or in or
out), and that the centre of rotation of the eye does
not have zero velocity. It has been reported that
the centre of rotation moves in a semicircle in the
plane of rotation.1 This locus has been called the
space centroid (Fig. 2.2).
Fig. 2.2 Position of space centroid as computed by Park and Park.
1
22 Theory and Practice of Squint and Orthoptics
However, for all practical purposes, the globe
can be considered to rotate around a fixed point.
In primary position, the centre of rotation lies
some 13.5 mm behind the apex of cornea, when
measured on the line of sight. This is in reality a
little behind the actual geometrical centre of the
globe; but from a practical standpoint, it will be
considered to coincide with the centre of the
globe. In big myopic eyes, the centre of rotation
is a bit farther posterior and in small hyperopic
eyes, it is a bit anterior to this ideal position.
FICK'S AXES
2
Fick described three axes (co-ordinates) to
analyse all movements of the globe around the
hypothetical centre of rotation. The proposed
three axes are perpendicular to each other and
intersect at the centre of rotation of the eye. In
Fick's system, these coordinates (Fig. 2.3A) are
as described below.3 Recently, oblique axis has
also been described.
X (horizontal) axis
It lies horizontally, when the head is in an
upright position. Rotation around the horizontal
(X) axis results in elevation (sursumduction) or
depression (deosursumduction) (Fig. 2.3B II and
VIII).
Y (anteroposterior) axis
The anteroposterior axes of the two eyes are
parallel to each other and perpendicular to the
horizontal axis. Rotation of the globe around the
anteroposterior (Y) axis (Fig. 2.3A) produces the
torsional movements named according to the
movement of the 12 o'clock meridian of the
cornea as extorsion (excycloduction) and intorsion
(incycloduction).
allow the eyeball to rotate obliquely up and
in (Fig. 2.3B I), up and out (Fig. 2.3B III), down
and in (Fig. 2.3 B VII) and down and out
(Fig. 2.3B IX).
Note. The X, Z and O axes lie in the same plane.
This plane passing through the centre of rotation
of the eye and containing the X, Z and O axes is
called Listing's plane (Fig. 2.3A and 2.3B V). The
eyeball can reach all positions of gaze by
rotations around the axes that are on Listing's
plane, i.e. Z, X and oblique axes (Fig. 2.3B).
TRANSLATORY AND ROTATORY MOVEMENTS
Translatory movements refer to the movements
of the eyeball as a whole in the orbit, with eye
remaining in primary position of gaze. These
include shift of the eyeball upwards or
downwards, anteriorly or posteriorly and
sideways. Rotatory movements occur along the
three axes of rotation around a fixed centre of
rotation as discussed above.
POSITION OF REST
Position of rest, i.e. a position without actions
of extraocular muscles is very hard to document.
Even in death, rigor mortis may make the
extraocular muscles pull the eye away from the
true rest position. It has been reported that the
eye positions of orthophoric normal individuals
under deep anaesthesia with curare paralysis
(measured by an accurate photographic
technique) are probably the closest approximation of the position of rest. Under such
experiments, an exotropia of 2.25° in each eye
which increases with age has been observed in
young adult eyes.
4
Z (vertical) axis
It extends vertically (direction of gravity), when
the head is in an upright position, and is
perpendicular to the X and Y axes. Rotations of
the globe around vertical (Z) axis are known as
adduction and abduction (Fig. 2.3B IV and VI).
Oblique axis
The Listing’s plane also contains two oblique
axes 0–0 and 0’–0’ (Fig. 2.3B I and III) which
MECHANICS OF ACTIONS OF EXTRAOCULAR MUSCLES
FACTORS INVOLVED IN MECHANICS OF
EXTRAOCULAR MUSCLE ACTIONS
1. Cross-sectional area of the muscle. Crosssectional areas of the extraocular muscles as
described by Volkmann5 and Nakagama6 are
shown in Table 2.1. On the basis of the crosssectional area of the muscles, following
comments have been made:
23Physiology of Ocular Motility
Fig. 2.3 (A) Fick's axes and Listing's plane; (B) Note that eyeball can reach all positions of gaze by rotations around axes
that are on Listing's plane, i.e. Z, X and O (oblique) axes.
24 Theory and Practice of Squint and Orthoptics
• Cross-sectional area of the horizontal recti is
maximum. This is sensible, since they alone
are horizontal movers.
• The vertical recti average about 75% and the
obliques about 50% the size of the horizontal
muscles.
• In general, antagonists such as medial and
lateral recti are similar in size, thereby
balancing opposing forces.
• Muscles exert force in proportion to their
cross-sectional area.
2. Length of the muscle. The average length of
various extraocular muscles is shown in
Table 2.1. It has been reported that for the
normal amplitude of rotation (45°–50° each way
from the primary position), approximately
10 mm (about 25% of the normal resting length)
change in muscle length is required in each
direction. Therefore, a sacrifice of muscle length
during resection of the muscles usually reduces
the amplitude of eye rotation.
3. Arc of contact. The distal portion of each
extraocular muscle or muscle tendon lies flush
against the globe for a variable distance before
it blends into the sclera (anatomic insertion). The
point at which the centre of the muscle or its
tendon first touches the globe is the tangential
point. It is also referred to as the physiologic or
effective insertion of the muscle, since from a
mechanical point of view, a tangent to the globe
at this point indicates the direction of pull of that
muscle. The position of this point changes, when
the muscle contracts or relaxes and the globe
rotates (Fig. 2.4).
The arc of contact is the distance on the scleral
circumference between the tangential point (T)
and the centre of anatomic insertion of the
muscle (A) on the sclera. The lengths of the
contact area for various extraocular muscles
in primary position of the gaze are given in
Table 2.1.
The arc of contact represents the lever arm in
the mechanical system. The arc of contact varies
with the position at tangential point, e.g. as the
eye is abducted, the arc of contact of lateral
rectus is reduced (Fig. 2.4B), while in adduction,
it is increased (Fig. 2.4C). However, the location
of the tangential point with respect to the centre
of rotation of the eye (C) and the point of origin
of the muscle (O) remains unchanged (Fig. 2.4A,
B and C); so that the torque (i.e. the force of
rotation) also remains constant. This holds true
Table 2.1 Lengths of the contact arc for various
extraocular muscles in primary position of the gaze
Muscles Lengths of contact arc
Lateral rectus 15 mm
Medial rectus 6 mm
Superior rectus 8.4 mm
Inferior rectus 9 mm
Superior oblique 5 mm
Inferior oblique 17 mm
Fig. 2.4 The arc of contact of the lateral rectus muscle.
till arc of contact is practically zero that is, the
tangential point coincides with the anatomical
insertion.
Since the power of a muscle is proportionate
to its length and arc of contact, retroplacement
of the insertion of muscle (recession) weakens
the action of the muscle by shortening its
effective length and by reducing its arc of contact
in the various positions of gaze. Advancement
of an extraocular muscle has a strengthening
effect because of the increase in the effective
length as well as its arc of contact.
4. Muscle plane. It is an imaginary plane which
passes through the midpoints of origin of the
muscle (O), anatomical insertion (A), functional
insertion, i.e. tangential point (T) and the centre
of rotation of the globe (C) (Fig. 2.5). Thus each
muscle plane runs through the long axis of the
muscle. The angle formed by a muscle plane
with the visual direction or line of fixation
depends on the position of the globe. When the
line of fixation lies in or parallel to the muscle
plane, the angle is zero.
The ocular muscles are paired, each pair
having a common muscle plane. These pairs are
the medial and lateral recti [their muscle plane
coincides with the horizontal plane of the globe],
superior and inferior recti [their muscle plane
makes an angle of 23° with the visual line in
primary position (Fig. 2.6)] and superior oblique
tendon from the trochlea to globe and the
inferior oblique [their muscle plane makes an
angle of about 51° with the visual line when the
eye is in the primary position (Fig. 2.7)].
25Physiology of Ocular Motility
Fig. 2.6 Relation of muscle plane of superior and inferior
recti with the visual line in primary position.
Fig. 2.5 Schematic presentation of muscle plane (TACO).
Fig. 2.7 Relation of muscle plane of superior and inferior
obliques with the visual line in primary position.
5. Muscle axis of rotation. It is perpendicular to
the muscle plane erected in the centre of rotation.
The individual muscle pulling on the eye will
rotate the globe around this axis through the
centre of rotation.