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376 Theory and Practice of Squint and Orthoptics
Fig. 13.4 Pathway for vertical gaze (upgaze) saccadic eye movements.
Supranuclear Control and Disorders of Ocular Motility
377
cortex plays the key supranuclear role in the
visual ocular reflex by way of projections to the
PPRF and riMLF. This reflex keeps a moving
image projecting on the fovea. There are specific
efferent fibres for horizontal, vertical, and
torsional movements.
Damage to only one side of the MT cortex
slows ipsilateral slow pursuit, requiring catchup saccades. Such lesions also temporarily
impair pursuit responses to fast targets in
moving in either direction.
ii. Medial superior temporal (MST) visual area:
In human, it is considered to lie superior and a
little anterior to MT area within the inferior
parietal lobe.
SUBCORTICAL CONTROL CENTRES
The brainstem control centres include:
• Paramedian pontine reticular formation
(PPRF)
• Rostral interstitial nucleus of medial longi-
tudinal fasciculus(riMLF)
• Convergence and divergence centre
• Posterior commissure
• Superior colliculus
• Vestibular apparatus
• Cerebellum
• Medial longitudinal fasciculus
1. Paramedian pontine reticular formation
Horizontal gaze centre. The paramedian pontine
reticular formation (PPRF) is the primary centre
responsible for generating horizontal conjugate
gaze. The PPRF is positioned ventral to the
medial longitudinal fasciculus (MLF). It extends
from the level of the trochlear nerve nucleus to
the abducens nerve nucleus (Fig. 13.1).
• Afferent connections. Most afferent connections
to the PPRF are through the vestibular nucleus.
It receives signals from both the frontal cortical
areas concerned with generation of saccades and
ipsilateral occipitoparietal cortical area concerned
with generation of pursuits directly (for voluntary
movements) and through the superior colliculus
(for involuntary movements). Vestibular input for
horizontal eye movements comes from the
contralateral vestibular apparatus by way of the
vestibular nuclei. An axon from the vestibular
nucleus crosses to the opposite abducens nucleus,
where it innervates a motor neuron and an
internuclear neuron for horizontal gaze in the
opposite direction (Fig. 13.2).
• Efferent connections. The centre for horizontal
gaze movements in turn is connected with the
homolateral abducent nucleus present next to
it. The axons of the internuclear neurons of the
abducent cross the midline and travel through
the medial longitudinal fasciculus (MLF) of the
opposite side to that part of nucleus of the
oculomotor nerve which innervates the medial
rectus muscle (Figs 13.2 and 13.5). Therefore,
impulses from this centre produce contraction
of the homolateral lateral rectus muscle and the
opposite medial rectus, hence binocular gaze
movements to the side of the stimulated centre.
2. Rostral interstitial nucleus of
medial longitudinal fasciculus
Vertical gaze centre. The vertical gaze centre is
the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) located at the level
of upper pole of the red nucleus. Slightly caudal
to the riMLF and directly connected to it lies the
interstitial nucleus of Cajal (INC). This nucleus
contains neurons which appear to be involved
in vertical gaze holding and vertical pursuit.
• Afferent connections. The vertical gaze centre
(riMLF) receives impulses from both the
frontal and occipital cortical ocular motor
centres as well as from the superior colliculus.
• Efferent connections. The riMLF nucleus
projects through the posterior commissure to
its equivalent on the other side of the
mesencephalon as well as directly to the nuclei
of III and IV cranial nerves supplying the
extraocular muscles concerned with the
vertical movements (Figs 13.3, 13.4 and 13.6).
3. Posterior commissure
Dorsal and rostral to the riMLF is the posterior
commissure, a fibre tract that contains some
scattered neuronal cell bodies. Lesions in this
region produce abnormalities of upward gaze.
It is likely that the fibres for upward gaze leave
the riMLF and pass through this region before
reaching the oculomotor and trochlear nuclei
(Fig. 13.7). Involvement of the posterior
commissure may be part of the dorsal midbrain
378 Theory and Practice of Squint and Orthoptics
Fig. 13.5 Neural pathway of horizontal pursuit eye movements.
syndrome (Parinaud syndrome). In this
syndrome, there is impairment of upwardly
directed saccades or, in extreme cases, loss of
all vertical movement. Other signs include
pupillary mydriasis and light-near pupillary
dissociation, corectopia, and convergenceretraction nystagmus.
4. Convergence and divergence centre
At present, nothing is known about the location
of a subcortical divergence centre. In fact, there
is no evidence that such a centre exists at all.
Similarly, a subcortical centre for convergence
also may not exist at all and convergence as well
as divergence may be purely cortical functions.
Supranuclear Control and Disorders of Ocular Motility
379
Fig. 13.6 Neural pathway of vertical persuit (downgaze) eye movements.
However, clinical evidence points that the
pretectal area is probable site for the subcortical
convergence centre; since lesions in this area
abolish convergence. The impulses to the socalled convergence centre come from the frontal
and the occipital ocular motor centres.
Convergence occurs only upon simultaneous
bilateral stimulation of either the frontal or the
occipital motor centres. These impulses are
relayed to the nuclei of both third nerves which
innervate the medial recti muscles (Fig. 13.8).
5. Superior colliculus
These structures in the dorsal midbrain play a role
in both ocular motor and sensory function. The
superior colliculus receives visual input directly
from branches of retinal ganglion cell axons.
Visual input also comes indirectly from the visual
cortex, the parietal and frontal lobes, and the
substantia nigra. There are efferent projections to
the brainstem premotor areas. The superior
colliculus can generate visually directed saccades
independently and may play a role in the control
380 Theory and Practice of Squint and Orthoptics
Fig. 13.7 Neural pathway of vertical persuit (upgaze) eye movements.
of pursuit eye movements. In primates, ablation
of both FEFs and both superior colliculi is
necessary to produce permanent saccadic defects.
6. Vestibular apparatus
Reflex eye movements that compensate for
changes in the position of head or body originate
from the vestibulum. They are called statokinetic
reflexes. If, for instance, the head is turned to
the left, the eyes perform an involuntary
compensatory movement to the right which
allows continuous fixation of the object. These
reflexes are innate and unconditioned, occurring
even in blind. The vestibular apparatus is a
receptor specialized to sense changes of
equilibrium and position. It is part of the inner
ear and is comprised of three semicircular
canals, the sacculus and the utriculus, all of
which belong to the membranous labyrinth. The
apparatus derives its name from the vestibulum,
which is the bony cavity housing the utriculus
and the sacculus.
Supranuclear Control and Disorders of Ocular Motility
Fig. 13.8 Presumptive pathway of convergence.
Semicircular canals
The semicircular canals contain receptor organs
called cristae that sense movements of the head.
Each of the canals is oriented perpendicular to
the other two so that a three-dimensional
structure is formed roughly coinciding with the
horizontal, vertical, and frontal planes. The
canals are filled with a watery fluid, the
endolymph. When the head moves, the
endolymph is subjected to inertia and exerts a
381
certain pull on the sensory hair of the cristae.
This stimulus excites the receptor cells and elicits
a nerve impulse that is transmitted through the
vestibular nerve (part of nerve VIII) to the
vestibular nuclei in the brainstem.
Sacculus and utriculus
The sacculus and the utriculus each contains a
macula that functions in a manner similar to that
of the cristae of the semicircular canals. Whereas
the cristae respond to head movements
(statokinetic reaction), the macula of the utricle
responds to gravity (static reaction). On top of
each macula rests a gelatinous plate into which
protrudes hair from the surface of the macula.
The plates, which contain mineral crystals, follow
the force of gravity and slide to which ever side
is dependent, thereby pulling on the hair. This
leads to stimulation of the terminal branches of
the vestibular nerve that supply the maculae. The
function of the sacculus is not known.
Vestibular nerve
The vestibular nerve (part of nerve VIII) enters
the brainstem at the level of the lower end of
the pons and terminates in the vestibular nuclei.
The vestibular nuclei, in turn, are connected
directly to the abducens nuclei and to the nuclei
of the ocular motor nerves through the medial
longitudinal fasciculi (Figs 13.3–13.7).
Stimulation of a labyrinth or of a vestibular
nerve causes conjugate deviation of the eyes to
the opposite side. Depending on the nature of
the stimulus, the eyes may remain in the deviate
position or a nystagmus may result. A
nystagmus is an oscillatory movement of the two
eyes. Both eyes turn in the same direction and
are suddenly pulled back into the primary
position by a fast, jerky movement, where upon
the cycle begins again with conjugate deviation.
Usually, the first phase of nystagmus, the
conjugate deviation, is distinctly slower than the
second phase in which the corrective movement
takes place. Diagnostically, vestibular nystagmus
can be produced to test the integrity of the
vestibular reflex mechanism. For this, the patient
is either submitted to rotation on a revolving
chair (rotatory vestibular nystagmus) or his/her
external auditory canals are irrigated with hot
or cold water (caloric vestibular nystagmus).
382 Theory and Practice of Squint and Orthoptics
•
Fig. 13.9 Connections of medial longitudinal fasciculus (MLF).
7. Medial longitudinal fasciculus
The medial longitudinal fasciculus (MLF) is a
fibre tract that extends from the spinal cord to
the oculomotor nerve nucleus. It contains
primarily ascending fibres, the majority of which
arise in the superior and medial vestibular
nuclei. The MLF is in close proximity to the
ocular motor nuclei and influences both
ipsilateral and contralateral nuclei.
Functions of MLF. The medial longitudinal
fasciculus plays an important role in the
pathway of ocular movements. Its main
functions can be summarized as follows
(Fig. 13.9):
Supranuclear Control and Disorders of Ocular Motility
383
• It connects the oculomotor nuclei with one
another.
• It transmits signals from the subcortical–
horizontal gaze centre for the horizontal
versions to the opposite medial rectus muscle.
• It transmits impulses originating from the
vestibular nucleus (in response to statokinetic
stimulation) to the ocular motor nuclei as well
as to nucleus innervating muscles of head and
neck.
• It relays signals from the proprioceptors of the
head and neck muscles to the ocular motor
nuclei.
Lesions of MLF. An abnormality of the MLF
causes problems with horizontal and vertical
gaze co-ordination of the two eyes. The clinically
most important connection passing through the
MLF links the contralateral abducens nucleus
with the ipsilateral medial rectus subnucleus.
Abnormalities of this tract produce an
internuclear ophthalmoplegia. Such a lesion
produces slowed or complete loss of adduction
of the ipsilateral eye and abducting nystagmus
of the fellow eye.
8. Cerebellum
While often associated with motor coordination
and balance, the cerebellum also plays a role in
fine-tuning and adjusting eye movements. It
receives sensory feedback about the ongoing eye
movements and contributes to their accuracy and
precision. The cerebellum appears to be involved
in the immediate modulation of ongoing eye
movements, as well as in the long-term adaptive
processes that compensate for ocular motor
dysmetria. The cerebellum controls and adjusts
the size of saccades. The latter ability is essential
for maintaining accurate ocular motor
performance during growth and aging, during
and after ocular motor disease, or even while
using spectacles. For instance, the use of anisometropic spectacles produces a varying
anisophoria in different directions of gaze, which
must be compensated in each direction of gaze.
Hemicerebellectomy produces ipsilateral
saccadic and contralateral pursuit defects, while
total cerebellectomy creates persistent saccadic
dysmetria and abolishes smooth pursuit. The
cerebellum has numerous connections to
nuclear and supranuclear ocular motor centres.
SUPRANUCLEAR EYE MOVEMENT
SYSTEMS
Supranuclear eye movements refer to the control
and coordination of eye movements that occur
above the level of the cranial nerve nuclei in the
brainstem. These movements are under the
influence of higher brain centers, particularly in
the cerebral cortex and associated structures.
Supranuclear eye movements are essential for
precise and voluntary control of the eyes to focus
on objects of interest, explore the visual
environment, and perform various visual tasks.
Following supranuclear eye movement systems
have been recognized:
• Saccadic eye movement system
• Smooth pursuit movement system
• Vergence movement system
• Vestibular eye movement system
• Optokinetic system
• Position maintenance system
All these systems perform specific functions
and each one is controlled by a different neural
system but share the same final common path,
i.e. the motor neurons that supply the
extraocular muscles.
SACCADIC EYE MOVEMENT SYSTEM
Saccades are sudden, jerky conjugate eye
movements that occur as the gaze shifts from
one object to another. Thus they are performed
to bring the image of an object quickly on the
fovea. Though normally voluntary, saccades
may be involuntary aroused by peripheral,
visual or auditory stimuli. The saccades
include:
• Horizontal saccades, and
• Vertical saccades.
Detailed features of saccadic eye movements are
described on page 35.
Neural pathway
Cortical areas
The pathway originates in the premotor cortex
of the frontal motor area. From there, the fibres
384 Theory and Practice of Squint and Orthoptics
for voluntary saccades pass directly and for
involuntary saccades through the superior
colliculus to the contralateral horizontal gaze
centre in PPRF (Fig. 13.2).
Pathways involved in the cortical generation
of saccades
It appears that there are three pathways
involved in the cortical generation of saccades:
i. Ventral pathway. The ventral pathway
projects by way of the posterior portion of the
anterior limb of the internal capsule and the
medial part of cerebral peduncle to reach the
pons, where there is a partial decussation and
termination in the PPRF.
ii. The dorsal pathway passes from the FEF
through the thalamus, the pulvinar, the pretectal
nuclei, and the superior colliculus to reach the
brainstem.
iii. The intermediate pathway extends from the
FEF to the rostral ocular motor nuclei and the
interstitial nucleus of Cajal.
Brainstem pathway
Recent evidence suggests that the saccades
(horizontal as well as vertical) are generated by
groups of neurons located in the brainstem and
are controlled by higher frontal system (for
voluntary saccades) and collicular system (for
involuntary saccades). The brainstem neurons
concerned with generation of saccades form the
final premotor circuits. These neurons are of
three types:
• Excitatory burst neurons (EBN)
• Inhibitory burst neurons (IBN), and
• Pause neurons (PN).
Brainstem pathway for saccades is described
below.
Pathway for horizontal saccades
For horizontal saccades, the excitatory neurons are
located in horizontal gaze centre in paramedian
pontine reticular formation (PPRF) and project
to the ipsilateral abducens nucleus. The axons
from these cells synapse in the abducens nucleus
on motor neurons that innervate the ipsilateral
lateral rectus and on the interneurons that
innervate contralateral medial rectus subnucleus
by way of the contralateral MLF (Fig. 13.2).
Pathway for vertical saccades
For the vertical saccades, the excitatory neurons
are located in the vertical gaze centre formed
by rostral interstitial nucleus of medial
longitudinal fasciculus (riMLF) and other
neurons in the region of posterior commissure.
• For downward saccades (Fig. 13.3), the activated
neurons in the riMLF send impulse directly
through the fibres that synapse upon the
inferior rectus subnucleus of the ipsilateral
IIIrd nerve and contralateral IVth nerve
nucleus for superior oblique muscles. riMLF
nuclei of both sides are connected by a
commissure which projects into the interstitial
nucleus of Cajal (INC) and to the ipsilateral
IIIrd nerve nucleus.
• For upward saccades (Fig. 13.4), the activated
neurons in the riMLF send impulse through
the fibres that synapse upon the inferior
oblique subnucleus of the ipsilateral IIIrd
nerve; and through the fibres which pass via
posterior commissure and synapse upon the
superior rectus subnucleus of the contralateral IIIrd nerve.
Activities of brainstem involved in generation
of saccades neurons
As mentioned above, three types of neurons are
involved in generation of saccades: Excitatory
burst neurons (EBN), inhibitory burst neurons
(IBN) and pause neurons (PN). These neurons
generate saccades by a ‘pulse-step’ innervation
system (Fig. 13.10). The ‘pulse’ is created by
sudden firing of the neurons to the extraocular
muscles. After the eyeball is moved to the new
position; to keep it in the same position,
sustained contraction of the muscle is required.
This is called a step and is affected by tonic
contraction of muscles due to continuous
discharge from neurons.
Excitatory burst neurons
The excitatory burst neurons (EBNs) discharge
at high frequencies just prior to and during the
saccades and provide the eye velocity
commands known as the pulse (Fig. 13.10). Burst
cells discharge, only when there is need for a
fast eye movement and do not discharge during
fixation, pursuit or vergence eye movements.
Supranuclear Control and Disorders of Ocular Motility
Fig. 13.10 Showing relation between the three sets of neurons (excitatory burst neurons, pause neurons and inhibitory
burst neurons) concerned with generation of saccades.
385
The EBNs send impulses to the neurons of
cranial nerve nuclei supplying to yoke muscles
for the gaze movements.
Inhibitory burst neurons (IBNs)
The inhibitory burst neurons (IBNs) send impulses
through the medullary reticular formation to the
neurons of cranial nerve nuclei supplying to the
antagonist muscles of the yoke muscles for the
concerned gaze movement and thus inhibit these
muscles and allow the gaze movement to occur.
Their firing rate is inversely proportional to
the excitatory burst cells.
Pause neurons
These neurons discharge tonically, except just
before and during saccades, when they pause.
They appear to exert an inhibitory influence on
the burst neurons preventing extraneous
saccades occurring during fixation. These cells
inhibit the burst cells within the ipsilateral PPRF.
These cells are important during fixation and
smooth pursuit. Abnormalities of these cells lead
to opsoclonus and ocular flutter.
SMOOTH PURSUIT EYE MOVEMENT SYSTEM
Smooth pursuit movements are tracking
movements of the eye as they follow moving
objects. These occur voluntarily, when the eyes
track moving objects but take place involuntarily, if a repetitive visual pattern is
displayed continuously. Their features are
described on page 35. When the velocity of the
moving object is more, the smooth pursuit
movement is replaced by small saccades (catchup saccades).
Neural pathway for pursuit movements
originates in the cortex of the perito-occipitotemporal (POT) junction. The fibres then
descend and terminate in the ipsilateral PPRF
for horizontal pursuits (Fig. 13.5) and the
ipsilateral mesencephalic reticular formation for
the vertical pursuits (Figs 13.6 and 13.7); and
then possibly directly to the ocular motor nuclei.
The right occipital lobe, therefore, controls
pursuits to the right and the left occipital lobe
those to the left. The cerebellum is closely
associated with normal pursuit movements. The
FEF and the superior colliculi paly a modulating
role in the production of pursuit eye movements
by POT junction. Lesions in the POT area
produce ipsilateral pursuit defects.
VERGENCE MOVEMENT SYSTEMS
Vergence movements are eye movements that
involve the simultaneous inward (convergence)
or outward (divergence) rotation of both eyes
to maintain single binocular vision at different
distances. These movements help adjust the
focus and alignment of the eyes for near and far