describe the law governing ocular motility and add a note on synkinetic movements
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Laws Governing Ocular Motility
Background: The Extraocular Muscle Framework
Each eye has six extraocular muscles that work in coordinated pairs. Before the formal laws can be understood, three terms must be defined:
- Agonist: the primary muscle moving the eye in a given direction (e.g., right medial rectus for adduction of the right eye)
- Antagonist: the muscle of the same eye acting in the opposite direction (e.g., right lateral rectus, which opposes the right medial rectus)
- Synergists: muscles of the same eye acting in the same direction (e.g., right superior rectus and right inferior oblique both elevate the right eye)
- Yoke muscles (contralateral synergists): a pair of muscles - one from each eye - that produce conjugate movement. For example, the yoke of the right lateral rectus is the left medial rectus; the yoke of the left superior oblique is the right inferior rectus
The six muscles are yoked in pairs so that both eyes move conjugately (versions) to maintain alignment of the visual axes. The medial and lateral recti act purely in the horizontal plane. The vertical recti act as pure vertical movers when the eye is abducted 23 degrees, and the obliques act as pure vertical movers when the eye is adducted approximately 51 degrees.
1. Sherrington's Law of Reciprocal Innervation
"Increased innervation to an extraocular muscle is accompanied by a reciprocal, simultaneous decrease in innervation to its antagonist."
When the agonist contracts, its ipsilateral antagonist automatically relaxes, and vice versa. This reciprocal inhibition is what allows smooth, precise, unopposed eye movement without the two opposing muscles fighting each other.
Example: When the right medial rectus receives increased innervation to adduct the right eye, the right lateral rectus receives a simultaneous, equal decrease in innervation and relaxes.
Scope: Sherrington's law applies to both versions (conjugate movements) and vergences.
Clinical relevance - muscle sequelae: When a muscle becomes paretic, the Sherrington law drives several downstream changes:
- The ipsilateral antagonist (the unopposed muscle) overacts and eventually undergoes contracture. In a left superior oblique palsy, the ipsilateral antagonist is the left inferior oblique, which overacts.
- The contralateral antagonist is secondarily inhibited via the Hering law (see below).
2. Hering's Law of Equal (Dual) Innervation
"During any conjugate eye movement, equal and simultaneous innervation flows to the yoke muscles of both eyes."
The amount of neural signal sent to both eyes during a version movement is always equal and is always determined by the fixating eye. This is why both eyes move together in conjugate gaze - neither eye can receive a different motor command from the other during a version.
Example: When looking to the right, the right lateral rectus and its yoke - the left medial rectus - receive equal simultaneous innervation.
Clinical relevance - primary vs. secondary deviation:
In a paretic squint (e.g., left lateral rectus palsy):
-
Primary deviation: The normal right eye fixates. The paretic left eye deviates inward because its antagonist (left medial rectus) is unopposed. The deviation is relatively small because normal innervation flows to both eyes.
-
Secondary deviation: The paretic left eye is now forced to fixate. Extra innervation must be sent to the weak left lateral rectus to achieve fixation. By Hering's law, this same excess innervation flows equally to the right medial rectus (the yoke muscle), causing overaction of the right medial rectus - resulting in excessive adduction of the right eye.
Key rule: In a paretic squint, the secondary deviation always exceeds the primary deviation.
The Full Sequence of Muscle Sequelae
The interaction of both laws produces a predictable cascade in any muscle palsy. Using a left superior oblique palsy as the model:
| Step | Muscle affected | Mechanism |
|---|---|---|
| 1 | Left superior oblique (paretic) | Primary underaction |
| 2 | Right inferior rectus (yoke) | Secondary overaction - Hering's law |
| 3 | Left inferior oblique (ipsilateral antagonist) | Secondary overaction, then contracture - Sherrington's law |
| 4 | Right superior rectus (contralateral antagonist) | Secondary inhibition - combined Hering + Sherrington laws |
This full pattern is critical for distinguishing a recently acquired palsy from a longstanding one, as the muscle sequelae take variable time to develop.
Note on Synkinetic Movements
A synkinesis (synkinetic movement) refers to an involuntary movement of one muscle group that occurs simultaneously and automatically with a voluntary movement of a different muscle group, due to shared or aberrant neural pathways.
In the context of ocular motility, synkinetic movements arise in two broad settings: (a) physiological synkineses that are normal components of conjugate gaze reflexes, and (b) pathological synkineses due to aberrant nerve regeneration or misdirected innervation.
Physiological Synkineses
1. Ocular Counter-Rolling (Ocular Tilt Reflex)
A normal synkinetic movement in which tilting the head to one side produces a compensatory counter-rotation of the eyes to maintain horizontal orientation of the visual environment. When the head tilts left:
- The left eye rises and intorts
- The right eye falls and extorts
This response has two phases: a dynamic (phasic) phase driven by semicircular canal stimulation, and a sustained (tonic) phase maintained by the otolith organs. Disruption of these otolithic-ocular pathways in the brainstem results in the pathological Ocular Tilt Reaction (OTR) - a syndrome of spontaneous skew deviation, cyclotorsion of both eyes, paradoxical head tilt, and displacement of the subjective visual vertical.
2. Near Triad (Synkinesis of Convergence)
On looking at a near object, a synkinetic triple response occurs: convergence, accommodation, and pupillary miosis - all three linked through a shared supranuclear command. Spasm of the near reflex exploits this synkinesis, producing pseudo-abducens palsy with miosis.
Pathological Synkineses
1. Aberrant Regeneration of the Third (Oculomotor) Nerve
This is a classic example of pathological synkinesis. After an oculomotor nerve injury (from trauma, aneurysm, or long-standing compressive lesions in the cavernous sinus), regenerating axons misfire into incorrect muscle branches:
- Pseudo-von Graefe phenomenon: The upper lid elevates on downward gaze (instead of drooping) - the levator palpebrae receives aberrant signals from inferior rectus fibers
- Lid-gaze synkinesis: The lid elevates on adduction
- Adduction on attempted elevation or depression: The medial rectus receives mis-directed innervation from vertical rectus branches
- Pupil constricts on adduction or downgaze instead of (or in addition to) light - this is near-light dissociation from aberrant sphincter pupillae innervation
- Czarnecki sign: Segmental constriction of the iris sphincter on attempted gaze in a third nerve direction, visible at the slit lamp
Aberrant regeneration is seen after trauma, aneurysm, congenital third nerve palsy, and syphilis. It virtually never follows ischemic third nerve palsy. Primary aberrant regeneration (without a prior acute palsy) occurs with slowly progressive cavernous sinus lesions such as meningiomas and trigeminal neuromas.
2. Marcus Gunn Jaw-Winking Phenomenon
A congenital ptosis in which opening the mouth (or moving the jaw sideways, engaging the lateral pterygoid) causes the ptotic eyelid to abruptly elevate. This represents a synkinesis between the motor division of the trigeminal nerve (CN V, supplying pterygoid muscles) and the levator palpebrae superioris (supplied by CN III), from a congenital miswiring in the motor nucleus. - Adams and Victor's Principles of Neurology, 12th ed.
Its inverse - the Wartenberg sign (inverse Marcus Gunn) - is a congenital synkinesis between trigeminal and facial nerve fibers, in which jaw movement causes involuntary ipsilateral eyelid closure.
3. Duane Retraction Syndrome
A congenital cranial dysinnervation disorder where the abducens nucleus is absent or hypoplastic, and the lateral rectus is instead innervated by a branch of the oculomotor nerve. This produces a synkinetic co-contraction of the medial and lateral recti on attempted adduction, causing globe retraction into the orbit and narrowing of the palpebral fissure - both eyes moving together on a normally unilateral command. The lid-gaze synkinesis in Duane syndrome is of opposite direction to that seen in aberrant CN III regeneration. - Localization in Clinical Neurology, 8th ed.
4. Crocodile Tears (Gustatory Lacrimation)
After facial nerve palsy, aberrant regeneration causes visceromotor fibers originally innervating the salivary glands to re-innervate the lacrimal gland. The result is involuntary tearing whenever the patient eats - a synkinesis between salivation and lacrimation pathways.
5. Congenital Trochlear-Oculomotor Synkinesis
A rare congenital anomaly in which attempted trochlear nerve activation triggers oculomotor responses. One documented case showed bilateral palpebral fissure widening of the adducting eye and narrowing of the abducting eye without globe retraction. - Localization in Clinical Neurology, 8th ed.
Summary Table
| Law/Concept | Statement | Applies to |
|---|---|---|
| Sherrington's Law | Agonist contraction = simultaneous antagonist inhibition | Same eye, ipsilateral pair |
| Hering's Law | Equal simultaneous innervation to yoke muscles | Both eyes, conjugate versions |
| Synkinetic counter-rolling | Head tilt drives compensatory eye torsion | Physiological, otolithic pathway |
| Aberrant CN III synkinesis | Lid/pupil moves with gaze due to misdirected regeneration | Pathological, post-injury |
| Marcus Gunn jaw-winking | Jaw movement opens ptotic lid via trigeminal-oculomotor synkinesis | Pathological, congenital |
| Duane syndrome | Lateral rectus co-contracts with medial rectus due to CN III misdirection | Pathological, congenital |
Sources: Kanski's Clinical Ophthalmology (10th ed.), Bradley and Daroff's Neurology in Clinical Practice, Adams and Victor's Principles of Neurology (12th ed.), Localization in Clinical Neurology (8th ed.)
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