extraocular muscle 3 Flashcards
what is Sherrington’s law?
- whenever an agonist receives an impulse to contract, an equivalent inhibitory input is sent to its antagonist, which relaxes and actually lengthens
- so when an impulse is sent to the right lateral rectus to contract , inhibitory impulse is sent to right medial rectus to relax
what is ipsilateral muscles ?
- muscles in the same eye
- e.g. right lateral rectus and right medial rectus
what is contralateral muscles ?
- muscles in opposite eye
- right superior rectus and left inferior oblique
what are synergists ( also known as yoke muscles ) ?
- muscles that are working together in order to pull eyes in same direction
- e.g. left lateral rectus and right medial rectus
what are contralateral synergists ?
- two muscles moving the eye in the same direction
what is Hering’s law?
- when an impulse is sent to a muscle to contract a simultaneous and equal impulse is sent to its contralateral synergist to also contract
what is Hering’s law properties ?
- binocular law ( 2 eyes together)
- aids maintenance of BSV
- applies solely to the extra ocular muscles (EOM)
what are the two types of normal binocular eye movements ?
- conjugate movement (version)
2. disjugate movement (vergence )
what is conjugate movement (version)?
- two eyes move in the same direction so that the visual axis remain more or less parallel
what is disjugate movement (vergence) ?
- two eyes move in equal but opposite directions so that the visual axes do not remain parallel
- convergence ( right eye looking to the left and left eye looking to the right)
what happens in disjugate movement ?
- hering’s law still applies
- muscle paring have changed
- convergence both MR
- divergence both LR
what happens when looking in any direction of gaze ?
- agonist ( the muscle producing the movement contracts)
- contralateral synergist ( muscle in other eye pulling in the same direction as agonist )
- direct ( or ipsilateral ) antagonist
- the maximally relaxing muscle
- primary action is in the opposite direction to agonist - contralateral antagonist
- maximally stretched muscle in other eye
- primary action in the opposite direction to the contralateral synergist
what happens when px looks to the right ?
- right LR contracts (agonist)
- contralateral synergist LMR contracts
- ipsilateral antagonist RMR relaxes
- contralateral antagonist LLR relaxes
what happens when px looks to left ?
- right MR contracts (agonist)
- contralateral synergist LLR contracts
- ipsilateral antagonist RLR relaxes
- contralateral antagonist LMR relaxes
what happens when px looks into dexto elevation ( right and up ) ?
- right SR contracts ( agonist)
- contralateral synergist LIO contracts
- Ipsilateral antagonist RIR relaxes
- contralateral antagonist LSO relaxes
what happens when px looks into dexto depression ( right and down) ?
- right IR contracts (agonist )
- contralateral synergist LSO contracts
- Ipsilateral antagonist RSR relaxes
- contralateral antagonist LIO relaxes
what happens when px looks into laevo elevation ( left and up) ?
- RIO contracts (agonist )
- contralateral synergist LSR contracts
- ipsilateral antagonist RSO relaxes
- contralateral antagonist LIR relaxes
what happens when px looks into laevo depression ( left and down)?
- RSP contracts ( agonist )
- contralateral synergist LIR contracts
- ipsilateral antagonsit RIO relaxes
- contralateral antagonist LSR relaxes
what happens if one muscle has defective input ?
- the muscle will have reduced innervation
- this muscle will not be effective as it should be
- it will under-act compared to the other eye
- there will be a consequent or sequential effect on other muscle actions ( muscle sequelae )
what is muscle sequela ?
- muscle sequelae develops to restore concomitance ( make things better)
- first and second step happens quickly the rest takes months and years
- muscle sequelae develops with time
- the spread of muscle sequelae varies between patient
how does a muscle sequelae happen when you have a right superior rectus palsy?
- primary under-action
innervation to muscle reduced ( RSR) - over action of the contralateral synergist ( left inferior oblique)
- when the affected eye is forced to look at the target. the unaffected eye will move further than the affected eye to try to move both eyes into that location - over-action ( and contracture) of the ipsilateral (direct) antagonist
- right inferior rectus will start to contract and over act because nothing is acting against it
- the muscle will have normal input but less opposition than normal, so the muscle will over-act and eventually contract
- secondary inhibitional palsy ( under-action) contralateral antagonist
- when the palsied eye is forced to fixate. the over-action ( and contracture) of the ipsilateral antagonist ( RIR ) means the contralateral antagonist will relatively under-act ( LSO)
what happens in R IR palsy ?
- R IR is under action
- over action of contralateral synergist L SO
- over-actions ( and contracture) of the ipsilateral antagonist ( RSR )
- contracture of ipsilateral antagonist (right SR) is vertically opposite muscle - under-action of contralateral antagonist ( left IO)
what happens in L LR palsy ?
- L LR is under action
- over-action of contralateral synergist (R MR)
- over-action and contracture of the ipsilateral antagonist (L MR)
- under-action contralateral antagonist ( R LR )
what happens in RSR palsy?
- R SR is under-action
- over-action of contralateral synergist (L IO)
- over-action and contracture of the ipsilateral antagonist ( right IR)
- under action contralateral antagonist ( left superior oblique )
what happens in a muscle sequelae following a muscle palsy ?
. step 2: its contralateral synergist will over-act ( due to herings law)
. step 3: its direct antagonist will over-act ( due to sherrington’s law)
. step 4: the contralateral antagonist will under-act ( due to Hering’s law)
how to investigate binocular eye movements?
. field of BSV ( only HES) . synoptophore ( only HES) . ocular motility . diplopia charts . hess charts
what is the fourth nerve
- fourth nerve palsy is the trochlear nerve which supplies superior oblique
how to to interpret a fourth nerve palsy?
- inside green line is field of BSV
what is binocular field of fixation ?
- small central field more useful than larger field displaced upwards
- a narrow field indicates a mechanical problem
- a good fusion range can increase the size of the field
- enables progression to be monitored
- uniform and repeatable record
- plots a larger area than the hess chart
