In: Anatomy and Physiology
1.Name the cranial nerves tested and the extraocular muscles tested when the subject is asked to follow the eraser on a pencil as you move it in a one foot circle, two feet from their face.
Define corneal–retinal potential (CRP) and explain its relation to electrooculography and the electrooculogram.
1. This is a test done to note if there is any deformity in extraoccular muscle dfunction or neve integrity.
There are 6 extraoccular muscles present in each eye-4 recti(superior rectus, medial,lateral,inferior rectus)and 2 oblique_(superior and inferior).
Nerve supply_via cranial nerves 3,4&6.
Abducens_6th cranial nerve supply lateral rectus,
Trochlear _4th cranial nerve supply superior oblique,
Occulomotor_3rd cranial nerve supply other 4 muscles.
Other supranuclear eye movement systems include,saccadic system,smooth pursuit system,vergence system,vestibular system.
Here binocular movements occour,For conjugate movements_a.Dextroversion_movement of both eyes to the right(simultaneous contraction of rt latral rectus and lt medial rectus)
b.Dextroelevation_rt and upside_rt superior rectus and lt inferior oblique.
c.supraelevation_upward movement of both eyes_simultaneous contraction of b\l suprior recti and inferior obliques,
d.Levoelevation_left and upside_lt SR and rt IO
e.Levoversion_both eyes to the left_lt LR and rt MR.
f.Levodepression_left and down side_lt iferior rectus (IR)and rt superior oblique(SO)
g.Infraversion_downward movement of both eyes_simultaneous contractionof b\l inferior recti and superior obliques.
h.Dextrodepression_right downside_rt IR,lt SO.
Likewise with muscles nerves are involved All these movements are possible due to concurrent function of involved muscle and nerves.
2.corneal retinal potential(CRP) is the resting membrane potential difference between cornea (+ve)and back of the eye(-ve)with the long axis of the eye acting as a dipole.
Electroocculography used to record eye movements .
Electrophysiological tests allow objective evaluation of retinal functions.these include(ERG_electroretinography,EOG_electro occulography,VER_visually evoked response).
Electrode placed over orbital margin near medial and lateral canthi.the patient is asked to move the eyes side ways and keep there for few seconds,during which recording is done(every 12 minutes)This procedure is performed first indark adapted stage(decreases)and then repeated for light adapted stage(peak)
Results of EOG are interpreted by finding out Arden ratio=(max height of light peak\minimum height if dark trough)×100.
Normal curve values are 185 and above,subnormal curve <150,flat curve value<125.
Since EOG reflects presynaptic function of retina, any disease that interferes with functional interplay between the retinal pigment epithelium and photoreceptorrs will produce abnormal or absent light rise in the EOG.Thus EOG is affected in diseases such as retinitispigmentosa,detachments and retinopathies.ERG serves as a test that is suplimentary and complimentary to EOG in diagnosis of different diseases such as bests disease.