electrophysiology Flashcards
what is electrophysiology
- study of electrical properties of cells and tissues
- based on the prinicple of resting membrane potentials and action potentials
requires the use of electrodes to detect action potentials and a monitor to display changes in electrical current
- name a example of a electrophysiological test
- ecg - allows cliniian to asess conduction of current in the heart and is widely used
can show different cardiac pathology
myocardinal infarcatrion
arthymias- atrial fibrilation , ventricular fibrilation
conduction abnormalities -
what is a eeg
useful in assessing a variety of neurological conditions e.g. epilepsy
emg - elctromyoography
used to asess both muscular activity as well as nerve supply
malfunction of skeletal muscle by either muscle or nerve will result in differences in graphs
electrophysiology of the eye
the eye has a different electrical charge at the front
compared to the back
the cornea has a relative postive charge compared to the retina which is relativley negativley charged
positive charge anterioly and negative charge posterioly
the eye is a dipole
why is the eye described as a dipole
because the conrea has a relative postive charg and the retina has a relative negative charge
what happens when a dipole moves
when dipoles move - (something with positive charge at one end and
negative charge at the other end -
moving the eye left to right or up and down creates a electrical deflection that we can measure
what does a eog do
a electrooculogram
records the electrical diopole between the front and the back of the eye
- you place electrodes in the medial and lateral canthus and then you ask the patient if they can look from left to right and you try to measure the changes in the electrical activity
reflects rpe activity
can help distinguish localised from diffuse retinal disease e..g rod dystrophyies or cone dystrophyes which effect the entire retina
e.g. in best viteliform macular dystrophy - erg is normal but eog light peak is markedly reduced
what does the eog do
the eog measure the conreoretial standing potential - the eye is a diopole postive charge at the cornea and negative at retina - by using lateral eye movements in conditions of varying luminance
measurements between pairs of electrodes at medial and lateral canthi are taken in scotopic conditions followed by mesopic conditions
signal amplitude is minimum in the dark and maximum in light
the light peak to dark trough is calculated = arden ratio
a normal ratio is more than 1.80
a ratio less than 1.65 is significantly subnormal
what is the arden ratio
- the amplitude of the change from light to dark conditions is called the arden ratio
you calculate the light peak by the dark trough
- in a normal eye you would expect it to be over 1.8
if its below 1.6 it is abnormal
what does a electroretinogram do
erg isolates the retinal function
electrical mass response of the retina to a light stimulus
affected by
intensity of stimulus
duration of stimulus
stimulus wavelength
stimulus pattern
corneal electrodes place via a contact lens
eog electrodes also used on medial and lateral canthi
what are the graphical components of a erg
a wave - corneal -ve deflection - signifies hyperpolarisation of rods and cones (mainly outer retina)
b wave - corneal +ve deflection - signifies depolarisation of on centre bipolar cells (mainly inner retina)
what do the a wave and b waves of an erg represent
a wave represents outer retinal function and the b wave represents inner retinal function
if you have a crao what would you see on the erg
you would see the b wave affected by the cra supplies the inner retina- a wave would not be affected because the outer retina recieves its blood supply from the choroidal circulation
what is a negative erg
a negative erg is where you lose the b wave - you have a flat b wave- and you still have the a wave
