Lecture 2 Flashcards
what is a neuron
cell of nervous system which conducts electrical impulses - these typically have an axon, soma (cell body) and dendrites which synapse with other cells
what fibres is the peripheral nervous system divided into
the peripheral nervous system is divided into sensory and motor fibres
e..g your skin has nerve endings which travel along the sensory nerve fibre and to your posterior root ganglion and then to the muscle so that you retract your hand i.e. if your touching something hot
fibres are bundles up together into nerve trunks (nerves) which can have up to 20 000 fibres in a structure and 3mm in diameter
what is the nervous system divided into
divided into a cns and a pns
the cns= brain and spinal cord and the ons can be further divided into autonomic and somative nervous system - the autonomic ns is part of the nervous system that is responsible for functions that argent consciously directed e.g. heart beat peristalsis breathing
somatic = for voluntary movements and can be divided into afferent and efferent motor fibres
what is a membrane potential
in order for nerve fibres to carry electrical impulses - a resting membrane potential must be established
when impulsess are arrived the number of excitatory or inhibitory impulses are summed if the men potential = less negative = depolarised and then the nerve cell will propagate an all o nothing acition potential along its axons towards the synapses
this= mediated by flow of na+ and k+ through specialised pumps and channels
what does digital transmission avoid
digital transmission avoids cross talk and external interference - the impulses last for about 1ms
the body uses up to 100 impulses per second
describe nerve fibres
nerve fibres are long and thin
10um diameter and 1m long
fibres may be myelinated or non myelinated
myelin insulates axon and forces action potentials to jump between the nodes of ranvier , this decreases the surface area to be depolarised and increases conduction velocity , myelination increases condition velocity by x10 and up to 70ms Max i..e 150mph
membrane capacitance is proportional to exposed area - if you have a unmyelinated nerve - you have more surface area - therefore the time taken to depolarise will be longer
conduction is also affected by age
conduction velocity decreases with age
what is another factor which affects conduction velocity
membrane capacitance is proportional to exposed area
the time taken to depolarise the next section of the nerve is proportional to resistance capacitance - therefore by decreasing c and or r
(fatter fibres) increases conduction velocity
how are frequency and acition potentials related
information is frequency modulated
more intense sensation or greater force required both result in a higher frequency of impulses per second
how are nerves stimulates electrically
current is applied to surface or needle electrodes in pulses per -100usec
a current of at least 20ma is needed to stimulate through the skin
this requires up to 250 volts because dry skin has a high electrical resistance as governed by ohms law v= ir where r = resistance and I= ionic current
in practice surface electrodes are used to stimulate and record superficial nerves whilst needle electrodes are required for measurement of deeper nerves
what is latency
latency = total time from stimulation to an observed muscle twitch - termed latency - latency includes the transit time across neuromuscular junction - it is necessary to stimulate at two positions and measure at 1 position in order to get true motor conduction
velocity hence proximal and distal stimulation points
advantage of recording from a muscle is that a muscle takes a very longtime to respond you have a delay across the nmj and that means that the muscle twitch is seen very distinctly which makes it much easier to make a measurement
how would you conduct conduction velocity
you stimulate two points because you can subtract one time from another
stimulate proximal point of stimulation and distal point of stimulation - you shorten the distance that the signs has to travel
what are photreeceptors an example of
photoreceptors are an example of transducer - converting light into a signal that our brains are able to interpret - the electrical signals recorded from the retina are able tell us about abnormal/normal physiology
what is the erg and eog
erg= a recording of electrical responses of retina to flashes or light patterns
eog - electrical responses of the retina to changes in steady stage illumination
what is the source of the erg
photoreceptors have a dark current within them and when a photon hits photopigment it it becomes depolarised
light dependent decrease in rod and cone dark current gives ‘a’ wave plus release of k+
muller cells absorb extracellular k+ resulting gin part of the b wave rest of the b wave comes from the bipolar cells
what are the origins of the erg components
a wave - from rods and cones - ap wave from the rods and as waves from the cones
b wave from bipolar cells and muller cells
oscillatory potentials from amacrine cells
what are the different electrodes used to record erg’s
ergs are recorded using different types of electrodes
fibre electrodes dlls - used in adult outpatients
infraorbital gel skin patch used for infants
and contact lens electrode for patients in theatre under ga
an ag/agcl ground electrode is placed on the forehead and another as a ref electrode on the ipsilateral temple
loca anesthetic oxybuporcraine 0.04% = necessary for comfort with contact lens electrode
what dilating drops are used for erg’s
- tropicimaide 1% phenlephrine 2.5% esnure pupils are non reactive and therefore the same diameter for all stimuli and regardless of stimulus brightness
- needed for consistent and reliable stimulation
what are recorded ergs the smallest and biggest with
smallest with - infraorbital skin patches
biggest with - contact lens electrodes (most invasive
amplitudes of approximately 300uv are commonly achieved with fibre electrodes
how may responses need to be averaged for a clear recording and what is bandwidth needed
10 responses need to be averaged for a clear recording
- bandwidth is set up to 0.3hz - 300hz for full response or 100 hz- 300hz to examine the oscillatory potentials alone
what are jet electrodes
jet corneal electrodes are used to record erg’s under general anesthetic if the patient were awake they would need topical anaesthetic - the electrode may occlude some of the pupil - these are disposable and about £9 each
what are dtl electrodes
dtl electrodes are used routinely at clinic - no anaesthetic is required drops may be given for comfort don’t interfere with vision and are suitable for all diffuse and structured stimuli
These are also disposable voting £8 each
what stimuli is used to record erg’s
stimuli are delivered via a ganzfield (full field stimulator)
standard flash luminance is defined as 3cd-sm-2 which is quite brightt especially if you have dilated pupils
how is a dark adapted (scotopic) response recorded
to record a dark adapted (scotopic response) eyes are adapted for 20mins under dim red lights - red = poor stimulus for rods prior to recoding
a dim flash 0.01 cd is used to produce a rod response -2.5 long units below of standard flash luminance and then standard flashes are used to produce a mixed response from rods and cones
how is a light adapted (photooptic) response recorded
stable conditions your eye is in to allow as much light In as possible
the rods are suppressed by a background light of 30cd for 10 mins before testing and during recording then standard flashes are used to elicit a cone response
what are measurements made from the erg
most clinical information comes from the amplitudes of the responses
a wave amplitude normally measured from the baseline to the first negative trough
b wave amplitude normally measured from a wave negative trough to the next positive peak
however timing is an important factor
time from light stimulus being applied to response peak occurring gives information about the response time and is known as the implicit time (it)
what are the clinical uses of erg
no erg response - retinitis pigments , ophthalmic arerty occlusion , total retinal detachment
reduced a and b waves = rod and cone dystrophy
drug toxicity
retinal degernation
normal a wave and reduced b wave
congenital stationary night blindness
juvenile retinoschisis (splitting of retinal layers)
melanomas associated retinopathy
batten disease (neuronal cored lupofuscinos)
central retinal artery occlusion
what does a normal scotopic response and a abnormal photooptic responses
cone dysfunction/ dystrophy
what does a normal photopic response abnormal scotopic response
rod dysfunction
what does diminished ocuallatory potentials indicate
early retinal dysfunction in diabetes
what would you expect to see in normal flash erg’s
dim flash - dark adapted response lower amplitude and and long implicit times - 0.01 cd- sm -2
small a wave system - just rod system
standard flash 3cd dark adapted larger amplitude , larger a wave medium implicit times - rod and cone systems
standard flash 3cd-sm2 light adapted lower amplitude short implicit times just cone system
what are the recording methods used for eog
eog= recording of the electrical responses in he retina which result from changes in steady state illumination and can inform us about the health of the retinal pigmented epithelium
standing potential are difficult to measure because of the uncertainty concerning baseline position e.g. electrode offset potentials
to overcome this difficulty and help establish baseline the signal of interest is made to vary with time by voluntary eye movements
to help patients/ subjects eye movements two lids in ganzfield subtending an arc of 30 degrees are illuminated atlerantley for 1 second and the subject asked to track them
what is the general eog set up
recording electrodes are placed on the naison and lateral canthus with a reference electrode placed equidistant from recording electrodes
some labs and electrodes use earlobe as the position for the reference electrode however some studies have pointed this as source if vaiancwe and at rah a skin gel patch electrode on the forehead is typically employed
signal size is approx 1mv
bandwidth of signal is approx 0.01hz- 3ohz
record response for 10 seconds for every 1 min (to avoid fatigue) for 16mins under dark adaptation conditions and 16mins during light adaptation
i.e. recording 10 seconds alternating saccades every minute for 16 minutes in the dark followoing 16 minutes with a background light switched on
what is a light peak/ dark trough ratio an index of
decreased light peak/ dark trough ratio is an index of retinal function
best vittelfrom macular dystrophy
pretty much essential for the diagnosis
retinal epithelium disease
central retinal artery occlusion - flat but erg is more informative
acute quinine toxicity
retinitis pigmentosa
azoor = supernormal
but parallel the erg
what is the Arden ratio
arden ratio= light peak/ dark trough - a value of greater than 1.85= regarded as normal
what are other uses of the eog
eng - electronystagmography
saccadic velocity - horizontal angle of gaze with electrodes either side of the eye
vertical angle of gaze - with electrodes above and below the eye
position of gaze - a vector derived from horizontal and vertical angles
what are the pros and cons of thee og
although many pathologies result in changes in the eog - these changes are often also reflected in the erg
the eog= complicated and requires that participants are able and willing to make smooth saccades - rarely employed unless there is a suspicion of best disease in which case eog= greatest diagnostic power
