Lecture 2

Question 1

what is a neuron

Answer 1

cell of nervous system which conducts electrical impulses - these typically have an axon, soma (cell body) and dendrites which synapse with other cells

Question 2

what fibres is the peripheral nervous system divided into

Answer 2

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

Question 3

what is the nervous system divided into

Answer 3

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

Question 4

what is a membrane potential

Answer 4

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

Question 5

what does digital transmission avoid

Answer 5

digital transmission avoids cross talk and external interference - the impulses last for about 1ms

the body uses up to 100 impulses per second

Question 6

describe nerve fibres

Answer 6

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

Question 7

what is another factor which affects conduction velocity

Answer 7

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

Question 8

how are frequency and acition potentials related

Answer 8

information is frequency modulated

more intense sensation or greater force required both result in a higher frequency of impulses per second

Question 9

how are nerves stimulates electrically

Answer 9

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

Question 10

what is latency

Answer 10

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

Question 11

how would you conduct conduction velocity

Answer 11

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

Question 12

what are photreeceptors an example of

Answer 12

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

Question 13

what is the erg and eog

Answer 13

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

Question 14

what is the source of the erg

Answer 14

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

Question 15

what are the origins of the erg components

Answer 15

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

Question 16

what are the different electrodes used to record erg’s

Answer 16

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

Question 17

what dilating drops are used for erg’s

Answer 17

• 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

Question 18

what are recorded ergs the smallest and biggest with

Answer 18

smallest with - infraorbital skin patches

biggest with - contact lens electrodes (most invasive

amplitudes of approximately 300uv are commonly achieved with fibre electrodes

Question 19

how may responses need to be averaged for a clear recording and what is bandwidth needed

Answer 19

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

Question 20

what are jet electrodes

Answer 20

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

Question 21

what are dtl electrodes

Answer 21

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

Question 22

what stimuli is used to record erg’s

Answer 22

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

Question 23

how is a dark adapted (scotopic) response recorded

Answer 23

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

Question 24

how is a light adapted (photooptic) response recorded

Answer 24

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

Question 25

what are measurements made from the erg

Answer 25

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)

Question 26

what are the clinical uses of erg

Answer 26

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

Question 27

what does a normal scotopic response and a abnormal photooptic responses

Answer 27

cone dysfunction/ dystrophy

Question 28

what does a normal photopic response abnormal scotopic response

Answer 28

rod dysfunction

Question 29

what does diminished ocuallatory potentials indicate

Answer 29

early retinal dysfunction in diabetes

Question 30

what would you expect to see in normal flash erg’s

Answer 30

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

Question 31

what are the recording methods used for eog

Answer 31

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

Question 32

what is the general eog set up

Answer 32

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

Question 33

what is a light peak/ dark trough ratio an index of

Answer 33

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

Question 34

what is the Arden ratio

Answer 34

arden ratio= light peak/ dark trough - a value of greater than 1.85= regarded as normal

Question 35

what are other uses of the eog

Answer 35

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

Question 36

what are the pros and cons of thee og

Answer 36

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