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Electrophysiology > Lecture 1 > Flashcards

Lecture 1 Flashcards

1
Q

What is the peripheral nervous system divided into?

A

Sensory and motor nerve fibres that bundle together into nerve trunks (‘nerves’) that can have to up 20,000 fibres in a cable-like structure around 3mm in diameter

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2
Q

What is the reflex arc?

A

Reflex arc is a neural pathway controlling a reflex. Most sensory neurons don’t pass directly into the brain but synapse at the spinal cord. This allows for faster reflex actions to occur by activating the spinal motor neurons without routing signals to the brain.

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3
Q

What are afferent fibres and where do they go?

A

Afferent = Sensory = Arrives at CNS

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4
Q

What are efferent fibres and where do they go?

A

Efferent = Motor = Exit at CNS

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5
Q

How thick and long are nerve fibres?

A

10um in diameter and 1m long

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6
Q

What does myeline do in terms of depolarisation?

A

Myelin on some fibres insulated them except at the small gaps, decreasing the area of membrane that needs to be ‘depolarised’, forcing the current to jump the gaps. Speeds up conduction of nerve signal through the Nodes of Ranvier, jump the gaps of insulation by the myelin.

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7
Q

Where in the body is purely unmyelinated?

A

In the gut

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8
Q

What is the conduction velocity of myelinated nerve fibres (in m/s and mph)?

A

70m/s or 150mph

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9
Q

What is the conduction velocity of unmyelinated nerve fibres (in m/s and mph)?

A

7m/s or 15mph

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10
Q

What affects conduction velocity?

A
  • Myelination
  • Membrane Capacitance (membrane capacitance is proportional to exposed area therefore myelination decreases membrane capacitance and increases conduction velocity)
  • Resistance of Axon (fatter fibres have less resistance thus increasing conduction velocity)
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11
Q

What is membrane capacitance?

A

Membrane capacitance is proportional to exposed area therefore myelination decreases membrane capacitance and increases conduction velocity

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12
Q

What is the resistance of axon?

A

Fatter fibres have less resistance thus increasing conduction velocity

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13
Q

What is membrane capacitance proportional to?

A

Proportional to exposed area

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14
Q

What is the time taken to depolarise the next section of a nerve proportional too?

A

Time taken to depolarise next section of nerve is proportional to RC (resistance of action x capacitance of membrane)

Therefore, decreasing C and/or R (fatter fibres) increases conduction velocity

So fatter nerve fibres with myelin are fast

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15
Q

How many times does myelination increase conduction velocity?

A

Myelination increases conduction velocity by approx. 10x, up to 70m/s i.e. 241kph (150mph)

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16
Q

How are nerve signals transmitted?

A

Digitally

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17
Q

How does a sodium-potassium potential work?

A

Na+ channels open causing sodium-potassium potential from -70mV to -55mV allowing more sodium in until depolarisation happens at +30mV

When the Na+ open, K+ channels open. Since K+ channels much slower to open the depolarisation has time to complete, with the K+ channels open, the membrane begins to repolarise back towards resting potential but hyperpolarises first (more potassium goes in than needed to -90mV). Seems counterproductive but is temporary, prevents neuron from receiving another stimulus for a while to raise the threshold for this to occur (= recovery time)

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18
Q

Why do we have digital transmission?

A

Digital transmission avoids crosstalk and external interference. Impulses either occur or do not occur giving an “all or nothing” last about 1ms and the body uses up to 100ips (impulses per second down a nerve fibre).

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19
Q

How long does an action potential last?

A

Impulses either occur or do not occur giving an “all or nothing” last about 1ms and the body uses up to 100ips (impulses per second down a nerve fibre).

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20
Q

Why are action potentials frequency modulated?

A

This information however is frequency modulated so the more intense sensation, or greater force is required, both resulting in more impulses per second (when we’re thinking of sensory fibres). If we want a weaker signal = weaker impulses down each nerve fibre. This is the same in motor fibres = when less contraction wanted.

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21
Q

How does electrical nerve stimulation happen with electrodes?

A

Have anode and cathode. Current applied via surface or needle electrodes. Pulses of 100usec (microseconds) and need 20mA to stimulate through the skin which is up to 250V because of dry skin having high electrical resistance (Ohm’s law)

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22
Q

What mA do we need to stimulate through the skin?

A

20mA

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23
Q

What voltage do we need to stimulate through the skin?

A

200V up to 250V (if dry skin as having high electrical resistance)

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24
Q

What is Ohm’s Law?

A

A law stating that electric current is proportional to voltage and inversely proportional to resistance

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25
Q

How do we measure conduction velocity?

A
  1. Stimulate a proximal and distal point on the arm (stimulates superficial nerves)
  2. Using electrodes, measure muscle contraction that occurs when the proximal point is stimulated then when the distal point is stimulated
  3. Subtract the latency to peak when distal point is stimulated by the latency to peak when the proximal point is stimulated
  4. Measure the distance between the proximal and distal points
  5. Speed = distance/time therefore you get the conduction velocity
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26
Q

What is the total time from stimulation to muscle ‘twitch’ known as?

A

Latency - includes transit time over the neuromuscular junction

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27
Q

How do we find the true motor nerve conduction velocity (remove the neuromuscular junction transit time)?

A

Need to eliminate this to find true motor nerve conduction velocity so need to stimulate two point i.e. electrode at elbow and electrode at hand. From this we do distance (i.e. from electrode 1 to electrode 2) /time (time 1 minus time 2) = velocity. This eliminates neuromuscular junction transit time.

28
Q

How does the neuromuscular junction transit time change as we age?

A

Typically, conduction velocity for myelinated fibres is 50m/s. Speed of conduction increases over the first 12 months of life thought to be due to myelination process in these months. It then declines with age beyond the peak velocity of childhood.

28
Q

Why do we use 2 stimulation points in electrophysiology?

A

The latency (time taken from stimulation to observed muscle twitch) includes the transit time across the neuromuscular junction (NMJ). transmission across NMJ takes longer than electrical conduction therefore by stimulating 2 points and taking the time difference, you can remove the time taken for transmission across the NMJ.

Having only 1 stimulation point will not give you conduction velocity as it will also include the time taken for transmission across NMJ

29
Q

What are the two constants in electrophysiology?

A

1) Range by which you’d expect rate to vary and 2) The rate of decline is constant.

30
Q

How is intensity information transmitted down sensory and motor nerve fibres?

A

Stimulus intensity is transmitted via afferent sensory nerves, with strong sensations resulting in high-frequency firing of action potentials. This is then transmitted to efferent motor nerves where high frequency of action potentials cause stronger contractions more frequently

*if there’s more intense sensation or greater force required then there will be higher frequency of impulses per second

31
Q

Which type of electrophysiology is the largest and what are their amplitudes?

A

ECG and EOG are the largest with amplitudes in the order of 1mV

Focal ERGs are low as a few nV

32
Q

What is the main purpose of electrodes?

A

To convert the ionic flow of current in the body to an electronic flow along a wire to a recorder

33
Q

What are the 3 common groups of electrodes?

A

1) Microelectrodes,
2) Needle Electrodes,
3) Surface Electrodes

34
Q

What do microelectrodes do?

A

Measure the potential either inside or very close to a single cell (not usually used clinically)

They open at tip, containing saline. New tungsten-in-glass electrode 12um at the tip

35
Q

What do needle electrodes do?

A

Pass through the skin and record potentials from a small area such as a motor unit within a muscle (for deep nerves). Must be sterilised before use and must not be used for 48 hours following sterilisation (for spore tests to be completed and any absorbed gas to be cleared)

Bipolar the most conventional, has an electrode running down the needle to allow you to record from a tiny area of muscle
Also used for BT
Or can be a sharp wire with Teflon to insulate it

36
Q

What do surface electrodes do?

A

Applied to the surface of the body and used to record signals such as ECH/EEG (for superficial nerves)

Used for recording gross potentials for heart or bicep for example. Usually a distance away from the source we want to record.

37
Q

Why can’t the metal of electrodes touch the skin?

A

Metal not allowed to make contact with skin due to feedback from movement. Use conductive gel to interface between this and the skin so it stabilises the connection so any movement is reduced in terms of the feedback coming off of it.

38
Q

What do we use if the metal can’t touch the skin when using electrodes?

A

Gel interface reduces both electrode offset potential and movement artefact

39
Q

What is a DTL ‘fibre’ electrode?

A

Fine thread of silver into outer canthus and comes out of inner eyelid. Stuck with two sticky pads

40
Q

What are Henke’s electrodes?

A

LoVac contact lens electrode - Keeps eye open. Need to use topical anaesthetic and interferes with vision.

41
Q

What is noise?

A

Any unwanted signal from a sound system such as:
- Hiss caused by random motion of electrons in electronics (intrinsic noise generated within the equipment)

  • Hum caused by magnetic fields from electrical machinery
  • Breakthrough from radio signals
42
Q

Why do we have amplifiers in electrophysiology?

A

Electrophysiological signals are amplified prior to recording because many are exceedingly small, some less than 5 nV. Sensitive low-noise differential amplifiers are almost always required.

43
Q

How big are electrodes?

A

Typically about 1 to 1.5m, make good antennas for picking up interference

44
Q

What are differential amplifiers?

A

Differential amplifiers, however, have two inputs, one inverting and the other non-inverting.

This allows any extraneous signals (radio, mains etc.) picked up in the electrode leads to be greatly reduced, without losing any of the wanted signal.

45
Q

How are differential amplifiers different from ordinary amplifiers?

A

Have 2 inputs to help exclude extraneous noise like radio signals (because we’re usually about a meter away from the patient). They do this without losing the wanted signal (see next slide)

46
Q

What is the gain of an amplifier?

A

Ratio of the output and input voltage

47
Q

What is the output of a differential amplifier?

A

Difference between inputs, known as the differential signal, multiplied by the ‘gain’ of the amplifier

48
Q

What signals does a differential amplifier reject?

A

Any signal common to both inputs, known as the common mode signal, will be rejected.

49
Q

How does a differential amplifier cancel out signals?

A

If you put signal into input 1 = comes in upside down and amplified

Input 2 = comes out right way up and amplified

If put signal between 2 inputs comes out amplified. But if signal coming between the two signals is the same then nothing comes out because one input is cancelling the other out so the signal is rejected. If they’re say, between eye and nose then they’d get different signals and not cancel each other out but if both pick up radio signal this would be cancelled out.

50
Q

Why must electrode leads be kept together for their full length?

A

Note that the electrode leads must be kept together for their full length, otherwise the interfering signals they pick up may differ and will not be blocked!

51
Q

What is the Common Mode Rejection Ratio (CMRR)?

A

If patient is next to mains socket would have the voltage from here coming through. 100x reduction in extraneous noise therefore not enough so rely on other techniques.

52
Q

What are repetitive signals considered as?

A

Joseph Fourier: “…any repetitive signal can be considered as the summation of sine waves.”

53
Q

How can we synthesise repetitive waveforms?

A

By adding sine waves (wave form synthesis to get different shapes)

54
Q

What signal does an ECG give off?

A

The ECG is a periodic signal whose lowest frequency component is the heart rate

If the heart rate is 60 per minute, i.e. 1Hz, then the lowest frequency component is 1 Hz

55
Q

What determines the shape of ECGs?

A

The amplitude of the components will determine the shape of the ECG

56
Q

Why do we have filters?

A

Thanks to our differential amplifier, we have our wanted signal with almost every bit of interference that was picked up by the electrode leads having been rejected.

Unfortunately, our electrodes themselves have picked up activity from other parts of the body, and we are not interested in those.

Fortunately, sometimes, they are higher or lower in frequency and we can use a filter to get rid of them.

57
Q

What is a high pass filter?

A

Allows high frequencies to pass through and holds back lower frequencies

58
Q

What is a low pass filter?

A

Allows low frequencies to pass through and holds back higher frequencies

59
Q

What is a band pass filter?

A

Combination of a high-pass and low-pass filter where only frequencies within a range are allowed to pass. Lower and higher frequency noise is attenuated

60
Q

What is the bandwidth of a filter?

A

Combining a high-pass filter with a low-pass filter creates a band-pass filter.

It is defined as the frequency range that is not less than 3dB below the maximum gain (-3dB). At this frequency range, the gain remains constant and all components are amplified equally.

This bandwidth MUST encompass our wanted signal. Has to encompass the frequencies that we’re interested in. Removes the high and low freqs as much as we can.

61
Q

What is an analogue to digital converter?

A

Samples the signal several thousand times a second and feeds it to the computer for further processing

A chip which samples the filtered signal several thousand times a second, feeding each measurement into a computer for further processing, analysis, storage and publishing.

62
Q

What Hz range do ECG, EEG, EMG, NAP, Smooth Muscle EMG & Hi-Fi cover?

A

ECG: 0.5 Hz to 100 Hz

EEG: 0.5 Hz – 75 Hz

EMG: 10 Hz – 5 kHz

NAP: 10 Hz – 10kHz

Smooth muscle EMG (gut): 0.05Hz – 10 Hz

Hi-Fi: 20 Hz – 20kHz

63
Q

What is the basic arrangement of an evoked potential system?

A
  • Electrodes
  • Amplifier
  • Filter
  • Analog to Digital Converter
  • Computer (or Visual Stimulator if needed)
64
Q

What is signal averaging?

A

Used to remove noise after recording we use signal averaging

Average successive responses. Noise different each time the flash goes off but the retina responds the same because average of noise is zero

65
Q

How does averaging reduce the noise in a signal?

A

By a factor of the square root of n

Electrophysiology (1 decks)

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