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7 Cards in this Set
- Front
- Back
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How do action potential propagate/how are they made? |
Sodium is in high concentration outside the nerve or the brain and low concentration inside. Ion channels are voltage sensitive. Ion channels open, which lets more sodium ions in and creates a potential difference which is detected by the next channel along. The next channel opens and so on - the action potential propagates. |
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EEG - brain |
20 electrodes on the head, each one images small area of brain below it. There is a dipole on the surface of the brain - it is negative outside and positive inside as they all depolarize simultaneously. Can tell when there is an unusual signal such as outbursts every few seconds or lots of activity - epilepsy. Low frequency signals -indicate brain damage. |
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EMG/Nerve conduction studies electromyography - myo - muscle |
Motor nerve stimulation - stimulate the nerve and measure the voltage produced at the muscle. Stimulate muscle and measure signal over the nerve. EMG - use needle attached to wire - put needle in muscle to see where the muscle is innervated. |
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EIT (lungs) |
Doesn't rely on body sources. Inject sine wave current and which generates a voltage. Need to demodulate the voltage to get the signal of interest - removes carrier frequency. Also remove high frequencies with low pass filter. repeat the signal to reduce noise as noise is uncorrelated between measurements. Used to image lungs |
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Why is voltage measured and not current? |
Cannot measure the current directly as the current spreads out and net current is random through a volume conductor which is how the body acts so measure the voltage which is analogous to the pressure - the source of the current. |
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Why do you use a differential amplifier? |
The physiological signals are around a million times smaller than the mains signal so need differential amplifier to get the difference between the two signals and extract the signal of interest. |
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Why is an earth connection needed? |
If there is no earth connection the small signal can go to ground through stray capacitance. The differential amplifier is not perfect - perfect would be to have infinite CMRR which in reality it is around 10000. Need earth to ensure we see the signal. |
