Excitable Tissues and Biologic Potentials

Question 1

Describe the waveform of a biological potential?

Answer 1

They are sine wave forms with different frequencies and phases.

Question 2

What factors effect the measured potential from excitable tissues?

Answer 2

1. Mass of excitable tissue and the
2. Extent of mass separating the excitable tissue from
   the recording electrode.

Question 3

What is the measured potential range and frequency range for an ECG?

Answer 3

1-2mV*
3-100Hz

*The resting potential of myocytes is -90mV but the voltage measured potential by the ECG is only 1-2mV due to the extent of mass separating the electrode from the excitable tissue

Question 4

How are the complex waveforms of an ECG analysed?

Answer 4

ECG can be analysed using Fourier analysis which breaks down a composite wave into its simpler component

Question 5

What is the measured potential range and frequency range for an EEG?

Answer 5

For an Electroencephalogram (EEG) the measured potential is even lower than that of an ECG due to reduction from impedance from the meninges, CSF, and skull.

The measured potentials are ~50 μV
The frequency range is 3-100Hz

Question 6

What are the main waveforms seen in an EEG?

Answer 6

1. Beta: awake and concentrating (12-25Hz)
2. Alpha
3. Theta
4. Delta: Slow wave sleep (3Hz)

Question 7

What is an EMG, what factors affect the measured potential, what is the measured potential range and frequency range?

Answer 7

An electromyogram which measures electrical activity in skeletal muscle.

Activity is dependent on the number of motor units stimulated and is between 50 μV up to 30 mV.

The short repolarisation leads to a frequency between 5Hz to 100Hz which is greater than that in cardiac muscle.

Question 8

What components are needed to measure biological potentials?

Answer 8

Electrodes
An amplifier* (e.g an Op-amp)
A display unit (containing an analogue to digital converter)

*Amplifiers increase the biological signal amplitude through gain and is measured in decibels. The bandwidth describes the range of frequencies this device will function satisfactorily.

The interference is removed by differential amplifiers that use the property of common-mode rejection ratio (CMRR).

Question 9

What are issues which can arise in non invasive electrodes?

Answer 9

1. Poor contact which will lose the signal display.
2. Chemical changes in the recording electrode from the biological signal. This is called polarisation and leads to altered performance of the sensor.
3. Moisture trapping – battery effect where the
   recording system itself generates a potential.

Question 10

How are modern electrodes constructed to avoid the issue of moisture trapping?

Answer 10

Modern electrodes contain silver (Ag) electrodes in contact with silver chloride (AgCl) and a conductive gel rich in Cl- ions.

An adhesive disc surrounds it to provide best contact with the skin.

Therefore moisture cannot get trapped between the electrode component and the skin.

Question 11

Describe the different types of clinical devices we use to stimulate excitable tissues?

Answer 11

Defibrillator.
Electroconvulsive therapy.
Peripheral nerve stimulators and needle nn stimulators.
Evoked potentials.
Cardiac pacemakers.
Deep brain/vagal nerve stimulators.

Question 12

In basic terms explain how a defibrilator and ECT works?

Answer 12

Defibrilator:
It is a capacitor that collects and discharges electrical charge.

The energy discharged is described in Joules which is directly proportional to the charge across the capacitor plates which is directly proportional to the voltage applied.

An inductor modulates the current prolonging the time of the charge being discharged increasing the success rate.

Gel pads attached to the patient reduce impedance.

ECT:
Works in the same fashion but across the brain with much smaller currents and applied in pulses.

Question 13

How do peripheral nerve and needle nerve stimulators work (include the relevant amplitudes)?

Answer 13

Peripheral nerve stimulators are used to assess degree of neuromuscular blockade. Gel electrodes stimulate the peripheral nerve via the skin at a current of 50-80mA.

Needle nerve stimulators are used to assist with regional anaesthesia. A needle capable of conducting a current is used. As the needle gets closer to the nerve less amplitude is needed (<2mA) to stimulate the nerve and induce a contraction of the innervated muscle.

If stimulation to cause contraction occurs at a very low current i.e. <0.3mA, the tip may be within the nerve. This is a good test to prevent intraneural injections.

Question 14

What are visual evoked potentials?

Answer 14

Nerve conduction studies.

Nerves are induced and the potential is measured in the neural circuit to give a visual reading.

Any abnormalities may indicate damage to the conducting pathway.

Question 15

What are deep brain stimulators used for and explain in basic terms how they work?

Answer 15

Used to treat movement disorder.

Placed in close proximity to the centres that regulate movement, such as the thalamus.

This electrode is then connected to an implantable pulse generator which delivers a set current to stimulate the relevant part of the brain.

Provided it is correctly placed, this does not result in activation of other areas of the cortex.