Biological Electrical Potentials: Their display and recording Flashcards
Compare the amplitude of the signal in the conducting system of the heart to that of the electrocardiogram. Explain this.
Amplitude of the cardiac action potential is 90mV
Amplitude of QRS complex: 1 - 2 mV
This biological signal is attenuated by the impedance of the tissues that exist between the conducting system of the heart and the skin electrode.
Why does the QRS complex have a larger amplitude versus the p wave
Atrial walls thinner, less muscle mass = smaller potential
How do EMG potentials differ from ECG potentials and why
EMG –> much shorter duration - 5 -10 ms (versus ECG QRS 110 ms)
–>Skeletal muscle depolarises more rapidly than cardiac
EMG –> spikes versus complex pattern of ECG
–> Skeletal muscle cells do not depolarise in synchrony with adjoining cells unlike the the cardiac functional syncytium.
What determines the amplitude of the skeletal muscle EMG spike
Depends on the number of muscle fibres simultaneously stimulated. This, in turn, depends on the number of fibres stimulated by the motor neurone
(e.g. eye muscle: ± one motor neurone to 8 muscle fibres versus limb muscle: one motor neurone to 200 muscle fibres
Differentiate the 1.Frequency distribution and 2. range of potentials of the EMG, ECG and EEG
EMG: 1 Hz - 1000 Hz and 10 uV - 50mV
ECG: 0.05 Hz - 100Hz and 100 uV - 5mV
EEG 1 Hz - 50 Hz and 20 uV - 200uV
Describe the names, frequencies and LOC for the various types of EEG waves
Good BAT Dad (mnemonic)
Gamma: 35 Hz - Concentration / focus
Beta: 14 - 30 Hz - Awake with active ext. attention
Alpha: 8 - 14 Hz - Awake and resting
Theta: 4 - 8 Hz - Sleep
Delta: < 4 Hz - Deep Sleep
Define the ‘black box’ concept
Black box 1: Input transducer
Black box 2: Amplifier
Black box 3: Recorder / Display device
These boxes simplify the collection of electrical components for the measurement and display of biological signals.
As long as voltage, current and frequency range or compatible between the boxes the system will work.
Define an electric cell
This is a device that converts stored chemical energy into electricity
What is an electrode in contact with the skin
Metal - gel - skin (+electrolytes within) form an electric cell which generates a potential
What are the problems that arise at skin surface electrodes
- Change in impedance if electrode moves
- Loss of contact
- Polarization (by biological signal current flowing through) –> fixed using a good electrode (silver chloride based)
Why is it useful that biological signals from the heart, brain and skeletal muscle have disparate cell types and hence difference frequency and voltages?
Various amplifiers and filters can be used so that specific biological signals from specific organs can be transduced, isolated and displayed with the minimization of interference from the biological signals emitted from other cell types.
How does the biological signal from the brain differ from that of the heart
ECG (0.1 - 100 Hz and 1mV)
Heart: myocardial cells contract in synchrony (via the functional syncytium formed by cardiac myocytes. therefore a biological signal of relatively higher amplitude is formed ( ±1mV)
EEG (1 - 50 Hz and 0.02 - 0.2 mV)
Brain:
1. Produces smaller and more complex signals
2. Combined potential of many post-synaptic potentials from sheets of large symmetrically arranged pyramidal cells in the cortical layers 3 and 4.
3. Delta: 0 - 4 Hz. Theta 4 - 8 Hz. Alpha 8 - 13 Hz. Beta > 13Hz
EMG (1 - 1000 Hz and 0.01 mV - 50 mV)
Skeletal muscle
1. Spikes (no organized complexes as cels not interconnected like myocardium)
What is the function of an EMG
It is used to investigate spontaneous or evoked electrical potential from muscle Can differentiate between 1. Primary muscular disorders 2. Neuromuscular junction abnormalities 3. Nerve disease or lesions
What is an amplifier and why are they necessary when measuring biological signals
Devices that convert a small weak electrical signal to a more powerful output. with as little signal distortion as possible.
Essential for interpreting biological signals as the amplitude of the signal at the surface of the skin is too small with too much noise and interference.
Increase amplitude and filters in the amplifier
Filter components within the amplifier device filter out allow the amplifier to concentrate on the range most relevant to the biological signal of interest
What is the bandwidth of an amplifier
The range of frequencies which an amplifier works over with minimum loss of signal amplification (or gain) is called the bandwidth
What is a differential amplifier
A differential amplifier has two inputs.
- Inverts one of the input signals –> changing its sign from positive to negative or vice versa
- Sums the two inputs together
This means that signals of equal magnitude common to both inputs such as mains signals and other noise will sum to zero. Therefore any signal which is the same to both sources e.g. 50 Hz mains interference will be eliminated and the remaining signal will be amplified, hopefully the signal you are interested in. This is known as common mode rejection
What is common mode rejection
Common mode rejection is carried out by differential amplifiers.
Differential amplifiers have two inputs: One of the input signals is inverted, changing its sign from negative to positive or vice versa. The inputs are then summed together. This means that signals of equal amplitude common to both inputs such as mains signals and other noise sum to 0. Therefore any signal which is the same to both sources e.g. 50Hz mains interference will be eliminated and the remaining signal will be amplified, hopefully the signal you are interested in.
What is the common mode rejection ratio CMRR
The ratio of the unwanted signal: wanted signal
E.g. CMRR of 10 000 : 1 would require an interference signal at both electrodes 10 000 times greater than the desired signal at one electrode to achieve the same output as the biological signal on the display
What are filters and why are they needed if common mode rejection works to remove interference
Despite common mode rejection, some interference with signal frequencies outside the bandwidth of interest may still be present in the signal to be amplified. Filters are essential to enhance the quality of the signal to be analysed and reject all other currents.
What are high pass filters
What are low pass filters
High pass
- Eliminate all currents with a low frequency allowing currents with high frequency to pass through to be analysed
Low pass
- Eliminate all currents with a high frequency
allowing currents with low frequency to pass through to be analysed
What is a band pass filter
A filter allows through a specific bandwidth. For example between 10 - 100 HZ eliminating frequencies above and below
What is a notch filter
A filter that rejects a specific band of frequency
E.g. ECG machines –> eliminate frequencies between 48 to 52 Hz to remove interference from mains electricity
What are the most common sources of interference
Electrical interference is the most common
- Mains 50 Hz (ECG or EMG interefernce)
- Radio
- Mobile phones
What is the frequency range of surgical diathermy and what filter can be used to remove interference from this device
0.5 - 1 MHz
Easy to eliminate with a low pass filter
