Biopotentials Flashcards
How is field potential formed
It arises between the recording electrode and the reference electrode as the current flow between them in the resistive bathing medium
Why biopotentials can be measured extracellularly
The activity of excitable cells leads to flow of ionic currents into the extracellular space and behaves approx. as a current source
Measurement of extracellular fields depends on
- The spatial and temporal characteristics of the locally-generated extracellular ionic currents
- The conductive characteristics of the fluid or tissue between the excited cells and the electrodes, referred as a volume conductor
Volume-conductor field-potentials include
- Electroneurogram (ENG)
- Electrocardiogram (ECG)
- Electroencephalogram (EEG)
- Electromyogram (EMG)
- Electroretinogram (ERG)
What is bipolar recording
- It is a measure of potential differences between 2 closely spaced electrodes on active sites (both respect to a remote reference electrode).
- Records local signals and is insensitive to far-field signals
- Noises or interferences common to both electrodes are reduced
- Recording is sensitive to direction of the signal wave
What is unipolar recording
- It is a measure of potential differences between a single electrode placed on active site and a remote reference electrode.
- Records both local and far-field signals
- The recording field is infinite and uniform in all directions, hence no directional sensitivity
Describe electroneurogram (ENG)
Sensory nerve action potentials evoked from median nerve of a healthy subject at elbow and wrist after stimulation of index finger with a ring electrode.
The difference in magnitude and waveshape of the potentials is due to the size (the radial distance of the measurement point from the neural source) and composition difference of the volume conductor at each location.
Describe electromyogram (EMG)
- Signal generated by muscle cells
- Amplitude: 0.1 - 5 mV
- Frequency range: 20-1000Hz
- Measurement: surface or needle electrodes
- Single motor unit (SMU): smallest area of muscle tissue that can be activated by a volitional effort.
SMU generates a triphasic potential having an amplitude of 20-2000mV lasting 3-15ms. Firing rate of SMU 6-30 per second. The shape of SMU potentials can be considerably modified by diseases.
Applications of EMG
- Assess muscle functions
- Diagnosis of neuromuscular diseases
- For prosthesis (e.g. EMG controlled prosthetic arm)
- For ergonomic assessment
Describe electrocardiogram (ECG)
- The heart pumps blood when the muscle cells making up the heart wall contract, generating their action potentials .
- This potential creates electrical current that spread from the heart throughout the body.
- The spreading currents create differences in electrical potential between various locations in the body, and these potentials can be recorded through surface electrodes attached to the skin.
- Generated in the heart
- Amplitude range: 0.5 - 4mV
- Frequency range: 0.1 - 250 Hz
- Measurement: surface electrodes
- The electrical activity of the heart can be represented by a net equivalent current dipole
Describe cardiac activation sequence
- The sinoatrial (SA) nodal cells are self-excitatory, pacemaker cells
- They generate an action potential at the rate of about 70 per minute
- From the SA node, activation propagates throughout the atria but cannot propagate directly across the boundary between atria and ventricles
- The atrioventricular node (AV node) is located at the boundary between the atria and ventricles; it has an intrinsic frequency of about 50 pulses/min; delays the propogation of excitation so that there is sufficient time delay for blood pump
- If the AV node is triggered with a higher pulse frequency, it follows this higher frequency.
- In a normal heart, the AV node provides the only conducting path from the atria to the ventricles
- Propagation from the AV node to the ventricles is provided by a specialized conduction system called bundle of His
- The His bundles ramify into Purkinje fibers that diverge to the inner sides of the ventricular walls
- Propagation along the conduction system takes place at a relatively high speed once it is within the ventricular region, but prior to this (through the AV node), the velocity is very slow
- From the inner side of the ventricular wall, many activation sites cause the formation of a wavefront which propagates through the ventricular mass toward the outer wall
- This process results from cell-to-cell activation through gap junctions
- After each ventricular muscle region has depolarized, repolarization occurs
Each wave and its depolarization or repolarization of a certain region of the heart
- P: atrial depolarization
- QRS: ventricular depolarization
- T: ventricular repolarization
- P-R Interval: due to conduction delay by AV node
- S-T Interval: average duration of the AP plateau of ventricular cell
Applications of ECG
- Reveals rhythm problems such as the cause of a slow or fast heart beat, or arrhythmia (irregular heartbeat)
- Assess if the patient has had a heart attack
- Diagnose diseases in the coronary arteries
- Identify thickening of a heart muscle (left ventricular hypertrophy)
What types of atrioventricular block are there
- Complete heart block where the cells in the AV node are seriously impaired and not able to pass excitation from atria to ventricles -> atria and ventricles beat independently, ventricles being driven by an ectopic (other-than-normal) pacemaker
- AV block wherein the node is diseased (e.g. include rheumatic heart disease and viral infections of the heart) - although each wave from the atria reaches the ventricles, the AV nodal delay is greatly increased -> first-degree heart block
Describe electroretinogram (ERG)
- Measures electrical response of retina to a visual stimulus
