#5: Electromyography (EMG) Flashcards
Skeletal Muscle
Most are attached to bones. Their contractions are responsible for moving and supporting the skeleton. Each muscle is made up by many individual fibers organized in fascicles. Each individual fiber is innervated (supplied with nerves) by a motor axon.
Motor Unit
The contraction of skeletal muscle is initiated by impulses in the neuron to the muscle. When an action potential is generated in a motor neuron, it activates all of the muscle fibers innervated by that one motor neuron.
The motor neuron together with all the individual muscle fibers that it innervates is termed a motor unit.
Activation of Neuromuscular Junction: Step 1
Once the motor neuron is activated, the action potential propagates along the axon.
Activation of Neuromuscular Junction: Step 2
The events of step 1 activates the voltage-cated Ca+2 channels, causing a calcium influx. Calcium is critical for exocytosis of neurotransmitters.
Activation of Neuromuscular Junction: Step 3
Calcium binds to proteins that enables the membrane of the acetylcholine containing vesicles to fuse with the neural plasma membrane to allow acetylcholine release.
Activation of Neuromuscular Junction: Step 4
Acetylcholine diffuses from the axon terminal over a short distance to the muscle membrane, where it binds to receptors.
Membranes found on the muscle membrane are called nicotinic cholinergic receptors.
Activation of Neuromuscular Junction: Step 5
Binding to the nicotinic cholinergic receptors by acetylcholine opens a nonspecific monovalent cation channel, allowing both sodium and potassium ions to pass through the channel. Sodium ion moves in, potassium moves out.
Since it’s more permeable to sodium, more sodium moves in, adding more positive ions inside. This causes the membrane to depolarize.
Activation of Neuromuscular Junction: Step 6
The voltage change caused in the previous step will then open voltage-gated Na+ channels, and will cause further depolarization of the muscle membrane.
This voltage change is called the end-plate potential (EPP). This action potential generated by the muscle fibers are electrical activities recorded using EMG.
Electromyogram (EMG)
A technique to record electrical signals from muscle fibers during muscle contraction.
Two Types
- Intramuscular (inserting needle or fire-wire through the skin and into the muscle). Provides more insight.
- Surface (placing electrodes on surface of skin).
Voluntary Contractions
Force changes with increased demand.
Reciprocal Activation: there is a simultaneous activation of analogous and inhibition of antagonist muscle. Inhibition is not complete, as antagonist muscle still fire action potentials, depending on the nature of the action. Phenomena called coactivation.
Cocontraction: simultaneous contraction of both analogous and antagonist muscle around the joint to hold a stable position.
Clinical Applications
Size and shape of wave give info on the ability of the muscles to respond when the nerves are stimulated.
Can be used together with a nerve conduction study to detect neuropathies and myopathies and many others.
Transducer
If the single of interest is not an electrical signal, it must be converted to an analog voltage, done by a transducer.
PowerLab
Takes a voltage signal, it’s then monitored by a hardware called PowerLab. It performs data acquisition, signal conditioning, and pre-processing.
If the signal is too small, it gets amplified. If other noises contaminate the signal, it gets filtered. In general, this process is called signal conditioning.
After single is conditioned, the analog voltage is sampled at regular intervals, called analog to digital conversion. Now this digitized signal can be recognized by a computer.
LabTutor
Can use to control the heartrate sampling, display the signal, and allow you to analyze the data.
Raw EMG records the potential difference detected at the electrode. We use integration to display the overall activity level of the signal. Integrated EMG is a running average of the raw EMG. It can be used to quantify the signal.
Electode Placement
Ensure good electrical contact.
Away from tendon. It generates weak electrical signals.
Avoid hairy areas.
Ground electrode should be placed as far away as possible from the recording electrodes.
