Physiology: EMG Practical Flashcards
(23 cards)
What is an EMG?
Electromyography (EMG) is a technique used to record electrical activity in skeletal muscles.
How is the electrical activity generated in an EMG?
Electrical activity in skeletal muscle is generated by stimulation from motor nerves.
Why is an EMG used medically?
Electromyography (EMG) is a diagnostic procedure that evaluates the health condition of muscles and the nerve cells that control them.
What is it important to appreciate before considering the EMG test?
Before considering the EMG test - it is important to appreciate that some cells e.g. nerve and muscle cells, have the capacity to be electrically excited and generate action potentials.
What are compound action potentials (CAPs)?
Action potentials are generated in individual cells, and compound action potentials (CAPs) are the sum of action potentials recorded from a group of nerve or muscle cells.
During depolarisation, which channels open and why?
During depolarisation voltage gated sodium ion channels open due to an electrical stimulus.
What must be reached for an action potential to occur? What is this called?
Action potentials will only occur if a threshold is reached, as such they are described as “all or nothing“.
Where does the resting potential for a neuron typical lie?
The resting potential for neurons typically sits between -50 and -75mV.
What are the features of voltages in electrically active cells and tissues? How is this electrical activity conducted and how can it be detected?
The voltages generated by electrically active cells and tissues are often very small (in the μV range), but this electrical activity is conducted through the fluids of the body and can be detected using specialised recording equipment and electrodes placed on or just under the skin.
What are the two kinds of recording electrodes?
There are two kinds of recording electrodes: surface (like an ECG pad) and intramuscular (needle) electrodes.
Summarise the mechanism of the EMG.
Muscle activity is recorded using
surface and
needle electrodes placed on the muscle itself. The EMGs correspond to the electrical potential detected on the surface of the skin, immediately above the muscle tissue, with respect to that measured with a
reference electrode located at bony regions on the arm.
The term motor unit refers to the
motor neuron and muscle fibres that it contacts. Motor unit recruitment refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle.
During the bicep EMG recording, as shown in the video, the subject lifts progressively larger weights and the
amplitude of the EMG increases meaning that more motor units are firing. The EMG data can be processed post recording to smooth the signals which more closely reflects the force generated by the muscle.
Which two ways can electrical activity in the muscles and nerves be examined?
- Voluntary muscle contractions - one basic function of EMG is to see how well a muscle can be activated. E.g. Performing a maximal voluntary contraction (MVC) of the muscle (as shown in Part 1).
- Evoked stimulations - produced by the external electrical stimulation of a motor nerve that is supplying a muscle.
What is the name for a single efferent neuron and all of the muscle fibres it controls?
A motor unit
What is an example of anatomical variation in the upper limb?
In some people the abductor pollicis brevis is innervated by the ulnar nerve instead of the median nerve
How does an evoked EMG work in the upper limb (using the median nerve)?
Percutaneous electrical stimulation of the median nerve at the wrist results in the contraction of the abductor pollicis brevis muscle, causing the thumb to twitch. The electrical activity (compound muscle action potential CMAP) generated in the muscle is recorded on the screen.
What does the compound motor action potential (CMAP - obtained from evoked EMG) represent?
It represents the summated action potentials of all stimulated motor endplates (see image on the slide - time along the x axis and voltage along the y axis).
What are two features to not on the CMAP?
There are some features to note:
Latency - the time taken by impulse to travel from site of the stimulation to site of response
Amplitude - muscle action potential amplitude which reflects the number of muscle fibres that are depolarised.
What might appear at the beginning of an evoked EMG recording? What is this a result of?
Sometimes a stimulus artifact appears at the beginning of recording (as shown on the slide) - this is a result of the stimulation itself and not to be confused with the CMAP which is recorded subsequently.
As shown in the video, the muscular/recurrent branch of the median nerve supplies which muscle in the hand?
Abductor pollicis brevis
The CMAP is a summated voltage response from the individual muscle fibre action potentials. True or false?
True
The latency of the CMAP is the time from stimulus to end of the response. True or false?
False - it is from the time of stimulus to the ONSET of response
The CMAP is a biphasic response usually with an initial upward deflection followed by a smaller downward deflection. True or false?
True
In an EMG, what technique is used to eliminate electrical noise?
Differential recording
