EMG Flashcards
Describe the EMG
recording of the action potentials occurring in skeletal muscle fibres
Extra-cellular recording: both electrodes outside the muscle fibres. Record the emf (potential) between 2 locations both outside the cells
How do we understand what is happening in the brain
To understand what is happening in the brain we need to analyse distal sites, such as the fingers and muslces.
Describe some other examples of extra-cellular recordings
ECG (EKG), electrocardiogram, recording action potentials from the heart. Electrodes on limbs, or chest.
EEG, electroencephalogram, recording action potentials from the brain. Electrodes on the scalp.
Describe intracellular recordings
one electrode inside the cell.
Measure voltage between inside and outside cell.
Describe extra-cellular recordings
both electrodes outside the muscle fibres
Measure voltage between 2 sites outside the fibres
What happens when an action potential occurs
The outside of the membrane becomes negative. May see a downward or upward deflection (depending on which electrode is the reference) this then comes back down to the resting value as the action potential moves away from that site.
Is an EMG extra-cellular or intra-cellular recording
Extra-cellular
When stimulating the nerves that innervate the muscles in the hand , for example the ulnar nerve, what two responses should you expect
Motor response- movement of the hand
Sensory response- the patient feels the stimulus
If no activity is seen, what could this indicate
You may have set the equipment up incorrectly, increase the stimulus.
Why will the patient feel the stimulus prior to the motor response
The sensory axons are large, and myelinated and so are more accessible to the stimulus, meaning that they will be activated at a lower strength stimulus than the motor neurons, which are small and myelinated.
Why are pain fibres only activated at larger stimuli
They are small and unmyelinated, and so are only accessible at larger strength stimuli.
What is used to measure the force
A transducer
How can this transducer be used to measure the force
The more the thumb pulls on the transducer (signal) converted into the electrical signal, which represents the force
What is an EMG representative of
A visual representation of the electrical activity inside a muscle.
What happens as you contract more
Electrical activity of the muscle increased amplitude on EMG. This corresponds with an increase in force generated too.
What can form the EMG
Voluntary & electrical stimulation, force and EMG
Is the action potential caused by brain activity
No, it is just a stimulation of the nerve by the stimulus at that point along the axon.
How does the force change as stimulus frequency is increased?
The twitch force increases
Why does the twitch force produced increase as the stimulus frequency is increased
As the electrical activity of the muscles increases, more muscle fibres are recruited to contract, hence more force is produced.
What happens if you stimulate the nerve quicker than the time it takes for the muscle to contract and relax
Takes longer for the force to be produced than the action potential
Hence you get amplification of the force, does not relax, this is summation. The force adds to the previous one
Describe tetanic contraction
Cannot see individual twitches, as the stimulus occurs so frequently. The twitches fuse, tetanic contraction, will continue to a maximum force as long as the stimulus is still present. The muscle will relax once the stimulus disappears.
At what frequency do tetanus and summation occur
Summation- 10Hz
Tetany- 20Hz
Mains supply is 50Hz, which is why touching live wires is dangerous- causes tetany.
Can we generate the maximum force voluntarily
Yes
How is our body able to generate a smooth fused contraction
Sends lots of action potentials down lots of different nerves at the required frequency to generate a smooth maximum force, why we don’t fluctuate and why the EMG looks messy
