7. Electromyography

Question 1

Extra-cellular recording

Answer 1

both electrodes outside the muscle fibres.

Record the emf (potential) between 2 locations both outside the cells

Question 2

3 types of extracellular recording

Answer 2

EMG: Electromyography (APs from skeletal muscle fibres)
EEG: Electroencephalogram (APs from brain)
ECG: Electrocardiogram (APs from heart)

Question 3

Intracellular recording

Answer 3

Cells cultured in a lab
1 electrode inside the cell.
Measure voltage between inside and outside cell.

Question 4

How does an extracellular recording show on a graph of voltage?

Answer 4

Start: 0: Flat line as no difference between 2 sites
AP arrives at 1st electrode: Voltage changes as difference between inside and out at 1st electrode is different to 2nd
AP passes 1st electrode: voltage returns to 0 as no difference between 2 sites
AP arrives at 2nd electrode: Voltage changes as difference between inside and out at 1st electrode is different to 2nd

Question 5

Nerves contain many axons: some back to CNS, some towards muscle. What happens when stimulated?

Answer 5

Nerves will conduct electricity in both directions:

feel stimulus and muscle being innervated moves

Question 6

Single stimulus at different stimulus strengths:

How does the force change as stimulus strength is increased?

Answer 6

As stimulus strength increases, force increases

Twitch gets bigger as more muscle fibres are recruited and force is greater

Question 7

Records made at different stimulus frequencies (number of stimuli per s). The stimulus strength was the same in all cases.
How does the force change as stimulus frequency is increased?

Answer 7

Force takes longer to be generated and adds onto previous (summation)
Fused/ tetanic contraction: can’t see individual twitches as happen so quickly and closely together, as long as stimulator is on, muscle is contracting

Question 8

Record of a voluntary contraction and a record made during stimulation at a frequency of 20 stim/s.
What are the similarities & differences?

Answer 8

Voluntary:
Many AP’s down many nerves, arriving at muscles and activating muscles at a frequency that is sufficient to cause a fused contraction
Generates maximum smooth force
Electrical: just electrical stimulation of nerve
Maximum force generated