Muscles and motor units (wk 3) Flashcards
Describe the 4 chain of events in muscle contraction:
- AP stimulates the release of a neurotransmitter across the neuromuscular junction
- AP spreads across sarcolemma/ muscle membrane and into fibre along the T-tubules (EMG signal)
- Causes release of calcium from the sarcoplasmic reticulum
- Calcium binds to muscles and causes cross-bridge cycling
-One action potential generates a signal twitch
How do you measure the effect of a single motor unit action potential (MUAP)?
Force produced by a single motor unit is tiny, buy can be seen with averaging.
How do twitch properties depend upon fibre type?
Twitch properties are also affected by fatigue and temperature. This requires changes in motor unit firing rates to compensate (‘muscle wisdom’).
How do you measure muscle activity?
AP propagate along the sarcolemma, starting at the neuromuscular junction to the ends of the muscle fibre. This electrical signal can be recorded, either by inserting a needle electrode into the muscle, or a surface electrode at the level of the skin. This technique is termed Electromyography (EMG).
What is the relationship between muscle activity and force?
-The amplitude of the sEMG signal is proportional to the force produced by the muscle
-Muscles act as low-pass filters: Neural input signal is high frequency, force output is low frequency.
-AP from numerous motor units summate to produce the surface EMG signal
Muscle fibre types and EMG:
-> Compared with slow muscle fibres, fast fibres have:
* Higher resting membrane potentials
* Greater density of sodium channels
* Faster action potentials
+ Therefore, fast fibres generate larger electrical responses
+ Biceps made up of a mixture of fast and slow twitch fibres. Fast twitch may become more dominant at higher forces, therefore producing more EMG.
EMG/ Force relationship depending on muscle lengthening/ shortening:
-> Not only there is more force during an eccentric contraction, but less EMG. This is due to possible preventing muscle/ tendon/ joint damage.
EMF/ Force relationship depends on joint angle:
-> EMG-force relationship is also affected by joint angle. This can be explained by mechanics and the length-tension relationship.
How do motor units act together to produce force output:
-The Motor Unit -> One alpha motor neuron and all of the muscle fibre it activates.
-Multiple action potentials generate continuous force
-Twitch fusion and motor unit recruitment -> Continuous smooth muscle contraction is generates by the fusion of multiple twitches. Muscles generate extra force by:
a) Recruiting more motor units
b) Increasing the frequency of firing
-Both of these factors also increase the electrical activity generates i.e. increase EMG activity
How is movement precision determined by the number of motor units?
-> Muscles differ greatly in both numbers of fibres and numbers of motor neurons. More motor neurons = finer force modulation = better control. Innervation number = fibres/ motor neurons. Lower innervation number = more control. Example: Calf muscles are large, but have a relatively poor degree of control and Eye muscles are much smaller, but have a high degree of control.
Describe EMG and force during fatiguing contraction:
During 50% maximum voluntary contraction (MVC), more motor units are recruited to compensate for failing contraction in other fibres. Force is maintained. More fibres are recruited at a higher intensity. During 100% MVC there is no scope for further recruitment. Therefore force and EMG also drops off. Twitch force becomes slower in the fatiguing stage and it also becomes prolonged.
Describe changes in fusion frequency after fatigue:
Muscle fatigue causes prolonged twitch duration, due to biochemical changes in the muscle.
What is muscle wisdom?
Frequency of motor units firing falls over time to compensate for prolonged twitch time. This causes a drop in the MEG amplitude (both single umots and also surface EMG). The brain knows the periphery level of the muscle (the firing rate and fatigue of the muscle).
How does the nervous system know when to reduce motor neuron firing rate:
-3 mechanisms
- Peripheral detection of the build of metabolites (e.g. lactate) and pain via group 3 and 4 receptors.
- Spinal suppression of motor neuron firing rates
- Change in voluntary activation
Why does the nervous system reduce the firing rate?
-2 reasons
- Possible to reduce the likelihood of neuronal fatigue e.g. failure of neuromuscular transmission
- More likely to maintain fine motor control (if firing rate is way above fusion frequency, force modulation is limited)
