Skeletal muscle - single-fibre and whole-muscle mechanisms (wk5) Flashcards
Describe muscle tension and load:
-Tension -> The force that a muscle exerts/produces on the joint when it is contracting is called the tension of a muscle
-Load -> The force that is exerted on a muscle by an object is called the load of the muscle
-Muscle tension must exceed the load in order for the muscle fibres to shorten, and therefore move the object that is responsible for the load
-If muscle tension does not exceed the load, then the muscle will either remain at the same length, or it will lengthen
Describe the different types of contraction:
Types of contraction:
-Concentric contraction;
* Shortening contraction
* Constant load, muscle shortened
* Tension (larger than)> Load
-Isometric contraction;
* Constant muscle length
* Free object: load = tension
* Fixed object: load => tension
-Eccentric contraction;
* Lengthening contraction
* Muscle length increases
* Load (larger than)> tension
-Excitation-contraction coupling and twitch generation -> propagation of action potential and the release of calcium (etc)
Describe the mechanisms of a muscle twitch:
-Single-fibre contractions -> The mechanical response of a muscle fibre to a single action potential is known as a twitch. After the action potential there is a latent period (few ms) before the tension in the muscle fibre begins to increase. The time interval from the beginning of tension development (at the end of the latent period) to the peak tension is the contraction time.
-In a shortening contraction, an increasing load causes;
1. The latent period to increase (electromechanical delay)
2. The velocity of shortening to slow down
3. The total duration of the twitch to become shorter
4. The distance shortened to become less
Describe the load-velocity relationship:
-In the absence of a load, a shortening contraction reaches its maximum shortening velocity
-When the load increases to the point where the muscle is not able to move it, then the contraction becomes isometric
-When the load increases beyond the peak tension that a muscle can produce, the contraction becomes lengthening (eccentric)
-Muscle contraction speed (shortening velocity) depends on the rate of cross bridge cycling, which depends on the myosin heavy chain isoform. Shortening results in changes in the I band but not the A band.
-When we don’t have a load, you can perform your fastest contraction, however once the load starts increasing, there can be a contraction of the muscle but there may not be a movement (isometric contraction)
-Negative velocity exerts less speed.
-Type 1 – slowest shortening velocity, type 2a – medium/moderate length of velocity, type 2b – high level of velocity
Explain the frequency-tension relationship:
-A single twitch = a single reaction
-Numerous twitches will summate and lead to a larger reaction over time
-Stimuli are separated by more time than total contraction time. All three tensions are similar (green)
-A stimulus (S3) is introduced halfway during the contraction time causing the muscle tension to rise further (orange)
-Two stimuli are introduced very close together resulting in a higher tension with a smooth curve (purple). However if they are 50m/s apart, then there will be one big twitch seen, not singular twitches.
Explain the frequency-tension relationship
-Unfused and fused tetanus (use graph from 23/10)
-When successive stimulations result in a sustained contraction, the contraction is called tetanus
1. Unfused tetanus -> The muscle fibre has time to partially relax before the next stimulation. The development of tension oscillates. Few or little stimuli means that the body processes rigid and unsmooth movements
2. Fused tetanus -> The muscle fibre has no time to relax between stimulations. The development of tension is continuous and smooth. More stimuli added, means that a more smooth movement can take place
Describe the length-tension relationship:
-Long muscle length (short) -> cannot shorten muscle much due to the extreme overlap of actin and myosin over each other. A force cannot be produced from this
-This differs from the resting/optimal length of the muscle at 90 degrees, where the muscle can exert maximal force (due to the length, which has an optimal length of actin and myosin overlapping one another)
-Actin and myosin filaments are so far away from each other when the muscle length is too long and requires too much effort and energy to produce a contraction in the muscle. This means that the muscle is too long to perform a contraction as there are not enough formed cross-bridges and overlap between the myosin and actin filaments.
Explain and describe energy metabolism and fatigue:
-Fatigue is not associated with ATP depletion;
1. Conduction failure -> Caused by potassium accumulation in the T-tubules Fast recovery (prevent fast recovery)
2. Lactic acid buildup -> Acidic environment in muscle affects the physiological functioning of proteins and the mechanisms involved calcium release and re-uptake (create acidic environment and creates fatigue)
3. Inhibition of cross-bridge cycling -> Accumulation of ADP and Pi in muscle fibres slows down the cross-bridge cycling by preventing the release of cross-bridges from actin molecules (slow down cross-bridge mechanism)
4. Fuel substrates -> Muscle glycogen, blood glucose, dehydration
5. Central command fatigue -> Failure to propagate signals from the brain to the motor neurons (lack of communication decreases force in the muscle)
-Fatigue combines both neural and motor mechanisms/neurons
Describe the further characteristics of muscle fibre:
I -> Slow-twitch, slow-oxidative fibres
-Muscle-fibre type, force and fatigue -> Most muscles have a mixed composition. Referred to as fast (II) or slow (I) twitch. Fast twitch equally sub-divided into type 2a and type 2b.
-On average 45-55% of type 1 fibres are in arms and leg muscles
-No gender differences in fibre distribution
-Large intra-individual variation
-Trend in distribution consistent across muscle groups
-Muscle fatigue -> Muscle fibres are categorised based on how fast they contract and the metabolic pathways that they use to utilise to produce ATP. Use stimuli and identify how fast the twitches are, which can measure the fatigue in muscles.
-Muscle biopsy includes; CSA, mitochondrial content around the muscle fibres
-Techniques to determine fibre types -> colour of fibre (1900), EMG identification of motor units (1950), fibre speed and oxidative capacity (1970), genomic nomenclature (2000), muscle biopsy in humans
