Making muscles contract: single-fibre and whole-muscle mechanism

Question 1

Definition of ‘tension’?

Answer 1

The force that a muscle exerts on the joint when it is contracting is called the tension of the muscle

Question 2

Definition of ‘load’?

Answer 2

The force that is exerted on a muscle by an object is called the load of the muscle

Question 3

What is the relationship between muscle tension and load?

Answer 3

Muscle tension must exceed the load in order for the muscle fibres to shorten and therefore move the object responsible for the load. If muscle tension doesn’t exceed the load, then the muscle will either remain at same length or lengthen.

Question 4

What is the ‘latent period’?

Answer 4

After the AP, there is a latent period (few Ms) before the tension in the muscle fibre begins to increase

Question 5

What is the ‘contraction time’?

Answer 5

The time interval from beginning of tension development (at the end of latent period) to the peak tension

Question 6

During a shortening contraction, what can an increasing load cause?

Answer 6

• Latent period to increase
• Velocity of shortening to slow down
• Total duration of twitch to become shorter
• Distance shortened to become less
  (See graph)

Question 7

Define ‘summation’

Answer 7

The increase of muscle tension from successive AP’s is called SUMMATION

Question 8

Define ‘tetanus’

Answer 8

When successive stimulations result in a sustained contraction, the contraction is called TETANUS

Question 9

Define ‘unfused tetanus’

Answer 9

The muscle fibre has time to partially relax before the next stimulation. The development of tension oscillates

Question 10

Define ‘fused tetanus’

Answer 10

The muscle fibre has no time to relax between stimulations. The development of tension is continuous and smooth

Question 11

How are muscle fibres categorised?

Answer 11

Muscle fibres are categorised based on how fast they contract and the metabolic pathways that they utilise to produce ATP.

Question 12

What are the mechanisms involved in muscle fatigue?

Note: Fatigue is NOT associated with ATP depletion

Answer 12

1) Conduction failure - caused by k+ accumulation in T-tubules
2) Lactic acid build up - acidic environment in muscle affects physiological functioning of proteins and mechanisms involved in calcium release and re-uptake
3) Inhibition of cross-bridge cycling - accumulation of ADP + Pi in muscle fibres slows down cross-bridge cycling by preventing the release of cross-bridges from actin molecules
4) Fuel substrate
5) Central command fatigue - Failure to propagate signals from the brain to the motor neurons