Muscle mechanics

Question 1

Discuss Isometric & Isotonic Contraction.

Answer 1

1. Isometric contractions: tension is developed but the overall length of the muscle does not change.
   - Isometric = same length.
2. Isotonic contractions: tension increases until it equals the weight to be lifted.
   - Then the muscle shortens and the tension stays constant.
   - isotonic = same tension.

Question 2

Discuss isolated contraction.

Answer 2

Question 3

What is the length retention relationship?

Answer 3

• When a muscle contracts isometrically, the tension developed depends on the initial length of the muscle before it contracted.
• By varying the stretch of the muscle beforehand we can construct a length-tension curve.
• Initial length is determined by the preload with the stop removed
• Afterload is the load is only encountered by the muscle when it contracts.
• It is the extra load placed on the lever after the stop has been replaced.
• A large afterload ensures that the muscle will contract isometrically.

Question 4

What is the Huxley sliding filament model?

Answer 4

• The overlap between the thin and thick filaments determines the number of cross-bridges that can form.
• The optimal length for a muscle is the pre-stretch length at which tension generation is maximal.
• At rest, skeletal muscle is usually at (or near) optimum length.

Question 5

What is the basis of the length tension relationship?

Answer 5

• At low initial stretch, areas of the thin
  filament are unavailable for cross-bridge
  formation: z-discs also close together and minimal room for shortening.
• As preload is increased, more binding sites on the actin become available: so a stronger contraction is possible.
• As stretching continues beyond the optimal stretch, overlap between
  the filaments begins to decrease
• Fewer cross-bridges means less force and eventually overlap will cease
  altogether.

Question 6

What is the Starling Law of the heart?

Answer 6

• The stroke volume of the left ventricle will increase as the left ventricular volume increases due to the myocyte stretch causing a more forceful systolic contraction.

Question 7

How does the Starling Law of the heart explain length tension relationship?

Answer 7

• Based on the link between the initial length of myocardial fibres and the force generated by contraction.
• There is a predictable relationship between the length between sarcomeres and the tension of the muscle fibres.
• May explain decrease in diaphragm function following hyperinflation in conditions like COPD.
• This is important to avoid airway collapse but decreases the force able to be generated.
  *NB: error in diagram. Switch 1 and 2.

Question 8

Discuss summation.

Answer 8

• In nerves, individual stimuli are unlikely to evoke an action potential.
• Requires temporal or spatial summation of EPSPs.
• In a muscle on the other hand, each stimulation will produce a twitch.
  -The size and duration of the twitch depends on the frequency of stimulation.
• If the frequency is sufficiently high, summation of the twitches will occur.

Question 9

Discuss summation with respect to tetanus.

Answer 9

• The muscle cell twitch can have a duration of a few hundred milliseconds and is never refractory.
• The muscle can be activated to contract many times, even before it has had time to relax.
• At high frequencies of activation, there is a build up of tension, this is summation.
• When it reaches a maximum level of tension is called complete Tetanus.

Question 10

Discuss the frequency of muscle twitches.

Answer 10

• Occur at low frequencies
• There is time between each stimulation
  for the muscle to achieve full relaxation.
• As such there is no overall increase in
  tension.

Question 11

How does relaxation prevent tetanus?

Answer 11

• Increasing the frequencies further, tension summates to maximum tension.
• Despite the increased frequency increase
  there is time for the muscle to start to
  relax between stimulation.
• This relaxation prevents complete
  tetanus.

Question 12

What happens during maximum tension?

Answer 12

• Increasing the frequencies further, tension summates to maximum tension.
• Here there is no time for relaxation so muscle develops steady tension (complete tetanus).
• Here almost all cross bridges are formed.
• Over time contraction will fade due to
  fatigue

Question 13

Summary of muscle stimulation.

Answer 13

Question 14

Discuss the staircase phenomenon (Treppe).

Answer 14

• Repeat stimulation at frequencies below that which cause summation produce Treppe.
• With each twitch there is a progressive increase in tension until a maximum twitch is reached.

Question 15

What is tetanus?

Answer 15

• Caused by the bacterium Clostridium tetani.
• Toxin blocks inhibitory synapses in the CNS: GABA and glycine neurons.
• Neurons act as a brake on motor neurons so their inhibition leads to increased activity of the motor nerves to skeletal muscle.
• This gives the characteristic, uncontrollable skeletal muscle spasms: often starts at the jaw giving the other name for the condition: lockjaw.

Question 15

What is tetanus?

Answer 15

• Caused by the bacterium Clostridium tetani.
• Toxin blocks inhibitory synapses in the CNS: GABA and glycine neurons.
• Neurons act as a brake on motor neurons so their inhibition leads to increased activity of the motor nerves to skeletal muscle.
• This gives the characteristic, uncontrollable skeletal muscle spasms: often starts at the jaw giving the other name for the condition: lockjaw.