Topic 20 - Macroscopic events of muscle contraction

Question 1

Words to include

Answer 1

• Contractile components
  
  • Sarcomere
• Serial elastic components
• Parallel elastic components
• Twitch
  
  • Contraction-relaxation cycle
  • Virtual latency
• Isometric contraction
• Isotonic contraction
• Mixed contraction
  
  • Auxotonic contraction
  • Preload
  • Afterload
• All-or-none law
• AP frequency
  
  • Cross-bridge cycle
  • Ca²⁺
• Quantal summation
• Contraction (frequency) summation
• Staircase effect / treppe
  
  • Warming up
  • IC Ca²⁺
• Tetanus
  
  • Maximal contraction state

Question 2

Components

Answer 2

• Contractile components
• Serial elastic components
• Parallel elastic components

1. During stimulation, serial elastic components will reach equlibrium with load because of the contraction of contractile components
2. This is followed by constant tension

Question 3

Graph

Answer 3

Question 4

Define muscle twitch

Answer 4

• Muscle twitch: a singel contraction-relaxation cycle
• An appropriate stimulus results in a contraction: muscle twitch occurs

Question 5

Define virtual latency

Answer 5

The sum of two latencies

Question 6

Name the different types of contraction

Answer 6

1. Isometric contraction
2. Isotonic contraction
3. Mixed contraction
   
   • Auxotonic contraction
   • Preload
   • Afterload

Question 7

Define isometric contraction

Answer 7

• Only tension is changed, but not the length of the muscle
• Occurs when muscle tries to lift such heavy load which it is not able to move

Question 8

Define isotonic contraction

Answer 8

• Muscle shortens with constant tension
• Regular physological activity

Question 9

Define mixed contraction

Answer 9

• Combination of isometric and isotonic contraction
• Can further be divided into:
  1. Auxotonic contraction
  2. Preload
  3. Afterload

Question 10

Mixed contraction

Define auxotonic contraction

Answer 10

• In natural conditions, muscle shortens and in the same time, tension is also increasing in it
• Example: when a muscle works against a spring

Question 11

Mixed contraction

Define preload

Answer 11

1. After stimulation contractile machinery will first stretch serial elastic components elements (isometric period)
2. When tension reaches equilibrium with the load, contraction becomes isotonic, twitch is continued with shortening of the muscle

• Most of the contraction in association with locomotion

Question 12

Mixed contraction

Define afterload

Answer 12

• If we block the free movement of the muscle with a frame, then no more shortening is possible from a certain level, but muscle is still able to increase tension
• At the beginning it is isotonic, then an isometric period follows

Question 13

Summation

Answer 13

• Addition of skeletal muscle contraction forms caused by different reasons
  
  • Will increase the contractile capacity of individual fibers and/or recruit more fibers into the contraction

1. All-or-none law
2. AP frequency
3. Quantal summation
4. Contraction (frequency) summation
5. Staircaise effect / treppe (warming up)​
6. Tetanus

Question 14

Summation

All-or-none law

Answer 14

A single fiber under constant metabolic conditions contracts according to the “all-or-none” law = to an adequate stimulus, response is maximal, to smaller stimulus there is no response

Question 15

Summation

AP frequency

Answer 15

1. Can increase
2. Prolonged Ca²⁺ release
3. More cross-bridge cycles
4. Stronger contraction

Question 16

Summation

Quantal summation

Answer 16

• If the increase of tension is caused by the participation of more and more fibers (addition of elementary units)
• If the demand is higher, a more frequent AP recruits more and more fibers
• Repetitive stimulus in a muscle (not in an induvidual fiber) with recruitment of more fibers: spatial, quantal summation occurs

Question 17

Summation

Contraction (frequency) summation

Answer 17

• Repetitive stimuli may cause increasing contraction, for the previous calcium transient may not be completed when a new stimulus elicits additional calcium release.
  
  • The amplitude of contraction ↑

Question 18

Summation

Staircase effect / treppe

Answer 18

• Warming up
• New stimuli applied shortly after the end of a twitch may elicit new contractions with gradually increasing amplitudes
  
  • Caused by IC calcium, which has no time to be removed in between stimuli

Question 19

Summation

Tetanus

Answer 19

1. Stimuli with increasing frequency we enhance possible summation modes
2. Muscle reaches maximal contraction state
3. Tetanus is formed