Structural properties and activation of muscle

Question 1

What is a tetanus?

Answer 1

The mechanical response to multiple stimuli (lots of twitches)

Question 2

What happens to calcium during and after activation of muscles?

Answer 2

• The half removal of calcium takes ~80 ms (long time)

- At 10Hz, calcium conc builds up leading to partially fused tetanus

Question 3

What happens to force?

Answer 3

• Reaches plateau and max force
• A high rate of impulses results in consistently high levels of calcium in the cytoplasm
• This permits CB cycling to continue “uninterrupted”

Question 4

What is the importance of mitochondria?

Answer 4

• Power stations of cells
• Sites of ox. phos.
• Enzymes for fat metabolisation (ß-oxidation), the Krebs cycle, the e- transport chain are located inside the mitochondria
• Produces lots of ATP bc using O2 and e-

Question 5

What is the subsarcolemmal unit?

Answer 5

• Just under the sarcolemma

- Membrane of muscle fibres

Question 6

What is the intermyofibrillar unit?

Answer 6

• Within muscle fibres

- Sarcomere w/ mitocondira

Question 7

What modulates mitochondria content?

Answer 7

Exercise.
E.g. Mo Farah vs Usain Bolt
MF has more mitochondria bc contracting muscles for longer

Question 8

What is the motor unit?

Answer 8

• Composed of a single motor neurone + all the muscle fibres it innervates
• Number of muscle fibres in a MU varies

Question 9

What are gastronemius and extraocular muscles?

Answer 9

G = 2,000 muscle fibres per motor neurone (imp. to stand)
EM = < 10 muscle fibres per motor neurone (related to eye movement - bc need force adjustments)

= depends on function of muscle

Question 10

What are the two neural mechanisms responsible for neural recruitment?

Answer 10

1) Spatial recruitment

2) Temporal recruitment

Question 11

What does spatial recruitment involve?

Answer 11

• Recruit minimum number of MUs needed (size principle - Henneman principle)
• Smallest = type I recruited first
• Midsized = type IIa recruited second
• Largest = type IIx recruited last

E.g. lifting a pen vs lifting a dumbbell (dependent on force required)

Question 12

What does temporal recruitment involve?

Answer 12

• Rate coding: refers to the MU firing rate (every single MU w/ an AP that moves along the nerve + that spreads along the muscle fibres, can increase the amount of AP of each single MU e.g. FR, no. of impulses)
• Active MUs can discharge at higher frequencies to generate greater tensions
• Smaller muscles (ex: first dorsal interosseous) rely more on rate coding - which need fine adjustment
• Larger muscles of mixed fibre types (ex: deltoid) reply more on recruitment

Question 13

Muscle activation summary:

Answer 13

1) Motor axon propagation
2) NMJ Ach release causes membrane depolarization
3) Voltage sensitive proteins allow calcium ions to diffuse out the SR into cytoplasm
4) Calcium binds to troponin causing conformational change in tropomyosin
5) Myosin CBs bind to actin. Release of phosphate during ATP hydrolysis powers the working stroke

Question 14

What is a contraction?

Answer 14

Everything which occurs from the brain to the NMJ.

Brain > spinal cord > motor neuron

Question 15

What is the muscoskeletal system?

Answer 15

Muscle/bone/CT

Question 16

What is the composition of muscle?

Answer 16

• 75% water
• 20% protein (myosin 40%, actin 20%, tropomyosin, etc)
• 5% inorganic salts and other substances

Myosin and actin regulate muscle contraction