Structural Properties and Activation of Muscle

Question 1

Name as many functions of a muscle as you can.

Answer 1

Convert chemical energy (stored in ATP bonds) to mechanical work
Breathe, talk, support structures, movement, posture, brake, heat, dynamic metabolic store, protective padding

Question 2

What is the composition of muscle?

Answer 2

75% water
20% protein (60% myosin)
5% inorganic salts etc

Question 3

Describe the structure of a muscle

Answer 3

Many myofibrils (actin and myosin) join to form a muscle fibre, covered by the endomysium. Many muscle fibres are bundled into fasicles covered by perimysium. Many fasicles join to form a muscle covered in epimysium. 
Muscles join to bones via tendons.

Question 4

How is the structure of a muscle related to its activation?

Answer 4

NMJ - highly folded post synaptic membrane, close proximity
SR; source of Ca++
Sarcomere; functional unit of contraction

Question 5

How is the structure of the sarcomere related to contraction and its fibre integrity?

Answer 5

Titin; keeps myosin betweeen Z lines, controls # of myosin and elastic movement
Nebulin; controls # of actin monomers joined
Desmin; forms connections between adjacents sarcomeres of different myofibrils
Actinin; binds actin at z lines
Sarcoglycan complex; anchors filaments to sarcolemma and includes a # of proteins including dystrophin and laminin
C and M proteins;

Question 6

Ratio of tropomyosin to actin and troponin

Answer 6

1 tropomyosin for every 7 actin monomers

1 tropomyosin for every troponin

Question 7

How is muscle structure related to energy supply?

Answer 7

Many mitochondria; oxidative phosphorylation, aerobic

Muscle, CV and respiratory systems all work togethers

Question 8

Satellite cells?

Answer 8

Stem cells, activated in damage and proliferate and differentiate

Question 9

What is a motor unit?

Answer 9

A motor nerve and all the muscle fibres it innervates.

Question 10

Explain how depolarisation of the sarcolemma results in contraction.

Answer 10

DHP (voltage senstive) receptors respond to depolarisation
Ryandodine binds to RyR
Mechanical coupling of end foot protein - channel unplugged and ca++ can diffuse out of sarcolemma and bind to troponin (TNC component) which causes a conformation change and tropomyosin moves to reveal to actin binding site…X bridge cycling

Question 11

Cross bridge cycling?

Answer 11

AM - ATP binds, and is hydrolysed. Myosin ATPase - myosin binds in weak state to actin with the products of hydrolysis. Chnage from weak to strong state, power stroke, Pi is released - filament sliding. ADP released.

5-10nm per step, 5pN fprce uses 100pJ energy

Question 12

Twitch?

Answer 12

Mechanical response to a single electrical impulse.

Question 13

Tetanus?

Answer 13

Mechanical response to lots of stimuli - if increase frequency of stimuli - start to ass together = partially fused tetanus

Question 14

Why is there a force frequency relationship?

Answer 14

High rate of impulse = consistently high [Ca++] in cytoplasm - cycling can continue uninterrupted and allow the electrical elements to be stretched.

Question 15

Muscle relaxation?

Answer 15

Ca++ to SR requires ATP (CaATPase)
high [Pi] and [ADP] reverse the pump and slow relaxation
Takes 2-3x longer than release.