Sliding Filament Model

Question 1

Define sliding filament model and describe what happens to the different sections of a sarcomere during muscle contraction?

Answer 1

Sliding filament model – movement of actin and myosin in relation to each other to cause contraction

• Light band is narrower
• Z-lines come closer
• H-Zone becomes narrower
• Dark band is same width

Question 2

What is the structure of myosin?

Answer 2

• Myosin filaments have hinged globular heads that allow them to move back and forth
• On each head, is a binding site for actin and ATP
• The tails of 100s of myosin molecules are aligned to make a myosin filament

Question 3

What is the structure of actin and how does its structure change during muscle contraction?

Answer 3

• Has binding sites for myosin heads – called actin-myosin binding site
• Binding sites blocked by a protein called tropomyosin which is held in place by the protein troponin

When the muscle is in resting state (relaxed) the actin-myosin is blocked by tropomyosin (myosin heads cannot bind) so the filaments cannot slide past one another

Once a muscle is stimulated to contract…

1. Myosin heads form bonds with actin filaments – actin-myosin cross bridges
2. Myosin heads flex in unison to pull the actin filament along the myosin filament
3. Using ATP, the myosin head detaches from the actin and the head returns to original position
4. Myosin reattaches further along the filament and repeats 1.-3.

Question 4

Define:

neuromuscular junction

motor unit

Answer 4

Neuromuscular junction: Point where a motor neurone and a skeletal muscle fibre meet

Motor unit: All muscle fibres that are supplied by a single neuromuscular junction

Question 5

Why are there many neuromuscular junctions as opposed to one?

Answer 5

• Ensures a muscle fibre contracts simultaneously
• If one neuromuscular junction existed, the muscle fibres wouldn’t contract together, it would be slow as a wave on contraction would need to be transmitted

Question 6

What occurs at a neuromuscular junction?

Answer 6

1. Action potential reaches NM junction, stimulates calcium ion channel openings
   
   1. Ca²⁺ diffuses from synapse to synaptic knob causing the synaptic vesicles to fuse with the presynaptic membrane
2. ACh released into synaptic cleft by exocytosis and diffuse across synapse
3. Binds to receptors on sarcolemma (post-synaptic membrane)
   
   1. Opens Na⁺ channels causing depolarisation
4. Ach broken by acetylcholinesterase into choline and ethanoic acid
   
   1. Prevents over-stimulation of muscle
5. Choline and ethanoic acid diffuse back to presynaptic neurone to make Ach using energy provided by mitochondria

Question 7

How do the muscle fibres receive the depolarisation from the motor neurone?

Answer 7

• Sarcolemma depolarisation travels deep into the muscle fibres due to T-tubules
  
  • T-tubules are in contact with the sarcoplasmic reticulum – stimulates Ca²⁺ from the SR (SR absorbs calcium from sarcoplasm) to be released

Question 8

What occurs in the sarcoplasm to stimulate muscle contraction?

Answer 8

1. Action potential reaches SR, stimulates calcium ion channels to open to they diffuse down their conc. gradient flooding the sarcoplasm
2. Calcium ions bind to troponin causing a conformational change pulling the tropomyosin from the actin-myosin binding sites so an actin-myosin cross-bridge can form
3. Myosin head flexes, pulling actin filament along
   
   • ADP is released from the myosin head, an ATP molecule can bind
4. Causing the myosin head to detach from the actin filament
5. Calcium ions in the sarcoplasm activate ATPase activity of myosin
   
   • Hydrolyses ATP to ADP + Pi
   • Releasing energy to unflex the myosin head
6. Myosin head can bind to another actin-myosin binding site and cycle is repeated

Question 9

What are the main 3 ways ATP is generated for muscle contraction?

Answer 9

Aerobic Respiration

• Most ATP for muscle contraction is regenerated from ADP during oxidative phosphorylation

Anaerobic Respiration

• Oxygen used more quickly than replaced in a very active muscle, so ATP must be generated anaerobically
• ATP made in glycolysis, but the pyruvate becomes lactate which builds up in the muscle causing fatigue

Creatine Phosphate

• It is stored in the muscle and produces ATP
• To form ATP, ADP is phosphorylated, creatine phosphate is a reserve supply of phosphate
• It is used to form ATP quickly, but the phosphate store is used up quickly, it replenishes when a muscle is contracted taking the phosphate from ATP