Sliding Filament Theory

Question 1

SFT Step 1

Answer 1

Electrical simtulus (axial terminal)

An action potential signal arrives at the axon terminal of the presynaptic neuron, simultaneously reaching many neuromuscular junctions. Then, the action potential causes voltage-gated calcium ion channels on the presynaptic knob to open, driving an influx of calcium ions (Ca2+).

Question 2

SFT Step 2

Answer 2

Voltage calcium channel

The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine (ACh) into the synaptic cleft. Acetylcholine is a neurotransmitter that tells the muscle to contract. ACh diffuses across the synaptic cleft and binds to ACh receptors on the muscle fibre, resulting in depolarisation (more negative charge) of the sarcolemma (cell membrane of the muscle cell).

Question 3

SFT Step 3

Answer 3

Triggers vesicles to do exocytosis

The action potential then spreads along the T tubules made by the sarcolemma. These T tubules connect to the sarcoplasmic reticulum. Calcium channels on the sarcoplasmic reticulum open in response to the action potential they receive, resulting in the influx of calcium ions (Ca2+) into the sarcoplasm.

Question 4

SFT Step 4

Answer 4

Ach neurotransmitter released (synaptic cleft)

Calcium ions bind to troponin C, causing a conformational change that leads to the movement of tropomyosin away from actin-binding sites.

Question 5

SFT Step 5

Answer 5

Ach receptor (ligand-gated channel)

High-energy ADP-myosin molecules can now interact with actin filaments and form cross-bridges. The energy is released in a power stroke, pulling actin towards the M line. Also, ADP and the phosphate ion dissociate from the myosin head.

Question 6

SFT Step 6

Answer 6

Sodium goes into the muscle fiber (local depolarization)
- flips more negative outside and more positive inside
- ligand-gated channel
- Ach E

As new ATP binds to the myosin head, the cross-bridge between myosin and actin is broken. Myosin head hydrolyses ATP to ADP and phosphate ion. The energy released returns the myosin head to its original position.

Question 7

SFT Step 10

Answer 7

In response to the decrease in calcium ion concentration within the sarcoplasm, tropomyosin moves and blocks the actin-binding sites. This response prevents any further cross bridges from forming between actin and myosin filaments, resulting in muscle relaxation.

Question 7

SFT Step 7

Answer 7

Myosin head hydrolyses ATP to ADP and phosphate ion. The energy released returns the myosin head to its original position. Steps 4 to 7 are repeated as long as calcium ions are present in the sarcoplasm (Figure 4).

Question 8

SFT Step 9

Answer 8

As the nerve impulse stops, calcium ions pump back into the sarcoplasmic reticulum using the energy from ATP.

Question 9

SFT Step 8

Answer 9

Continued pulling of actin filaments towards the M line causes the sarcomeres to shorten.

Question 10

SFT requires

Answer 10

Calcium and ATP

Question 11

SFT key components

Answer 11

Tropomyosin (Longer, like the word)
Troponin (Shorter, like the word)