Question

describe the role each of the following plays in skeletal muscle function

• troponin
• tropomyosin
• calcium ions (ca²⁺)
• acetylcholine (ach)
• acetylcholinesterase (ache)
• atp

number the following steps into the correct order (#1 has been done for you)
-ca²⁺ is released from the sarcoplasmic reticulum (sr)
-atp detaches the myosin heads from actin, breaking the cross bridge
-atp lifts myosin heads up to join with actin, forming a cross bridge
-neuron impulse arrives at the axon terminal
-troponin slides tropomyosin away, exposing the myosin binding sites
-myosin head pivots (power stroke), the muscle fiber contracts
-ca²⁺ binds with troponin
-the depolarization spreads to the t - tubules and then into the sarcoplasmic reticulum (sr)
-ach is released from the axon terminal into the synaptic cleft
-ach depolarizes (switches \on\) the sarcolemma

Explanation:

• Troponin: Binds calcium ions, causes tropomyosin shift to expose myosin - binding sites on actin.
• Tropomyosin: Covers myosin - binding sites on actin in relaxed muscle.
• Calcium ions ($Ca^{2 +}$): Bind to troponin, initiate muscle contraction process.
• Acetylcholine (ACh): Neurotransmitter that depolarizes sarcolemma to start muscle contraction.
• Acetylcholinesterase (AChE): Breaks down ACh to terminate muscle contraction signal.
• ATP: Provides energy for myosin - actin cross - bridge cycling and detachment.

For the step - ordering:

1. Neuron impulse arrives at the axon terminal
2. ACh is released from the axon terminal into the synaptic cleft
3. ACh depolarizes (switches "ON") the sarcolemma
4. The depolarization spreads to the T - tubules and then into the sarcoplasmic reticulum (SR)
5. $Ca^{2+}$ is released from the sarcoplasmic reticulum (SR)
6. $Ca^{2+}$ binds with troponin
7. Troponin slides tropomyosin away, exposing the myosin binding sites
8. ATP lifts myosin heads up to join with actin, forming a cross bridge
9. Myosin head pivots (power stroke), the muscle fiber contracts
10. ATP detaches the myosin heads from actin, breaking the cross bridge

Answer:

• Troponin: Binds $Ca^{2+}$, exposes myosin - binding sites.
• Tropomyosin: Covers myosin - binding sites in relaxed state.
• Calcium ions ($Ca^{2 +}$): Initiate contraction by binding troponin.
• Acetylcholine (ACh): Starts contraction by depolarizing sarcolemma.
• Acetylcholinesterase (AChE): Terminates ACh signal.
• ATP: Powers cross - bridge cycling.
• Step - ordering:

1. Neuron impulse arrives at the axon terminal
2. ACh is released from the axon terminal into the synaptic cleft
3. ACh depolarizes (switches "ON") the sarcolemma
4. The depolarization spreads to the T - tubules and then into the sarcoplasmic reticulum (SR)
5. $Ca^{2+}$ is released from the sarcoplasmic reticulum (SR)
6. $Ca^{2+}$ binds with troponin
7. Troponin slides tropomyosin away, exposing the myosin binding sites
8. ATP lifts myosin heads up to join with actin, forming a cross bridge
9. Myosin head pivots (power stroke), the muscle fiber contracts
10. ATP detaches the myosin heads from actin, breaking the cross bridge