Question
4. Explain how excitation of a muscle cell's membrane (sarcolemma) initiates a contraction of the muscle cell. Include the following key terms: action potential, -Tubules, sarcoplasmic reticulum (SR), Ca++ channels, Ca++ ions.
Answers
GPT-4o mini
Excitation of a muscle cell's membrane, known as the sarcolemma, initiates contraction through a well-coordinated series of events that involve the generation of an action potential and the subsequent release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). Here’s how this process works:
1. **Action Potential Generation**: The process begins when a motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction. This neurotransmitter binds to receptors on the sarcolemma, leading to depolarization of the membrane. If the depolarization reaches a certain threshold, it triggers an **action potential**.
2. **Propagation of Action Potential**: The action potential travels along the sarcolemma and then spreads into the muscle cell through invaginations in the membrane called **T-tubules** (or transverse tubules). This allows the action potential to penetrate deep into the muscle fibers, ensuring that the depolarization reaches all areas of the cell.
3. **Activation of the Sarcoplasmic Reticulum**: The arrival of the action potential at the T-tubules triggers the **sarcoplasmic reticulum (SR)**, a specialized type of smooth endoplasmic reticulum surrounding the myofibrils, to respond. The action potential causes the SR to open **Ca++ channels** located in its membrane.
4. **Release of Calcium Ions**: When the Ca++ channels in the SR open, Ca++ ions are released into the cytoplasm of the muscle cell. The increase in intracellular calcium concentration is critical for muscle contraction.
5. **Muscle Contraction**: The released Ca++ ions bind to the protein troponin, which causes a conformational change in another protein, tropomyosin, that normally blocks binding sites on the actin filaments. This allows myosin heads to attach to the exposed binding sites on actin, forming cross-bridges. The interaction between actin and myosin leads to the sliding filament mechanism of contraction, resulting in muscle shortening and tension.
In summary, the excitation of a muscle cell's membrane leads to the generation of an action potential that travels along the sarcolemma and through T-tubules, activating the sarcoplasmic reticulum to release Ca++ ions. This increase in Ca++ concentration ultimately triggers the contraction of the muscle cell through the interaction of actin and myosin filaments.
1. **Action Potential Generation**: The process begins when a motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction. This neurotransmitter binds to receptors on the sarcolemma, leading to depolarization of the membrane. If the depolarization reaches a certain threshold, it triggers an **action potential**.
2. **Propagation of Action Potential**: The action potential travels along the sarcolemma and then spreads into the muscle cell through invaginations in the membrane called **T-tubules** (or transverse tubules). This allows the action potential to penetrate deep into the muscle fibers, ensuring that the depolarization reaches all areas of the cell.
3. **Activation of the Sarcoplasmic Reticulum**: The arrival of the action potential at the T-tubules triggers the **sarcoplasmic reticulum (SR)**, a specialized type of smooth endoplasmic reticulum surrounding the myofibrils, to respond. The action potential causes the SR to open **Ca++ channels** located in its membrane.
4. **Release of Calcium Ions**: When the Ca++ channels in the SR open, Ca++ ions are released into the cytoplasm of the muscle cell. The increase in intracellular calcium concentration is critical for muscle contraction.
5. **Muscle Contraction**: The released Ca++ ions bind to the protein troponin, which causes a conformational change in another protein, tropomyosin, that normally blocks binding sites on the actin filaments. This allows myosin heads to attach to the exposed binding sites on actin, forming cross-bridges. The interaction between actin and myosin leads to the sliding filament mechanism of contraction, resulting in muscle shortening and tension.
In summary, the excitation of a muscle cell's membrane leads to the generation of an action potential that travels along the sarcolemma and through T-tubules, activating the sarcoplasmic reticulum to release Ca++ ions. This increase in Ca++ concentration ultimately triggers the contraction of the muscle cell through the interaction of actin and myosin filaments.