Question

The steps of muscle contraction, starting with the initial action potential in a motor neuron, are as follows: 1. action potential in motor neuron 2. acetylcholine release 3. acetylcholine binds to receptors 4. muscle cell depolarizes to threshold 5. action potential spreads down t-tubules 6. release of calcium from sarcoplasmic reticulum?

Answer 1

Muscle contraction begins with an action potential in a motor neuron that releases acetylcholine at the neuromuscular junction, leading to muscle fiber depolarization.

Step-by-step explanation:

The Steps of Muscle Contraction

Muscle contraction is a complex process that is initiated when an action potential travels along the axon of a motor neuron to the neuromuscular junction (NMJ). At the NMJ, the neurotransmitter acetylcholine (ACh) is released from the axon terminal and binds to ACh receptors on the muscle fiber's plasma membrane, or sarcolemma.

This binding causes the sarcolemma to depolarize, generating an action potential that spreads across the membrane and travels down the T-tubules deep into the muscle fiber.

The spreading action potential triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR), the calcium storage site within the muscle fiber. The influx of Ca2+ into the sarcoplasm interacts with the troponin-tropomyosin complex on the actin filaments, causing the tropomyosin to shift and expose the myosin binding sites on the actin.

Myosin heads then form cross-bridges with these sites, and the myosin heads pivot, pulling the actin filaments toward the center of the sarcomere in a 'power stroke'. This process requires ATP and leads to the shortening of the sarcomere, resulting in muscle contraction.