Choose the answer that places the following key steps of skeletal muscle contraction into their appropriate order 1. Acetylcholine binds receptor. 2. Acetylcholine is released from neuron. 3. Calcium ions released by sarcoplasmic reticulum. 4. Sarcolemma action potential is propagated. 5. Sodium enters the muscle cell.

1. Acetylcholine is released from neuron. 2. Acetylcholine binds receptor 3. Calcium ions released by sarcoplasmic reticulum. 4. Sarcolemma action potential is propagated. 5. Sodium enters the muscle cell.

Explanation:

A neural signal is the electrical trigger for calcium release from the sarcoplasmic reticulum into the sarcoplasm. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. Electrical signals called action potentials travel along the neuron's axon, which branches through the muscle, connecting to individual muscle fibers at a neuromuscular junction. The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the motor-end plate. The end of the neuron's axon is called the synaptic terminal; it does not actually contact the motor-end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor-end plate.

Because neuron axons do not directly contact the motor-end plate, communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses junctional folds: folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors. The receptors are sodium channels that open to allow the passage of Na + into the cell when they receive neurotransmitter signal.