When you move, your brain sends signals to your skeletal muscle cells in the form of electrical impulses carried by nerve cells.

Here's a breakdown:

1. The Brain's Command: Your brain decides to move, generating an electrical signal.

2. Nerve Cells: The signal travels down a motor neuron, a specialized nerve cell that connects to muscle fibers.

3. Neuromuscular Junction: At the neuromuscular junction, the point where the motor neuron meets the muscle fiber, the signal triggers the release of a chemical messenger called acetylcholine.

4. Muscle Fiber Activation: Acetylcholine binds to receptors on the muscle fiber, causing a change in its electrical potential. This triggers a cascade of events within the muscle cell, ultimately leading to the sliding of protein filaments within the muscle fibers, causing them to contract.

5. Movement: The contraction of multiple muscle fibers working together generates the force needed for movement.

So, the electrical impulses sent by the brain, carried by nerve cells, and transmitted across the neuromuscular junction are what tell your muscles to move.