The Brain's Involvement in Creating a Muscle Contraction: A Brief Overview of the Neuromuscular Process 

In the previous section on How Does a Muscle Contract? , you learned about the "binding and sliding" mechanism of the myofilaments(tiny little fibers). There was some mention of ATP and Calcium ions and how they assist in the whole contraction process...

But what we haven't discussed is the process that initiates the contraction...basically what happens before the binding and sliding occurs? Recall that skeletal muscle is voluntary, meaning that it needs to receive a signal from the brain to contract. In other words, it can be controlled consciously.

It all begins with the brain sending a nerve impulse, known as an action potential(AP), down a network of nerves that reaches the muscle fiber. Basically, a nerve is a large collection of neurons(nerve cells), which are specialized cells that carry nerve impulses(signals) to and from the brain.

Neuron(specialized nerve cell)

So the brain sends an action potential(impulse) that eventually reaches a motor neuron, a specific type of nerve cell that controls muscle fibers. Remember, nerves are made up of neurons(nerve cells), and a "motor neuron" is simply a special type of nerve cell in charge of a muscle fiber.

Now, a motor neuron doesn't actually touch the muscle fiber. There's a small gap between the motor neuron and the muscle fibers it controls. Keep in mind that a single motor neuron controls many fibers. This small gap between the neuron and fiber is called a synapse, or more appropriately the neuromuscular junction.

The neuromuscular junction(synapse), in other words, is the space between a motor neuron and the fibers its in charge of. So when an action potential(AP) reaches this junction, its message has to cross over and get into the fiber. In order to cross over, the message of the AP(action potential) is basically transmitted by a neurotransmitter, a chemical that bridges the gap between the neuron and the fiber, passing through the neuromuscular junction.

The neurotransmitter is created by the motor neuron and its main purpose is to convey the message of the nerve to the muscle fiber by crossing over the neuromuscular junction(synapse). The name of the specific neurotransmitter that is created in this case is acetylcholine. At this point, the entire process gets a bit complicated so here's a very simplified overview of what happens...

The acetylcholine binds to the muscle fiber and causes an increase in electrical activity measured by voltage. This electrical activity, or increase in voltage, creates another action potential from inside the muscle cell. This action potential inside the muscle cell travels rapidly through the sarcoplasm(membrane) and hits the fiber(myofilaments), causing calcium ions(Ca++) to flood the sarcoplasm(membrane).

Recall that the sarcoplasm is the membrane inside of a muscle fiber, surrounding the myofibrils and myofilaments. This flooding of calcium into the sarcoplasm is what allows the crossbridge cycle to occur, which is the process of thin & thick filaments binding and sliding to create a contraction.

As long as the sarcoplasm is flooded with calcium, the contraction continues and it allows you to generate continuous force. Of course, ATP is also needed to keep the contractions going, and this is why it is essential to have an adequate supply of ATP and calcium ions in your muscles.

The other thing to keep in mind is acetylcholine, the neurotransmitter(chemical) that crosses over into the muscle fiber to signal the whole process. For this reason, you also need sufficient amounts of the raw compounds that create acetylcholine.