The Nervous Systems's Role in Muscular Activity

In order for a voluntary muscular contraction to occur, a series of electrical 'command' signals must first be generated in the brain. These signals, which are also known as action potentials, then pass down the spinal cord to the motor nerves and eventually to the appropriate muscle.

Motor nerves contain bundles of individual fibers known as motor neurons. Each motor neuron branches a number of times and each branch attaches to an individual muscle fiber approximately halfway along its length at a point known as the neuromuscular junction. When an action potential reaches the end of the motor nerve it causes the release, from the nerve ending, of a chemical transmitter substance known as acetylcholine. Acetylcholine, which diffuses across the small gap between the nerve ending and the outer membrane of the muscle fiber, then triggers the initiation of another action potential within the muscle fiber itself. The muscular action potential then passes along the muscle fiber in both directions and enters the T-tubules which lie adjacent to each sarcomere's sarcoplasmic reticulum. In response to these events the sarcoplasmic reticulum releases calcium into the fluid which bathes the myofibrils, thus initiating cross-bridge cycling and tension development. Once the series of action potentials stops the calcium is pumped back into the sarcoplasmic reticulum and the muscle relaxes.

A fundamental characteristic of action potentials is that once they are successfully initiated they always travel the whole way along a nerve or muscle fiber. This characteristic is known to physiologists as the 'all or none' law and is further evidence that no one section of a muscle fiber can develop tension while other sections are relaxed.

Summary of the contraction process
1. Action potentials generated in the brain travel down the spinal cord and along the motor nerve to muscle.
2. Acetylcholine is released from the nerve ending which results in the initiation of action potentials within the muscle fiber.
3. The muscular action potentials pass along the fiber and into the transverse tubules.
4. The passage of the action potentials into the t-tubule stimulates the release of calcium from the sarcoplasmic reticulum.
5. Calcium interacts with the acting filament and cross-bridge cycling is initiated. The muscle develops tension as long as the series of action potentials continues to arrive.
6. As soon as the series of action potentials stop's the calcium is drawn back into the sarcoplasmic reticulum and the muscle relaxes.