Update: During the past few weeks, I have discussed skeletal muscles. I explained how they contract and what effect the various fiber types have on performance.
Today, I would like to briefly discuss how the nervous system works, because skeletal muscles can't contract unless they receive a signal from the nervous system.Anatomically, the nervous system consists of the brain and spinal cord (the central nervous system), and all the nerves that carry information to and from various parts of the body (the peripheral nervous system). The basic unit of the nervous system is the neuron or nerve cell. It is this unit that allows information to be sent from one place to another in the body.
The neuron works because the cell membrane or outer covering of the cell is able to maintain a small electrical charge (about -75 millivolts). This charge is maintained by "pumps" that pump small sodium ions (a charged particle of potassium) into the cell. Because the cell is more permeable to potassium ions, there are more positive charges on the outside of the cell than on the inside.
When a signal needs to be sent, a chemical transmitter substance opens some sodium channels on the membrane, and the membrane "depolarizes" or changes polarity; and this depolarization sequence occurs all the way along the neuron until it reaches another neuron, organ or muscle that was to receive the signal. The membrane repolarizes immediately after the signal passes so that new signals can be generated as needed.
There are motor or movement areas in the brain that contain giant neurons responsible for signaling muscles to contract. For instance, if we want to move our hands to the keyboard, the neurons responsible for these movements depolarize and the signal is transmitted from the brain down the spinal cord (on the opposite side) to the level where the motor neurons to the arms are located. The signal is passed to smaller interneurons in the spinal cord which signal the motor neuron to tell the muscles to contract - and your arms move.
Signals also come back from receptors in muscles to tell the brain what they are doing. These signals are processed by a part of the brain called the cerebellum which compares what you told the muscles to do with what the muscles are really doing. This allows you to make very accurate movements so that you can accomplish the tasks of life.
The neurons that carry signals to muscles are called motor neurons. An individual motor neuron and all the muscles that it innervates is called a motor unit. Motor units of the hands and of the tongue, where you need extremely find control, have only a few muscle fibers per motor neuron; whereas motor units that go to muscles in the front of the legs may have thousands of muscle fibers per motor neuron because little fine control is needed.
Riding a bicycle at a moderate pace would involve only about half of the motor units of the cycling muscles and the muscle fibers in these units would be red or type I fibers that are very resistant to fatigue. If we decided to ride up a hill without changing the gears (to make it easier), we would add more motor units, and we would use more and more white, type IIa or IIb fibers to be able to do the added work. Because these white fibers fatigue more easily, we soon have to change gears or stop the activity.