The all-or-none law of skeletal muscle contraction states that an individual muscle fiber contracts with equal force in response to each action potential. However, if this is true, how can muscles contract with different strengths? It would seem, according to the law we've just heard, that this is impossible. But there are other factors involved in the strength of a muscles contraction. In this paper, we'll discover these hidden factors and see, first, how they apply to your muscle tone at different times and, second, how they affect diseases like tetanus that involve the muscles.
First, we'll try to understand what the all-or-none law of skeletal muscle contraction is saying. The all-or-none law basically states that, when an action potentials travel down a single motor unit, all of the muscle fibers in the motor unit will contract with exactly the same strength for each action potential they receive.
But how does this help explain the fact that muscles can contract with different strengths? Muscles contract with different strengths in part because every motor unit does not always receive an action potential. And even if all the motor units happen to receive action potentials at the same time, they do not always receive the same number per unit. But the main reason why a muscle is able to contract with different strengths is simply because it is comprised of several motor units. As you know, each motor unit controls its own part of the muscle and is composed of one axon of a motor neuron which branches to communicate with a number of muscles fibers. The motor neuron carries action potentials, a kind of electrical message, to the muscle fibers. But, in order to carry action potentials, the neuron must first receive a stimulus. It is the strength of this stimulus that determines the frequency with which action potentials will be transmitted to the muscle fibers. And it is the frequency of the action potentials which determines how strongly the muscle contracts. All action potentials are the same size; it is by measuring their frequency that the strength of the stimulus can be determined.
The strengths of stimuli have been classified into five levels. The first, a subthreshold stimulus, refers to "a stimulus too small to create even one action potential in a neuron". The second level, a threshold stimulus, is defined as "a stimulus strong enough to create one action potential in a neuron". The third stimulus strength, the submaximal stimuli, speaks of "stimuli of increasing strength that create more action potentials along more neurons". Maximal stimulus, the fourth level, is "a stimulus which is strong enough to create action potentials in all the motor neurons innervating a whole muscle". Finally, the supramaximal stimulus refers to any stimulus more than maximal stimulus, producing no further effect.
Now that we hopefully understand how the all-or-none principle works, let's see how this knowledge can help us learn more about muscle tone. Muscle tone is defined as "the state of partial contraction in a muscle, even when the muscle is not being used." Muscle tone is the reason why you stay seated even when asleep, instead of slithering off the chair to land in a puddle on the floor. According to the all-or-none principle, it's impossible for one muscle fiber to partially contract, but it is possible, however, for only some of the motor units in a muscle to contract at once. Muscle tone, then, is the muscles' constant state of partial contraction. So, basically, it is because the all-or-none principle exists that muscle tone exists.
Tetanus is a disease whose symptoms are closely related to muscle tone and stimulus strength. The toxin that causes tetanus functions by suppressing the muscles' inhibiting influences on the motor neurons, causing the muscles, especially those of the neck and jaw, to contract continuously and violently. The muscles are receiving constant stimuli and the contractions build until they reach a single, sustained contraction, at which time the person is said to have tetanus. How is this related to muscle tone? Well, as we've seen, muscle tone is the constant state of partial contraction in a muscle. Tetanus is a constant state of complete contraction in a muscle, so, essentially, tetanus is just a very large increase in muscle tone.
The all-or-none principle governs how all muscles work. By mastering this principle, anyone can gain a better knowledge of how muscles work, and consequently, how their amazingly crafted body functions.