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Good Morning Dr. Schuster and Classmates,
Welcome to A & P Week 3! This week I’ll be teaching us about the All or None Law for our Nerves and Muscles. Physiologist Henry Pickering Bowditch is accredited with first describing this process in explaining how the heart contracts, in 1871. Cherry, 2019, states that “the all-or-none law is a principle that states that the strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus.” A muscle fiber or nerve will fire, if a certain threshold is met by the stimulus, usually around -55mV. The action potential happens, and a neuron transmits data to an axon, toward the synapse, when a stimulus has enough strength. “The action potential is always a full response. (Cherry, 2019).” It is an all-or-nothing process; no weak or strong action potential. It is both irreversible and nondecremental. Irreversible in that once a neuron reaches its threshold, there is no turning back, it cannot be stopped; and nondecremental, meaning they do not get weaker with distance. Saladin, 2017 states that, “the last action potential at the end of a nerve fiber is just as strong as the first one in the trigger zone, no matter how far away.” The action potential process allows the least amount of information loss along the way.
How does our body determine if a stimulus has enough power? Our nervous system depends on the rate at which a neuron fires and how many neurons fire. Neurons that fire fast and in quicker succession are stronger, and vice versa. An example of which is how fast we pull our hand away from hot water. If it is too hot but not scalding, we have a fast response, but if the water is scalding, we will respond faster, to remove it from further harm or damage.
Let me break it down in a different way. Let’s compare it to firing a gun. If you simply touch the trigger, the gun will not shoot. But if you apply either a small amount of pressure or give it a firm tug, the gun will shoot. The bullet will travel at the same speed, regardless of how hard you pulled the trigger; if you applied enough pressure. Same is true with the all or none principle.
Cherry, K. (2019). https://www.verywellmind.com/what-is-the-all-or-none-law-2794808
Saladin, K. (2017). Anatomy & Physiology, The Unity of Form and Function. New York: McGraw Hill Education