• Introduction
  • The basics of the all-or-none law
  • How does the all-or-none law work?
  • History of the all-or-none law
  • Final thoughts
  • Sources
  • Action potential: a brief reversal of electric polarization of the membrane of a nerve cell or muscle cell.
  • Axon: a long projection of a nerve cell that sends signals away from the cell body.
  • Cell body: located in the spinal cord, the cell body maintains the neuron’s structure, carries genetic information, and provides energy to drive activities. The cell body also contains a nucleus.
  • Central nervous system: the nervous system consisting of the brain and spinal cord.
  • Cross-bridge: the attachment of myosin and actin within the muscle cell to produce a contraction.
  • Dendrite: branch-like extensions at the ends of the neuron responsible for receiving, processing, and transferring signals from other neurons to the cell body.
  • Motor unit: a motor neuron and all muscle fibers innervated by it.
  • Motor pool: a collection of motor units.
  • Rate coding: the frequency with which the muscle fibers are stimulated by their motor neuron.
  • Soma: cell body of a neuron.

Introduction

Muscle contraction starts from a motor neuron located in the brain. When stimulated, this motor neuron sends a signal to the muscle fibers (a long, tubular muscle cell), causing them to contract and create movement. This combination of muscle fibers and a single motor neuron is called a motor unit. The amount of muscle fibers innervated by the same motor neuron depends on the function of the muscle.

A skeletal muscle consists of hundreds or even thousands of muscle fibers. They are responsible for turning chemical energy into mechanical output – muscle movement. To conserve energy, not all muscle fibers are fired at once. By altering the firing rate and activating muscle fibers with different metabolic properties, the body is able to utilize energy more efficiently. This phenomenon is known as multiple motor unit summation, which also gives each muscle their ability to contract with varying levels of force.

There are two main ways how muscle fibers are activated; the size principle, and the all-or-none law. This post focuses on the latter, and explains why it is so significant for human movement.

The basics of the all-or-none law

The all-or-none law states that “the strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus”. If a stimulus exceeds a certain threshold, all the muscle fibers within the motor unit will contract simultaneously, and to the maximum possible extent. In simple terms, the motor unit will always give a maximal response or none at all. 

The strength exerted by a muscle is dependent on several factors, including how many motor units are recruited for a given movement, the rates at which they discharge their action potentials (rate coding), and the amount of contractable muscle mass available. The greater the strength required, the greater the number of motor units (and muscle fibers) are recruited.

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The All-Or-None Law

Describes how motor units are activated according to a stimulusEither provides a maximal response or none at allThe stimulus must exceed an activation threshold for the motor unit to fireAlso known as excitability

How does the all-or-none law work?

Neurons (nerve cells) are the basic building block of the nervous system. When these neurons send information throughout the body, a part of the transmission process involves an electrical impulse. This is known as an action potential. 

On a cellular level, an action potential describes a temporary shift (from negative to positive) in the neuron’s membrane potential as a result of ions flowing in and out of the neuron. This process, which takes place when neurons fire, allows a nerve cell to send an electrical signal down the axon (a long projection of a nerve cell that sends signals away from the cell body) toward other cells. This signals the muscles to contract in response.

The size of the action potential is always the same for any given neuron – there is no such thing as a “weak” or “strong” action potential. A neuron will either reach its threshold and provide a maximum response, or not respond at all. Interestingly, only neurons and muscle cells are capable of generating action potentials. This is more commonly known as excitability. 

History of the all-or-none law

The all-or-none law was first introduced by American physiologist Henry P. Bowditch in 1871. While studying the contraction properties of the heart, he discovered that “an induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.”

This was originally thought to be specific to cardiac and other specialized tissues. However, it was later established that nerves and muscle fibers also respond to stimuli according to the all-or-none law. 

Final thoughts 

The all-or-none law is considered one of the cornerstones of human biology. Together with the size principle, it explains how muscles are recruited by the nervous system in order to perform specific motor tasks.

Did you learn anything new about the all-or-none law? Let us know in the comments.

Sources

  • Douglas L. Smith; Basic Concepts in Physiology: II. Keith Lucas and the Nerve-Muscle Response. The American Biology Teacher 1 December 1963; 25 (8): 610–615. doi: https://doi.org/10.2307/4440465
  • Martini F, Nath JL. Anatomy & Physiology. Benjamin Cummings; 2010.
  • Pareti G. The "all-or-none" law in skeletal muscle and nerve fibres. Arch Ital Biol. 2007 Jan;145(1):39-54. PMID: 17274183.

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    All-Or-None Law Motor Unit