All-or-none Principle (Law)
All-or-none principle: A nerve or muscle cell's response to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential (threshold of an action potential ), the nerve or muscle cell responds completely; otherwise, there is no response. That is, a nerve cell will depolarize completely and propagate a signal the entire length of an axon or not all. Similarly, a muscle cell that is stimulated will depolarize along the entire sarcolemma and stimulate all sarcomere within that muscle cell.
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Examples:
- You Cannot Target Half a Muscle: You cannot contract half the length of a muscle fiber; the entire muscle fiber must contract. Muscle fibers generally extend from one end of a muscle to the other (from origin to insertion). This fact should have dispelled myths about targeting your "distal biceps brachii," "medial pectoralis major ," or "lower gluteus maximus ," etc; however, these myths persist. Note that it is possible to target different motor units when they contribute to different ranges of motion. For example, the sternal head of the pectoralis major exhibits more activity than the clavicular head when performing incline pushing motions and/or shoulder flexion. However, the muscle cells within the sternal head will contract completely across their entire length.
- Motor Unit Recruitment: When a motor unit in the biceps brachii is stimulated, it contracts the same way, whether lifting a 10 lb or 50 lb dumbbell. The difference in force output depends on the number, type, and size of muscle fibers involved, which are organized into motor units. Each motor unit consists of a motor nerve and all the muscle fibers it innervates (which are of a similar type and size). Once the motor nerve reaches its threshold potential, the resulting action potential activates all the muscle fibers within that motor unit.