**Question:** Twitch of a single motor unit is called

A. Basic action potential
B. Motor unit action potential
C. Motor unit discharge
D. Motor unit potential

**Core Concept:** In neuromuscular physiology, a motor unit consists of a single motor neuron and the muscle fibers it innervates. The contraction of a single motor unit is characterized by a specific electrical event.

**Why the Correct Answer is Right:** A twitch of a single motor unit is referred to as a "motor unit action potential" (MUAP). A motor unit action potential is the summation of individual motor neuron action potentials that occur due to the interaction between the motor neuron and the muscle fiber. This summation results in a larger and more complex waveform compared to a single motor neuron action potential.

**Why Each Wrong Option is Incorrect:**

A. "Basic action potential" is incorrect as it refers to the basic electrical event generated by a single motor neuron, not the overall event of a single motor unit contraction.
B. "Motor unit discharge" is a broader term that encompasses both the action potentials and other electrical events within a motor unit, which makes it less specific than "motor unit action potential."
C. "Motor unit potential" is a similar term to "motor unit action potential" but is not commonly used, making it less familiar to examinees.

**Clinical Pearl / High-Yield Fact:** Understanding motor unit action potentials is crucial for understanding muscle strength and fatigue. A decrease in motor unit action potential duration or amplitude indicates muscle weakness, while a decrease in motor unit number indicates muscle fatigue. This knowledge aids in clinical examination and diagnosis of neuromuscular disorders.

**Correct Answer:** D. Motor unit action potential