**Question:** A twitch of motor unit is called

A. Faciocerebral reflex
B. Autonomic reflex
C. Limbic reflex
D. Spinal reflex

**Correct Answer:** D. Spinal reflex

**Core Concept:** A twitch of a motor unit is a brief, involuntary contraction of a single motor neuron and its muscle fibers. This occurs as a result of a stimulus or input reaching the motor neuron cell body in the spinal cord or the anterior horn cells.

**Why the Correct Answer is Right:** A spinal reflex is a reflex arc involving the spinal cord, which consists of a sensory receptor stimulus, the corresponding sensory neuron, and the motor neuron. When the stimulus reaches the spinal cord, it triggers an action potential in the sensory neuron and the motor neuron, leading to a rapid contraction of the muscle involved. In this case, the twitch of a motor unit represents the contraction of a single motor neuron and its muscle fibers.

**Why Each Wrong Option is Incorrect:**

A. Faciocerebral reflex (A) is a reflex involving the brain and face muscles, which is not related to a twitch of a motor unit.

B. Autonomic reflex (B) is a reflex involving the autonomic nervous system, regulating involuntary functions like heart rate, blood pressure, and digestion, not related to a twitch of a motor unit.

C. Limbic reflex (C) is a reflex related to limbic system functions like learning, memory, and emotion, not relevant to a twitch of a motor unit.

**Clinical Pearl:** Spinal reflexes, like the twitch of a motor unit, are essential components of the nervous system involved in reflex arc activation and reflex responses. Understanding these concepts helps in assessing reflexes in clinical practice, particularly during neurological examinations, to evaluate the integrity of the spinal cord and peripheral nerves.