**Question:** Which of the following is NOT a physiologic characteristic of cardiac muscle?

A. Contraction occurs in a fixed sequence (ex: A-V-V-A)
B. Contraction is non-directional (can contract in any direction)
C. Contraction is dependent on calcium ions
D. Contraction is directly influenced by the autonomic nervous system

**Core Concept:** Cardiac muscle, also known as myocardium, is the muscle tissue found in the heart. It has unique physiologic characteristics that allow it to efficiently pump blood throughout the body.

**Why the Correct Answer is Right:** Cardiac muscle contraction occurs in a fixed sequence, ensuring a coordinated and synchronized pumping action. This fixed sequence is crucial for maintaining an efficient cardiac contraction. In contrast, option B states that cardiac muscle contraction is non-directional, which contradicts the specific contraction sequence that allows for unidirectional blood flow.

**Why Each Wrong Option is Incorrect:**

Option A (A-V-V-A) is correct, as the cardiac muscle's contraction follows a specific sequence to ensure efficient blood flow.

Option B (non-directional contraction) is incorrect because cardiac muscle contraction has a specific direction (unidirectional blood flow) and is not non-directional like the correct answer suggests.

Option C (contraction is dependent on calcium ions) is correct, as calcium ions play a vital role in the contraction process of cardiac muscle cells.

Option D (contraction is directly influenced by the autonomic nervous system) is incorrect, as cardiac muscle contraction is primarily regulated by the sinoatrial (SA) node, rather than the autonomic nervous system. The autonomic nervous system does influence cardiac muscle function, but its direct influence is not responsible for contraction initiation and regulation.

**Clinical Pearl / High-Yield Fact:** The fixed contraction sequence in cardiac muscle allows for a more efficient pumping action compared to a non-directional contraction. Understanding the role of calcium ions in cardiac muscle contraction is essential for understanding the mechanism and function of the heart. The autonomic nervous system does influence cardiac muscle function, but its direct influence is not responsible for contraction initiation and regulation. Cardiac muscle contraction is primarily regulated by the sinoatrial (SA) node.

**Correct Answer:** B (non-directional contraction) is NOT a physiologic characteristic of cardiac muscle.