Multiple Choice
In the context of monitoring autonomic nervous system activity, what is one appropriate action to take if a flat line appears on an electrocardiograph (ECG)?
14. The Autonomic Nervous System
Introduction to the Autonomic Nervous System
Multiple Choice
Which statement best describes phase 4 of the action potential in sinoatrial (SA) node cells?
A
It is the rapid repolarization phase caused by potassium efflux.
B
It is the initial rapid depolarization due to sodium influx.
C
It is the spontaneous depolarization phase that determines the heart's pacemaker activity.
D
It is the plateau phase maintained by calcium influx.
Verified step by step guidance1
Step 1: Understand the phases of the action potential in sinoatrial (SA) node cells. The SA node is the heart's natural pacemaker, and its action potential is unique compared to other cardiac cells. It consists of three main phases: phase 4 (spontaneous depolarization), phase 0 (rapid depolarization), and phase 3 (repolarization).
Step 2: Focus on phase 4 of the SA node action potential. This phase is characterized by spontaneous depolarization, which is crucial for the pacemaker activity of the heart. During this phase, the membrane potential gradually becomes less negative due to the activity of specific ion channels.
Step 3: Identify the key ion channels involved in phase 4. The 'funny' current (If), carried by sodium and potassium ions, plays a major role in initiating the depolarization. Additionally, transient calcium channels (T-type Ca²⁺ channels) contribute to the gradual depolarization as the membrane potential approaches the threshold.
Step 4: Compare the descriptions provided in the problem. The correct description of phase 4 is 'It is the spontaneous depolarization phase that determines the heart's pacemaker activity,' as this phase is responsible for generating rhythmic electrical impulses that regulate the heartbeat.
Step 5: Eliminate the incorrect options. The other descriptions refer to phases of action potentials in different types of cardiac cells or phases other than phase 4 in SA node cells. For example, rapid repolarization due to potassium efflux occurs in phase 3, and the plateau phase maintained by calcium influx is characteristic of ventricular myocytes, not SA node cells.
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