Chapter 19 - Section 19.4 - Electrical and Contractile Activity of the Heart - Apply What You Know-2 - Page 724: 1

The falling phase of a myocardial (cardiac muscle cell) action potential differs from that of a neuron's action potential in terms of the ions involved and the mechanisms underlying repolarization. Here's a comparison of the two: **Myocardial Action Potential:** 1. **Ion Movements:** In a myocardial action potential, the falling phase, or repolarization, is primarily driven by the efflux of **potassium ions (K+)** out of the cell. 2. **Repolarization Mechanism:** During the rising phase of the myocardial action potential, voltage-gated sodium channels open, allowing sodium ions to enter the cell and initiate depolarization. However, unlike in neurons where inactivation of sodium channels is a key factor for repolarization, in myocardial cells, the **inactivation of calcium channels** plays a significant role. Calcium influx during depolarization triggers the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction. As repolarization occurs, these calcium channels close, and the efflux of potassium ions takes place, bringing the membrane potential back to its resting level. **Neuron Action Potential:** 1. **Ion Movements:** In a neuron's action potential, the falling phase, or repolarization, is primarily due to the efflux of **potassium ions (K+)** out of the cell, similar to myocardial cells. 2. **Repolarization Mechanism:** During the rising phase of the neuron's action potential, sodium channels open, allowing sodium ions to rush into the cell and cause depolarization. Inactivation of these sodium channels and activation of **voltage-gated potassium channels** are key factors in repolarization. As the cell depolarizes, potassium channels open more slowly, and the efflux of potassium ions leads to the restoration of the resting membrane potential. In both cases, the repolarization phase involves the movement of potassium ions out of the cell, contributing to the re-establishment of the resting membrane potential. However, the details of the mechanisms and the involvement of other ions (such as calcium) set the myocardial and neuron action potentials apart. It's important to note that while there are similarities in the general process of repolarization, the specific ion channels and dynamics can vary between different types of neurons and different regions of the heart.

The falling phase of a myocardial (cardiac muscle cell) action potential differs from that of a neuron's action potential in terms of the ions involved and the mechanisms underlying repolarization. Here's a comparison of the two: **Myocardial Action Potential:** 1. **Ion Movements:** In a myocardial action potential, the falling phase, or repolarization, is primarily driven by the efflux of **potassium ions (K+)** out of the cell. 2. **Repolarization Mechanism:** During the rising phase of the myocardial action potential, voltage-gated sodium channels open, allowing sodium ions to enter the cell and initiate depolarization. However, unlike in neurons where inactivation of sodium channels is a key factor for repolarization, in myocardial cells, the **inactivation of calcium channels** plays a significant role. Calcium influx during depolarization triggers the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction. As repolarization occurs, these calcium channels close, and the efflux of potassium ions takes place, bringing the membrane potential back to its resting level. **Neuron Action Potential:** 1. **Ion Movements:** In a neuron's action potential, the falling phase, or repolarization, is primarily due to the efflux of **potassium ions (K+)** out of the cell, similar to myocardial cells. 2. **Repolarization Mechanism:** During the rising phase of the neuron's action potential, sodium channels open, allowing sodium ions to rush into the cell and cause depolarization. Inactivation of these sodium channels and activation of **voltage-gated potassium channels** are key factors in repolarization. As the cell depolarizes, potassium channels open more slowly, and the efflux of potassium ions leads to the restoration of the resting membrane potential. In both cases, the repolarization phase involves the movement of potassium ions out of the cell, contributing to the re-establishment of the resting membrane potential. However, the details of the mechanisms and the involvement of other ions (such as calcium) set the myocardial and neuron action potentials apart. It's important to note that while there are similarities in the general process of repolarization, the specific ion channels and dynamics can vary between different types of neurons and different regions of the heart.