Cell Membrane and Cardiac Excitability Quiz

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Questions and Answers

What membrane potential range corresponds to the low threshold type of fast Na channel?

-89 to -80 (correct)

-79 to -70

-70 to -65

-80 to -75

During phase 1 of the action potential in a pacemaker cell, what primarily happens?

Na channels open and cause depolarization

Na channels begin to close (correct)

Minimum opening of Ca channels

K channels are closing

What occurs during the plateau phase of the pacemaker action potential?

There is a significant K efflux

Rapid Na channels close completely

Maximum Na influx occurs

Calcium channels open while K channels are active (correct)

What is the primary ion associated with the initial depolarization in phase 0 of the action potential?

Sodium (Na) Signup and view all the answers

What is the main effect on the membrane potential during phase 2 of the pacemaker cell action potential?

Plateau with no significant change Signup and view all the answers

What is the resting membrane potential for cardiac muscles and ventricles?

-90 Signup and view all the answers

At what membrane potential is the threshold reached?

-65 Signup and view all the answers

What change occurs to the Na gates during Phase 0?

2.5 Signup and view all the answers

Which state corresponds to 'no excitability' in relation to gate activation?

2 Signup and view all the answers

Which condition indicates that a cell membrane is being stimulated?

-65 Signup and view all the answers

In which state are the H gates open and the M gates closed?

1 Signup and view all the answers

What effect does a change in cell membrane permeability have?

1.5 Signup and view all the answers

Which phase is characterized by the opening of Na channels?

0 Signup and view all the answers

What occurs if both the SA node and AV node are damaged?

The Purkinje fibers take charge at a much slower rate. Signup and view all the answers

Why are the atrial and ventricular muscles electrically separated?

To prevent backward conduction of impulses from ventricles to atria. Signup and view all the answers

What is the typical heart rate when the AV node takes over as the pacemaker?

40-60 beats per minute. Signup and view all the answers

What makes the SA node the normal pacemaker of the heart?

It generates the highest rate of spontaneously generated action potentials. Signup and view all the answers

What would happen if the AV bundle is damaged?

The heart would experience complete cardiac arrest. Signup and view all the answers

What is the inherent risk of a heart rate falling to 15 beats per minute?

It cannot sustain life effectively. Signup and view all the answers

What role does the fibrous barrier between atrial and ventricular muscle play?

It serves as an insulator preventing non-forward conduction. Signup and view all the answers

What happens to the action potential generation if the AV node is functioning?

It generates action potentials at a rate of 40-60 beats per minute. Signup and view all the answers

What occurs when Na channels activate rapidly in cardiac muscle?

Tetanic contraction Signup and view all the answers

What is the consequence of failure in ventricular relaxation?

Interference with cardiovascular function Signup and view all the answers

During phase 3 repolarization, which state do Na channels transition to first?

Closed state Signup and view all the answers

What must occur for another action potential to be generated during the relative refractory period?

A larger than normal depolarization Signup and view all the answers

What defines the state of Na+ channels during the inactivated period?

Channels cannot be reopened immediately Signup and view all the answers

What is the relative refractory period characterized by?

Altered threshold for action potential generation Signup and view all the answers

How does the inactivation of Na+ channels affect the cardiovascular system?

It causes failure in ventricular relaxation Signup and view all the answers

What happens when some Na+ channels close during the transition back to resting potential?

Channels with higher threshold become excitable Signup and view all the answers

What is the primary effect of sympathetic stimulation on the cardiac rhythm?

Increases conduction velocity through the heart Signup and view all the answers

Which adrenergic receptors are primarily stimulated by sympathetic nerve endings?

Beta-1 receptors Signup and view all the answers

What is the effect of maximal sympathetic stimulation on heart contraction strength?

It doubles the contraction strength. Signup and view all the answers

Which of the following correctly describes the role of the vagus nerve in heart rhythm?

It primarily innervates the SA and AV nodes. Signup and view all the answers

How does sympathetic stimulation specifically affect heart rate?

It can triple the heartbeat frequency. Signup and view all the answers

What term describes the effect of sympathetic stimulation that increases heart rate?

Chronotropic effect Signup and view all the answers

Which statement accurately reflects the distribution of the parasympathetic nerves?

They are mainly distributed to the SA and AV nodes. Signup and view all the answers

What is the primary neurotransmitter released by sympathetic nerve endings affecting heart function?

Norepinephrine Signup and view all the answers

Study Notes

Cell Membrane and Excitability

Changes in cell membrane permeability affect ion movement, thereby altering membrane voltage.
Resting membrane potential in cardiac muscles is -90 mV, indicating the muscle is not stimulated.
Threshold voltage for action potential initiation is -65 mV; reaching this decreases negativity in the inner cell environment.

Sodium Channel Activation and Inactivation

Sodium (Na) channels activate rapidly within 0.1 msec and enter an inactivated state after 1-2 msec, preventing action potential (AP) generation.
Tetany in cardiac muscle can lead to failure in ventricular relaxation, disrupting heart function and blood circulation.
During phase 3 (repolarization), Na channels that initially close have a higher threshold, followed by lower threshold channels, leading to the relative refractory period.

Fast Na Channels

Types of fast Na channels based on threshold:
- Low threshold opens from -89 to -80 mV.
- Moderate threshold opens from -79 to -70 mV.
- High threshold opens from -69 to -65 mV.

Phases of Action Potential

Phase 0 (Depolarization): Na channels open, causing a rapid influx of Na ions and a decrease in membrane potential.
Phase 1 (Rapid Repolarization): Na channels close while K channels open, resulting in increasing K ion efflux.
Phase 2 (Plateau): Calcium channels open, maintaining Ca influx as K channels simultaneously allow minimal K efflux.

Cardiac Conduction System

If the SA node is damaged, the AV node can assume pacing with a heart rate of 40-60 beats per minute.
Damage to both SA and AV nodes leads to Purkinje fibers pacing at a much slower rate of 15-40 beats per minute, insufficient for life.
SA node is the primary pacemaker due to its highest rate of spontaneously generated action potentials.

Sympathetic Stimulation Effects

Sympathetic stimulation increases heart rate and force of contraction through norepinephrine release at sympathetic nerve endings.
Activation of beta-1 adrenergic receptors increases conduction velocity and enhances overall cardiac activity.
Maximal sympathetic stimulation can triple heart rate and double contraction strength.

Nerve Contributions to Cardiac Function

Parasympathetic nerves (mainly the vagus nerve) predominantly influence SA and AV nodes, with minimal effect on ventricular muscle.
Sympathetic nerves are widely distributed throughout the heart and strongly influence ventricular muscle function.
Cardiac responses are categorized:
- Chronotropic: Increase in heart rate.
- Inotropic: Increase in force of contraction.
- Dromotropic: Increase in conduction velocity.

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Description

Test your knowledge on cell membrane dynamics and excitability, specifically within cardiac muscles. This quiz covers resting membrane potential, action potentials, and the role of sodium channels in cardiac function. Understand the implications of ion movement and channel activation in heart physiology.