lecture 7 Cardiac Muscle and Electrical Conduction Quiz

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Questions and Answers

What primarily causes the rapid depolarization during Phase 2 of the action potential in cardiac muscle cells?

Inward movement of Cl-

Outward movement of K+

Influx of Na+ (correct)

Influx of Ca++

Which phase of the cardiac action potential features an influx of Ca++ into the cell?

Phase 3

Phase 4 (correct)

Phase 1

Phase 5

What mechanism describes the process where Ca++ entry induces further Ca++ release from the sarcoplasmic reticulum?

Calcium influx facilitation

Calcium reabsorption pathway

Calcium-induced calcium release (correct)

Calcium-exchange mechanism

What is the primary result of the long refractory period in cardiac muscle compared to skeletal muscle?

Prevention of rapid repeated contractions Signup and view all the answers

During which phase of cardiac action potential does the outward movement of K+ occur?

Phase 5 Signup and view all the answers

What initiates the action potential in the SA node under normal conditions?

Leakage of Na+ inside the cell Signup and view all the answers

How do sympathetic fibers affect the pacemaker potential of the heart?

They speed up the time required to reach the threshold. Signup and view all the answers

What is a significant difference between action potentials in cardiac muscle and skeletal muscle?

Cardiac muscle has a longer refractory period than skeletal muscle. Signup and view all the answers

What contributes to the heart's ability to function as a 'functional syncytium'?

Gap junctions allowing electrical activity propagation Signup and view all the answers

What is the consequence of the 'all or none rule' in cardiac muscle contraction?

Contraction occurs or does not occur at all. Signup and view all the answers

What is the primary role of the sino-atrial (SA) node in the heart?

To determine the heart contraction rhythm Signup and view all the answers

Which type of heart cell is responsible for the majority of cardiac function?

Contractile cells Signup and view all the answers

What is the function of gap junctions in cardiac muscle?

To transmit electrical activity between cells Signup and view all the answers

What allows for the coordinated contraction of the heart chambers?

The electrical conduction system's organized timing Signup and view all the answers

Which structure slows down the electrical signal before it reaches the ventricles?

Atrio-ventricular (AV) node Signup and view all the answers

How does the autonomic nervous system influence cardiac function?

It alters heart rate and strength of contraction Signup and view all the answers

What is the clinical significance of the electrocardiogram (ECG)?

It determines heart that rhythm abnormalities Signup and view all the answers

What is the process that follows the self-excitation of the sino-atrial (SA) node?

Propagation of action potentials through the atria Signup and view all the answers

What is the effect of the sympathetic nervous system on the heart?

Stimulates heart rate and increases contraction force Signup and view all the answers

Which neurotransmitter is involved in the actions of the parasympathetic nervous system on heart function?

Acetylcholine Signup and view all the answers

What is the primary purpose of an electrocardiogram (ECG)?

To measure heart membrane potential during the cardiac cycle Signup and view all the answers

What does the P wave in a standard ECG represent?

Depolarization of the atria Signup and view all the answers

Which condition is characterized by a slower heart rate?

Bradycardia Signup and view all the answers

What occurs when there is an abnormally long P-Q interval on an ECG?

AV conduction problem Signup and view all the answers

What effect does the sympathetic nervous system have on Ca2+ transport in cardiac cells?

Increases the speed of Ca2+ transport Signup and view all the answers

Which of the following statements is true regarding the effects of acetylcholine on autorhythmic cells?

It causes hyperpolarization, increasing the time to reach AP threshold Signup and view all the answers

Study Notes

Cardiac Muscle Composition

Myocardium is composed of two types of muscle cells
- Contractile cells: 99% of total cells, responsible for contractions and require action potentials
- Autorhythmic cells: modified non-contractile cells, concentrated in parts of the heart, capable of spontaneous action potential generations, act as pacemakers

Gap Junctions

Adjacent cells are connected by water-filled pores forming open connections called gap junctions
Allow ions to freely move between cells, facilitating the electrical conduction system

Electrical Conduction System

Initiated by pacemaker cells (autorhythmic)
Sino-atrial (SA) node acts as the command center, determining heart contractions, with rhythmic self-excitation
Atrio-ventricular (AV) node possesses autorhythmic capability but at a slower pace, therefore controlled by the SA node, serves as the electrical gateway between atria and ventricles
Bundle of His and Purkinje fibers facilitate swift electrical activity propagation from the AV node to the rest of the ventricles

Significance of the Electrical Activity

The electrical system contributes to
- Maintaining proper heart rate
- Coordinating the contractions of the atria and ventricles
- Synchronizing individual chamber contractions
Electrocardiogram (ECG) can be used to determine heart rhythm
Conduction problems can lead to abnormal rhythm, also known as arrhythmia

Sequence of Excitation

The action potential sequence is as follows:
- SA node generates its own action potential
- Action potential spreads through atria, triggering atrial contraction
- AV node receives the action potential, transmitting it to the bundle of His and Purkinje fibers with a slight delay (allows ventricles to fill with blood and AV valves to close)
- Action potential reaches ventricles, inducing ventricular contraction

Generation of Action Potential in the SA Node

Under normal circumstance:
- SA node cells gradually depolarize due to sodium ion influx and reduced potassium ion diffusion
- Once the threshold is reached, an action potential is generated
- The cycle repeats itself, creating a pacemaker potential
- Controlled by the autonomic nervous system
Sympathetic fibers: reduce the time required to reach the threshold, resulting in faster pace
Parasympathetic fibers: extend the time required to reach the threshold, leading to slower pace

Cardiac Muscle Contraction

Autorhythmic cells action potentials propagate to contractile cells
Contractile cell action potentials cause muscular contractions due to gap junctions
The heart functions as a "functional syncytium" due to gap junctions, resulting in an all-or-none contraction rule

Action Potential of Contractile Cells

Distinguishable from autorhythmic cell action potentials by a stable resting membrane potential
Distinct from skeletal muscle action potentials in terms of
- Duration: skeletal muscle action potentials are short (milliseconds), cardiac muscle action potentials are long (hundreds of milliseconds)
- Refractory Period: Skeletal muscle's refractory period is shorter than cardiac muscle
The functional impact of these differences is the prevention of rapid repeated contractions in the heart, preventing a tetanic state.

Excitation - Contraction Coupling

Calcium-stimulated-calcium-release mechanism: - Depolarization occurs - Voltage-gated calcium channels open - Calcium ion entry triggers a surge in calcium release from the sarcoplasmic reticulum, resulting in significant intracellular calcium increase - The heightened intracellular calcium concentration triggers contraction - Calcium is pumped back to its pre-stimulation compartment

Control of Heart Contraction

Sympathetic nervous system:
- Stimulates heart rate by accelerating SA node firing and increasing AV node conduction velocity (shorter delay)
- Increases force of contraction by enhancing calcium release from sarcoplasmic stores
- Reduces contraction time through increased calcium transport speed (reducing plateau length)
- These actions are mediated by epinephrine/norepinephrine binding to beta-adrenergic receptors present in all cardiac cells
Parasympathetic nervous system (Vagus nerve):
- Decreases heart rate by slowing down SA node firing and lowering AV node conduction velocity (longer delay)
- Actions are mediated by acetylcholine binding to muscarinic receptors in autorhythmic cells
- Increases potassium permeability
- Hyperpolarization increases the time needed to reach the action potential threshold

Electrocardiogram (ECG)

ECG measures the collective heart membrane potential during the cardiac cycle
Multiple heart cells fire action potentials synchronously, generating a measurable electrical current
Electrodes placed on the skin detect the electric current wave progression
Standard ECG consists of three waves:
- P wave: atrial depolarization
- QRS wave: ventricular depolarization, appears larger due to bigger muscle mass
- T wave: ventricular repolarization

Use of ECG

Assess heart rate and rhythm:
- Heart rate: measured by intervals between cycles
- Contraction force: determined by wave amplitude
- Rhythm: measured by intervals between waves
Identification of abnormalities:
- Heart rate (under resting condition):
  - Bradycardia: slower rate
  - Tachycardia: faster rate
- Rhythm:
  - Abnormally long P-Q interval indicates an AV conduction problem
  - "Ectopic" beats (extrasystole): action potentials generated independently of the SA node, resulting in extra contractions

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Description

Test your knowledge on the composition of cardiac muscle, focusing on the roles of contractile and autorhythmic cells. Understand the importance of gap junctions and the electrical conduction system dominated by the SA and AV nodes. This quiz will help clarify how these components work together to maintain heart rhythm.