24 Questions
What occurs during the absolute refractory period?
The excitability of the heart is lost
What is the characteristic of the membrane potential of a rhythmic cell?
It is unstable with no resting membrane potential
What is the discharge rate of the SAN?
90 beats per minute
What causes the depolarization of the rhythmic cell to the firing level of -40 mV?
The opening of transient Ca channels
What is the peak of the action potential in a rhythmic cell?
+10 mV
What determines the heart rate in a rhythmic cell?
The slope of the pre-potential
What occurs during the relative refractory period?
The excitability of the heart is weak
What is the characteristic of the membrane of a rhythmic cell?
It is more permeable to Na and Ca
What is the term for the initiation of regular impulses independent of nerves?
Rhythmicity
What is the term for the ability of the cardiac muscle to generate an action potential?
Excitability
What is the primary ion responsible for the rapid depolarization of the cardiac muscle cell during phase 0 of the action potential?
Sodium
What is the term for the process by which the action potential leads to muscle contraction?
Excitation-Contraction Coupling
During which phase of the action potential does the membrane potential rapidly repolarize to the resting membrane potential?
Phase 3
What is the duration of the plateau phase of the action potential?
200 msec
During which phase of the action potential does the mechanical response of the cardiac muscle coincide with the period of depolarization?
Systole
What is the resting membrane potential of the cardiac muscle cell?
-90 mV
What is the effect of sympathetic stimulation on heart rate?
Increase
What is the result of parasympathetic stimulation on heart rate?
Decrease
What is the rate of conduction in the Atrial pathway?
1 m/sec
What is the significance of the slow conduction at the AVN?
To prevent abnormal rhythm originated in atria to reach ventricles
What is the definition of Contractility?
The ability of the cardiac muscle to contract to pump blood
What is the role of gap junctions in the conduction of cardiac excitation waves?
To increase the conduction rate
What is the conduction rate of the Purkinje fibers?
4-5 m/sec
What is the significance of the rapid conduction at the Purkinje fibers?
To cover all ventricles in a very short time to contract as one unit to eject blood
Study Notes
Action Potential and Excitability Change
- Absolute Refractory Period: coincides with the period of depolarization till the end of the plateau, where excitability is lost.
- Relative Refractory Period: coincides with the period of rapid repolarization, where excitability is weak.
Rhythmicity
- Defined as the ability of the heart to beat regularly and initiate its own regular impulse independently of any nerve supply.
- Rhythmic cells characterized by:
- Discharge spontaneously.
- Membrane potential is unstable with no resting membrane potential (RMP).
- No plateau.
- Membrane is more permeable to Na and Ca.
- Firing level = -40 mV.
- Peak of AP = +10 mV.
- SAN (Sinoatrial Node) discharge rate: 90/min.
- AVN (Atrioventricular Node) discharge rate: 60/min.
- Purkinje discharge rate: 30/min.
Ionic Bases of Rhythmicity
- Membrane potential decreases spontaneously from -60 mV to -50 mV due to Na influx.
- At -50 mV, transient Ca channels open, and Ca enters the cell, causing depolarization to the firing level (-40 mV).
- At -40 mV, long-lasting Ca channels (L-type) open, and Ca enters the cell, causing depolarization and closure of transient Ca channels and Na channels till +10 mV.
- Repolarization phase occurs due to K outflow through K channels and closure of L-type Ca channels.
- At -60 mV, inactivation of K channels and K efflux stops.
Action Potential and Mechanical Change
- Systole: coincides with the period of depolarization till the end of the plateau.
- Diastole: coincides with the period of rapid repolarization phase and continues in the rest phase, because the mechanical response lasts one and a half times as long as the action potential.
- Increase in slope: increases heart rate (tachycardia).
- Decrease in slope: decreases heart rate (bradycardia).
Conductivity
- Defined as the ability of cardiac muscle to transmit cardiac excitation waves.
- Excitation wave generated in SAN, then passes through the intermodal pathway to the far part of atria.
- Excitation wave reaches AVN, where there is a delay in transmission at a rate of 0.05 m/sec.
- Once excitation wave enters the A-V bundle, it spreads rapidly down to the branches and Purkinje fibers at a rate of 4-5 m/sec.
- Cardiac impulse is then conducted through ventricular muscle itself at a rate of 1 m/sec.
Significance of Conduction Rates
- Rate of conduction in different parts:
- Atrial pathway: 1 m/sec.
- AV node: 0.05 m/sec.
- Bundle of His: 1 m/sec.
- Purkinje fibers: 4-5 m/sec.
- Ventricular muscle: 1 m/sec.
- Significance of slow conduction at AVN:
- Delays in conduction allow for both atria to be excited and contract to empty their blood into ventricles.
- Prevents abnormal rhythms originating in atria from reaching ventricles.
- Significance of fast conduction at Purkinje:
- Rapid conduction allows for all ventricles to contract as one unit to eject blood.
Contractility
- Defined as the ability of the cardiac muscle to contract to pump blood.
- Ca release from sarcoplasmic reticulum initiates contraction.
This quiz covers the relationship between action potential and excitability changes in the heart, including the absolute and relative refractory periods and rhythmicity.
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