Lecture 8 Flashcards
Describe conductive system
-SA Node -AV Node - Slow conduction is caused mainly by diminished numbers of gap junctions along pathway resulting in an increase in the resistance to conduction.
Describe SA node
• Composed of special cardiac muscle fibers. • SA fibers connect directly to atrial fibers.
Describe AV node
- Receives signal from SA node .03 sec. after origin.
- Signal is delayed in the AV node for .09 sec.
- Due to small size of cells, low amplitude of action potential, and slow rate of depolarization during excitation.
- A final delay of .04 sec. occurs in the penetrating bundles.
- Therefore, there is a .16 sec delay from the initial origin of the signal until onset of ventricular contraction. Slow conduction is caused mainly by diminished numbers of gap junctions along pathway resulting in an increase in the resistance to conduction.
Draw organization of AV node
Draw cardiac impulse through heart
Describe membrane potentials of SA node
- Resting membrane potential of SA node fiber:
- -55 to -60 mV (Threshold ≈ -40 mv)
- Fast sodium channels are already inactivated (blocked).
- Inactivation gates close when membrane potential is less negative than -55 mV.
Describe gates of SA node
Only slow sodium-calcium channels can open.
- Therefore, atrial nodal action potential is slower to develop.
- Therefore, repolarization is also slower.
- There is a slow leak of sodium ions back into the cells.
- Membrane potential becomes more positive.
- At -40 mV, sodium-calcium channels become activated.
- Sodium-calcium channels are inactivated within 100-150 msec after opening.
- Large numbers of potassium channels open at the time the sodium calcium channels become inactivated. • Nodal cells become repolarized.
- Potassium channels remain open for a few tenths of a second.
What is sinus rhythm
Action potentials originating in the SA node
What is an ectopic focus
Action potentials originating anywhere other than SA node
Ventricular fiber resting potential
• -85 to -90 mV
Describe vagus nerve
- Distributed mainly to the SA and AV nodes
- Neurotransmitter = acetylcholine
- Involves muscarinic receptors
- Decreases rate of rhythm of SA node (negative chronotropic effect)
- Decreases excitability of the AV junctional fibers, slowing transmission of the cardiac impulse into the ventricles.
- Increases permeability of fiber membranes to potassium ions
- Hyperpolarization: -65 to -70 mV rather than -55 to -60 mV
Describe sympathetic innervation
- Distributed to all parts of the heart, mainly the ventricles
- Neurotransmitter = norepinephrine
- Stimulates beta-1 adrenergic receptors
- Increases depolarization rate (positive chronotropic effect)
- May increase permeability of fiber membranes to sodium and calcium ions.
Describe characteristics of an ECG
• An ECG measures extracellular potential; it is not the same as the potential recorded from an axon when recording the transmembrane potential. •
Deflection from 0 occurs only when there is current flow between regions of the heart (when there is variation in the membrane potential in different regions of the heart). Current flows between regions of different membrane potentials.
• Because the atrial musculature and ventricular musculature are electrically isolated, current flow does not occur when only the atria and the ventricles have different potentials.
The P-Q (P-R) time interval
0.16 seconds
The Q-T time interval
0.35 seconds
What does the P wave represent
The P wave represents phase 0 of the AP’s spreading through the atrial muscle
