cardiac structure conducting and cycle part 3 Flashcards
what is auto-rhythmicity of the heart
the heart generates its own action potential
- What are the two types of cardiac muscle cells
- Contractile cells
- Autorhythmic cells
- What percentage of cardiac muscle cells are contractile and what percentage are autorhythmic
contractile - 99%
Auto-rhythmic cells - 1%
- Describe 2 features of contractile cells
- Do the mechanical work of pumping
- Do NOT generation their own action potential
what are contractile cells in the heart called
myocytes
- Describe a feature of autorhythmic cells
- initiated conduct AP
- What region of the conduction system beats at the highest rate
Sinoartial node
- Describe the 4 stages of spread of Action potential in the heart
- sinoatrial node beats at highest rate in wall of RA
- from SA electrical impulse spreads across the left atrium and AV node, depolarising it
- after nodal delay, the depolarisation spreads down the bundle of His and purkinje fibres of the heart to the ventricles
- the depolarisation causes contraction of the atria first, then ventricles
- What is the range of bpm of the SA node, where is it located and what order pacemaker is it.
Range: 90-120 bpm
Location: Right atrial wall
Order pacemaker: 1st order
- What is the range of bpm of the AV node, where is it located and what order pacemaker is it
Range: 40 – 60 bpm
location: at base of right atrium near septum
Order pacemaker: 2nd order pacemaker
- What is the range of bpm of the Bundle of His, where is it located and what order pacemaker is it
Range: 20-40 bpm
location: inter ventricular septum
order pacemaker: 3rd order pacemaker
- What is the range of bpm of the purkinje fibres, where is it located and what order pacemaker is it
Range: 20-30 bpm
location: small terminal fibres spread throughout the ventricular myocardium
order pacemaker: 4th order pacemaker
- What is the 100 millisecond delay of the electrical impulse to the AV node known as?
nodal delay
- Why is action potential being conducted slowly through the AV node an advantage to the conducting system
allows time for complete ventricular filling before contraction occurs
- What is the resting membrane potential of all cells (terms not numbers) and what causes this
- it is electronegative
- caused by different concentrations of various ions inside and outside the cell
- What leads to depolarisation
If the permeability of the membrane to Na+ and Ca2+ ions increases
- What leads to repolarisation
If the permeability of the membrane to K+ ions increases
- What does action potential cause in muscles
contraction
- Outline the phases of the conduction and contraction in atrial/ventricular myocytes using the Acronym SPCP as a foundation
Summit
Plummet
Continue
Plumet
- Why is there no phase 1 or 2 in SA or AV nodes?
because there is no contraction only rhythm
- What happens to phase 4 in SA and AV nodes
it gradually depolarises until threshold potential is reached and another AP is generated
- When does heart rate increase and what is this also known as?
when the slope of the pacemaker potential is made more steep
- AKA sympathetic activation
- When does heart rate decrease?
- when the slope of the pacemaker potential is made less steep and/ or the “resting” membrane potential is made more negative
- AKA PARAsympathetic activation
- What does the rate of discharge and shape of AP depend on?
varies depending upon which ions carry the current and the channels they use
Give 3 stages of an Action Potential
- Rapid depolarisation due to Na+ inflow when voltage gated fast Na+ channels open
- Plateau due to Ca2+ inflow when voltage gated slow Ca2+ channels open an some K+ channels close
- Repolarisation due to K+ outflow when voltage gated K+ channels open
- When does contraction of ventricular cells occur
when Ca2+ ions enter from the interstitial fluid
- When is ventricular contraction at its maximum, what portion of an ECG does this correspond to and abnormalities of what segment of an ECG will show a problem with contraction?
corresponds to the middle of the ST-segment of the ECG
where does the aVR lead go?
on the right arm
where does the aVL lead go?
On the left arm
where does the aVF lead go?
Left leg
- What does the P wave of an ECG relate to in the cardiac cycle?
Atrial depolarisation
- What 2 things does the QRS complex of an ECG relate to in the cardiac cycle
ventricular depolarisation
- atrial depolarisation
- What does the T wave relate to in the cardiac cycle?
ventricular repolarisation
- Outline the steps of ventricular action potential in relation to an ECG
- Fast depolarization coincides with the QRS complex due to inward sodium current.
- Plateau = ST segment.
Except for a small early repolarization (a brief transient outward current), during most of the plateau phase only slow voltage changes occur, resulting from the balancing of small inward (sodium and calcium) and outward (potassium) currents. - AP terminates with a fast repolarization, T-wave in the ECG primarily from outward potassium currents (I Kr and I Ks ).
- When is atrial systole (contraction) on an ECG and how long is it
- Middle of P wave - middle of QRS complex, on the “spike going downwards”
- 0.1 seconds
- When is ventricular systole on an ECG and how long is it
Middle of QRS complex to the decline of the T wave
- 0.3 seconds
- when is the relaxation period on an ECG and how long is it.
- Decline of T wave to middle of next P wave
- 0.4 seconds
