Cardiovasvular Physiology 2 Flashcards
Describe the conduction system of the heart
The SA node is the primary pacemaker of the heart, which means that it normally dominates the control of the heart
- Only if it fails one of the other components of the conduction system takes over (AV node). For this reason they are called auxiliary pacemakers.
- SA node pacemakers have the fastest action potential firing rate of any cardiac muscle cells. (70/min). AV node acts as a subsidiary pacemaker (40/min)
What is the pathway of the Cardiac pacemaker current?
- Sinus node
- Atrioventricular node
- Bundle of His
- Split into bundles, bundles of His then go down bundle branches
- Purkinje fibers
Give the sequence of depolarization of the cardiac muscle
- AP generation in the SA node
- Right/left atrium depolarization leads to contraction
- AV node depolarization (delay as ventricles are being filled!)
- Bundle of His (same as AV bundle)
- Septum depolarization
- Apes depolarization
- Atria repolarization begins —> atria relaxation(atria relaxed while ventricles contract)
Summarize what happens at different 0arts of the heart
- SA node depolarizes
- Electrical activity goes rapidly to AV node via internodal pathways
- Depolarization spreads more slowly across atria. Conduction slows through AV node
- Depolarization wave spreads upwards from apex
When does contractions occur?
AFTER depolarization
What is peacemaking?
Peacemaking is the function of some unique ion channels expressed in pacemaker cell and the electrical connectivity of the cardiac muscle via ‘gap junctions’
Why does the heart have automaticity?
Action potentials are generated automatically by pacemakers
Gap junctions give electrical connectivity through out the heart, causing rgphythmic contraction
What is the resting membrane potent of the pacemaker?
Don’t have a pacemaker potential
What are the electrophysiology properties of cardiac pacemakers?
(A) the pacemaker potential gradually becomes less negative until it reaches threshold, triggering an action potential
(B) ion movements during an action potential and pace maker
(C) States of various ion channels
What is the pace maker potential?
The pacemaker potential is vital electrical property that infers rhythmically
Explain pacemaker depolarization
- During pacemaker potential(it is negative) there is net sodium ions in and calcium channels or Ca ions as potential becomes more positive and approaches threshold.-slowing down potassium efflux (IF-channels)(phase 4)
- Calcium brings it to threshold
- IF channels close, calcium channels open at threshold and fast calcium channels open(steep portion of curve)(phase 0)
- At peak, potassium channels open and K+ efflux occurs and calcium ions channels close(phase 3)
The heart primes itself because of this mechanism
Describe the contribution of ionic channels to the pacemaker potenti
Pacemaker potential is the sum of increased inward currents and decreased outward current:
IF channels: Non selective cation current that depolarizes and is activated by hyperpolarization
ICa2+ channels: Increased inward Ca2+ current depolarizes
Ik+ channels: Decreased outward K+ current depolarizes
What is the resting cardiac muscle potential?
-90 mV
Summarize ‘Driven’ Cardiac Action Potential
- resting membrane potential
- Sodium ion channels open and membrane potential rises to +20 mV(straight line upwards)
- Ca+ channels open; fast K+ channels close (here it plateaus but then mV curves down)(the dip before is because of decreased potassium(phase 1)
- Ca+ channels close; slow K+ channels open
- resting membrane potential
What are the 5 phases of Cardiac action potentials ?
Phase 0- Na+ channels open
Phase 1- Na+ channels close
Phase 2- Ca2+ channels open; fast K+ channels close
Phase 3- Ca2+ channels close; slow K+ channels open
Phase 4- Resting potential
Describe refractory period of cardiac muscle
Cardiac muscle fiber: the refractory period lasts almost as long as the entire muscle twitch
-Long refractory period in a cardiac muscle prevents tetanus
No summation, no tetanus
It is a pump-A.P. Needs to be timed for maximal blood transport
Describe refractory period in skeletal muscles
Fast twitch fiber: the refractory period (yellow) 8s very short compared with the amount of time required for the development of tension
-skeletal muscles that are stimulated repeatedly will exhibit summation and tetanus
Describe heart action potential duration
Cardiac action potential has a long duration (250 msec) (Neural action potential 1-2 msec duration)
Refractory period is also long
Action potential is almost as long as the contraction it causes
This leads to summation of contractions is impossible in the heart
-Ensures that a second action potential cannot be generated until after the muscle has relaxed
Briefly describe the excitation skeletal muscle cell
Troponin holds tropomyosin in position to block myosin-binding sites on actin(Relaxation)
Ca2+ binds to troponin, which changes the shape of the troponin-tropomyosin complex and uncovers the myosin- binding sites on actin(Contraction)
Describe excitation contraction coupling of cardiac muscles
- Action potential enters from adjacent cell
- Voltage-gated Ca2+ channels open. Ca2+ enters cell
- Ca2+ induces Ca2+ release through reading receptor channels (RyR)
- Local release causes Ca2+ spark
- Summed Ca2+ sparks create a Ca2+ signal
- Ca2+ ions bind to troponin to initiate contraction
- Relaxation occurs when Ca2+ unbind from troponin
- Ca2+ is pumped back into the sarcoplasmic reticulum for storage
- Ca2+ is exchanged with Na+
- Na+ gradient is maintained by the Na+-K+-ATPase
What are the differences of cardiac and skeletal muscles?
Some differences:
- Action potent arrives from current generated by pacemakers
- Step 3: cardiac muscle has Ca2+ induced Ca2+ release
- Step 9: Cardiac muscle had additional Ca2+ removal via Na+/Ca+ exchanger
Cardiac myocyte contractions can be graded depending on Ca2+ influx from E.C.F. In skeletal muscles it is all or none response
What are the properties of cardiac muscles?
- Strength of contraction in a cardiac muscle is graded, in which the fiber can vary the force it can generate
- This ability is dependent on the amount of Ca2+ available -more calcium available, more bind8bg of actin-myosin, more forceful contraction
- Force generated depends on the number of active crossbridges
- In skeletal muscle it is all or none response
What is the difference between EKG and ECG?
Same thing son
What is a distant ECG?
- The ECG is a distant extracellular recording of the electrical activity of the heart
- The electrodes are so far apart that we can treat the heart as a ‘dipole’ (battery)
- As the heart goes through a cardiac cycle currents change direction. Therefore, the dipole rotates and turns on and off
- That is a simplistic way of thinking of why the ECG looks the way it does
How does and an EKG work?
Electrodes are attached to the skin surface
-A lead consists of two electrodes, one positive and negative
If you have a lead in the same direction of current, it will have max deflection
If lead and current then it will cancel out
An electro potential difference must exist(must have a negative and positive poor )must have a positive and negative pole and have a difference between them- current moves from negative to positive poles
What are the general points of ECGs?
- electrocardiogram or ECG is recording of the electrical activity of the heart
- It is a timely sequence of depolarizations and repolarizations in the heart. Because the atrium depolarizes before the ventricle the ECG shows the atrial signal before the ventricular signal
- The strength of the signal generated at any given time is determined by the number of muscle fibers depolarizing or repolarizing simultaneously at this moment.
- Because the ventricle is much bigger than the atrium, more muscle fibers de/repolarizes simultaneously, the signal it generates is stronger than the atrial signal
What does the strength of ECG depend on?
The strength of the signal recorded in an ECG depends on two factors:
- the strength of the signal generated by the atrium or ventricle.
- The method of recording the signal, e.g. the placement of the electrodes
ECGs show only electrical events , not mechanical events
-Under pathological conditions it is possible that a heart depolarizes and repolarizes without beating, that is without any muscle contraction
What are the leads of an EKG?
The standard EKG done has 12 leads
- 3 limbs leads
- 3 augmented leads
- 6 chest leads
Back, front, etc all provide different views
This provides a 3 dimensional view of the electrical activity of the heart
In the emergency room- if there is only one lead, you’re seeing the parallel lead to the general direction. This is Lead 2
Whaat are the uses of EKG?
- rate
- rythym(may be third beat does feel the same for example)
- conduction velocity (the speed)
- muscle mass
- injury to myocardium
- electrolyte abnormalities
Describe the ECG and Einthoven method
Connecting three leads to both arms and one leg provides best results, because of an equilateral triangle with the heart close to its centre and same distance to the leads
The right leg is connected to a ground lead to prevent electrical interferences
Lead 1 attaches the right arm(negative pole) and left arm is positive
Lead 2 attaches right arm(negative pole) and left leg (positive pole)
Lead three attaches left arm(negative pole) and left leg(positive pole)
Which is the most parallel lead?
Lead 2
What are chest leads particularly useful for?
MIs
