Electrophysiology Flashcards
What is the purpose of the activation sequence of the heart?
Action potential will spread in a precise way to make sure the heart contracts in the right way at the right moment in time
What is the activation sequence of the heart?
Start of the electrical activity at the sinoatrial node (SA node) at the top right of right atrium then it spreads like waves to the atrioventricular valve and through the bundle of His (or His bundle or atrioventricular bundle). It then separates into the right bundle branch and the left bundle branch which are in the septum but very close to the edge of their respective ventricles. The action potentials start in left branch and move to the right because the right branch is well insulated, encapsulated in connective tissue within the septum. The septum is the first cardiac muscle to get activated (does so from left to right). The bundle branches branch into thin fibers that reach subendocardially, right at the base of the myocardium all at the same time/simultaneous activation/ simultaneous distribution of action potentials from inside to outside in an orderly fashion. in both ventricle free walls thanks to the branching/tree-like form. These thin fibers are known as the Purkinje fibers.
What are the yellow parts in the heart at slide 79 of CVS?
These are all cardiac cells, not nerves or neurons. They are specialized cells that make up that 0.01% of cardiac cells. They are non-contractile, their job is to generate action potentials. Also internodal atrial pathways do not exist.
How are cardiac cells organized? How does this help for orderly contraction?
They are organized as intercalated discs which are particularly significant for end to end communications, not side to side communications. It is quite wavy, not a straight line and the interstitial space is so tiny it eventually barely exists. At this point where no interstitial liquid is an even more specialized part of the intercalated disc which is the nexus or gap junction. This junction contains special gap-junctional channels, the main communication channels for the spread of contraction.
What is a gap-junctional channel specifically?
They are made up of six identical subunits. Each cell membrane has a hemi-channel (connexon) and at that nexus/gap junction, where there is no interstitial liquid, the hemi-channels dock onto each other and make full channels. The ions can then move through that space to transmit the action potential.
What is the mechanism of action potential spread?
- Through local circuit currents which slowly depolarize the next cell until the threshold for firing of sodium is reached
- A depolarized cell (has an action potential/positive on the inside) will wanna get rid of + ions into the next cell that has a negative cytoplasm through the gap-junctional channels. This is usually due to the movement of K+ because lots of K+ in a cell. Since the outside of the action potential cell is negative, the + ions from the outside of the cell next to it will wanna move there. Usually this is the movement of Na+ since abundant outside of the cell. This creates a loop of current (local circuit current) at the gap junction which helps the spread of action potentials. All species of ions move though (neg and pos) in such a way that the cell that must get depolarized gets depolarized. Neg ions will flow in the opposite direction to follow the local current circuit. A repolarization wave will follow the depolarization with a flipped local circuit current. Both intra and extra-cellular flows of current necessary for propagation to occur.
What is the voltage of the cell?
Voltage is the difference of charge between the inside and outside of the cell. It is directly proportional to the number of ions.
What is excitation-contraction coupling?
Translate electrical activity to mechanical contraction following a series of steps. We need an AP to generate contraction.
What are the steps of excitation-contraction coupling for cardiac muscle contraction?
- Depolarization of the membrane is going to open the Ca2+ channels clustered at the end of the T-tubules (transverse tubule) (real site of action) so it is actually quite close to the receptors on the sarcoplasmic reticulum (SR) for quick reaction
- [Ca2+] very low in cytoplasm nearly 0 before the AP plateau but the SR is very full of Ca2+. Ca2+ is going to bind to ryanodine receptor on the SR which is also a Ca2+ channel releasing a flow of Ca2+ in the cytoplasm (low on Ca2+) and will reach myosin/actin to form the Ca2+-troponin complex
- through multiple steps will end up in a contraction of the muscle cell
What is the relationship between AP and Ca2+ concentration?
A little delay between the upstroke of the AP and the opening of the calcium channels (calcium can be measured with fluorescent tags)
What can be said about excitation vs contraction?
- tension developed in a time-course parallel to that of the internal concentration of Ca2+, also highest peak a little bit after the plateau of the AP (opening of the Ca2+ channels)
- Mechanical activity lags behind electrical activity
- Activation is not equal to contraction, AP is not equal to contraction
- When there is electro-mechanical dissociation, we will observe a pulseless electrical activity
