Final-Chapter 12 Part 2 Cardiac Physiology Flashcards
Describe pacemaker cells.
Pacemaker cells do not have a steady resting membrane potential. They spontaneously depolarize and fire action potentials. Unique in that they don’t have a steady resting Membrane Potential. There is an AP that is produced and then at the very end of that AP, the undershoot phase results in another AP is produced. These cells are constantly producing APs – this is good because you don’t want these cells to take a break on you. A hyperpolarization results in a depolarization and back and forth. The Membrane potential never steadies out – one AP followed by the next.
Describe pacemaker action potentials?
Cardiac pacemaker cells spontaneously depolarize because of the activity of “funny” channels. “Funny” channels are activated by hyperpolarization of the membrane. “Funny” channels = HCN channels (HCN = hyperpolarization-activated and cyclic nucleotide-gated channels).
What is the first step in creating an action potential in pacemaker cells.
- Pacemaker potential (orange/bottom of slow depolarization): Following an AP, at the end of the undershoot phase, the potassium channels are closing and funny channels open. Funny channels are permeable to Na and K [Na in, K out] due to the activity of HCN/Funny channels. Voltage-gated K+ channels close. Permeability for Na increased and permeability for K decreases – this is because K channels were open before. These cells have a lot more voltage.
What is the second step in creating an action potential in pacemaker cells.
- Top of slow depolarization (Yellow): T-type voltage-gated calcium channels open (T for transient) purpose of T channels is to get the cell to threshold quickly. Funny channels close. The cells’ permeability to sodium decreases and permeability to Calcium increase. Purpose of T-type channels is to bring membrane to threshold.
Why would the cell shut down permeability to Na instead of Ca?
The goal is the get the cell to threshold as quickly as possible. Calcium is twice as positive as sodium, so more positive charge is going into the cell, also, the driving force is way larger for Ca than Na.
What is the third step in creating an action potential in pacemaker cells.
- Rapid depolarization (blue):
T-type channels close and voltage gated L-type Ca channels open. L type channels mean we have a LOT of positive charge going in, which gives us such a steep riding phase. Voltage-gated Na channels open causing the inside to get really positive real quick and really permeable to Ca than is to Na.
What is the fourth step in creating an action potential in pacemaker cells.
- Peak of AP causing repolarization (purple): voltage-gated K channels open and voltage-gated L-type Ca channels close. Voltage-gated Na channels inactivate and no there is a low permeability to Na and Ca.
A high permeability to potassium is the falling phase. Very large potassium current, because of how far the membrane potential is from equilibrium potential of potassium which is why you get such a steep falling phase.
What are the special ions of cardiac muscle cells?
Cardiac Pacemakers cells have “Funny” Channels (HCN Channels) which allow allow for spontaneous depolarization. T-Type voltage gated calcium channels allow membrane to reach AP threshold.
Contractile Cells have L-Type voltage gated calcium channels allowing for long duration APs.
Why must Cardiac APs be so long?
Long refractory period makes summation (tetanus) of contraction impossible. AP lasts about the same amount of time as contraction. Potassium is leaving but Calcium channels are still open, so Ca is leaking in, creating a sort of plateau phase, which gives a long AP. The length of the contraction is about the same length as the AP and during this time, you cannot produce another AP. The heart has time to contract and relax before it gets told to relax again. You will never get summation [or tetanus] in the heart. If this AP was short, this cell could potentially get another AP while it was already contracting, that would NOT good.
Describe excitation contraction coupling in cardiac muscle.
- The current spreads through gap junctions to contractile cell. The presynaptic cell could be a pacemaker, contractile, or whatever – current spreads through gap
junction, produces an AP. - AP travels along membrane and makes it way down T tubule. As it propagates down the membrane, it opens Ca channels down the membrane and Ca enters from the outside.
- Ca2+ channels open in plasma membrane and SR. Calcium induced Calcium release – the calcium coming inside the cells, opening up the channels on the
SR and causes more calcium to leave the SR. 95% Ca2+ released from SR, 5% comes from outside of cell. - Ca2+ induces Ca2+ release from SR.
- Ca2+ binds to troponin, exposing myosin-binding sties.
- Crossbridge cycle begins (muscle fiber contracts).
- Ca2+ is actively transported back into SR and ECF.
- Tropomyosin blocks myosin-binding sites (muscle fiber relaxes).
Three ways Ca levels are lowered:
SERCA pumps, Ca2+ pump on plasma membrane moves Ca2+ out of cell using ATP, An exchanger doesn’t use ATP. Uses Na going down it’s gradient to move Ca2+ against its gradient.
Describe Excitation Contraction coupling in Skeletal muscles vs. Cardiac muscles.
In Skeletal muscles, AP triggered by motor neuron and ACh release. DHP voltage sensors on plasma membrane open ryanodine receptors on SR. Ca is released from SR. Calcium is removed from cytosol by SERCA pumps on SR. In Cardiac muscle AP triggered by positive current spread through gap junctions. DHP voltage sensors open ryanodine receptors on SR. Voltage-gated Ca channels on plasma membrane also open. 95% Ca released from SR, 5% comes from outside cell. Ca-induced Ca release (CICR) and Ca removed by SERCA pumps, pm Ca ATPase, and pm Na/Ca exchanger.
Describe the cardiac cycle.
The cardiac cycle Involves rhythmic changes in Valve opening and closing, Atrial, ventricular, and aortic pressure, and Ventricular blood volume. Patterns of opening and closing of valves in response to changes of pressure, which results in changes of volume of blood in the heart chambers. Composed of 4 stages, 2 stages are diastole and 2 are systole. Diastole are ventricular relaxation. Systole is ventricular contraction. Important to note: IT ALL HAS TO DO WITH THE VENTRICLES. Each of the phases are classified by what the
ventricles are doing.
What are the phases of the cardiac cycle.
- Ventricular filling and Atrial Contraction [diastole]. Two different phases in one.
- Isovolumetric Contraction [systole]
- Ventricular Ejection [systole], also known as the payoff phase – what we’re aiming for. If phase 3 doesn’t go well, the other phases don’t matter.
- Isovolumetric relaxation [diastole]. Phases 2 and 4 – isovolumetric [means that the volume of blood in the ventricles is not changing]. WHEN YOU SEE THIS TERM, IMMEDIATELY THINK THAT ALL VALVES ARE CLOSED.
Important to note!
Pressure is higher in the ventricles than in the atria to close the AV valves. The pressure is higher in the aorta than in the ventricles to close Semilunar valves. To increase pressure add blood or contract.
Describe the valves?
Atrioventricular valves found between atrium and ventricles. Aortic and pulmonary semilunar valves separated the ventricles from the arteries that come out of them. Pressure causes these valves to open.
The AV valves: When the pressure in the atria is higher than the pressure in the ventricles the AV valves open. They close when pressure in ventricles is higher than the pressure in the atria.
Both semilunar valves: SL valves open when pressure in ventricles is higher than pressure in the arteries. They are closed when pressure in the arteries is higher than pressure in the ventricles.
