Cardiac Muscle Flashcards
What are the three ways cardiac muscle APs differ from skeletal muscle APs?
they’re self generating
they conduct directly from cell to cell
they have a long duration
What are the three most important ions for cardiac APs?
sodium, calcium, potassium
On what side of the membrane is sodium concentration higher? calcium? potassium?
sodium and calcium are higher in the interstitial fluid (so they want to move into the cell)
Potassium is high in th eintracellular fluid, so it wants to move out
What structures in the membrane are responsible for the resting membrane potential and rapid changes?
ion channels
What are the three possible states of ion channels?
open, closed, inactivated
What is the resting membrane potential for cardiac cells? Closes to what ion equilibrium?
-90 mV - which is potassium’s equilibirum
What are the two types of myocardial cells?
myocardial contractile cells
myocardial autorhythmic cells = pacemaker cells
Contractile cell APs are similar to neurons exept for what important difference?
much longer AP due to extended Ca2+ entry
How can pacemakers spontaneously generate APs?
their membrane potential is inherently unstable
Which cells produce the “slow response” and which produce the “fast response”
pacemaker cells = slow response
contractile cells = fast response
How many phases are there in a contractile cell AP?
5, but they’re numbered 0-4
For contractile cells, what is the first step in firing an AP?
the Na channels open and the K+ channels close. THis means K+ can’t leave the cell and Na+ rushes in. This causes a rapid depolarization.
For contractile cells, what happens at the peak - the overshoot?
the Na+ channels close and FAST K+ channels open. K+ is now wanting to enter the cell based on the electrochemical gradient - this is what makes the initial repolarization happen briefly.
It doesn’t last long though - the K+ channels close
After the fast K+ channels close, what opens?
the slow inward Ca2+ channels open. Ca 2+ enters the cell very slowly, halting the repolarization and causing the characteristic plateau
What causes the repolarization after the plateau?
the Ca2+ channels close and the delayed rectifier slow K+ channels open, allowing K+ to rush out of the cell causing the rapid repolarization
What slows the rapid repolarization such that the resting membrane is reached and maintained?
the slow K+ channels close and the inward rectifier channels open when you reach the K+ resting membrane potential
What current characterizes the pacemaker cells
the funny current
what are the phases of the apcemaker cell AP/
three phases: 0, 3, and 4 to coincide with the contractile
What makes the funny current funny?
both Na and K are going in and out
What causes the potential to gradually rise?
a gradual influx of Na
When the potential reaches a threshold, what happens?
you have a transient opening of the Ca2+ channels and you get an influx of Ca2+ to further depolarize
How does the influx of Ca2+ essentially sustain itself?
It triggers long-lasting Ca2+ channels to open
What causes the depolarization to stop and then result in repolarization?
opening of the K+ channels so K leaves the cell
So what ion primarily mediates phase 0 in contractile cells/ in pacemaker cells?
Na in contractile
Ca in pacemaker
What keeps the contractile cells from going into summation or tetanus?
they have a really long refractory period
What are the structures that connect the ends of two adjacent cardiac contractile cells?
intercalated disks
In the intercalated disks, what forms the firm mechanical attachments?
desmosomes with adherens proteins
In the intercalated disks, what causes the electrical connection between the cells?
gap junctions with channels formed by connexin proteins
What do the gap junctions allow for?
THey allow positive charged ions to flow away from the depolarized membrane of the first cell over to the hyperpolarized membrane of the adjacent cell.
This triggers a depolarization in the adjacent cell, leading to an AP
How do we measure electrical events of the cardiac cycle/
electrocardiogram
What is happening during the P wave?
atrial depolarization
What does the PR interval represent?
the conduction time through the atria and the AV node
What is the QRS complex?
ventricular depolarization
What is the QT interval?
the duration of ventricular systole, or contraction
What does the ST segment signify?
duration of the plateau phase in the contractile cells
What does the T wave represent?
ventricular repolarization
Why can’t you see repolarization on an EKG?
Because is occurs during ventricular depolarization, which has the enormous QRS segment
THe parasympathetic fibers from the vagus nerve release what NT? To what effect?
ACh
It increases the permeability of the resting membrane to K+, leading to a hyperpolarization - this means you have a slower rate of spontaneous depolarization - a negative chronotropic effect; heart slows down
What do sympathetic fibers release? to what effect?
NE
it increases the spotnaneous depolarization of the membrane - a positive chronotropic effect; the heart speeds up
What is the ion that really drives the actual contraction?
Ca2+
What happens when you have an action potential in a contractile cell?
you get an opening of a voltage-gated Ca2+ channel, which allows Ca2+ to enter the cell
What does the Ca2+ do in the cell?
It binds to ryanodine receptors on the sarcoplasmic reticulum, which triggers more Ca2+ channels to open, allowing stored Ca2+ to flow out of the SR into the cell further increasing Ca2+ concentration
What does this high Ca2+ concentration cause in the cell?
It binds to troponin and allows actin and myosin to interact, leading to a cotnraction
Why does relaxation occur?
Because Ca2+ will be pumped back into the SR and out of the cell after about 200 ms
What’s the difference between isometric and isotonic contraction?
isometric is when the length is fixed by having the ends of hte muscle held rigidly - means it can develop tension but not shorten
Isotonic is when the tension is fixed - an unrestrained muscle causes it to shorten without force development
Active tension developed during isometric contraction depens on what?
the muscle length at which contraction occurs
too short - no contraction
too long - no contraction
appropriate length is the most myosin-actin interactions possible = contraction
What is an inotrope? Example?
something that influences the amount of tension the muscle can develop, or the amount of shortening a muscle can achieve
NE is a positive inotrope because it increases isometric tension
Mechanical response of the myocyte depends upon what?
preload, afterload, and contractility
Cardiac myocyte length-tension relationshipsa re correlated with what?
changes in volume and pressure in the intact ventricle
