Chapter 9.2: Cardiac Muscle Continued Flashcards
During the creation of a pacemaker potential, what occurs during the FIRST HALF of the SLOW DEPOLARIZATION step?
There is a net entry of NA+ through HCN voltage gated Na+ channels.
At the same time, there is a progressive reduction in the passive OUTWARD FLUX of K+ because Cardiac K+ channels that open during the falling phase of the previous action potential SLOWLY CLOSE at negative potentials.
THEREFORE, K+ efflux slowly declines at the same time as the inward leak of Na+ (adds to the drift towards threshold)
When does an HCN channel open up?
whent he membrane potenital of the pacemaker cells becomes more negative at the end of repolarization from the previous action potential.
When the previous action potential ends, HCN channels open, causing an inward Na+ current and membrane potential moves closer to threshold potential.
During the creation of a pacemaker action potential, what occurs during the SECOND HALF of the SLOW DEPOLARIZATION step? (before threshold is reached)
HCN channels start to close and transient Ca2+ (T-TYPE Ca2+ Channels) channels open.
This causes a brief influx of Ca2+ which further depolarizes the membrane, bringing it to threshold
Why is there K+ efflux during the first half of the slow depolarization of pacemaker potentials?
because Cardiac K+ channels that open during the falling phase of the previous action potential SLOWLY CLOSE at negative potentials.
What kind of channel is responsible for finally pushing a pacemaker membrane to threshold? When do these channels close?
T-type Ca2+ channels that open after Na+ influx and K+ efflux-slowing has commenced.
These channels close once threshold is reached.
At threshold of a pacemaker potential (self induced action potential), ___-type ___ channels open. What does this cause?
L-Type Ca2+ channels open at threshold, producing a large influx of Ca2+ leading to the rapid rising phase of the action potential.
After an action potential is induced in the cardiac pacemaker cells (myogenic cells), what causes the falling phase?
falling phase caused by OPENING of voltage gated K+ channels and CLOSURE of the L-type Ca2+ channels.
the HCN channel is also known as the ____ channel
funny channel
Main locations of pacemaker cells. (nodes). What side of the heart are they on?
1) SA node: small, specialized region in the RIGHT atrial wall (in fish its the sinus venosus)
2) AV node: small bundle of specialized cardiac muscle cells located at the base of the RIGHT atrium near the seotum, just above the junction of the atria and ventricles.
Which node is the main pacemaker? Why?
SA node is the main pacemaker of the heart because it exhibits the FASTEST rate of autorhythmicity.
If the AV node also is myogenic, how come it does not depolarize and cause contractions on its own?
the AV node does not assume its own slower rate because it is ACTIVATED by action potentials generated from the SA node before it reaches threshold at their own slower rhythym.
why is it important that atrial excitation and contraction is complete before the onset of ventricular contraction?
completion of atrial contraction is salient to ensure complete ventricular filling before the ventricle contracts.
What is fibrillation
when contraction of the heart becomes random and uncoordinated. Normal spread of excitation ensures that excitation of cardiac muscle fibers is coordinated to ensure that each heart chamber contracts as a unit to accomplish efficient pumping.
Initial depolarization and impulse created by SA node facilitates ____ excitation. Afterwards, the impulse is propagated between the atria and the ventricles through the ____ node. The impulse travels along two tracts, the __ ___ ___ and the ___ ___, to facilitate ventricular excitation.
Initial depolarization and impulse created by SA node facilitates ATRIAL excitation.
Afterwards, the impulse is propagated between the atria and the ventricles through the AV node. The impulse travels along two tracts, the BUNDLE OF HIS and the PURKINJE FIBERS, to facilitate ventricular excitation.
What is the AV nodal delay?
in the AV ndoe, the impulse from the SA node is delayed momentarily which enables the atria to become completely depolarized and to contract, emptying their contents into the ventricles.
3 steps of the orchestration of the spreading of electrical impulse in the heart
1) atrial excitation
2) transmission between teh atria and ventricles (AV nodal delay)
3) ventricular excitation.
Outline the events of the rising phase of an action potential in a cardiac myocyte (contractile cell, no longer a pacemaker cell)
when stimulated by the pacemaker cells, voltage gated Na+ channels get activated and membrane potential rapidly climbs to about +20 to +30mV
At peak potential, (+30), permeability to Na+ plummets (like in neurons)
outline the events of the Peak phase of an action potential in a cardiac myocyte.
at the peak, a subclass of K+ channels TRANSIENTLY open, causing K+ EFFLUX. This brings a brief and small repolarization as the membrane becomes slightly less positive
outline the events of the Plateau phase of an action potential in a cardiac myocyte
plateau phase is the maintenance of a positive potential for several hundred msec.
plateau phase in maintained by two voltage-dependent permeability changes triggered by the sudden voltage change during the rising phase of the action potential.
1) slow L-type Ca2+ channels get ACTIVATED. causes Ca2+ influx
2) Transient AND LEAKY K+ channels close and the cell experiences a DECREASE in K+ permeability (PREVENTS K+ EFFLUX)
purpose of plateau phase
prolongs contraction for squeezing the heart chambers.
outline the rapid falling phase of an action potential in a cardiac myocyte
(L type) Ca2+ channels become inactivated and the “ordinary” voltage gated K+ channels activate (like in the neuron), causing K+ EFFLUX RAPID
Ca2+ influx stops, K+ efflux occurs, causes membrane potential to FALL. Cell returns to resting potential.
At rest, the voltage gated K+ channels close, but leaky K+ channels remain open
SEE CARDIAC DIAGRAM #3
Where will you find majority of the L-type Ca2+ channels in a cardiac muscle cell?
in the T TUBULES! (recall, an AP spreads on the surface of a muscle cell (also seen in skeletal muscle), and can penetrate deeper into the cell through the T Tubules)
Outline the mechanism in which an AP brings about a contraction in cardiac muscle
1) L-type Ca2+ channels lie primarily in the T tubules
2) Ca2+ moves into the cytosol from the ECF acros the T tubule membrane (DIFFERNENT THAN IN SKELETAL MUSCLE)
3) entering Ca2+ causes ryanodine Ca2+ release channels in the LATERAL SAC OF THE SR to open
4) a Ca2+ induced Ca2+ release occurs. SR releases Ca2+ into cytosol.
5) the Ca2+ spark turns on the contractile machinery (interacts with toponin and tropomyson, like in skeletal muscle)
6) More Ca2+ means that the contraction can last longer than in skeletal muscle
In cardiac muscle, he extent of cross bridge activity varies with the amount of ___ ____
cytosolic Ca2+
