RAT 7 Flashcards
what is an action potential?
a quick temporary change in the membrane potential of a cell in a single region of the plasma membrane
how is an action potential related to muscle contraction?
the action potential causes the muscle to contract in response to the electrical excitation
what is required for a skeletal muscle to have an action potential?
stimulation
is stimulation required for cardiac muscles? why or why not?
no because their electrical activity is coordinated by pacemaker cells
what are pacemaker cells?
a cell that depolarizes spontaneously and triggers action potentials in neighboring cells
what are contractile cells?
triggered by pacemaker cells to also have action potentials
are most cells in the heart contractile cells or pacemaker cells?
contractile cells
what is autorhythmicity?
sets its own rhythm without a need for input from the nervous system
do cardiac muscle cells have striations?
yes
what causes striations?
cells have alternating light and dark bands when viewed under a microscope
list some ways that cardiac muscle cells differ from skeletal muscle cells
- do not form long fibers (they are short and wide and contain a single nucleus)
- contain lots of myoglobin
- contain intercalated discs
what are intercalated discs?
join adjacent cardiac muscle cells (pacemaker to contractile) (contractile to contractile)
what types of cell junctions are found in intercalated discs?
- desmosomes
- gap junctions
how does the action potential of a pacemaker cell differ from the action potential of a skeletal muscle cell?
- pacemaker cells cause an electrical tracing
- depolarization occurs much slower
- the action potential oscillates (never at resting level)
why does hyperpolarization trigger a new action potential in a pacemaker cell?
channels open when the membrane hyperpolarizes and that opening starts a slow depolarization which leads back to step 1
why does hyperpolarization trigger a new action potential in a pacemaker cell?
channels open when the membrane hyperpolarizes and that opening starts a slow depolarization which leads back to step one
what structure is responsible for triggering a new action potential in a pacemaker cell?
HCN channels
what is a cardiac conduction system?
group of interconnected pacemaker cells
what is the function of a cardiac conduction system?
spread cardiac action potentials quickly through the heart
what are the three populations of pacemaker cells that compose the cardiac conduction system?
- sinoatrial node (SA node)
- atrioventricular node (AV node)
- purkinje fiber system
where is the sinoatrial node located?
in the right atrium slightly inferior and lateral to the opening of the superior vena cava
what can influence the rate that the SA node depolarizes?
the sympathetic and parasympathetic nervous systems
where is the AV node located?
posterior and medial to the tricupsid valve
what are the components of the purkinje fiber system?
- AV bundle
- right and left bundle branches
- terminal branches
which component of the cardiac conduction system has the fastest intrinsic rate of depolarization?
SA node
why would the SA node set the rhythm of the heart beat?
the fastest rhythm “wins” and sets the heartbeat
what is sinus rhythm?
electric rhythms generated and maintained by the SA node
what happens if the SA node malfunctions?
the AV node can successfully pace the heart, but its slower
what happens if the AV bundle malfunctions?
the SA node cannot pace the ventricles, the purkinje fiber system can pace the heart but only for a short time
what is AV node delay?
conduction slows considerably due to low number of gap junctions between AV nodal cells and presence of the nonconducting fibers skeleton that surround the AV node
how is AV node delay beneficial?
allows the atria to depolarize (and contract) before the ventricles, giving the ventricles time to fill with blood; prevents backflow
what proportion of cardiac muscle cells are contractile cells?
99%
how are action potentials of pacemaker cells transmitted to contractile cells?
intercalated discs
what is the biggest difference between a skeletal muscle action potential and a cardiac contractile cell action potential?
there is no plateu phase in a cardiac contractile cell action potential
why is the difference in skeletal muscle action potential and cardiac muscle action potential so critical to the proper functioning of the heart?
the plateu phase lengthens the cardiac action potential which slows heart rate, providing the time required for the heart to fill with blood
what is the effective refractory period?
prevents tetany (sustained contraction) from occurring in the heart
why is the effective refractory period beneficial to the function of the heart?
allows the heart to relax and the ventricles to refill with blood before the cardiac muscle cells are stimulated to contract again
what is the most important difference between the contraction of these two cell types?
many of the calcium ions needed for contraction diffuse into the cell from the extracellular fluid through calcium ions channels in the T-tubules
how does an action potential lead to contraction in cardiac contractile cells?
sliding filament mechanism: depolarization propagates through sacrolemma, divides along the t-tubules, causing release of calcium. these bind and a cross-bridge cycle begins
what does the ECG measure?
electrical activity occurring in the cardiac muscle cells over a period of time
what does the P wave represent?
atrial depolarization
what does the QRS complex represent?
ventricular depolarization
what does the T wave represent?
ventricular repolarization
compares the pressures in the left and right ventricles during the cardiac cycle
Right - max pressure that must be generated in order to push open the pulmonary valve and eject blood is 28 mmHg
Left - max pressure is 118 mmHg
aorta - 80 to 118 mmHg
what events are occurring in ECG in relation to ventricular filling?
SA node fires an action potential, which propagated through the atria and delayed at the AV node, leading to the P wave and Q wave
what events are occurring in ECG in relation to isovolumetric contraction?
depolarization spreads through the AV node to the ventricles, leading to the R and S waves
what events are occurring in ECG in relation to ventricular ejection?
depolarization spreads through the ventricles, enters the plateu phase then begin repolarization, leading to the T wave
what events are occurring in ECG in relation to isovolumetric relaxation?
slow depolarization
explain S1 in relation to the cardiac cycle.
heard as the AV valves close during the isovolumetric contraction phase and is still heard at the end of ventricular ejection phase
explain S2 in relation to the cardiac cycle
heard as the SV valves close during isovolumetric relaxation phase
explain pressure changes in ventricular filling.
aortic pressure decreases, atrial pressure remains slightly higher than ventricular pressure
explain pressure change in isovolumetric contraction.
ventricular pressure rises rapidly until it equals aortic pressure, ventricular pressure rises above atrial pressure causing the AV valve to close
explain pressure change in ventricular ejection.
ventricular pressure rises above aortic pressure and the aortic valve opens, atrial pressure increases as the atria fills with blood
explain pressure change in isovolumetric relaxation.
ventricular pressure falls below aortic, causing the aortic valve to close, ventricular pressure falls below atrial pressure, causing the mitral valve to open
explain volume change in ventricular filling
ventricular volume rises rapidly as blood drains in from the atria
explain volume change in isovolumetric contraction
volume remains constant
explain volume change in ventricular ejection
ventricular volume rapidly declines as blood is ejected
explain volume change in isovolumetric relaxation
volume remains constant
define HR
- heart rate
- 60-80 cardiac cycles or beats per minute
define CO
- cardiac output
- the amount of blood pumped into the pulmonary and systemic circuits in one minute
define SV
- stroke volume
- the amount of blood pumped in one heartbeat
express SV using an equation
EDV - ESV = SV
what is the equation for CO
HR * SV = CO
list three factors that influence SV
- preload
- contractility
- afterload
what factors increase SV?
- preload increases
- contractility increases
- afterload decreases
what factors decrease SV?
- preload decreases
- contractility decreases
- afterload increases
what is the Frank-Starling Law of the Heart?
- the more ventricular muscle cells that are stretched, the more forcefully they contract
- explains the relationship between preload and stroke volume
what is a positive chronotropic agent?
anything that increases the rate at which this node fires
what is a negative chronotropic agent?
anything that decreases the rate at which this node fires
how does the sympathetic nervous system influence cardiac output?
release of norepinephrine increases cardiac output with both positive chronotropic and inotropic effects
how does the parasympathetic nervous system influence cardiac output?
release of acetylcholine affects the SA node, decreasing its rate of action potential generation
how would epinephrine and norepinephrine influence cardiac output?
norepinephrine and epinephrine increase cardiac output
if a hormone increases water retention in the body, how would cardiac output be influenced?
increase in cardiac output
the blood returning from the head, neck, thorax, and upper limbs drain in to which two veins?
the 2 brachiocephalic veins
what vein is formed when the 2 brachiocephalic veins merge?
superior vena cava
what vein drains blood from the lower limb?
external iliac vein
what vein drains blood from the pelvis?
internal iliac vein
what vein is formed when the external iliac vein and the internal iliac vein merge?
common iliac vein
what two veins merge to form the inferior vena cava?
right and left common iliac veins
what are the three main veins that drain the head and neck?
- internal jugular vein
- vertebral vein
- external jugular vein
what is a dural sinus?
venous channel located between two layers of dura mater and drain the cerebral veins of the brain
what is a portal system?
circuit in which veins feed a capillary bed
which veins drain in to the hepatic portal vein (and not into the inferior vena cava)?
splenic, gastric, superior and inferior mesenteric veins
where does the hepatic portal vein travel? why?
the liver in order to form another set of capillary beds, the liver can process and detoxify the blood before it reaches the rest of the circulation
how does venous blood exit the liver?
hepatic veins
where does venous blood travel to next?
inferior vena cava
