Cardiac Muscle Contraction Flashcards
what is preload
amount of stretch before contraction
what is afterload
load during active shortening
is recruitment a factor in cardiac muscle?
no, every cardiac muscle cell is recruited during contraction
what is contractility
increased work in the heart for the same preload
does skeletal muscle have calcium current during AP?
no
describe excitation contraction coupling in cardiac muscle
actin and myosin are regulated by troponin C but cardiac cells rely on extracellular Ca for contraction through Calcium influx during AP
which is the most reliable measure of left ventricular preload
muscle fiber length, but it is not practical, so left ventricular volume during ECHO
order of reliability of measure of left ventricular preload
- muscle fiber length
- left ventricular volume
- left ventricular diameter
- left ventricular end diastolic pressure
- left atrial pressure
- pulmonary capillary wedge pressure
- pulmonary artery pressure
- central venous pressure
how to determine left ventricular diameter
xray
problem with using leeft ventricular end diastolic pressure as a measure
if the heart or vessels around it are stiff , there would be high pressure in the ventricle itself, but the ventricle could actually not be stretched
how is force-velocity relationship altered in cardiac muscle?
extrinsic regulatory mechanisms like sympathetic nerves – would increase curve, parapsympathetic would decrease curve
what occurs if preload increases
increased force of contraction and ejection
what occurs if preload decreases
decreased force of contraction and ejection
what is ionotropic
effects on force of contraction by external influences (physiological or pathological)
what is contractility in cardiac muscle
extrinsic control of heart by autonomic nerves and hormones that regulate response to preload and afterload
what is the primary pacemaker
SA node
secondary pacemaker
AV node
how does cardiac relaxation occur?
- SERCA pumps Ca back to SR
2. Ca efflux by active transport (Na/Ca exhanger)
what regulates cardiac force
calcium influx during AP
what is DHPR
dihydropyridine receptors which are L type voltage gated channels that respond to Ca influx. they open and trigger release of Ca from SR by binding to RYR
what is RYR
calcium gated calcium channel, releases Ca from SR
describe calcium induced Ca release from SR
1, cardiac AP
- Ca enters cell during plateau
- Ca binds to DHPR
- DHPR opens RYR
- Ca leaves SR
- Ca binds to troponin C
- cross bridge
what is a calcium pulse
rise in Ca to trigger contraction via DHPR (L type voltage gated channels)
what does calcium pulse indicate in terms of cardiac relaxation?
Ca must be actually removed from cell, not just resequestered in the SR
how does cell remove Ca
Na/Ca exchanger, sends 3 Na into the cell to bring 1 Ca out
what is Na/Ca
secondary active transport, bc follows Na concentration gradient that has been established via Na/K ATPase
what occurs if there is ischemia or mitochondrial damage?
lower ATP means transport of Ca is limited (Na/K ATPase not doing concentration), so Ca builds up in the cell and interferes with normal resting potential and AP
what does increased Ca in cardiac muscle do
interferes with normal membrane potential and AP, so you see reduced force and inadequate relaxation
why is duration of AP, calcium pulse and active force important
refractory periods extend well into force production to prevent tetany and limits ability of increased frequency to increase force
what controls force production in cardiac muscle
calcium influx
what would be seen in hypocalcemia?
decreased AP amplitude, decreased Ca influx and decreased force
what occurs if Na/K pump is inhibited by digitalis?
Na would build up in the cell. therefore, concentration gradient of Na not established, and Na/Ca pump could not work. Ca would increase in the cell and cause increased force of contraction. (positive ionotropic effect)
why is preload important for contractility
- actin and myosin have more overlap/drawn closer together when stretched aka more potential to interact
- stretch increases Ca sensitivity of troponin
what is intrinsic regulatino
preload and afterload
what is extrinsic regulation
amount of response to preload and afterload
what is positive ionotropic effect
increased contractility due to increased intracellular Ca
what is negative ionotropic effeect
decreased calcium or decreased ability to produce force
what is chronotropy
factors that change HR
what is lusitropy
rate of relaxation
what would occur if preload decrease, as with hemorrhage
sympathetic system would provoke contractility to keep alive
what is frank starling law of heart
input must equal output, that is cardiac output must match venous return
how does one change response to preload to match frank starling?
change contractility
what is compliance?
ability to be stretched without permanent damage of shape, but takes time to return to size
what is the equation for compliance
change in volume/change in pressure
what is elasticity
production of tension as a material is stretched (hard to stretch makes increased tension, but snaps back fast)
equation for elasticity
change in pressure/change in volume
what occurs in the case of a hypertrophic heart?
increased mass of heart means decreased compliance. which takes more pressure to fill
what occurs in the case of a dilated heart
high compliance except for at very high volumes
what is the la place relationship
Tension = hydrostatic pressure x radius
calculate wall stress
hydrostatic pressure x radius/2xwall thickness
describe dilated heart in terms of wall tension and la place
dilated heart would have increased radius and decreased thickness. therefore, tension is increased, and wall stress is also increased. this means it has to work harder to eject the same amount of blood
what does cardiac muscle get its energy from
fatty acid oxidation, dependent on oxygen
what is the impact of oxygen on cardiac muscle
CARDIAC FORCE DEPENDENT ON OXYGEN SUPPLY; there is a direct relationship between cardiac work, cardiac oxygen consumption and coronary blood flow
what is the direct effect of ichemia on the heart? (post MI_)
decrease in oxygen leads to a decrease in active force and work within 30 seconds of occlusion
what is the impact of loss of ATP on the heart
slight ATP depletion means huge depletion of ATP in heart.
- loss of allosteric control of L type Ca channels and a transient decrease of Ca influx.
- low ATP inhibits SR Ca release
- reduces energy of hydrolysis of ATP required for force production
- inability to resequester or pump Ca out of cell poisons mitochondrial energy production
- excess Ca increases risk of free radical damage
what sympathetic receptors are on cardiac cells
adrenergic beta 1
what does SNS do to alter ion conductance?
cAMP
what does cAMP do in the heart?
increases Ca influx and release of Ca from SR leading to increased transport and resequesteration of Ca.
makes stronger, faster, shorter duration (more frequent)
parasympathetic receptors on cardiac cells
muscarinic M2 receptors (Ach)
what do muscarinic receptors do to cAMP?
decrease cAMP
what does reduced cAMP do?
weaker, slower, longer duration
what does hypertension do to the heart
has chronic high afterload, meaning concentric left ventricular hypertrophy to overcome afterload
what are examples of heart remodeling
dilation and hypertrophy
what is remodeling due to
altered gene expression
