Cardiac Muscle Flashcards
Cardiac muscle structure
. Striated . Rigid sarcomere structure . extensive T-tubule and SR . Tons of mitochondria . Dense capillary supply . Gap junctions btw cells
Characteristics of cardiac muscle
. SR and extracellular sources of activating Ca
. Troponin site of Ca regulation
. Slow speed of contraction
. Spontaneous production of APs in pacemaker cells
. No tone
. Can be excited or inhibited by nerve stimulation
. Controlled by hormones
Intercalated disk
. Site of gap junctions
. Allow for rapid conduction of electrical activity from 1 cardiac m. Fiber to another
. Makes it so cardiac m. Acts as one unit (functional syncytium)
. Single depolarizing stimulus results in contraction of entire myocardium
. Cardiac m. Is fully recruited at all times
T/F ANS modulates cardiac contractile performance
T
SNS effect on cardiac cells
. Activate beta-1 receptors
. Inc. contractility
PNS effect on cardiac cell
. Slow HR and conduction
. Effect on contractility is normally small
Difference btw skeletal and cardiac mm. E-C coupling
. Ca in skeletal m. Comes from SR
. Ca in cardiac m. Comes from both SR and from outside
Main regulatory site for cardiac function
L-type Ca channel
Ca-induced Ca release in cardiac cells
. Extracellular Ca can activate contractile protein directly or trigger release from SR
. Ca release is positive feedback loop
. Ca channels are stimulated at low cytosolic levels and inhibited at high cytosolic levels
Peak level of cytoplasmic Ca depends on ____
. Amount of Ca stored in SR
. HR
. Amount of Ca entering from extracellular space
Resequesteration of Ca
. Primary mechanism to return Ca to resting levels
. ATP-dependent reuptake into SR via Ca-ATPase
. High levels Ca stimulate pump
Extrusion of Ca in cardiac muscle
. Na-Ca exchange: removes 1 Ca from intracellular space for 3 Na moving into cell
. Ca-ATPase: not as important as Na-Ca exchanger
Na-Ca exchanger
. 1 Ca out for 3 Na in
. Electro genie (net intracellular gain of 1 pos. Charge)
. Not directly ATP-requiring pumps indirectly dependent on Na gradient
Major mechanisms for gradation of force in cardiac mm.
. Change in contractility
. Change in initial fiber length
Recruitment of cardiac muscle cells
. Cardiac muscle is fully recruited
. Considered analogous to 1 giant motor unit
. Activity initiated by SA node
Summation of cardiac muscle cells
. Inc. frequency of stimulation (inc. HR) does not result in summation and titanic contractions like it does in skeletal muscle
Refractory period in cardiac cells
. Nearly as long as contractile process
. Cell can’t be re-excited in time to cause summation of contractile force
Contractility
. Performance of heart at given period
. Ability of muscle tog enervate force from given resting fiber length
. Inc. in contractility (positive inotropic effect): inc. in tension developed at given fiber length
. Negative inotropic effect: dec. in tension development at given fiber length
Role of Ca in changes in contractility
. Alteration of Ca flux is almost always predominant mechanism for alteration in contractility
. Ca flux regulated on beat-to-beat basis
. Ca flux from 1 AP is enough to activate 1/2 crossbridges
. Additional cytoplasmic Ca will result in additional force production
Heart rate effect on Ca flux
. Inc. frequency of stimulation inc. contractility (Bowditch, Staircase, or Treppe effect)
. Intrinsic property of cardiac m.
. HR inc., number of APs per unit inc., inc. Ca current per unit time
. Less time btw beats to extrude excess Ca that entered
. Excess Ca is sequestered by SR and thus more Ca is available for release in subsequent beats
. If interbeat interval is too short, the basal level of intracellular Ca may inc.
Length-tension relationship in cardiac muscle
. Operates below Lmax
. Significant role in modulation of force production bc large changes in initial cardiac m. Length (sarcomere length) can occur
. Normal operates on ascending part of relationship curve (normal initial resting lengths are below optimal length for producing force)
What determines initial cardiac muscle length in the intact heart?
. Blood contained in ventricles just prior to beginning of contraction (end-diastolic volume) determines what degree ventricles are distended
. Greater EDV, greater force produced
How does inc. In muscle fiber length result in increased force of contraction?
. Number of functional crossbridges inc.
. Ca-sensitivity to Tn-C is length-dependent (inc. in length results in inc. affinity of Tn-C for Ca)
Starling’s law of the heart
. Force of contraction is dependent on initial fiber length
Cardiac glycosides (digitalis)
. Drugs that treat HF
. Positive inotropic agents that inc. myocardial contractility
. Inhibits NA-ATPase causes accumulation of intracellular Na that is exchanged for Ca
. Inc. intracellular Ca inc. contractility
. Too high levels of drugs can cause arrhythmia
Negative inotropic agents
. Ca channel blockers
. Limited use
