Muscle VII Flashcards
What is cardiac muscle made of
- cardiac myocytes (myocardial muscle cells)
- all cells interconnected by intercalated disks
- larger t-tubules
- smaller amounts of SR
- ## abundance of mitochondria (1/3 cell volume)
What is main role of cardiac muscle
Generates force to move blood throughout whole blood
What are cardiac myocytes
Shorter, branched cells and usually contain single nucleus
What are two types of intercalated disks
- desmosomes link mechanically (uniform contraction)
- gap junctions link them electrically (uniform propagation)
What type of calcium does cardiac muscle require more than skeletal
Extracellular calcium
What type of metabolism for cardiac muscle for ATP production
Oxidative metabolism
How many cardiac muscle cells contract in average life span
- almost 3 billion times without resting
- 70x/min
What is cardiac contraction initiated by
Autorhythmic cells (pacemakers)
What are autorhythmic cells
1% of myocardial cells are specialized non-contracting cells
- generate APs spontaneously
- smaller and contain few contractile fibers
- pass conduction out to contractile cells
Where does depolarization occur in heart
Sinoatrial node (main pacemaker of heart)
What are gap junctions for in heart
AP travels through
- calcium and sodium diffuses to adjacent cell causing depolarization
- auto-rhythmic cells are electrically coupled to auto-rhythmic cells and then coupled to contractile cells and contractile to each other
Why do APs in autorhythmic cells occur spontaneously
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels
- depolarize at negative membrane potential
Where are HCN channels
Only in autorhythmic cells
What is plateau of cardiac muscle AP for
Prevents summation
- voltage gated Na+ channels in inactive state
- creates long refractory period
- don’t want summation, rest so chamber can fill and then twitch again
How much calcium from SR and from ECF in cardiac cells
SR 90%
ECF 10%
What is necessary for excitation-contraction coupling in cardiac muscle
L-type Ca channels (DHP receptors) are not mechanically coupled to ryanidine receptors
- Ca entry is necessary (Ca induced Ca release)
What are steps of excitation-contraction coupling in cardiac muscle
- AP enter from adjacent cell
- Voltage gated Ca channels open (caused by DHP receptors sensing depolarization) Ca enters cell
- Ca induces Ca release through RyR
- Local release causes Ca spark
- Summed Ca sparks create Ca signal
- Ca ions bind to troponin and initiate contraction
What triggers contraction in cardiac muscle
Increase in Ca removing the inhibition of cross bridge cycling (same as skeletal)
- once at Ca signal cycle is the same
Relaxation of cardiac muscle
Similar to skeletal
- removal of Ca to ECF is more important, more activity of sarcolemma Ca ATPase and Na-Ca exchanger
- reuptake to SR is still primary mechanism
What is the SERCA pump regulated by in cardiac muscle
Phospholamban (PLN)
How does PLN regulate SERCA pump in dephosphorylated state
Inhibits SERCA
How does PLN regulate SERCA pump in phosphorylated state
Ca2+ pump inhibition is removed, enhancing relaxation rates contractility
- rate and amount of Ca uptake is increased, causing quicker relaxation and larger store of Ca for subsequent contractions
What are 2 ways to enhance contractile force in cardiac muscle
- Increase in intracellular Ca in cardiac myocytes
- Length tension relationship: generates greater force when slightly stretched (stretched when ventricle full)
What happens when there is an increase in intracellular calcium in cardiac myocytes
- capable of graded single twitch contractions (force can change)
- is Ca is low some actin remains covered by tropomyosin
- increased Ca additional troponin complexes activated so more cross bridge formation
How does neuronal input in the hear modify conduction/contraction
By ANS:
- sympathetic increases heart rate/conduction and contractility (autorhythmic and contractile) (frequency and force)
- parasympathetic decreases HR/conduction (autorhythmic) (frequency)
What are the steps of sympathetic modulation of contraction
- Phosphorylation of Ca channels increase Ca conductance during APs
- Phosphorylation of RyR enhances sensitivity to Ca, increasing release of Ca from SR
3.increase rate of myosin ATPase - Phosphorylation of SERCA (PLN) increases the speed of CA re-uptake which increases storage
What does norepinephrine or epinephrine act on in sympathetic modulation
Acts on B1 adrenergic receptor which acts on adenylate cyclase
What does calcium reuptake increase in cardiac muscle
Increase oxygen delivery and frequency and force of HR to increase blood delivery
