Comparative Muscle Flashcards
Cardiac Muscle Structure
Structure of Cardiac Muscle is very similar to that of skeletal muscle (thin to thick = 2:1) and clearly visible sarcomeres separated by Z lines
Contraction is like skeletal muscle via sliding filament mechanism
Cardiac muscles are highly branched and contain intercalated discs which are low resistance pathways that allow gap junction to spread excitation syncially
Excitation originating in the SA node is transmitted thru internodal paths and thru the atria, into the Ventricles via the AV node and thru the ventricles thru the Purkinje fibers after the perkinje fibers the excitation spreads from muscle cell to muscle cell thru intercalated discs
Spontaneous Electrical Activity of cardiac tissues
SA node is the pacemaker that dictates the contraction rate for the entire heart. SA nodal cells have a very depolarized membrane potential (close to threshold) than other cardiac muscle cells and exhibit spontaneous depolarization to reach threshold to trigger the next AP and contraction. SA rate of firing can be dictated by the vagus, and sympathetics
Purkinje fibers and ventricular muscle cells have a very negative Vm during diastole and exhibit rapid depolarization followed by plateau phase of depolarization and then a rapid recovery before the next beat.
Prolonged duration of AP in the heart prevents reactivation of cardiac cells and is responsible for the absolute refractory period (no excitation) and the relative refractory period (when a really strong stimulus is needed for excitation).
Refractory periods prevent premature excitation and maintain coordinated contraction
Ions involved in Cardiac muscle
An explosive increase in sodium conductance and inward sodium current occurs during the rapid depolarization phase of AP, depolarization inactivates Na channels and triggers a slow increase of outward K current terminating the Na influx
During the plataue phase, K current is reduced and Ca inward current starts (due to voltage gated Ca
A delayed increase in K current occurs (repolarizing the Vm) the repolarization also stops Ca from coming in
Exitation Contraction Coupling of cardiac tissue
Extracellular Ca influx via voltage gated Ca channels play an important role in excitation coupling in cardiac muscle. SR not so important and not close to the T tubule system as those in the skeletal muscle, so that Ca mediated Ca release plays a major role, via direct activation of the ryanodine sensitive Ca plugs (not by junctional foot proteins like skeletal muscle)
Increasing Ca via (Na Ca enxchanger)- regulates Ca inside (can counter or co transport during diastole or systole)
The exchanger is activated by high levels of intracellular Na (due to APs), Ouabain and digitalis inhibt exchanger to increase contraction
Removal of Ca from cyto: SERCA, Plasma PMCA (serca for membrane), NA/CA exchanger (not so important in smooth muscle)
Force velocity for Cardiac muscle, skeletal, and smooth muscle
skeletal muscle fixed (Vmax)
smooth muscle: variable
Cardiac: variable
