Can be self generating; can be conducted directly from cell to cell; long durations
List the three ways in which cardiac muscle action potentials differ from those in skeletal muscle.
____ allows the heart to sustain rhythmic contractions without CNS input.
Cell to cell conduction
____ helps the muscle contract as a unit.
____ prevents fusion of individual twitch contractions.
What is the potassium equilibrium potential in cardiac muscle cells?
What is the sodium equilibrium potential in cardiac muscle cells?
K+; depolarized; Na+
____ is the main contributor to membrane potential, but the resting membrane potential is ____ compared to its equilibrium potential and this difference is the ____ contribution.
During a fast response AP, depolarization occurs so quickly, that the activation gate is opened and ____ influxes before the inactivation gate has a chance to close; the depolarization slowly causes ____ activation gate to open before the inactivation gate closes.
During a slow response AP, depolarization is so slow that the ____ gate has a chance to close before Na+ influxes into the cell; the Ca2+ ____ gate still opens and allows for Ca2+ influx.
K+; fast; inactivation; slow
A higher than normal ____ extracellular concentration would result in a less-negative resting membrane potential, increasing the cardiac cell's excitability (because less depolarization will be needed to reach threshold). If such conditions are extreme and persist, the chronic depolarization can inactivate the ____ channels by closing ____ gates, leaving only the possibility of ____ APs. Large, sustained depolarizations, however, can inactivate both the fast and slow channels, making cardiac muscle cells inexcitable.
Provide mechanical connections and gap junctions (for low resistance electrical conduction).
What are the dual functions of intercalated disks?
Local current flow will flow through the ____ and depolarize the membrane of the adjacent cell and cause an action potential.
SA node; AV node
The AP is normally initiated at the ____ and spreads through the atria, to the ____, then to the conducting system which spreads the signal to the ventricular walls.
slow; ventricular filling before contraction
AV node has ____ conduction due to small-diameter cells which cannot conduct as quickly as bundle of His, bundle branches, and Purkinje fibers. This allows ____.
extremely rapid; unified ventricular contraction
Because of sharply rising APs and large cell diameters, electrical conduction is ____ throughout the His bundle, bundle branches, and Purkinje system. This allows for ____.
The ____ is the pacemaker of heart because spontaneous depolarization of resting membrane is most rapid and reaches threshold potential before cells elsewhere.
AV node; SA node
____ is the latent pacemaker of the heart because it has a faster spontaneous depolarization during the resting period than other cells except ____, and will also sometimes control the heart rhythm.
P wave; QRS complex; T wave
The ____ corresponds to atrial depolarization, the ____ corresponds to ventricular depolarization, and the ____ corresponds to ventricular repolarization.
Intrinsic heart rate
The uninfluenced rate of firing from the SA node (100 bpm).
Resting heart rate
Heart rate with some parasympathetic involvement, 70 bpm.
vagus; ACh; muscarinic; G protein; K+; cAMP; Na+; hyperpolarization; slow
Parasympathetic nervous system uses the ____ n. which releases ____ on SA nodal cells which binds to ____ receptor, leading to inhibitory ____ (causes increase in ____ conductance and suppression of adenylate cyclase which causes a fall in intracellular ____ which reduces inward ____). Functions to cause initial ____ of resting membrane potential by bringing it closer to the K+ equilibrium potential and ____ the rate of spontaneous depolarization of resting membrane.
norepinephrine; β1-adrenergic; adenylate cyclase; cAMP; Na+; Na+ & Ca2+
Sympathetic nerves release ____ on cardiac cells (binds to ____ receptors on SA nodal cells which cause stimulatory G protein. G protein increases ____ which causes increased ____ → increased open-state probability of ____ current channel) which increases inward current carried by ____ during diastolic interval.
Positive chronotropic effect
Factor that increases heart rate.
Negative chronotropic effect
Factor that decreases heart rate.
Positive dromotropic effect
Factor that increases conduction velocity.
Negative dromotropic effect
Factor that decreases conduction velocity.
sarcolemma; L-type Ca2+; Ca2+; SR; Ca2+
The excitation-contraction coupling of heart cells involves the depolarization of the ____ (running along t-tubules) and the opening of ____ channels. The subsequent influx of ____ causes further release from the ____. The dramatically increased ____ cytoplasmic concentration allows crossbridging to occur.
20%; Ca2+/Na+; Ca2+ ATPase; 80%; Ca2+ ATPase
Recovery (relaxation) is quickly initiated by pumping ____ of the Ca2+ out of the cell via ____ pumps and ____, and pumping ____ of the Ca2+ into the SR via ____.
____ is an inhibitor of cardiac muscle sarcoplasmic reticulum Ca2+-ATPase in the unphosphorylated state, but inhibition is relieved upon ____ of the protein.
Actin/myosin interactions can be limited/enhanced through variations in Ca2+ cytoplasmic concentration.
Explain why excitation-contraction coupling in cardiac muscle makes it possible for cardiac contractility to be modified.
A cardiac glycoside that slows down the Na+/K+ pump and therefore reduces the sodium gradient which causes an increase in intracellular Ca2+, thus increasing contractility.
Duration of cardiac muscle contraction is about the same as the action potential, which means the electrical refractory period is not over until the response is over, thus cannot be rapidly activated to produce tetanic contraction.
Explain why the cardiac muscle cell cannot exhibit tetanic contraction.
Fixed length contraction during which the muscle develops tension.
A contraction during which a muscle shortens against a constant load.
The maximal velocity at which a muscle can contract.
A force required to stretch a resting muscle to a desired length.
Tension developed when muscle is stimulated to contract while length is held constant.
Sum of resting and active tension.
The length at which the active tension is at a maximum.
Where does cardiac muscle operate in relation to Lmax?
Extent of overlap between thick and thin filaments in the sarcomere at rest; length-dependent change in calcium sensitivity; increase in amount of calcium released with excitation when the resting length is increased.
What are the three mechanisms that may be responsible for the length-tension relationship in cardiac muscle?
afterload isotonic contraction; preload; total load
An ____ is a complex contraction during which the ____ (load on muscle at rest) and the ____ (load on muscle during contraction) are different.
Positive inotropic effect
Intervention which increases the peak isometric tension that a muscle can develop at a fixed length that subsequently increases cardiac muscle contractility.
At a constant length, ____ increases the isometric tension developed by the muscle, it is said to cause an increase in contractility.
Positive lusitropic effect
Increase in the rate of relaxation.
NE; phospholamban; K+
____ causes phosphorylation of the regulatory enzyme, ____, which results in greater activity from the SR-bound Ca2+-ATPase and increases the rate that calcium is retrapped, Also, it increases ____ permeability which shortens the plateau phase and causes quicker repolarization.
Prevents compromise of diastolic refilling time.
What is the significance of NE effects at higher heart rates?
Staircase (treppe) phenomenon
A sudden increase in heart rate is followed by a progressive increase in contractility.
As the heart rate increases, more calcium enters each beat and the intracellular store builds up thus increasing contractility.
How is the treppe phenomenon explained?
T = P x r
Describe the effect of ventricular radius (or volume) on the relationship between intraventricular pressure and the cardiac muscle tension needed to produce that pressure.