cardiac muscle Flashcards
the properties of cardiac muscle.
- Excitability
- Conductivity
- Automaticity
- Rhythmicity
- Contractility
- Refractory period
- infatigability
Excitability
- Process of eliciting an action potential
- Resting membrane potential and action potential
- Pacemaker potential and Ventricular muscle action potential
✓RMP - potential difference existing across the cell membrane when the cell is “at rest”.
✓RMP in neuron or nerve fiber is –70 mV. (i.e the inner surface of the membrane is 70 mV negative as compared to the exterior surface).
✓RMP in Cardiac muscle = – 90 mV ✓RMPinSkeletalmuscle= –85to–90mV
Ventricular Action Potential (graph)
Phase 0: Rapid Depolarization
Opening of voltage-gated sodium channels: so Influx of Na+.
Positive charge enters the cell causing depolarization.
Phase 1 : Initial, Brief Repolarization
Opening of voltage gated K+ channels: so Mild K+ Efflux .
Phase 2: Plateau phase
Opening of voltage gated Ca++ channels.
Influx of Ca++ balances K+ efflux.
Sustained depolarization.
Causes ventricular contraction to last 15 times longer than skeletal muscle.
Phase 3: Rapid Repolarization
Inactivation of Ca++ channels.
Opening of K+ channels causes K+ efflux.
Strong K+ (positive charge) efflux brings membrane potential back to – 90 mV.
Phase 4: Resting Phase
RMP maintained at resting state.
Na+-K+ ATPase activated.
Intracellular Na+ and K+ concentrations back to normal.
Conductivity:
the ability of cardiac muscle fibers to conduct the cardiac impulses that are initiated in the SA-node
Components of conducting system
➢Sinoatrial node
➢Atrioventricular node
➢Inter-nodal pathways
➢Bundle of His
➢Purkinje fibers
The pacemaker of the heart: conductivity
= the SA- node.
♥ Contains the P- cells, which are probably the
actual pacemaker cells.
♥ Has the fastest rhythm (rate of discharge) of all parts of the heart, 90 impulses/min.
its fibers have an unstable RMP.
♥ Has spontaneous (w/out stimulation) depolarization, up to firing level.
The conduction velocities of the impulse:
SA-node -0.05 m/sec.
AV-node - 0.01 m/sec. … (slowest)
Bundle of His -1.00 m/sec.
Purkinje fibers -4.00 m/sec. …. (fastest)
Atrial & Ventricular muscles- 0.3 to 0.4 m/sec.
The slowest conduction velocity in AV-node:
■ because it has few no. of intercalated discs.
■ Importance: to allow sufficient time for ventricles to be filled w blood before they contract.
The fastest Conduction velocity in Purkinje fibers:
Importance: to allow the 2 ventricles to contract at the same time simultaneously.
The direction of the impulse
The impulse is conducted: 1st Atrial spread
■ from SA-node → conductive tissue → ventricles.
2nd Ventricular spread
■ from apex of the heart → base, via Purkinje fibers to the endocardial surface of ventricles.
Rhythmicity (automaticity):
the ability of cardiac muscle to contract in a regular constant manner
♥ myogenic
♥initiated by SA- node.
* Heart beats at regular intervals
* Rhythm denotes rate and site of production of impulse.
* Sinus rhythm – Cardiac impulse normally produced by SA node.
* In abnormal conditions when the SA node fails to produce an impulse, the other
structures may initiate the cardiac impulse.
* If the impulse is produced by the atrium, then it is called atrial rhythm.
* So we have a nodal rhythm and ventricular rhythm if the impulse production is from the node and ventricle respectively.
Automaticity
✓Self-excitation without any external stimulus.
✓Due to the presence of pacemaker in the heart.
✓Production of cardiac impulse is automatic.
Autorhythmicity
- Automaticity + Rhythmicity = Autorhythmicity
- Automaticity which is rhythmic - heart beats on its own without
external stimulus at regular intervals. - Other tissues like atria, ventricles and other parts of conducting system also has automaticity. In abnormal conditions like myocardial infarction or conduction blocks, they generate impulses.
Mechanism of contraction
- similar to that of skeletal muscles
-Excitation-contraction coupling is provided by Ca2+ ions
sources
1. Influx of small amounts of Ca++ from the ECF during AP generation (plateau phase) via DHP (dihydropyridine) channel - L-type voltage-gated Ca2+ channel (ICa-L) - Cannot trigger E-C coupling
- Triggers the release of Ca2+ from SR (via RyR) - calcium-induced Ca2+ release
2. SR (is less developed than in skeletal muscles): Ca2+ from SR enter the cytosol via ryanodine channel (RyR) and triggers E-C coupling
Contractile responses
- All or none law: Either full response or no response at any constant conditions is due to syncytial nature.
- Staircase phenomenon: When the frequency of stimulation increases, the
tension developed gradually increases [stepwise manner] to a new steady state. - Summation of subliminal stimuli: A contractile response elicited when a series of successive stimuli are applied at a rapid rate.
Refractory period
- Period of action potential in which a second
stimulus will not produce a contractile response - Two types: Absolute and Relative
- Cardiac muscle has a long refractory
period: 300 msec - Since the refractory period lasts whole of contraction and part of relaxation period, heart muscle cannot be tetanized.
- ARP + RRP = Total refractory period
dif between ARP AND RRP
