Question

Topic 7: Cardiac action potential.

Define and describe the differences in the location and function of cardiac autorhythmic and contractile cells. in detail draw and describe the action potential of these two cell types of making sure to include

• Voltages At which important events occur

  • Resting membrane potential

  • Threshold potential

  • Peak of the action potential

  • Repolarization and hyperpolarization phase

• types of different ion channels and their state throughout

  • When do they open and close?

• What ions are moving where and when

• any other relevant information

Answer 1

• Rhythmicity or autorhythmicity or self-excitation is the ability of a tissue to produce its own impulses regularly. heart has a specialized excitatory structure called pacemaker. From here, the impulses spread to other parts through the specialized conductive system.
• cardiac autorhythmic cells are composed of SA node, AV node and AV bundles.
• the auto rhythmic cells which serves as a pacemaker helps in initiating the cardiac cycle ie, pumping the blood from the heart to other body organs. autorhythmic cells can generate their own action potential

• contractile cells are those cells which are responsible for contractions that pump blood through the body.
• most of the muscle cells in the heart are contractile cells.
• contractile cells cannot generate their own action potential but can cause mechanical contraction.
• It is responsible for keeping the heart pumping and blood circulating around the body.

Resting membrane potential in SA node has a negativity of –55 to –60 mV.

threshold potential of –40 mV

peak of action potential occurs at +5mv

Depolarization starts very slowly and the threshold level of –40 mV is reached very slowly. After the threshold level, rapid depolarization occurs up to +5 mV. It is followed by rapid repolarization.

ionic basis of electrical activity in pace maker

PACEMAKER POTENTIAL OR RESTING MEMBRANE POTENTIAL

• sodium channels (I_f) open allowing sodium ions leak into the pacemaker fibers and cause slow depolarization.
• Transient (T-type) calcium channels (I_caT) open at -50mv, bringing the membrane closer to threshold.
• Once the threshold is reached, long-lasting (L-type) voltage-gated calcium channels (I_caL) open.
• Thus, the initial part of pacemaker potential is due to slow influx of sodium ions and the later part is due to the slow influx of calcium ions.

DEPOLARIZATION

• The depolarization occurs because of influx of more calcium ions.
• When the negativity is decreased to –40 mV, which is the threshold level, the action potential starts with rapid depolarization.

REPOLARIZATION

• It is due to the efflux of potassium ions (IK)from pacemaker fibers. Potassium channels remain open for a longer time, causing efflux of more potassium ions.
• leads to the development of more negativity, beyond the level of resting membrane potential
• exists only for a short period.
• Then, the slow depolarization starts once again.

CONTRACTILE CELLS ACTION POTENTIAL

1 = Depolarization, 2 = Initial rapid repolarization, 3 = Plateau, 4 = Final repolarization.

Resting Membrane Potential= – 85 to – 95 mV

threshold potential= -70mv

1. Initial Depolarization

• very rapid and it lasts for about 2 msec . Amplitude of depolarization is about + 20 mV
• Rapid opening of fast sodium channels and the rapid influx of sodium ions

2. Initial Repolarization

• Immediately after depolarization
• rapid repolarization for a short period of about 2 msec
• opening of potassium channels and efflux of a small quantity of potassium ions from the muscle fiber.
• the fast sodium channels close suddenly and slow sodium channels open, resulting in
  slow influx of low quantity of sodium ions.

3. Plateau or Final Depolarization

• Plateau is due to the slow opening of calcium channels.
• These channels are kept open for a longer period and cause influx of large number of calcium ions.

4. Final Repolarization

• Final repolarization is due to efflux of potassium ions.
• Number of potassium ions moving out of the muscle fiber exceeds the number of calcium ions moving in.

Restoration of Resting Membrane Potential
after repolarization, all sodium ions, whichhad entered the cell move out of the cell and potassium ions moveinto the cell, by activation of sodium-potassium pump.