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  • The heart is essentially a muscle that contracts and pumps blood.

  • It consists of specialized muscle cells called cardiac myocytes.

  • The contraction of these cells is initiated by electrical impulses, known as action potentials.

  • Unlike skeletal muscles, which have to be stimulated by the nervous system, the heart

  • generates its OWN electrical stimulation.

  • In fact, a heart can keep on beating even when taken out of the body.

  • The nervous system can make the heartbeats go faster or slower, but cannot generate them.

  • The impulses start from a small group of myocytes called the PACEMAKER cells, which constitute

  • the cardiac conduction system.

  • These are modified myocytes that lose the ability to contract and become specialized

  • for initiating and conducting action potentials.

  • The SA node is the primary pacemaker of the heart.

  • It initiates all heartbeats and controls heart rate.

  • If the SA node is damaged, other parts of the conduction system may take over this role.

  • The cells of the SA node fire SPONTANEOUSLY, generating action potentials that spread though

  • the contractile myocytes of the atria.

  • The myocytes are connected by gap junctions, which form channels that allow ions to flow

  • from one cell to another.

  • This enables electrical coupling of neighboring cells.

  • An action potential in one cell triggers another action potential in its neighbor and the signals

  • propagate rapidly.

  • The impulses reach the AV node, slow down a little to allow the atria to contract, then

  • follow the conduction pathway and spread though the ventricular myocytes.

  • Action potential generation and conduction are essential for all myocytes to act in synchrony.

  • Pacemaker cells and contractile myocytes exhibit different forms of action potentials.

  • Cells are polarized, meaning there is an electrical voltage across the cell membrane.

  • In a resting cell, the membrane voltage, known as the RESTING membrane potential, is usually

  • negative.

  • This means the cell is more NEGATIVE on the INSIDE.

  • At this resting state, there are concentration gradients of several ions across the cell

  • membrane: more sodium and calcium OUTSIDE the cell, and more potassium INSIDE the cell.

  • These gradients are maintained by several pumps that bring sodium and calcium OUT, and

  • potassium IN.

  • An action potential is essentially a brief REVERSAL of electric polarity of the cell

  • membrane and is produced by VOLTAGE-gated ion channels.

  • These channels are passageways for ions in and out of the cell, and as their names suggest,

  • are regulated by membrane voltage.

  • They open at some values of membrane potential and close at others.

  • When membrane voltage INCREASES and becomes LESS negative, the cell is LESS polarized,

  • and is said to be DE-polarized.

  • Reversely, when membrane potential becomes MORE negative, the cell is RE-polarized.

  • For an action potential to be generated, the membrane voltage must DE-polarize to a critical

  • value called the THRESHOLD.

  • The pacemaker cells of the SA node SPONTANEOUSLY fire about 80 action potentials per minute,

  • each of which sets off a heartbeat, resulting in an average heart rate of 80 beats per minute.

  • Pacemaker cells do NOT have a TRUE RESTING potential.

  • The voltage starts at about -60mV and SPONTANEOUSLY moves upward until it reaches the threshold

  • of -40mV.

  • This is due to action of so-calledFUNNYcurrents present ONLY in pacemaker cells.

  • Funny channels open when membrane voltage becomes lower than -40mV and allow slow influx

  • of sodium.

  • The resulting DE-polarization is known aspacemaker potential”.

  • At threshold, calcium channels open, calcium ions flow into the cell further DE-polarizing

  • the membrane.

  • This results in the rising phase of the action potential.

  • At the peak of depolarization, potassium channels open, calcium channels inactivate, potassium

  • ions leave the cell and the voltage returns to -60mV.

  • This corresponds to the falling phase of the action potential.

  • The original ionic gradients are restored thanks to several ionic pumps, and the cycle

  • starts over.

  • Electrical impulses from the SA node spread through the conduction system and to the contractile

  • myocytes.

  • These myocytes have a different set of ion channels.

  • In addition, their sarcoplasmic reticulum, the SR, stores a large amount of calcium.

  • They also contain myofibrils.

  • The contractile cells have a stable resting potential of -90mV and depolarize ONLY when

  • stimulated, usually by a neighboring myocyte.

  • When a cell is DE-polarized, it has more sodium and calcium inside the cell.

  • These positive ions leak through the gap junctions to the adjacent cell and bring the membrane

  • voltage of this cell up to the threshold of -70mV.

  • At threshold, FAST sodium channels open creating a rapid sodium influx and a sharp rise in

  • voltage.

  • This is the depolarizing phase.

  • L-type, or SLOW, calcium channels also open at -40mV, causing a slow but steady influx.

  • As the action potential nears its peak, sodium channels close quickly, voltage-gated potassium

  • channels open and these result in a small decrease in membrane potential, known as EARLY

  • RE-polarization phase.

  • The calcium channels, however, remain open and the potassium efflux is eventually balanced

  • by the calcium influx.

  • This keeps the membrane potential relatively stable for about 200 msec resulting in the

  • PLATEAU phase, characteristic of cardiac action potentials.

  • Calcium is crucial in coupling electrical excitation to physical muscle contraction.

  • The influx of calcium from the extracellular fluid, however, is NOT enough to induce contraction.

  • Instead, it triggers a MUCH greater calcium release from the SR, in a process known as

  • calcium-induced calcium release".

  • Calcium THEN sets off muscle contraction by the samesliding filament mechanism

  • described for skeletal muscle.

  • The contraction starts about half way through the plateau phase and lasts till the end of

  • this phase.

  • As calcium channels slowly close, potassium efflux predominates and membrane voltage returns

  • to its resting value.

  • Calcium is actively transported out of the cell and also back to the SR.

  • The sodium/potassium pump then restores the ionic balance across the membrane.

  • Because of the plateau phase, cardiac muscle stays contracted longer than skeletal muscle.

  • This is necessary for expulsion of blood from the heart chambers.

  • The absolute refractory period is also much longer - 250 msec compared to 1 msec in skeletal

  • muscle.

  • This long refractory period is to make sure the muscle has relaxed before it can respond

  • to a new stimulus and is essential in preventing summation and tetanus, which would stop the

  • heart from beating.

The heart is essentially a muscle that contracts and pumps blood.

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心臓の活動電位、アニメーション。 (Cardiac Action Potential, Animation.)

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    楊瑋珊 に公開 2021 年 01 月 14 日
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