A report on Action potential and Cardiac muscle

As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na+) and potassium- (K+) gated ion channels open and close as the membrane reaches its threshold potential. Na+ channels open at the beginning of the action potential, and Na+ moves into the axon, causing depolarization. Repolarization occurs when the K+ channels open and K+ moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.
Shape of a typical action potential. The membrane potential remains near a baseline level until at some point in time, it abruptly spikes upward and then rapidly falls.
3D rendering showing thick myocardium within the heart wall.
Approximate plot of a typical action potential shows its various phases as the action potential passes a point on a cell membrane. The membrane potential starts out at approximately −70 mV at time zero. A stimulus is applied at time = 1 ms, which raises the membrane potential above −55 mV (the threshold potential). After the stimulus is applied, the membrane potential rapidly rises to a peak potential of +40 mV at time = 2 ms. Just as quickly, the potential then drops and overshoots to −90 mV at time = 3 ms, and finally the resting potential of −70 mV is reestablished at time = 5 ms.
The swirling musculature of the heart ensures effective pumping of blood.
Action potential propagation along an axon
Cardiac muscle
Ion movement during an action potential.
Key: a) Sodium (Na+) ion. b) Potassium (K+) ion. c) Sodium channel. d) Potassium channel. e) Sodium-potassium pump. In the stages of an action potential, the permeability of the membrane of the neuron changes. At the resting state (1), sodium and potassium ions have limited ability to pass through the membrane, and the neuron has a net negative charge inside. Once the action potential is triggered, the depolarization (2) of the neuron activates sodium channels, allowing sodium ions to pass through the cell membrane into the cell, resulting in a net positive charge in the neuron relative to the extracellular fluid. After the action potential peak is reached, the neuron begins repolarization (3), where the sodium channels close and potassium channels open, allowing potassium ions to cross the membrane into the extracellular fluid, returning the membrane potential to a negative value. Finally, there is a refractory period (4), during which the voltage-dependent ion channels are inactivated while the Na+ and K+ ions return to their resting state distributions across the membrane (1), and the neuron is ready to repeat the process for the next action potential.
Illustration of a cardiac muscle cell.
When an action potential arrives at the end of the pre-synaptic axon (top), it causes the release of neurotransmitter molecules that open ion channels in the post-synaptic neuron (bottom). The combined excitatory and inhibitory postsynaptic potentials of such inputs can begin a new action potential in the post-synaptic neuron.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.
Dog cardiac muscle (400X)
In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.
Comparison of the conduction velocities of myelinated and unmyelinated axons in the cat. The conduction velocity v of myelinated neurons varies roughly linearly with axon diameter d (that is, v ∝ d), whereas the speed of unmyelinated neurons varies roughly as the square root (v ∝√d). The red and blue curves are fits of experimental data, whereas the dotted lines are their theoretical extrapolations.
Cable theory's simplified view of a neuronal fiber. The connected RC circuits correspond to adjacent segments of a passive neurite. The extracellular resistances re (the counterparts of the intracellular resistances ri) are not shown, since they are usually negligibly small; the extracellular medium may be assumed to have the same voltage everywhere.
Electrical synapses between excitable cells allow ions to pass directly from one cell to another, and are much faster than chemical synapses.
Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.
Giant axons of the longfin inshore squid (Doryteuthis pealeii) were crucial for scientists to understand the action potential.
As revealed by a patch clamp electrode, an ion channel has two states: open (high conductance) and closed (low conductance).
Tetrodotoxin is a lethal toxin found in pufferfish that inhibits the voltage-sensitive sodium channel, halting action potentials.
Image of two Purkinje cells (labeled as A) drawn by Santiago Ramón y Cajal in 1899. Large trees of dendrites feed into the soma, from which a single axon emerges and moves generally downwards with a few branch points. The smaller cells labeled B are granule cells.
Ribbon diagram of the sodium–potassium pump in its E2-Pi state. The estimated boundaries of the lipid bilayer are shown as blue (intracellular) and red (extracellular) planes.
Equivalent electrical circuit for the Hodgkin–Huxley model of the action potential. Im and Vm represent the current through, and the voltage across, a small patch of membrane, respectively. The Cm represents the capacitance of the membrane patch, whereas the four g's represent the conductances of four types of ions. The two conductances on the left, for potassium (K) and sodium (Na), are shown with arrows to indicate that they can vary with the applied voltage, corresponding to the voltage-sensitive ion channels. The two conductances on the right help determine the resting membrane potential.

The primary function of both muscle types is to contract, and in both cases, a contraction begins with a characteristic flow of ions across the cell membrane known as an action potential.

- Cardiac muscle

Using voltage-sensitive dyes, action potentials have been optically recorded from a tiny patch of cardiomyocyte membrane.

- Action potential
As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na+) and potassium- (K+) gated ion channels open and close as the membrane reaches its threshold potential. Na+ channels open at the beginning of the action potential, and Na+ moves into the axon, causing depolarization. Repolarization occurs when the K+ channels open and K+ moves out of the axon, creating a change in electric polarity between the outside of the cell and the inside. The impulse travels down the axon in one direction only, to the axon terminal where it signals other neurons.

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Overall

Sinoatrial node shown at 1. The rest of the conduction system of the heart is shown in blue.

Sinoatrial node

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Group of cells known as pacemaker cells, located in the wall of the right atrium of the heart.

Group of cells known as pacemaker cells, located in the wall of the right atrium of the heart.

Sinoatrial node shown at 1. The rest of the conduction system of the heart is shown in blue.
Figure 2: Low magnification stained image of the SA node (center-right on image) and its surrounding tissue. The SA node surrounds the sinoatrial nodal artery, seen as the open lumen. Cardiac muscle cells of the right atrium can be seen to the left of the node, and fat tissue to the right.
Figure 3: Sinoatrial node action potential waveform, outlining major ion currents involved (downward deflection indicates ions moving into the cell, upwards deflection indicates ions flowing out of the cell).
Schematic representation of the atrioventricular bundle

These cells can produce an electrical impulse (action potential) that travels through the electrical conduction system of the heart, causing it to contract.

The main role of a sinoatrial node cell is to initiate action potentials of the heart that can pass through cardiac muscle cells and cause contraction.

Drugs affecting the cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing efflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.

Cardiac action potential

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Drugs affecting the cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing efflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive calcium channels.
Different shapes of the cardiac action potential in various parts of the heart
Action potentials recorded from sheep atrial and ventricular cardiomyocytes with phases shown. Ion currents approximate to ventricular action potential.
Figure 2a: Ventricular action potential (left) and sinoatrial node action potential (right) waveforms. The main ionic currents responsible for the phases are below (upwards deflections represent ions flowing out of cell, downwards deflection represents inward current).
Figure 4: The electrical conduction system of the heart

The cardiac action potential is a brief change in voltage (membrane potential) across the cell membrane of heart cells.

The cardiac action potential differs from action potentials found in other types of electrically excitable cells, such as nerves.

Heart

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Muscular organ in most animals.

Muscular organ in most animals.

Human heart during an autopsy
Computer-generated animation of a beating human heart
The human heart is in the middle of the thorax, with its apex pointing to the left.
Heart being dissected showing right and left ventricles, from above
Frontal section showing papillary muscles attached to the tricuspid valve on the right and to the mitral valve on the left via chordae tendineae.
Layers of the heart wall, including visceral and parietal pericardium
The swirling pattern of myocardium helps the heart pump effectively
Arterial supply to the heart (red), with other areas labelled (blue).
Autonomic innervation of the heart
Development of the human heart during the first eight weeks (top) and the formation of the heart chambers (bottom). In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different pattern from that of an adult.
Blood flow through the valves
The cardiac cycle as correlated to the ECG
The x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.
Transmission of a cardiac action potential through the heart's conduction system
Conduction system of the heart
The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell; there is no resting potential.
3D echocardiogram showing the mitral valve (right), tricuspid and mitral valves (top left) and aortic valve (top right).
The closure of the heart valves causes the heart sounds.
Cardiac cycle shown against ECG
Heart and its blood vessels, by Leonardo da Vinci, 15th century
Animated heart
Elize Ryd making a heart sign at a concert in 2018
The tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depicts cell nuclei.
Basic arthropod body structure – heart shown in red
The human heart viewed from the front
The human heart viewed from behind
The coronary circulation
The human heart viewed from the front and from behind
Frontal section of the human heart
An anatomical specimen of the heart
Heart illustration with circulatory system
Animated Heart 3d Model Rendered in Computer

The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.

Cells in the sinoatrial node do this by creating an action potential.

Image showing the cardiac pacemaker or SA node, the normal pacemaker within the electrical conduction system of the heart.

Cardiac pacemaker

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Image showing the cardiac pacemaker or SA node, the normal pacemaker within the electrical conduction system of the heart.
Schematic representation of the sinoatrial node and the atrioventricular bundle of His. The location of the SA node is shown in blue. The bundle, represented in red, originates near the orifice of the coronary sinus, undergoes slight enlargement to form the AV node. The AV node tapers down into the bundle of HIS, which passes into the ventricular septum and divides into two bundle branches, the left and right bundles. The ultimate distribution cannot be completely shown in this diagram.

The contraction of cardiac muscle (heart muscle) in all animals is initiated by electrical impulses known as action potentials.

Isolated heart conduction system showing Purkinje fibers

Purkinje fibers

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The Purkinje fibers (often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium.

The Purkinje fibers (often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium.

Isolated heart conduction system showing Purkinje fibers
Purkinje fiber just beneath the endocardium.

They are larger than cardiomyocytes with fewer myofibrils and many mitochondria.

In short, they generate action potentials, but at a slower rate than the sinoatrial node.

Ventricular fibrillation (VF) showing disorganized electrical activity producing a spiked tracing on an electrocardiogram (ECG)

Arrhythmia

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Too fast or too slow.

Too fast or too slow.

Ventricular fibrillation (VF) showing disorganized electrical activity producing a spiked tracing on an electrocardiogram (ECG)
Broad classification of arrhythmias according to region of heart required to sustain the rhythm
Normal sinus rhythm, with solid black arrows pointing to normal P waves representative of normal sinus node function, followed by a pause in sinus node activity (resulting in a transient loss of heartbeats). Note that the P wave that disrupts the pause (indicated by the dashed arrow) does not look like the previous (normal) P waves – this last P wave is arising from a different part of the atrium, representing an escape rhythm.

Automaticity refers to a cardiac muscle cell firing off an impulse on its own.

All of the cells in the heart have the ability to initiate an action potential; however, only some of these cells are designed to routinely trigger heartbeats.

General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril

Muscle cell

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General structure of a skeletal muscle cell and neuromuscular junction: 1. Axon

2. Neuromuscular junction

3. Skeletal muscle fiber

4. Myofibril
Diagram of skeletal muscle fiber structure

A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells.

This contraction of the myocyte is triggered by the action potential over the cell membrane of the myocyte.

As long as the stimulus reaches the threshold, the full response would be given. Larger stimulus does not result in a larger response, vice versa.

All-or-none law

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As long as the stimulus reaches the threshold, the full response would be given. Larger stimulus does not result in a larger response, vice versa.

In physiology, the all-or-none law (sometimes the all-or-none principle or all-or-nothing law) is the principle that if a single nerve fibre is stimulated, it will always give a maximal response and produce an electrical impulse of a single amplitude.

It was first established by the American physiologist Henry Pickering Bowditch in 1871 for the contraction of heart muscle.