Cardiac muscle

3D rendering showing thick myocardium within the heart wall.
The swirling musculature of the heart ensures effective pumping of blood.
Cardiac muscle
Illustration of a cardiac muscle cell.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
Dog cardiac muscle (400X)

One of three types of vertebrate muscle tissue, with the other two being skeletal muscle and smooth muscle.

- Cardiac muscle

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Alpha

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.

Action potential

Action potential occurs when the membrane potential of a specific cell location rapidly rises and falls.

Action potential occurs when the membrane potential of a specific cell location rapidly rises and falls.

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.
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.
Action potential propagation along an axon
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.
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.
In pacemaker potentials, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.
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.

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

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

Sinoatrial node

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

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.

Skeletal muscle, with myofibrils labeled at upper right.

Myofibril

Basic rod-like organelle of a muscle cell.

Basic rod-like organelle of a muscle cell.

Skeletal muscle, with myofibrils labeled at upper right.
Muscle fibre organisation
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A diagram of the structure of a myofibril (consisting of many myofilaments in parallel, and sarcomeres in series)
Sliding filament model of muscle contraction

In striated skeletal and cardiac muscle tissue the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells).

The progression of atherosclerosis (narrowing exaggerated)

Atherosclerosis

Pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions.

Pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions.

The progression of atherosclerosis (narrowing exaggerated)
Atherosclerosis and lipoproteins
Micrograph of an artery that supplies the heart showing significant atherosclerosis and marked luminal narrowing. Tissue has been stained using Masson's trichrome.
Severe atherosclerosis of the aorta. Autopsy specimen.
Progression of atherosclerosis to late complications.
CT image of atherosclerosis of the abdominal aorta. Woman of 70 years old with hypertension and dyslipidemia.
Microphotography of arterial wall with calcified (violet color) atherosclerotic plaque (hematoxylin and eosin stain)
Doppler ultrasound of right internal carotid artery with calcified and non-calcified plaques showing less than 70% stenosis

One recent hypothesis suggests that, for unknown reasons, leukocytes, such as monocytes or basophils, begin to attack the endothelium of the artery lumen in cardiac muscle.

Smooth muscle

Involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations .

Involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations .

Smooth muscle tissue, highlighting the inner circular layer (nuclei then rest of cells in pink), outer longitudinal layer (nuclei then rest of cells), then the serous membrane facing the lumen of the peritoneal cavity
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
A series of axon-like swellings, called varicosities from autonomic neurons, loosely form motor units through the smooth muscle.
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Smooth muscle differs from skeletal muscle and cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling.

Myofilament

Myofilament

Myofilaments are the three protein filaments of myofibrils in muscle cells.

Myofilaments are the three protein filaments of myofibrils in muscle cells.

Myofilament
Muscle fiber showing thick and thin myofilaments of a myofibril.

Types of muscle tissue are striated skeletal muscle and cardiac muscle, obliquely striated muscle (found in some invertebrates), and non-striated smooth muscle.

Isolated heart conduction system showing Purkinje fibers

Purkinje fibers

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.

During the ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse from both the left and right bundle branch to the myocardium of the ventricles.

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Troponin

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Troponin activation. Troponin C (red) binds Ca2+, which stabilizes the activated state, where troponin I (yellow) is no longer bound to actin. Troponin T (blue) anchors the complex on tropomyosin.

Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle.

Skeletal muscle fiber, with T-tubule labelled in zoomed in image.

T-tubule

Skeletal muscle fiber, with T-tubule labelled in zoomed in image.

T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the centre of skeletal and cardiac muscle cells.

Leads aVL and aVF of an electrocardiogram showing atrial fibrillation. There are irregular intervals between heart beats. No P waves are seen and there is an erratic baseline between QRS complexes. The heart rate is about 125 beats per minute.

Atrial fibrillation

Abnormal heart rhythm (arrhythmia) characterized by rapid and irregular beating of the atrial chambers of the heart.

Abnormal heart rhythm (arrhythmia) characterized by rapid and irregular beating of the atrial chambers of the heart.

Leads aVL and aVF of an electrocardiogram showing atrial fibrillation. There are irregular intervals between heart beats. No P waves are seen and there is an erratic baseline between QRS complexes. The heart rate is about 125 beats per minute.
Normal rhythm tracing (top) Atrial fibrillation (bottom)
How a stroke can occur during atrial fibrillation
Non-modifiable risk factors (top left box) and modifiable risk factors (bottom left box) for atrial fibrillation. The main outcomes of atrial fibrillation are in the right box. BMI=Body Mass Index.
A 12-lead ECG showing atrial fibrillation at approximately 132 beats per minute
Diagram of normal sinus rhythm as seen on ECG. In atrial fibrillation the P waves, which represent depolarization of the top of the heart, are absent.
ECG of atrial fibrillation (top) and normal sinus rhythm (bottom). The purple arrow indicates a P wave, which is lost in atrial fibrillation.
3D Medical Animation still shot of Left Atrial Appendage Occlusion

All of these mutations affect the processes of polarization-depolarization of the myocardium, cellular hyper-excitability, shortening of effective refractory period favoring re-entries.