A report on Cardiac muscle and Myofilament

Myofilament
3D rendering showing thick myocardium within the heart wall.
Muscle fiber showing thick and thin myofilaments of a myofibril.
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)

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

- Myofilament

The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.

- Cardiac muscle

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Overall

A top-down view of skeletal muscle

Skeletal muscle

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Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton.

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

The other types of muscle are cardiac muscle which is also striated and smooth muscle which is non-striated; both of these types of muscle tissue are classified as involuntary, or, under the control of the autonomic nervous system.

The myofibrils are composed of actin and myosin filaments called myofilaments, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for muscle contraction.

Types of muscle contractions

Muscle contraction

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Activation of tension-generating sites within muscle cells.

Activation of tension-generating sites within muscle cells.

Types of muscle contractions
In vertebrate animals, there are three types of muscle tissues: 1) skeletal, 2) smooth, and 3) cardiac
Organization of skeletal muscle
Structure of neuromuscular junction.
Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions
Cross-bridge cycle
Muscle length versus isometric force
Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.
Swellings called varicosities belonging to an autonomic neuron innervate the smooth muscle cells.
Cardiac muscle
Key proteins involved in cardiac calcium cycling and excitation-contraction coupling
A simplified image showing earthworm movement via peristalsis
Asynchronous muscles power flight in most insect species. a: Wings b: Wing joint c: Dorsoventral muscles power the upstroke d: Dorsolongitudinal muscles (DLM) power the downstroke. The DLMs are oriented out of the page.
Electrodes touch a frog, and the legs twitch into the upward position

Unlike skeletal muscle, the contractions of smooth and cardiac muscles are myogenic (meaning that they are initiated by the smooth or heart muscle cells themselves instead of being stimulated by an outside event such as nerve stimulation), although they can be modulated by stimuli from the autonomic nervous system.

During a concentric contraction, contractile muscle myofilaments of myosin and actin slide past each other, pulling the Z-lines together.

Skeletal muscle, with myofibrils labeled at upper right.

Myofibril

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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

These proteins are organized into thick, thin, and elastic myofilaments, which repeat along the length of the myofibril in sections or units of contraction called sarcomeres.

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).

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.

A striated muscle fiber contains myofibrils consisting of long protein chains of myofilaments.

Micrograph of HPS stained skeletal striated muscle (fibularis longus).

Striated muscle tissue

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Micrograph of HPS stained skeletal striated muscle (fibularis longus).

Cardiac muscle (heart muscle)

Each muscle cell contains myofibrils composed of actin and myosin myofilaments repeated as a sarcomere.

Ribbon diagram of G-actin. ADP bound to actin's active site (multi color sticks near center of figure) as well as a complexed calcium dication (green sphere) are highlighted.

Actin

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Family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.

Family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.

Ribbon diagram of G-actin. ADP bound to actin's active site (multi color sticks near center of figure) as well as a complexed calcium dication (green sphere) are highlighted.
Fluorescence micrograph showing F-actin (in green) in rat fibroblasts
A merged stack of confocal images showing actin filaments within a cell. The image has been colour coded in the z axis to show in a 2D image which heights filaments can be found at within cells.
Structure of the C-terminal subdomain of villin, a protein capable of splitting microfilaments
The structure of a sarcomere, the basic morphological and functional unit of the skeletal muscles that contains actin
Diagram of a zonula occludens or tight junction, a structure that joins the epithelium of two cells. Actin is one of the anchoring elements shown in green.
Ribbon model of actin from rabbitmuscle. The four subdomains can be seen, as well as the N and C termini and the position of the ATP bond. The molecule is oriented using the usual convention of placing the - end (pointed end) up and the + end (barbed end) down.
F-actin; surface representation of a repetition of 13 subunits based on Ken Holmes' actin filament model
Ribbon model obtained using the PyMOL programme on crystallographs of the prefoldin proteins found in the archaean Pyrococcus horikoshii. The six supersecondary structures are present in a coiled helix “hanging” from the central beta barrels. These are often compared in the literature to the tentacles of a jellyfish. As far as is visible using electron microscopy, eukariotic prefoldin has a similar structure.
Ribbon model of the apical γ-domain of the chaperonin CCT
Microfilament formation showing the polymerization mechanism for converting G-actin to F-actin; note the hydrolysis of the ATP.
Atomic structure of Arp2/3. Each colour corresponds to a subunit: Arp3, orange; Arp2, sea blue (subunits 1 and 2 are not shown); p40, green; p34, light blue; p20, dark blue; p21, magenta; p16, yellow.
An actin (green) - profilin (blue) complex. The profilin shown belongs to group II, normally present in the kidneys and the brain.
The protein gelsolin, which is a key regulator in the assembly and disassembly of actin.
Principal interactions of structural proteins are at cadherin-based adherens junction. Actin filaments are linked to α-actinin and to the membrane through vinculin. The head domain of vinculin associates to E-cadherin via α-catenin, β-catenin, and γ-catenin. The tail domain of vinculin binds to membrane lipids and to actin filaments.
Structure of MreB, a bacterial protein whose three-dimensional structure resembles that of G-actin
Giant nemaline rods produced by the transfection of a DNA sequence of ACTA1, which is the carrier of a mutation responsible for nemaline myopathy
Position of seven mutations relevant to the various actinopathies related to ACTA1
Cross section of a rat heart that is showing signs of dilated cardiomyopathy
Image taken using confocal microscopy and employing the use of specific antibodies showing actin's cortical network. In the same way that in juvenile dystonia there is an interruption in the structures of the cytoskeleton, in this case it is produced by cytochalasin D.
Western blot for cytoplasmic actin from rat lung and epididymis
Nobel Prize winning physiologist Albert von Szent-Györgyi Nagyrápolt, co-discoverer of actin with Brunó Ferenc Straub
Chemical structure of phalloidin
Ribbon diagram of an actin monomer from rabbit skeletal muscle, with the molecule's surface shown semi-transparent. The four subdomains as well as the bound ATP and calcium ion are annotated.

Of these, two code for the cytoskeleton (ACTB and ACTG1) while the other four are involved in skeletal striated muscle (ACTA1), smooth muscle tissue (ACTA2), intestinal muscles (ACTG2) and cardiac muscle (ACTC1).

Realizing that Banga's coagulating myosin preparations contained actin as well, Szent-Györgyi called the mixture of both proteins actomyosin.

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

T-tubule

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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.

In cardiac muscle cells, across different species, T-tubules are between 20 and 450 nanometers in diameter and are usually located in regions called Z-discs where the actin myofilaments anchor within the cell.