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Muscle tissue

Last updated: May 5, 2021

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Muscle tissue is a soft tissue that is primarily composed of long muscle fibers. The coordinated interaction of the myofilaments actin and myosin within the myocytes gives muscle tissue the ability to contract. Depending on the intracellular arrangement of these myofilaments, muscle tissue is classified as either striated (skeletal and cardiac) or nonstriated (smooth) muscle. The myofilaments of striated muscle are arranged into sarcomeres while smooth muscle myofilaments lack a specific arrangement. The underlying mechanisms of contraction (excitation-contraction coupling and the sliding filament mechanism) are similar in all muscle types. Skeletal muscle is under voluntary control of the somatic nervous system. Smooth muscle is under involuntary control of the autonomic nervous system and external stimuli (e.g., chemical, mechanical). It possesses greater elasticity and is present in the walls of hollow organs (e.g., stomach, bladder, uterus), the walls of vessels, and the respiratory and urinary tracts. Cardiac muscle is also under involuntary control of cardiac pacemaker cells and forms the walls of the cardiac chambers (myocardium).

Overview [1]

Muscle cell structures [1]

Sarcolemma (myolemma)

Sarcoplasm

Sarcoplasmic reticulum (SR)

Myofilaments

  • See section below.

Comparison of muscle cell types [1]

Structural differences between muscle cell types
Muscle fiber Location Cell morphology Sarcolemma
Striated muscle fibers
  • Large elongated cells with multiple nuclei located at the periphery
  • Branched cells with a central nucleus
Nonstriated muscle fibers
  • Spindle-shaped cells with a central nucleus

Myofilaments are protein fibers consisting of thick (myosin) and thin (actin) filaments and are responsible for the contractile properties of muscle cells.

Actin myofilaments (thin filaments) [1]

Structure

Function

Troponin proteins in cardiac muscle have a different structure than in skeletal muscle. In myocardial infarction, cardiac myocytes are damaged as a result of absolute oxygen deficiency and release their intracellular content into the bloodstream. An increase in blood cardiac troponin levels thus indicates cardiac muscle tissue damage.

Myosin myofilaments (thick filaments) [1]

Structure

  • Contain ∼ 250 myosin molecules
  • Muscle myosin molecules (myosin II) [2]
    • Various isoforms occur in different muscle types and determine the speed of contraction
    • Consist of two heavy and two light protein chains
    • Domains
      • Head: has both an actin and ATP binding site (which also has ATPase activity)
      • Neck: Two light protein chains attach to the heavy protein chains, which serve to regulate the myosin head.
      • Tail: Myosin heavy chains are wound around each other in a coil-like structure, with myosin heads protruding on both sides.
    • Myosin heads have a binding site for actin and a binding site for ATP (which also has ATPase activity).

Function

Both striated and smooth muscle cells mediate contractions via actin and myosin.

Mutations in the genes encoding β-myosin heavy protein chain or cardiac myosin binding protein C are the underlying cause of familial autosomal dominant hypertrophic obstructive cardiomyopathy.

Comparison of contractile filaments between muscle cells [1]

Overview of contractile filaments in muscle cells
Striated muscle Smooth muscle
Arrangement
  • Disorganized

Interaction with calcium

  • Troponin: composed of the subunits troponin C, I, and T
  • Calcium interacts with troponin C.

Myosin-binding site blockage

Anchorage

  • Dense bodies

Phosphorylation of the light myosin chain required

  • No
  • Yes

Overview

  • Striated and smooth muscle have similar contraction mechanisms, but with a few important differences.
  • This section provides a general overview of the main principles of muscular contraction.
  • See the sections below for details on individual muscle types.

Excitation-contraction coupling [1][3]

At resting position, actin and myosin are unable to interact because they are inhibited by regulatory proteins. An initiating stimulus is needed to enable interaction between the myofilaments.

Sliding filament model [5][6]

Comparison of stimulation and contraction [1]

Comparison of stimulation and contraction between muscle types
Criteria Skeletal muscle Cardiac muscle

Smooth muscle

Initiation of contraction
Neural control of contraction
Special features of excitation-contraction coupling [3]
  • No AP required for contraction
  • Ca2+ influx in the cell, especially from the extracellular space
  • The light myosin chains require phosphorylation for interaction with actin.
Function as a unit
  • Organized transfer of the stimulus from cell to cell via gap junctions in intercalated disks

The glucose-alanine cycle provides skeletal muscle with glucose as a source of energy. The production of urea as a byproduct consumes a lot of energy, so it is less effective than the lactic acid cycle.

Overview [1]

  • Type: striated muscle tissue
  • Structure
    • Joined muscle cells form muscle fibers surrounded by endomysium.
    • Bundles of muscle fibers form fascicles surrounded by perimysium.
    • Multiple fascicles form a muscle surrounded by epimysium.
    • Muscles and the epimysium are covered by fascia.
    • Muscles attach to bones via tendons.
  • Contraction regulation
    • Under voluntary control of the somatic nervous system
    • Stimulation occurs at the motor endplate.
  • Function: responsible for skeletal movement

Microscopic anatomy [1]

Types of skeletal muscle fibers
Criteria Type 1 fibers Type 2 fibers
Appearance
Energy production
Myosin ATPase activity
  • Low
  • High
Contraction velocity
  • Slow twitch
  • Fast twitch
Activity period
  • Long-term activity (e.g., endurance training)
  • Short-term activity (e.g., sprinting, weight lifting, resistance training)
Example
  • Postural muscles

“Too (2) fast to last; light and white, no air to spare.” The most important features of type 2 skeletal muscle fibers are fast-twitching, short-term activity, white appearance, and anaerobic glycolysis (no air).

Sarcomeres [1]

Components on electron microscopy

The I band and H zone shorten during contraction, while the A band remains the same length.

Anchorage

Myofilament contraction [1][3]

Excitation-contraction coupling in skeletal muscle

Skeletal muscle contraction results from an influx of intracellular calcium from stores in the SR. This explains the ability of skeletal muscle to contract despite treatment with calcium channel blockers, which can block an influx of extracellular calcium through DHPRs but cannot affect DHPR voltage-sensing capabilities or the resulting intracellular calcium release.

A mutation in the ryanodine receptor gene of striated muscle cells results in a ryanodine receptor that can be activated by certain substances, such as inhalation narcotics (e.g., isoflurane). This activation leads to an uncontrolled release of Ca2+ from the SR, which results in a continuous contraction that increases the energy and oxygen consumption of the muscle cell enormously. Affected individuals present with lactic acidosis due to increased anaerobic glycolysis and hyperthermia due to increased muscle metabolism. This life-threatening condition is called malignant hyperthermia.

Steps of the contraction cycle (crossbridge cycling) [1]

  1. Crossbridge formation: released intracellular Ca2+ binds to troponin C and causes a conformational change → tropomyosin moves away from the myosin binding site of the actin filament myosin head binds actin at a 90° angle, forming a crossbridge
  2. Powerstroke of the myosin head: myosin head releases phosphate (Pi)→ myosin head tilts by 45°, pulling myosin along actin muscle shortens (contracts) → ADP is released
  3. Loosening of the crossbridge: new ATP binds to myosin head → myosin head detaches from the actin filament → myosin returns to its original position
  4. Reorientation of the myosin head: hydrolysis of ATP to ADP and Pi (both remain on the myosin head) → myosin head alters its conformation (changes to a “cocked state”) → myosin returns to its original position (ready to bind to actin again)
  5. Replication of the cycle
    • If the Ca2+ concentration in the muscle cell remains elevated, a new cycle begins with crossbridge formation.
    • Depending on ATPase activity (ATP cleavage rate per unit of time) of the myosin heavy chain, there may be ∼ 10–100 crossbridge cycles per second.
    • The more crossbridge cycles per unit of time, the faster and stronger the contraction.

Rigor mortis is the stiffening of the muscles after death, which is caused by persistent attachment of actin to myosin due to lack of ATP.

Types of muscle contraction

  • Isometric contraction: The muscle contracts and generates power but does not shorten or lengthen.
  • Isotonic contraction: The muscle length changes and muscle power remains constant.
    • Concentric contraction: muscle shortens
    • Eccentric contraction: muscle lengthens
  • Auxotonic contraction: simultaneous change in both muscle power and length

Reflexes

Skeletal muscle adaptations

Mechanisms of skeletal muscle adaptation
Hypertrophy Atrophy
Number of myofibrils Increased (sarcomeres are added in parallel) Decreased (due to degradation in proteasomes)
Number of nuclei Increased (due to activation and fusion of myosatellite cells) Decreased (due to apoptosis)

Overview [1]

Microscopic anatomy [1]

Types of smooth muscle cell units
Single-unit smooth muscle Multi-unit smooth muscle
Definition
  • Cell bundle consisting of several individual cells connected by many gap junctions
  • Cells contract together as a unit
  • Cell bundle consisting of individual cells separated by a basement membrane and not connected by gap junctions
  • Cells contract individually (must be stimulated individually)
Location

Myofilament structure [1]

Smooth muscle cells do not have sarcomeres.

Myofilament contraction and relaxation [1]

Excitation-contraction coupling

Smooth muscle cells do not have motor endplates.

Steps of contraction

  1. Stimulus opens L-type voltage-gated Ca2+ channels in the sarcolemma → influx of Ca2+ from the extracellular space into the smooth muscle cell (Ca2+ is also released from the SR, further increasing the intracellular concentration of Ca2+)
  2. Ca2+ binds to calmodulin in the sarcoplasm, leading to activation of calmodulin.
  3. Ca2+-calmodulin complex activates myosin light-chain kinase (MLCK).
  4. MLCK phosphorylates the light chain head of the myosin filament.
  5. ATPase activity in phosphorylated myosin enables crossbridge formation with actin, resulting in muscle contraction. (see “Crossbridge cycling” above)

The very slow attachment and detachment of crossbridges between actin and myosin allows smooth muscle to maintain prolonged tonic contraction while consuming little ATP and O2.

The Ca2+ concentration determines the force of smooth muscle contraction: The higher the Ca2+ influx, the more force is generated.

Steps of relaxation

  1. Stimuli
  2. Increased activity of myosin light-chain phosphatase (MLCP) and/or decreased MLCK activity
  3. MLCP dephosphorylates myosin myosin no longer interacts with actin contraction ceases

The sliding of filaments and the cleavage of ATP by myosin ATPase occur 100–1,000 times slower in smooth muscle than in skeletal muscle. The maximal contraction speed of smooth muscles is therefore significantly lower than that of skeletal muscle.

MLCK phosphorylates myosin, leading to smooth muscle contraction. MLCP dephosphorylates myosin, leading to smooth muscle relaxation.

Smooth muscle cells Contract with Ca2+ and relax when there's NO stress.

Overview [1]

Microscopic anatomy [1]

Myofilament arrangement [1]

Myofilament contraction [1]

Excitation-contraction coupling

Contraction

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