Muscle Tissue Anatomy and Function

Muscle Tissue Overview

• Muscle tissue is a specialized tissue comprised of elongated cells called myocytes.
• Myocytes are primarily responsible for contraction.
• Myofilaments (thin and thick) are the key structures mediating contraction.
• Thin filaments are composed of actin, a fibrous protein polymerized from globular actin subunits.
• Thick filaments are made of myosin II protein.
• Actin and myosin interact to cause muscle contraction.
• The cytoplasm of muscle cells is referred to as the sarcoplasm
• Other functions of actin and myosin include cytokinesis, exocytosis, and cell migration.

Muscle Tissue Classification

• Muscle tissue is categorized as striated or smooth.
• Striated muscle cells exhibit cross-striations.
• Smooth muscle cells lack cross-striations.
• Cross-striations are a consequence of the organized arrangement of actin and myosin myofilaments.
• Striated muscle is subdivided further, as skeletal, visceral and cardiac muscles.
• Skeletal muscle is attached to bone and responsible for movement.
• Visceral striated muscle is found in certain locations in the body and is often morphologically indistinct from skeletal muscle
• Cardiac muscle is found in the heart and is specialized for rhythmic contraction.

Skeletal Muscle Overview

• Skeletal muscle is composed of multinucleated cells called muscle fibers
• Muscle fibers are formed by the fusion of multiple individual myoblast cells.
• Muscle fibers vary in length.

Skeletal Muscle Structure

• Skeletal muscle fibers are organized and held together by connective tissues.
• Endomysium: connective tissue surrounding individual muscle fibers
• Perimysium: connective tissue surrounding bundles of muscle fibers (fascicles).
• Epimysium: connective tissue surrounding entire muscle bundles, forming muscles.

Skeletal Muscle Fiber Types

• Skeletal muscle fibers are generally categorized into three types, I, IIa, and IIb, with varying oxidative and glycolytic capabilities.
• Type I fibers are slow oxidative, appearing red and containing many mitochondria and a large quantity of myoglobin and cytochromes
• Type IIa fibers (fast oxidative)
• Type IIb fibers (fast glycolytic)

Myofibrils and Myofilaments

• Muscle fibers are composed of structural units called myofibrils, arranged longitudinally within the cells.
• Myofibrils are bundles of myofilaments (actin and myosin).
• Myofilaments are the actual contractile units in skeletal muscle.

Sarcomere Structure

• The sarcomere is the fundamental functional unit of a myofibril.
• Defined by the structure between successive Z lines
• A-bands and I-bands are visible under phase contrast microscopy.
• Z-discs traverse the I-bands.
• The M-line traverses and bisects the A-band.

Sarcomere Structure Details

• Myosin-containing thick filaments are located in the central part of the sarcomere, the A-band
• The thin actin filaments connect to the Z-lines and extend into the A-band.
• The I-band contains only thin filaments.
• Thin filaments are primarily composed of F-actin, tropomyosin, and troponin.
• Thick filaments consist only of myosin II.

Contractile Proteins

• Myosin is a motor protein with projections that allow it to "walk" along actin filaments.
• Myosin converts Adenosine triphosphate (ATP) into mechanical energy
• Thin filaments (actin), contain sites where myosin can attach
• Tropomyosin and troponin are accessory proteins associated with actin.
• In relaxed muscle, myosin binding sites on actin are blocked by tropomyosin.
• Calcium ion binding to troponin exposes the myosin-binding sites.

Structural Proteins (cont.)

• Titin stabilizes the position of myosin within the sarcomere.
• Titin accounts for much of the elasticity and extensibility of myofibrils.
• Dystrophin links thin filaments to the sarcolemma.

Contraction and Relaxation of Skeletal Muscle

• The sliding filament mechanism describes how muscle fibers contract.
• Myosin heads attach to and move along actin filaments.
• This progressively pulls the thin filaments towards the center of the sarcomere.
• Z-discs move closer together, shortening the sarcomere.
• The sarcomere shortening leads to the contraction of the entire muscle.

Contraction Cycle

• The contraction cycle begins with the SR releasing calcium ions into the muscle cell.
• Calcium ions bind to actin, opening myosin-binding sites.
• Myosin hydrolyzes ATP and reorients to generate force.
• Myosin heads attach to actin, and the power stroke rotates, sliding the filaments.
• Myosin detaches from actin when the next ATP binds.
• The cycle repeats as long as ATP is available and calcium ion level remains sufficient.

Excitation-Contraction Coupling

• An increase in calcium ions initiates contraction.
• Muscle cell membrane has calcium pumps to quickly return calcium ion levels.
• Decresing calcium ions initiates muscle relaxation.
• Action potentials trigger calcium release from the sarcoplasmic reticulum.
• Calcium movement away from myosin-binding sites causes muscle relaxation.

Muscle Metabolism

• Muscle contraction requires a substantial amount of ATP.
• ATP supplies the energy for contraction, and for pumping calcium ions back into the sarcoplasmic reticulum.
• Muscles have several mechanisms for ATP production: creatine phosphate, anaerobic respiration and aerobic respiration.
• ATP regeneration from creatine phosphate is immediate and fuels short-duration, high-intensity contractions.
• Anaerobic respiration occurs without oxygen, produces lactic acid, and supports moderate-intensity contractions.
• Aerobic respiration, requiring oxygen, occurs for prolonged activity and produces the most ATP per glucose molecule.

Muscle Fatigue

• Muscle fatigue is the inability to maintain muscle contraction.
• Factors like calcium ion release inadequacy, creatine phosphate depletion, glycogen depletion, oxygen insufficiency, buildup of lactic acid and ADP contribute to muscle fatigue

Oxygen Consumption After Exercise

• Oxygen consumption remains elevated after exercise to restore muscle cells to their resting state.
• Muscle cells use the additional oxygen to convert lactic acid to glycogen, synthesize ATP, and replenish oxygen stores in myoglobin.

Control of Muscle Tension - Twitch Contraction and Muscle Fiber Characteristics

• Twitch contraction is the response of a muscle fiber to a single action potential.
• The contraction period and relaxation period collectively define the duration of a twitch
• Latent period refers to the delay between stimulus and contraction
• Contraction period: duration of active cross-bridge activity (myosin pulling on actin).
• Relaxation period: duration of calcium reuptake into the sarcoplasmic reticulum.
• Isotonic contractions: muscle changes length while tension remains constant.
• Isometric contractions: muscle does not change length during contraction.

Smooth Muscle

• Smooth muscle is found in various tissues, and characterized by tapered ends and elongated fusiform cells.
• Smooth muscle cells often exhibit synchronous contractions due to gap junctions that connect the cells.
• Thin filaments of smooth muscle are associated with intermediate filaments (desmin and vimentin), which give the cells their structural support
• Labile thick myosin filaments that are easily lost during tissue preparation.

Smooth Muscle (cont'd)

• Smooth muscle is frequently regulated by the autonomic nervous system.
• Sometimes, smooth muscle will spontaneously contract or exhibit spontaneous activity

Muscle-Tendon Junction

• Oblique-cut skeletal muscle inserting into tendon is a common structural observation

Cardiac Muscle

• Cardiac muscle cells are striated similar to skeletal muscle
• Cardiac muscle fibers contain intercalated discs between cells, highly specialized attachments for coordinated contractions.
• cardiac muscle cells are not a syncitium (single large muscle fiber).
• Intercalated discs facilitate ionic flow between cells, enabling synchronized contractions.
• Cardiac muscle requires a large amount of ATP and has an abundance of mitochondria
• Cardiac muscle can spontaneously contract (pacemaker cells).

Cardiac Muscle Structure Details

• The sarcoplasmic reticulum (SR) in cardiac muscle is often simpler than in skeletal muscle.
• Cardiac myocytes have many large mitochondria with numerous cristae packed between the myofibrils.
• There are numerous glycogen granules between the myofibrils.
• The intercalated disc represents the attachment site between individual cardiac myocytes.

Intercalated Discs

• Components of the intercalated disc include inter-connecting transverse components and inter-connecting lateral components.
• Adhering junctions, forming part of the transverse component, structurally join the muscle cells and transmit the force generated during contractions.
• Gap junctions, which form part of the lateral component of the intercalated discs, allow ionic flow between the cells facilitating synchronized contraction in cardiac muscle.
• The SR network in cardiac muscle forms a single network along the sarcomere, extending from Z-line to Z-line.
• One T-tubule usually extends into the centre of cardiac muscle cells per sarcomere.

This quiz covers essential aspects of muscle tissue, including the structure and function of myocytes. Test your knowledge on striated muscle fibers, the role of intercalated discs, and the components that facilitate muscle contraction. Ideal for students studying anatomy and physiology.