ΜΥΙΚΟ ΣΥΣΤΗΜΑ. The formation and nuclei of a skeletal muscle (a) (b) (a) The formation...

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ΜΥΙΚΟ ΣΥΣΤΗΜΑ ΜΥΙΚΟ ΣΥΣΤΗΜΑ

Transcript of ΜΥΙΚΟ ΣΥΣΤΗΜΑ. The formation and nuclei of a skeletal muscle (a) (b) (a) The formation...

ΜΥΙΚΟ ΣΥΣΤΗΜΑΜΥΙΚΟ ΣΥΣΤΗΜΑ

The formation and nuclei of a skeletal muscleThe formation and nuclei of a skeletal muscle

(a)(a) The formation of muscle fibers by the fusion of myoblasts. Notice the multiple nuclei. (b)(b) A micrograph and diagrammatic view of one muscle fiber (LM X 612).

(F. Martini, A&P, 2004)

Levels of functional organization Levels of functional organization in a skeletal muscle fiberin a skeletal muscle fiber (F. Martini, A&P, 2004)

Photomicrograph of the capillary network surrounding skeletal muscle fibersPhotomicrograph of the capillary network surrounding skeletal muscle fibers

The arterial supply was injected with dark red gelatin to demonstrate the capillary bed. The long muscle fibers are stained orange (30X).

(E. Marieb, HA&P, 2004)

Connective tissue sheaths of skeletal muscleConnective tissue sheaths of skeletal muscle. (a) Each muscle fiber is wrapped with a delicate connective tissue sheath, the endomysium. Bundles, or fascicles, of muscle fibers are bounded by a collagenic sheath called a perimysium. The entire muscle is strengthened and wrapped by a coarse epimysium sheath. (b) Photomicrograph of a cross section of part

of a skeletal muscle (90X) (E. Marieb, HA&P, 2004).

Microscopic anatomy of Microscopic anatomy of skeletal muscle fiberskeletal muscle fiber

(a) Photomicrograph of portions of two isolated muscle fibers (700X). Notice the obvious striations (alternating light and dark bands).

(b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber.

(c) A small portion of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next.

(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.

(E. Marieb, HA&P, 2004)

Thick and thin filamentsThick and thin filaments

(a)(a) The structure of the thin filament, showing the attachment at the Z line.(b)(b) The organization of G actin subunits in an F actin strand, and the position of the troponin-tropomyosin complex.(c)(c) The thick filaments, showing the orientation of the myosin molecules.(d)(d) The myosin molecule. (F. Martini, A&P, 2004)

The components of thin filamentsThe components of thin filaments

G-actinG-actin is assembled into single-stranded F-actin. F-actinF-actin strands are twisted together to form the backbone of a thin filament. Double helixes of tropomyosinropomyosin lie on the F-actin strands. Each tropomyosin has a molecule of troponintroponin positioned at one end (D. Moffett, HP, 1993).

The components of thick filamentsThe components of thick filaments

Single myosin unitsmyosin units have a double globular head at the end of a long rod. The long rod is composed of two heavy two heavy polypeptide chainspolypeptide chains; the ends of the heavy chains, together with the four four light polypeptide chainslight polypeptide chains of myosin, form the heads. Myosin units self-assembled into thick filaments. The myosin heads project from the thick filament at the proper angles to interact with the hexagonal array of thin filaments (D. Moffett, HP, 1993).

Composition of thick Composition of thick and thin filamentsand thin filaments

(a) An individual myosin molecule has a rod-like tail, from which two heads “protrude”.

(b) Each thick filament consists of many myosin molecules, whose heads protrude at opposite ends of the filament.

(c) A thin filament contains a strand of actin twisted into a helix. Each strand is made up of actin subunits. Tropomyosin molecules coil around the actin filaments, helping to reinforce them. A troponin complex is attached to each tropomyosin molecule.

(E. Marieb, HA&P, 2004)

Relationship of the sarcoplasmic reticulum (SR) and T tubules to myofibrils of skeletal Relationship of the sarcoplasmic reticulum (SR) and T tubules to myofibrils of skeletal musclemuscle. The tubules of the SR encircle each myofiber like a “holey” sleeve. The tubules fuse to form communicating channels at the level of the H zone and abutting the A-I junctions, where the saclike elements called terminal cisternae are formed. The T tubules, inward invaginations of the sarcolemma, run deep into the cell between the terminal cisternae. Sites of close contact of these three elements (terminal cisterna, T tubule, and terminal cisterna) are called triads (E. Marieb, HA&P, 2004).

Levels of functional organization Levels of functional organization in a skeletal muscle fiberin a skeletal muscle fiber (F. Martini, A&P, 2004)

An overview of the process of An overview of the process of skeletal muscle contractionskeletal muscle contraction

(F. Martini, A&P, 2004)

Motor unitsMotor units

A motor unit is a group of skeletal muscle fibers controlled by a single motor neuron.

(A)(A) A schematic diagram of three different motor units in a single muscle, consisting of 3 to 5 individual muscle fibers.

(B)(B) The total tension in a muscle is given by summating the activity of all the active motor units.

(D. Moffett, HP, 1993)

A motor unit consists of a motor neuron and all the muscle fibers it innervates. A motor unit consists of a motor neuron and all the muscle fibers it innervates. (a)(a) Schematic view of portions of two motor units. The cell bodies of the motor neurons reside in the spinal cord, and their axons extend to the muscle. In the muscle, each axon divides into a number of axonal terminals that are distributed to muscle fibers scattered throughout the muscle. (b)(b) Photomicrograph of a portion of a motor unit (80X). Notice the diverging axonal terminals and the neuromuscular junctions formed by the terminals and muscle fibers (E. Marieb, HA&P, 2004).

The neuromuscular The neuromuscular junctionjunction

(E. Marieb, HA&P, 2004)

(c) Ach attaches to receptors, opening ion channels and initiating depolarization of the sarcolemma.

(b) ACh in vesicles is released into the synaptic cleft when the action potential reaches the axonal terminal.

Summary of events in the generation and propagation of an action potential in Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber a skeletal muscle fiber (E. Marieb, HA&P, 2004).

Excitation-Excitation-contraction contraction (E-C) (E-C) coupling coupling

(E. Marieb, HA&P, 2004)

Role of ionic calcium in the contraction mechanismRole of ionic calcium in the contraction mechanism. Views (a-d) are cross-sectional views of the thin (actin) filament. (a)(a) At low intracellular Ca2+ concentration, tropomyosin blocks the binding sites on actin, preventing attachment of myosin cross bridges and enforcing the relaxed muscle state. (b)(b) At higher intracellular Ca2+ concentrations, additional calcium binds to (TnC) of troponin. (c)(c) Calcium-activated troponin undergoes a conformational change that move the tropomyosin away from actin’s binding sites. (d)(d) The displacement allows the myosin heads to bind and cycle, and contraction (sliding of the thin filaments by the myosin cross bridges) begins (E. Marieb, HA&P, 2004).

Sequence of events involved in the sliding of the thin filaments during contraction (E. Marieb, HA&P, 2004)

Steps involved in of skeletal muscle contractionSteps involved in of skeletal muscle contraction

(F. Martini, A&P, 2004)

Steps involved in of skeletal muscle contractionSteps involved in of skeletal muscle contraction

(F. Martini, A&P, 2004)

The time relationships of the muscle action potential, rise and fall of The time relationships of the muscle action potential, rise and fall of cytoplasmic Cacytoplasmic Ca2+2+, and force development by sarcomeres during a twitch, and force development by sarcomeres during a twitch

(D. Moffett, HP, 1993)

The muscle twitchThe muscle twitch

(a) Myogram of an isometric twitch contraction, showing its three phases: the latent period, the period of contraction, and the period of relaxation.

(b) Comparison of the twitch

responses of extraocular, gastrocnemius, and soleus muscles.

(E. Marieb, HA&P, 2004)

Wave summation and tetanus in a whole muscleWave summation and tetanus in a whole muscle

(1)(1) A single stimulus is delivered, and the muscle contracts and relaxes (twitch contraction). (2)(2) Stimuli are delivered more frequently, so that the muscle does not have adequate time to relax completely, and contraction force increases (wave summation). (3)(3) More complete twitch fusion (unfused or incomplete tetanus) occurs as stimuli are delivered more rapidly. (4)(4) Fused or complete tetanus, a smooth, continuous contraction without any evidence of relaxation occurs (E. Marieb, HA&P, 2004).

The effect of sarcomere length on tensionThe effect of sarcomere length on tension

(a) Maximum tension is produced when the zone of overlap is large but the thin filaments do not extend across the sarcomere’s center.

(b) If the sarcomeres are stretched too far, the zone of overlap is reduced or disappears, and cross-bridge interactions are reduced or cannot occur.

(c) At short resting lengths, thin filaments extending across the center of the sarcomere interfere with the normal orientation of thick and thin filaments, reducing tension production.

(d) When the thick filament contact the Z lines, the sarcomere cannot shorten -the myosin heads cannot pivot- and tension cannot be produced. The light purple are represents the normal range of resting sarcomere length. (F. Martini, A&P, 2004)

Isotonic (concentric) contractionIsotonic (concentric) contraction

On stimulation, the muscle develops enough tension (force) to lift the load (weight). Once the resistance is overcome, the tension remains constant for

the rest of the contraction and the muscle shortens.

(E. Marieb, HA&P, 2004)

Isometric contractionIsometric contraction

The muscle is attached to a weight that exceeds the muscle’s peak tension-developing capabilities. When stimulated, the tension increases to the muscle’s peak tension-developing capability, but the muscle does not

shorten.

(E. Marieb, HA&P, 2004)

Factors influencing force, velocity, and duration of skeletal muscle Factors influencing force, velocity, and duration of skeletal muscle contractioncontraction

(E. Marieb, HA&P, 2004)

Muscle metabolismMuscle metabolism

(F. Martini, A&P, 2004)

Methods of regenerating ATP during muscle activityMethods of regenerating ATP during muscle activity

The fastest mechanism is direc phosphorylation (a); the slowest is the aerobic mechanism (c).

(E. Marieb, HA&P, 2004)

Energy system used Energy system used during sports at peak during sports at peak activity levelactivity level

Skeletal muscles rely largely on glycolysis (anaerobic respiration) for ATP synthesis. The initial activity burst is energized by ATP and CP reserves. Muscles operating at peak levels fatigue rapidly as lactic

acid accumulates.

(E. Marieb, HA&P, 2004)

Fast versus slow fibersFast versus slow fibers

(a)(a) The slender, slow fiber (R for red) has more mitochondria (M) and a more extensive capillary supply (cap) than does the fast fiber (W for white). (b)(b) Notice

the difference in the size of slow fibers, above, and of fast fibers, below.

(F. Martini, A&P, 2004)

The effect of changes in innervation on the contractile properties of muscleThe effect of changes in innervation on the contractile properties of muscle

The nerve running to a muscle composed predominantly of slow-twitch fibers is surgically switched with a nerve innervating a muscle composed predominantly of fast twitch fibers. The original twitch properties of the muscle are shown in A. The effects of crossed-innervation on twitch properties re shown in B. The influence is exerted both by chemical messengers that travel between the muscle and the nervous system via axoplasmic flow and by the frequency and pattern of recruitment of the muscles by the motor neurons (D. Moffett, H&P, 1993).