BIOL 250 EText Ch 10 - MUSCLE TISSUE

Introduction to Muscle Tissue

Muscle tissue is essential for active movement of the body and the transportation of materials within the body. It is composed of specialized cells capable of contraction. The three primary types of muscle tissue are skeletal muscle, cardiac muscle, and smooth muscle, each with distinct structures, functions, and locations within the body.

Types of Muscle Tissue

• Skeletal Muscle: This type of muscle is responsible for moving the skeleton and is under voluntary control, meaning it can be consciously controlled by an individual. Skeletal muscle fibers are long, cylindrical, and striated due to the organized arrangement of contractile proteins actin and myosin.

• Cardiac Muscle: Found exclusively in the heart, cardiac muscle is responsible for pumping blood through the circulatory system. It is involuntary and striated like skeletal muscle but consists of shorter, branched fibers with one or two central nuclei. Intercalated discs connect adjacent cardiac muscle cells, allowing for rapid electrical signal transmission and synchronized contractions.

• Smooth Muscle: Located in the walls of hollow organs and passageways, such as the intestines, blood vessels, and urinary bladder, smooth muscle controls involuntary movements, including the contraction of blood vessels and the movement of food through the gastrointestinal tract. It is spindle-shaped, contains a single nucleus, and lacks striations, making it distinct from the other two types.

Chapter Objectives

1. Explain the organization and classification of muscle tissue.

2. Describe the structures and functions of skeletal, cardiac, and smooth muscle.

3. Explain how muscles work with tendons to facilitate body movement.

4. Define muscle metabolism and the processes involved in muscle relaxation.

5. Explore the connections between exercise and muscle performance, including adaptation to training.

6. Outline the development and regeneration processes of muscle tissue throughout life.

Overview of Muscle Tissues

All muscle tissues share the property of excitability, which is the ability to respond to stimuli and undergo contractions as a result. Muscle membranes can change their electrical states and conduct action potentials, allowing them to contract. Unlike skeletal muscle, which is directly influenced by the nervous system, both cardiac and smooth muscles can also respond to hormonal signals and local stimuli.

Muscle Contraction Mechanism

The primary proteins responsible for muscle contraction are actin and myosin. The presence of calcium ions (Ca++) is crucial for the activation of the contraction mechanism in all muscle types. In skeletal muscle, contractions are initiated by neural signals transmitted from motor neurons, whereas the cardiac muscle can contract autonomously due to pacemaker cells embedded within the heart.

• Elasticity in muscle allows it to recoil

• Extensibility in muscle allows for it to stretch

• Contractility allows muscle tissue to pull on its attachment point and shorten with force

Skeletal Muscle Structure

• Control: Skeletal muscles are subject to voluntary control, allowing conscious movement.

• Composition: Skeletal muscle fibers are encapsulated by three connective tissue layers: epimysium (outer layer), perimysium (surrounding bundles of fibers), and endomysium (covering individual muscle fibers).

  • Fascicle are bundles of muscle fibers bundled up by the perimysium

  

• Striations: Myofibrils, located inside muscle fibers, are responsible for the striated appearance of skeletal muscles, resulting from the organized arrangement of actin and myosin filaments.

Functions of Skeletal Muscle

1. Movement: Facilitate voluntary movements and maintain posture.

2. Protection: Provide structural support and shield internal organs from injury.

3. Heat Production: Generate heat during contraction, which is critical for maintaining body temperature.

Structural Details

Skeletal muscle fibers are multinucleated and contain specialized structures:

• Sarcolemma: The plasma membrane of muscle fibers that participates in action potential propagation.

• Sarcoplasm: The cytoplasm of muscle fibers, containing organelles such as the sarcoplasmic reticulum (SR) for calcium storage.

• Sarcomeres: The functional unit of muscle contraction, organized into repeating units within the myofibrils.

  

Neural Control of Muscle Contraction

At the neuromuscular junction (NMJ), motor neurons communicate with muscle fibers. The action potentials generated by motor neurons lead to calcium release from the sarcoplasmic reticulum, initiating muscle contraction.

Contraction Cycle

Upon stimulation, calcium ions expose binding sites on actin filaments—this allows myosin heads to bind to actin and form cross-bridges, facilitating the sliding filament model of contraction. ATP is vital for both the contraction and relaxation phases of the muscle cycle.

• Cell membrane potential is the -60 to -90 mV, which is used to generate electrical signals

• action potential travels from axon to motor neuron, then terminates at the NMJ, leading to the release of acetylcholine, which binds to receptors on the sarcolemma, causing depolarization and the initiation of the action potential in the muscle fiber.

• This depolarization wave travels along the sarcolemma and down the T-tubules, triggering the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm, further promoting the contraction process.

• Muscle contraction stops when signaling from the motor neuron ends which repolarizes the sarcolemma and T-tubules

• Muscle needs ATP to continue contracting

• In contraction, the thin filaments slide past the thick filaments, a process known as the sliding filament theory, which shortens the sarcomeres and ultimately leads to muscle shortening.

  

• ATP is generated through creatine phosphate metabolism, anaerobic glycosis, fermentation and aerobic respiration

  

• oxygen debt is the amount of oxygen required to restore the body to its normal resting state after exercise, including replenishing ATP and creatine phosphate stores, clearing lactic acid, and restoring oxygen levels in the blood and muscle tissues.

Muscle Tension

• Muscle tension refers to the force generated by muscle fibers during contraction, which can vary in response to different stimuli. This tension is influenced by factors such as the number of motor units activated, the frequency of stimulation, and the length-tension relationship of the muscle fibers.

• isotonic contractions: involve the muscle tension staying constant while the muscle changes length, allowing movement to occur, such as lifting a weight or walking.

  • concentric contraction: involves muscle shortening to move a load

  • eccentric contraction: involves muscle tension diminishes as the muscle lengthens

• isometric contraction: produces muscle tension without changing the angle of skeletal joint, which is essential for maintaining posture or stabilizing joints during various activities.

  

• A twitch or single action potential goes through the phases of the latent period, contraction phase, and relaxation phase

  • latent period: the initial phase where the muscle fiber does not respond to stimulation, as it is preparing for contraction by releasing calcium ions from the sarcoplasmic reticulum.

  • contraction phase: calcium ions have bound to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites, allowing myosin heads to attach to actin and initiate the power stroke, which leads to muscle shortening.

  • relaxation phase: the period following contraction where calcium ions are reabsorbed into the sarcoplasmic reticulum, troponin returns to its original shape, and tropomyosin re-covers the actin binding sites, leading to the detachment of myosin heads from actin and allowing the muscle fiber to return to its resting length.

    

• muscle tone

  • hypotonia: a condition characterized by decreased muscle tone, resulting in reduced resistance to passive movement and often leading to weakness and floppiness in the affected muscles.

  • hypertonia: a condition characterized by increased muscle tone, resulting in heightened resistance to passive movement and often leading to stiffness and rigidity in the affected muscles.

Muscle Fibers

Cardiac Muscle Structure and Function

Cardiac muscle, responsible for heart contractions, displays the following characteristics:

• Location: Exclusively found within the heart.

• Structure: Striated with shorter, branched fibers and one or two central nuclei.

• Intercalated Discs: These specialized connections allow rapid electrical signal transmission and maintain structural integrity during contractions.

• Rhythmic Contractions: Controlled by pacemaker cells that generate electrical impulses autonomously.

Smooth Muscle Characteristics

Smooth muscle is present in various organs, displaying the following:

• Shape: Spindle-shaped with a single nucleus, lacking striations.

• Contraction Initiation: Not initiated by neural signals; can respond to various stimuli such as stretching and hormonal signals.

• Calcium Interaction: Contraction is regulated by calcium that interacts with calmodulin, a distinct mechanism compared to striated muscles.

Types of Smooth Muscle

1. Single-Unit Smooth Muscle: Cells connected by gap junctions, allowing synchronized contractions. Commonly found in internal organs.

2. Multi-Unit Smooth Muscle: Cells act independently, lacking gap junctions. Found in locations such as arterioles and respiratory passages.

Muscle Metabolism

The sources of ATP for muscle contraction include:

• Creatine Phosphate Metabolism: Provides an immediate but limited supply of ATP.

• Anaerobic Glycolysis: Produces ATP rapidly but is less efficient in the long term.

• Aerobic Respiration: Fuels prolonged activities and generates a greater yield of ATP over time.

Muscle Fatigue Overview

Muscle fatigue occurs due to:

• Depletion of ATP

• Accumulation of lactic acid

• Disturbance in ion balance within muscle cells

Development and Regeneration of Muscle Tissue

Muscle tissues originate from embryonic mesoderm. Satellite cells assist in the repair and regeneration of skeletal muscle, although it has limited regenerative capacity. Smooth muscle exhibits a better regenerative ability due to pericytes. Aging leads to muscle atrophy as muscle fibers decrease and are replaced by connective tissues.

Key Terms

• Acetylcholine (ACh): The primary neurotransmitter at the neuromuscular junction, facilitating nerve impulses to muscle fibers.

• Sarcoplasmic Reticulum (SR): Organelle that stores calcium ions, essential for muscle contraction signaling.

• Myofibril: Thread-like organelles within muscle fibers that contain actin and myosin.

• Tetanus: A sustained contraction resulting from a rapid series of stimuli without relaxation.

• Treppe: The staircase effect observed in muscle contractions where increased strength is noted with successive stimuli.