Skeletal and Smooth Muscle

Chapter 9: Skeletal and Smooth Muscle

Learning Objectives

  • L.O. 9.1 Identify the functions of skeletal muscle.

  • L.O. 9.2 Investigate the anatomy of skeletal muscle, and explain its movement, gross anatomy, microanatomy, neuromuscular system, tone, and length-tension relationship.

  • L.O. 9.3 Characterize the energetics of muscle activity, and describe muscle metabolism, fatigue, recovery, body temperature regulation, and aging.

  • L.O. 9.4 Summarize the macrostructure and microstructure of smooth muscle.

Overview of Muscle Tissue

  • Muscle Cells: Referred to as muscle fibers, which can transform chemical energy (ATP) into directed mechanical energy to exert force.

  • Specialized Terminology:

    • Prefixes like Myo-, Mys-, and Sarco- signify muscle-related terms.

    • For example, Sarcoplasm refers to the cytoplasm of a muscle cell.

  • Muscle Tissue Composition:

    • Muscle tissue constitutes nearly half of the body’s mass.

    • There are three main types of muscle tissue: Skeletal, Cardiac, and Smooth.

Muscle Tissue Types

Skeletal Muscle

  • Definition: Skeletal muscle tissue makes up most of the skeletal muscles, which are organs attached to bones and skin.

  • Characteristics of Skeletal Muscle Fibers:

    • Longest muscle fibers with striations (visible stripes).

    • Capable of rapid contraction but tire easily.

    • Known as voluntary muscles because they are consciously controlled.

Cardiac Muscle

  • Definition: Cardiac muscle tissue is found exclusively in the heart, forming the bulk of the heart walls.

  • Characteristics of Cardiac Muscle Fibers:

    • Striated and involuntarily controlled.

    • Contractions occur at a steady rate and can be adjusted by the nervous system.

Smooth Muscle

  • Definition: Smooth muscle tissue forms the walls of hollow organs (visceral organs), such as the stomach, alimentary tract, urinary bladder, and blood vessels.

  • Characteristics of Smooth Muscle Fibers:

    • Involuntarily controlled; not striated.

Functions of Muscle Tissue

  • Four critical functions of muscle tissue include:

    1. Produce Movement: Responsible for all locomotion and manipulation (e.g., walking, digesting food, pumping blood).

    2. Maintain Posture and Body Position.

    3. Stabilize Joints.

    4. Generate Heat: Muscles generate heat as they contract.

Primary Functions of Skeletal Muscle

  • Movement: By contracting, muscles pull on tendons that connect to bones.

  • Support: Abdominal muscles stabilize visceral organs and shield them from injury.

  • Posture: Continuous muscle contractions hold the body in stable positions, such as sitting or standing.

  • Temperature Regulation: Skeletal muscles, which constitute about 40% of the body mass, have a significant effect on body temperature.

  • Communication: Skeletal muscles facilitate various modes of interpersonal communication (e.g., speaking, typing, writing, making facial expressions, and gestures).

Skeletal Muscle Macrostructure

  • Facial Muscles: Important for expressions and communication.

  • Deltoids: Shoulder muscles pivotal for arm movements.

  • Masticatory Muscles: Involved in chewing.

  • Trapezius: Muscle that extends from the back of the head to the shoulder blades.

  • Pectoralis: Chest muscles aiding in various upper body movements.

  • Biceps and Triceps: Arm muscles working in opposition for flexion and extension.

  • Abdominal Muscles & Erector Spinae: Core muscles that support the spine and maintain trunk stability.

  • Gluteus Maximus & Quadriceps: Major muscle groups in the hips and thighs that aid in leg movements.

  • Hamstrings & Gastrocnemius: Key muscles for knee flexion and ankle movement.

Connective Tissue Sheaths

  • Every skeletal muscle and muscle fiber is covered in connective tissue that provides support and reinforcement to the muscle.

  • Layers of Connective Tissue from External to Internal:

    • Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.

    • Perimysium: Fibrous connective tissue surrounding a fascicle (a group of muscle fibers).

    • Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Gross Anatomy of Skeletal Muscle

  • Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.

  • Sarcoplasm is the cytoplasm of a muscle fiber.

  • Contains numerous glycosomes (for glycogen storage) and myoglobin (for oxygen storage).

  • Features of skeletal muscle fibers include:

    • Myofibrils: Contractile threads of the muscle fiber.

    • Sarcoplasmic Reticulum: Specialized smooth endoplasmic reticulum around myofibrils that stores calcium.

    • T Tubules: Extensions of the sarcolemma that penetrate into the fiber's interior.

Main Features of Skeletal Muscle Fibers

  • Contents: Specialized structures like sarcoplasm and sarcoplasmic reticulum.

  • Development: Myogenesis involves the fusion of myoblasts to form myocytes.

  • Size and Shape: Cylindrical cells with diameters ranging from 10-100 μm and lengths up to 23 inches.

  • Orientation: Fibers are typically positioned obliquely to the muscle's axis of force.

  • Satellite Cells: Myoblasts that did not fuse during development serve a regenerative purpose.

Interior Structure of a Muscle Fiber

  • Thin Filaments: Composed primarily of F-actin, along with regulatory proteins tropomyosin and troponin.

  • Thick Filaments: Composed mainly of myosin, resembling double-headed golf clubs.

  • The muscle’s contraction involves the interactions between actin and myosin.

Structure of the Sarcomere

  • The sarcomere is the smallest contractile unit of skeletal muscle, comprising repeating units of actin and myosin that enable muscle contraction via the sliding filament model.

  • Boundaries of the sarcomere are defined by Z-lines.

Motor Units

  • A motor unit consists of a single motor neuron and the muscle fibers innervated by it.

  • It sends impulses that travel through the spinal cord affecting the muscle fibers, leading to muscle action potential stimulation and subsequent sarcomere sliding.

Neuromuscular Junction

  • The neuromuscular junction (NMJ) is a chemical synapse between an alpha motor neuron and a muscle fiber, essential for muscle stimulation.

  • At the NMJ:

    • The axon terminal of the alpha motor neuron meets the muscle fiber membrane.

    • Stimulation causes the buildup of intracellular sodium ions (Na+Na^+) and the exit of intracellular potassium ions (K+K^+).

    • The resulting graded potentials lead to action potentials and calcium ion (Ca2+Ca^{2+}) release.

  • Ends of stimulation occur when acetylcholinesterase degrades acetylcholine (ACh) in the synaptic cleft.

Focus on Disease: Myasthenia Gravis

  • Myasthenia Gravis: An autoimmune disorder affecting the NMJ, characterized by the destruction of ACh receptors and muscle weakness.

  • Treatment: Includes the use of acetylcholinesterase inhibitors, which increase the availability of ACh at the NMJ.

Phases of a Muscle Twitch

  • Latent Phase:

    • Action potentials depolarize the sarcolemma.

    • Calcium ions are released from the sarcoplasmic reticulum (SR).

    • No tension produced yet.

  • Contraction Phase:

    • Muscle fiber tension peaks as cross-bridge interactions occur when actin binding sites are exposed.

  • Relaxation Phase:

    • Calcium levels decrease, covering actin binding sites.

    • Number of cross-bridges declines.

Muscle Contraction Summation

  • Multiple Twitches Summation:

  • Lower Rate of Stimulation: Muscles partially relax between twitches.

  • High Rate of Stimulation: Muscles do not relax between twitches, leading to greater force output.

Factors Affecting Muscle Tension

  • Greater muscle tension necessitates recruitment of more muscle fibers.

  • Recruitment of motor units culminates in increased force (e.g., from biceps recruitment).

Muscle Tone

  • Muscle Tone: Refers to the ongoing partial contraction of muscles, contributing to posture and joint stability.

  • Factors affecting muscle tone:

    1. Structure of the muscle, including its connective tissue and elastin component (titin).

    2. Active muscle tone; i.e., the number of motor units stimulated even when the muscle is at rest.

Length-Tension Relationship of Muscle Strength

  • Muscle tension is most effective at a specific muscle length due to the optimal alignment of actin and myosin, allowing for effective cross-bridge formation.

Sliding Filament Model of Muscle Movement

  • Process involves:

    1. Calcium ions bind to troponin on actin’s active site.

    2. Myosin binds to actin to form a cross-bridge (myosin in “cocked” formation).

    3. Release of phosphate causes the myosin head to shift to low-energy conformation, pulling actin towards the M line (referred to as the powerstroke).

    4. Binding of a new molecule of ATP replaces ADP, facilitating cross-bridge detachment.

    5. The cycle repeats itself as cross-bridges break and reform, allowing for continued muscle contraction.

Excitation-Contraction Coupling for Muscle Contraction

  1. ACh binds to muscle fiber to initiate the action potential.

  2. Action potential travels into T tubules.

  3. Calcium ion release from the SR occurs in response to the action potential.

  4. Calcium binds, leading to cross-bridge formation and muscle contraction, utilizing ATP.

  5. Acetylcholinesterase degrades ACh to terminate stimulation.

  6. Calcium is reabsorbed into the SR, disallowing further contraction as active sites become unexposed.

T-Tubules and Calcium Release

  • T-Tubules carry depolarization deep into muscle fibers and form a triad with two terminal cisternae and one T-tubule.

  • Linked by specialized proteins responsible for calcium release, such as ryanodine receptor (RyR) and dihydropyridine receptor (DHPR).

Energetics of Muscular Activity

Muscle Metabolism Overview

  • Resting Muscle: Produces more ATP than is necessary, which is transferred to creatine:
    ATP+creatine<br>ightarrowcreatinephosphate+ADPATP + creatine <br>ightarrow creatine phosphate + ADP

  • Contracting Muscle: Generates ATP from creatine phosphate using the reverse reaction:
    creatinephosphate+ADP<br>ightarrowATP+creatinecreatine phosphate + ADP <br>ightarrow ATP + creatine

  • ATP generation matches usage rate.

Energy Sources

  1. Short/Intense Exercise:

    • Initial energy from creatine phosphate.

    • Lasts about 5-10 seconds with lactic acid build-up thereafter.

  2. Prolonged Exercise:

    • Energy primarily from aerobic metabolism.

    • Oxygen consumption shifts towards longer sources, allowing for extended exercise duration.

Energy Use During Different Activity Levels

  • Direct Phosphorylation: Coupled reaction of creatine phosphate and ADP.

  • Anaerobic Pathway: Glycolysis with lactic acid formation.

  • Aerobic Pathway: Cellular respiration involving glucose, fatty acids, and amino acids, yielding the most ATP:

    • 32 ATP per glucose molecule in aerobic respiration.

Causes and Effects of Muscle Fatigue

  • Fatigue-inducing Situations: Include lactic acid build-up post high-intensity exercise and glycogen depletion during medium-intensity exercise over extended periods.

  • Fatigue occurs when muscles cannot continue contractions even under nervous stimulation.

Muscle Recovery

  • Lactic Acid Removal: High-intensity exercise leads to lactic acid recycling back to pyruvic acid for ATP generation or glycogen reformation.

  • The Cori Cycle: Shuttles lactic acid through the blood to the liver and back to the muscle.

  • Factors for improving muscle recovery include muscle tone, strength, endurance, and increased metabolic capacity.

  • Delayed-Onset Muscle Soreness (DOMS): Usually peaks 3-4 days after intense exercise, commonly associated with small muscle tears and connective tissue injury. The principle of overload suggests the body adapts and becomes stronger post-recovery.

Features of Muscle Aging

  • Muscles undergo various changes with age, including:

    • Thinning of muscle fibers.

    • Declining muscle strength and flexibility.

    • Decreased tolerance for exercise and recovery ability.

  • Sarcopenia: Age-related loss of muscle fiber size and number.

  • Fibrosis: Age-related fibrous tissue formation that hampers movement, affecting thermoregulation and increasing fatigue susceptibility. Excess fibrous tissue impairs optimal healing of injuries.

Smooth Muscle: Macrostructure and Microstructure

Structure of Smooth Muscle Cells

  • Smaller in size compared to skeletal muscle cells; spindle-shaped structure.

  • Nucleus: Centrally located within the cell.

  • Smooth muscle cells lack T-tubules or visible sarcomeres, are not fused during development, and are interconnected through junctions.

  • They have membrane invaginations known as caveolae.

Functions of Smooth Muscle Tissue

  • Lines the walls of hollow organs enabling involuntary movements by facilitating contraction/relaxation of organ passageways (e.g., respiratory systems, blood vessels).

  • Smooth muscle can produce more force and change size effectively, achieving contraction that is slower yet more sustainable.

Excitation-Contraction Coupling in Smooth Muscle

  1. Calcium ions enter the sarcoplasm and engage with calmodulin.

  2. Myosin light chain kinase (MLCK) phosphorylates myosin, allowing interaction with actin.

  3. Cross-bridges develop, generating muscle tension.

  4. Relaxation involves the removal of calcium ions and dephosphorylation of myosin.

Plasticity of Smooth Muscle Tissue

  • Plasticity: The ability of smooth muscle to maintain contractibility over an extensive range of lengths, which is vital for its functional adaptability in organ systems.