Chapter 9 – Muscles and Muscle Tissue

Skeletal Muscle Anatomyyne

Skeletal muscle tissue is composed of long, cylindrical cells called muscle fibers. The organization of these fibers and their associated structures is essential for muscle function.

• Functional Unit – Sarcomere: The sarcomere is the basic contractile unit of a skeletal muscle fiber. It is defined as the segment between two Z discs and is responsible for muscle contraction.

• Myofibrils: These are long, rod-like organelles within muscle fibers, composed of repeating sarcomeres.

• Sarcolemma: The plasma membrane of a muscle fiber, which plays a role in conducting electrical signals.

• Epimysium: A connective tissue sheath that surrounds the entire muscle. A sheetlike extension of the epimysium is called an aponeurosis, which connects muscles to the parts they move.

• Fascicle: A bundle of muscle fibers surrounded by perimysium.

Example: The biceps brachii muscle is surrounded by epimysium, which may form a tendon or aponeurosis at its ends.

The Sliding Filament Theory

The sliding filament theory explains how muscles contract at the molecular level, involving the interaction of thick and thin filaments within the sarcomere.

• Thick Filaments: Composed primarily of the protein myosin. Thick filaments are found in the A band of the sarcomere.

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

• Regulation of Contraction: In a resting muscle cell, the myosin-binding sites on actin are blocked by tropomyosin, preventing contraction.

• T Tubules: Invaginations of the sarcolemma (muscle cell membrane) that help transmit action potentials deep into the muscle fiber.

Example: During contraction, calcium ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

Muscle Contraction

Muscle contraction is a complex process involving electrical and chemical events that lead to the shortening of muscle fibers.

• Calcium Storage: In a resting muscle cell, most calcium ions are stored in the sarcoplasmic reticulum.

• Initiation of Contraction: When stimulated, calcium is released from the sarcoplasmic reticulum into the sarcoplasm, binding to troponin and initiating contraction.

• ATP Requirement: ATP must bind to myosin heads for them to detach from actin after a power stroke.

• Calcium Reuptake: After contraction, calcium is actively transported back into the sarcoplasmic reticulum.

• Types of Contractions:

  • Twitch: A single, brief contraction in response to a single threshold stimulus.

  • Summation: Increased force of contraction due to repeated stimulation.

  • Tetany: A sustained contraction resulting from rapid stimulation.

• Muscle Tension: The force exerted by a contracting muscle.

• Latent Period: The time between stimulation and the onset of contraction.

• Factors Affecting Force: The force of muscle contraction is affected by the size of muscle fibers, number of fibers stimulated, and degree of stretch, but not by the method of stimulation.

• Recruitment: Increasing the number of active motor units increases contraction strength.

Example: Lifting a heavy object requires recruitment of more motor units and greater muscle tension.

Muscle Metabolism

Muscle fibers require energy to contract, which is supplied by various metabolic pathways.

• Direct Energy Source: ATP is the immediate source of energy for muscle contraction.

• ATP Regeneration: Occurs via creatine phosphate, anaerobic glycolysis, and aerobic respiration.

• Muscle Soreness: Often associated with anaerobic glycolysis, which produces lactic acid.

• Fuel Sources: After about 30 minutes of exercise, fatty acids become the major source of fuel for muscle activity.

• Aerobic Endurance: Refers to the length of time a muscle can continue to contract using aerobic pathways.

Example: Marathon runners rely heavily on aerobic metabolism and fatty acid oxidation for sustained energy.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs and has unique structural and functional characteristics compared to skeletal muscle.

• Muscle Layers: Contraction of the longitudinal layer of smooth muscle results in shortening of the organ.

• Unitary Contraction: Smooth muscle cells can contract as a unit due to the presence of gap junctions, which allow electrical signals to pass between cells.

• Calcium Binding: In smooth muscle, calcium binds to calmodulin (not troponin) to initiate contraction.

• Dense Bodies: These structures in smooth muscle correspond to Z discs in skeletal muscle and anchor thin filaments.

• Stress-Relaxation Response: Allows organs like the stomach to stretch and then contract slowly, accommodating changes in volume.

Example: The bladder can expand to store urine and then contract to expel it, thanks to the stress-relaxation response.

Muscle Fiber Types

Muscle fibers can be classified based on their contraction speed and metabolic properties.

• Slow Oxidative Fibers: Characterized by high myoglobin content, many mitochondria, and a rich blood supply. They are fatigue-resistant and suited for endurance activities.

• Fast Glycolytic Fibers: Have large diameters, few mitochondria, and low myoglobin content. They fatigue quickly and are suited for short bursts of power.

Example: Postural muscles contain a high proportion of slow oxidative fibers, while muscles used for sprinting have more fast glycolytic fibers.

Table: Comparison of Muscle Fiber Types

Key Equations

• ATP Hydrolysis:

• Creatine Phosphate Reaction:

Additional info: Some explanations and the comparison table were expanded for clarity and completeness based on standard Anatomy & Physiology content.