Muscles and Muscle Tissue

Chapter 09: Muscles and Muscle Tissue

9.1 Muscle Tissue

  • Muscle tissue comprises nearly half of the body's mass.

  • Muscles convert chemical energy (ATP) into directed mechanical energy to exert force.

Types of Muscle Tissue

Overview

  • Prefixes: myo-, mys-, and sarco- refer to muscle tissue (e.g., sarcoplasm = muscle cell cytoplasm).

  • Types of Muscle Tissue:

    • Skeletal

    • Cardiac

    • Smooth

  • Muscle fibers: elongated muscle cells in skeletal and smooth muscle, not in cardiac muscle.

Skeletal Muscle

  • Packaged into muscles attached to bones and skin.

  • Longest muscle fibers, striated (striped), and voluntary (consciously controlled).

  • Rapid contraction; tires easily; powerful.

  • Key terms: skeletal, striated, voluntary.

Cardiac Muscle

  • Found only in the heart, forming the bulk of heart walls.

  • Striated and involuntary (cannot be consciously controlled).

  • Contracts at a steady rate due to its pacemaker, but can be modulated by the nervous system.

  • Key terms: cardiac, striated, involuntary.

Smooth Muscle

  • Located in walls of hollow organs (e.g., stomach, bladder, airways).

  • Non-striated and involuntary.

  • Key terms: visceral, non-striated, involuntary.

Characteristics of Muscle Tissue

  • Excitability: responds to stimuli.

  • Contractility: shortens forcibly when stimulated.

  • Extensibility: can be stretched.

  • Elasticity: recoils to resting length.

Muscle Functions

  • Movement: locomotion and manipulation.

  • Posture: maintains body position.

  • Joint Stability: stabilizes joints.

  • Heat Generation: generates heat during contraction.

9.2 Skeletal Muscle

Anatomy

  • Composed of various tissues including nerves, blood supply, and connective tissues (epimysium, perimysium, and endomysium).

9.3 Skeletal Muscle Fibers

  • Long and cylindrical with multiple nuclei; includes structures like sarcolemma, sarcoplasm, myofibrils, and T-tubules.

Myofibrils

  • Densely packed elements accounting for muscle cell volume.

  • Striations: caused by repeating dark and light bands (A bands and I bands).

  • Sarcomeres: smallest functional unit of muscle fiber containing A bands and I bands.

9.4 Motor Neurons

  • Motor neurons stimulate skeletal muscle fibers to contract through action potentials (AP).

  • Process of excitation-contraction coupling links muscle fiber excitation with contraction.

  • Events at the neuromuscular junction include neurotransmitter release (acetylcholine).

9.5 Whole Skeletal Muscle Contraction

  • Contraction generates muscle tension. Load is the opposing force.

  • Graded muscle contractions depend on frequency and strength of stimulation.

Graded Muscle Contractions

  • Muscle Twitch: simplest contraction from a single AP.

    • Phases: latent period, period of contraction, period of relaxation.

  • Recruitment: more motor units are engaged for stronger contractions.

9.6 ATP for Muscle Contractions

  • ATP is crucial for muscle contraction, required for cross bridge cycling and calcium management.

  • ATP is generated through direct phosphorylation, anaerobic and aerobic pathways.

Types of Smooth Muscle

  • Unitary: found in hollow organs, electrically coupled and have spontaneous action potentials.

  • Multi-Unit: less common, found in large airways and blood vessels; require neural input for contraction.

Conclusion

  • Understanding muscle types, their functions, and mechanisms of contraction is essential for grasping human physiology.