4.4 Muscle Tissue and Motion

Muscles are the body's movers and shakers. They come in three types: skeletal, cardiac, and smooth. Each type has unique characteristics and functions, from voluntary movements to involuntary organ contractions.

Muscle contraction is the key to movement. The sliding filament theory explains how muscles contract, while excitation-contraction coupling links electrical signals to mechanical action. Understanding these processes reveals how our bodies move and function.

Muscle Tissue Types and Functions

Types of muscle tissue

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• Skeletal muscle tissue forms muscles attached to bones via tendons, under voluntary control, and responsible for gross body movements (walking, lifting) and maintaining posture
• Cardiac muscle tissue forms the muscular walls of the heart, under involuntary control, and responsible for pumping blood throughout the body
• Smooth muscle tissue forms the walls of hollow organs (stomach, intestines) and blood vessels, under involuntary control, and responsible for the movement of substances within organs (peristalsis) and the regulation of blood flow (vasoconstriction, vasodilation)

Skeletal vs cardiac vs smooth muscle

• Skeletal muscle consists of long, cylindrical cells called muscle fibers that are multinucleated with nuclei located at the periphery, have a striated appearance due to the arrangement of myofilaments, are under voluntary control by the somatic nervous system, and contract rapidly and forcefully
  • Skeletal muscle fibers contain myofibrils, which are bundles of protein filaments responsible for muscle contraction
• Cardiac muscle consists of branched, interconnected cells with a single, centrally located nucleus, connected by intercalated discs that allow for coordinated contraction, have a striated appearance similar to skeletal muscle, are under involuntary control by the autonomic nervous system, and contract rhythmically and continuously
• Smooth muscle consists of spindle-shaped cells with a single, centrally located nucleus, lack striations due to the irregular arrangement of myofilaments, are under involuntary control by the autonomic nervous system, and contract slowly and maintain tension for extended periods

Muscle Contraction and Movement

Muscle contraction for movement

• The sliding filament theory explains how muscle contraction occurs through the interaction of actin and myosin filaments
  • Troponin and tropomyosin are regulatory proteins that control the interaction between actin and myosin during muscle contraction
• In skeletal muscle, motor neurons stimulate muscle fibers organized into motor units that contract simultaneously, generating force transmitted to bones via tendons, causing bones to move at joints and resulting in body movement (flexion, extension)
  • The neuromuscular junction is the site where motor neurons communicate with muscle fibers to initiate contraction
• In smooth muscle, contraction is regulated by the autonomic nervous system and local factors
  1. Contraction of smooth muscle in the walls of hollow organs helps to move substances through the organ via peristalsis (esophagus, stomach, intestines)
  2. Contraction of smooth muscle in blood vessel walls regulates blood flow and blood pressure through vasoconstriction (narrowing blood vessels, increasing resistance and blood pressure) and vasodilation (widening blood vessels, decreasing resistance and blood pressure)

Excitation-Contraction Coupling

• Excitation-contraction coupling is the process by which an electrical stimulus (action potential) is converted into a mechanical response (muscle contraction)
• This process involves the release of calcium ions from the sarcoplasmic reticulum, which then interact with regulatory proteins to allow for the sliding of actin and myosin filaments