Muscle Contraction Physiology

• The muscular system is responsible for producing body movements, maintaining posture, and producing body heat
• The muscular system consists of muscles attached to the skeleton by tendons

Functions of the Muscular System

• Body movement
• Maintenance of posture
• Respiration
• Production of body heat
• Communication
• Constriction of organs and vessels
• Heartbeat
• Walking and running are examples of body movements due to skeletal muscle contraction
• Sitting or standing erect requires skeletal muscles to constantly maintain tone
• Thorax muscles like the diaphragm and intercostal rib muscles facilitate breathing
• Heat is released as a byproduct of skeletal muscle contraction and is essential for thermoregulation
• Speaking, writing, gesturing, texting/typing, and facial expressions are all due to skeletal muscle contraction
• Smooth muscle contraction in internal organ walls helps move and mix food and water in the digestive tract
• Smooth muscle contraction also propels secretions from organs and regulates blood flow through blood vessels
• Cardiac muscle contraction causes the heartbeat and propels blood to all body parts

4 Major Functional Properties of Muscle

• Contractility refers to a muscle's ability to shorten forcefully
• Excitability is the capacity to respond to stimulus from nerves
• Extensibility is the ability to stretch to its normal resting point and beyond
• Elasticity is the ability to recoil to its original resting length after being stretched

Types of Muscle Tissue

• Skeletal muscle
• Smooth muscle
• Cardiac muscle

Skeletal Muscle Structure

• Skeletal muscles are composed of skeletal muscle cells

• Skeletal muscles include connective tissue, blood vessels, and nerves

• Epimysium, also known as fascia, is connective tissue that surrounds each skeletal muscle

• Perimysium is loose connective tissue surrounding a fascicle

• Fascicle is a muscle bundle composed of several muscle fibers

• Endomysium surrounds each muscle fiber in a fascicle

• Sarcolemma is the cell membrane of a muscle fiber/muscle cell

• Sarcoplasm is a muscle fiber's cytoplasm

• T or Transverse Tubules are tubelike invaginations of the sarcolemma that wrap around sarcomeres

• Sarcoplasmic Reticulum is a highly organized smooth ER that contains a high concentration of Calcium ions for muscle contraction

Myofibril and Myofilaments

• Myofibrils are threadlike structures contained in the sarcoplasm

• Myofilaments are protein filaments of actin and myosin contained in each myofibril

• Sarcomere is a skeletal muscle's basic structural and functional unit

• Sarcomere contains repeating units along a myofibril and extends from Z disk to Z disk

• The I or Light band consists of only actin myofilaments

• The A or Dark band is the central region of each sarcomere where actin and myosin overlap

• The H zone is a second light zone in the center of a sarcomere consisting only of myosin myofilaments

• The M line is where myosin myofilaments are anchored

• The Z disk is the attachment site for actin myofilaments

• Actin Myofilament is a thin myofilament resembling two pearl strands twisted together with molecules

• Troponin attaches at specific intervals and provides calcium binding sites on the actin

• Tropomyosin filaments are located along the groove between twisted actin strands and expose the attachment

• Myosin Myofilament is thick and resembles bundles of minute golf clubs

• Its heads can bind to the exposed attachment sites on the actin myofilaments

• The Motor Neuron is a nerve cell that sends action potential along a skeletal muscle fiber

• The Neuromuscular Junction, or Synapse, is the contact point of a motor neuron with a skeletal muscle

• The Neuromuscular Junction is composed of the presynaptic terminal, synaptic cleft, postsynaptic membrane, and synaptic vesicles

• Acetylcholine is a neurotransmitter found in synaptic vesicles

• A Motor Unit contains of one motor neuron innervating several skeletal muscle fibers

Muscle Contraction

• Occurs as actin and myosin myofilaments slide past one another, causing sarcomeres to shorten
• Many sarcomeres joined end-to-end form myofibrils
• Shortening of sarcomeres causes myofibrils to shorten shortening myofibrils causes an entire Muscle to shorten

Sliding Filament Model

• Actin and Myosin filaments in a relaxed or contracted muscle are the same lengths
• Myofilaments do not change length during muscle contraction
• During contraction, actin myofilaments slide past the myosin myofilaments toward each other
• The Z disks are brought closer together, causing the sarcomere to shorten
• As the actin myofilaments slide over the myosin filaments, the H zones (yellow) and the I bands (blue) narrow
• The A bands, which are equal to the length of the myosin myofilaments, do not narrow, because the length of the myosin myofilaments does not change
• In a fully contracted muscle, the ends of the actin myofilaments overlap at the sarcomere's center and the H zone disappears

Breakdown of ATP and Cross-Bridge Movement During Contraction

• During muscle contraction, Calcium binds to troponin molecules, and tropomyosin molecules move, exposing myosin attachment sites on myofilaments
• Myosin heads bind to the exposed attachment sites of actin myofilaments, forming cross-bridges, and phosphates are released from the myosin heads
• Energy stored in the myosin heads moves the myosin heads, causing actin myofilaments to slide past the myosin myofilament, ADP molecules are released from the myosin heads
• ATP molecules bind to the myosin heads
• ATP breaks down to ADP and phosphates, and the myosin heads release from the actin attachment sites
• Myosin heads return to their resting position, and energy is stored in the heads of the myosin molecules
• If Ca2+ remains attached to troponin, cross-bridge formation and movement are repeated
  • This cycle happens many times during a muscle contraction and not all cross-bridges form and release simultaneously