Muscle Tissue & Contraction

• Muscles enable body movement through contraction or shortening, making them the body's machine
• There are three primary types of muscles found in the body: skeletal, cardiac, and smooth muscle

Skeletal Muscle

• Skeletal muscles attach to bones via tendons
• Cells are multinucleate, containing multiple nuclei
• Skeletal muscles have visible banding, known as striations, hence striated muscles
• Subject to conscious control, this means they are voluntary
• Cells are surrounded and bundled by connective tissue, which facilitates great force

Smooth Muscle

• Smooth muscles lack striations
• Cells are spindle-shaped
• Each cell has a single nucleus
• Not under conscious control, therefore involuntary
• Predominantly found in the walls of hollow organs such as stomach, intestine, uterus, urinary bladder, arteries as well as other internal organs
• Contractions are slow

Cardiac Muscle

• Forms the contractile wall of the heart, existing only in the heart
• Cardiac cells are branched
• Cardiac muscles exhibit striations
• Typically possesses a single nucleus
• Not under conscious control, so involuntary

Muscle Functions

• Muscles facilitates movement
• Muscles are responsible for maintaining position
• Muscles stabilize joints
• Muscles generate heat

Skeletal Muscle Organization

• Sarcolemma represents the fiber cell membrane
• Sarcoplasm makes up the cytoplasm of the fiber
• Fiber cells contain organelles similar to other cells which include myoglobin, an oxygen-binding protein
• Cells are punctuated by transverse tubules (T-tubules), these are narrow tubes extending into the sarcoplasm at right angles, filled with extracellular fluid
• Myofibrils are cylindrical structures with muscle fibers containing bundles of protein filaments called myofilaments
• There are two types of Myofilaments:
  • Actin filaments (thin filaments)
  • Myosin filaments (thick filaments)
• When a myofibril shortens, the muscle contracts

Sarcomere

• The sarcomere represents the contractile unit of the muscle
• Sarcomere feature alternating bands of thin (actin) and thick (myosin) filaments

Thin and Thick Filaments

• Thin filaments are made of actin
• Tropomyosin: a protein that lies on actin
• Strands of actin and tropomyosin are coiled together
• Troponin molecules are protein, binding to tropomyosin fibers
• Thick filaments are made of myosin
• Myosin molecules are proteins with a globular head (cross bridge)

Interaction of Thick & Thin Filaments

• Cross bridges are connections forming between myosin heads (thick filaments) and actin (thin filaments)
• Connections between thick and thin filaments cause the muscle to shorten, which means to contract

Mechanism of Muscle Contraction

• A nerve impulse reaches the myoneural junction
• Acetylcholine, a neurotransmitter, is released from a motor neuron
• Acetylcholine binds with receptors in the muscle membrane, allowing sodium to enter
• Sodium influx generates an action potential in the sarcolemma
• The action potential travels down the T tubule
• The sarcoplasmic reticulum then releases stored calcium (Ca+2)
• At rest tropomyosin blocks myosin-binding sites on actin
• Calcium binds to troponin complex, causing a shape change and movement of the troponin complex
• Change reveals myosin-binding sites on actin
• Myosin globular heads attach to the exposed binding sites, creating a power stroke, leading to muscle contraction
• ATP detaches myosin heads and readies them for another contraction

Muscle Response

• Muscle force is reliant on the number of fibers stimulated
• More fibers contracting equal to greater muscle tension
• Muscles are able to continue to contract unless they run out of energy

Roles of muscle

• Agonist: the muscle provides the major force needed to complete movement, also known as the prime mover
• Antagonist: muscle opposes or reverses a prime mover
• Synergist: muscles work together to achieve a common effect
• Muscles work by contracting; skeletal muscles pairs up to be antagonistic (flexor vs extensor)
• Muscles contracting = shortening and parts move
• Tendons: Connects bone to muscle
• Ligaments: Connects bone to bone

Energy for Muscle Contraction

• Initially muscles use stored ATP for energy
• Bonds of ATP are broken to release energy
• Muscle only stores 4-6 seconds worth of ATP
• After using the initial ATP other pathways must be used for more production of ATP
• Direct phosphorylation sees that muscle cells contain creatine phosphate (CP)
• CP is a high energy molecule that tranfers energy to ADP to regenerate ATP
• CP supplies are exhausted in about 20 seconds
• Anaerobic glycolysis breaks down glucose without oxygen
• Glucose is broken down to produce some ATP
• Pyruvic acid is converted to lactic acid
• This is not efficient
• Lactic acid causes muscle fatigue
• The common reason for muscle fatigue is oxygen debt
• Aerobic respiration is a series of metabolic pathways that occur in the mitochondria
• Glucose is broken down to carbon dioxide and water, thus releasing energy
• It requires continuous oxygen and is a slower reaction

Test your knowledge of muscle tissue types and contraction mechanisms. Learn about the unique characteristics of cardiac, smooth, and skeletal muscle. Understand the roles of key components like T-tubules, myoglobin, and ATP in muscle function.