Muscles and Motor Locomotion

Muscle Movement * Muscles do work by contracting * Skeletal muscles come in antagonistic pairs * Flexor vs. extensor * Contracting = shortening * Move skeletal parts * Tendons * Connect bone to muscle * Ligaments * Connect bone to bone
Structure of Striated Skeletal Muscle * Muscle fiber * Muscle cell * Divided into sections = sarcomeres * Sarcomere * Functional unit of muscle contraction * Alternating bands of thin (actin) and thick (myosin) protein filaments
Actin * Complex of fibers * Brain of actin molecules and tropomyosin fibers * Tropomyosin fibers secured with troponin molecules
Myosin * Single protein * Myosin molecule * Long protein with globular head
Thick and thin filaments * Myosin tails aligned together and heads pointed away from center of sarcomere

Interaction of Thick and Thin Filaments * Cross bridges * Connections formed between myosin heads and actin * Cause muscle to shorten
Muscle Cell Organelles * Sarcoplasm * Muscle cell cytoplasm * Contains many mitochondria * Sarcoplasmic reticulum * Organelle similar to ER * Network of tubes * Stores Ca2+ * Ca2+ released from sarcoplasmic reticulum through channels * Ca2+ restored to sarcoplasmic reticulum by Ca2+ pumps * Pump Ca2+ from cytosol * Pumps use ATP
Muscle at Rest * Interacting proteins * At rest, troponin molecules hold tropomyosin fibers, so that they cover the myosin-binding sites on actin * Troponin has Ca2+ binding sites
Motor Neurons * Motor neuron triggers muscle contraction * Release acetylcholine (Ach) neurotransmitter
Nerve Trigger of Muscle Action * Nerve signal travels down T-tubule * Stimulates sarcoplasmic reticulum of muscle cell to releases stored Ca2+ * Flooding muscle fibers with Ca2+
Ca2+ Triggers Muscle Action * At rest, tropomyosin blocks myosin-binding sites on actin * Secured by troponin * Ca2+ binds to troponin * Shape change accuses movement of troponin * Releasing tropomyosin * Exposes myosin-binding sites on actin
How Ca2+ Controls Muscle * Sliding filament mode * Exposed actin binds to myosin * Fibers slide past each other * Ratchet system * Shorten muscle cell * Muscle contraction * Muscle doesn’t relax until Ca2+ is pumped back into sarcoplasmic reticulum * Requires ATP
How It All Works * Action potential causes Ca2+ release from sarcoplasmic reticulum * Ca2+ binds to troponin * Troponin moves tropomyosin, uncovering myosin binding site on actin * Myosin bonds actin * Uses ATP to ratchet each time * Releases and bonds to next actin * Myosin pulls actin chain along * Sarcomere shortens * Z discs move closer together * Whole fiber shortens * Contraction * Ca2+ pumps restore Ca2+ to sarcoplasmic reticulum
Fast Twitch and Slow Twitch Muscles * Slow twitch muscle fibers * Contract slowly, but keep going for a long time * More mitochondria for aerobic respiration * Less sarcoplasmic reticulum * Ca2+ remain in cytosol longer * Long distance runner * “Dark” meat = more blood vessels * Fast twitch muscle fibers * Contract quickly, but get tired rapidly * Store more glycogen for anaerobic respiration * Sprinter * “White” meat
Muscle Limits * Muscle fatigue * Lack of sugar * Lack of ATP to restore Ca2+ gradient * Low O2 * Lactic acid drops pH which interferes with protein function * Synaptic fatigue * Loss of acetylcholine * Muscle cramps * Build up of lactic acid * ATP depletion * Ion imbalance * Massaging or stretching increases circulation
Diseases of Muscle Tissue * ALS * Amyotrophic lateral sclerosis * Lou Gehrig’s disease * Motor neurons degenerate * Myasthenia gravis * Autoimmune * Antibodies to acetylcholine receptors
Botox * Bacteria Clostridium botulinum toxin * Blocks release of acetylcholine * Botulism can be fatal
Rigor Mortis * No life, no breathing * No breathing, no O2 * No O2, no aerobic respiration * No aerobic respiration, no ATP * No ATP, no Ca2+ pumps * Ca2+ stays in muscle cytoplasm * Muscle fibers continually contract * Eventually, tissues breakdown and relax * Measure of time of death