Musculoskeletal System and Neuromuscular System

·      Musculoskeletal system- composed of bones, joints, muscles and tendons

·      Muscles pull against bones

·      Bones rotate about joints

·      Force is transmitted by pulling and rotation

·      Bony levers + muscle-pulling forces = pulling or pushing forces on external objects

·      Skeletal system- around 206 bones in the body that provide leverage, support, and protection

·      Axial-skull sternum ribs and vertebral column and appendicular skeleton- shoulders, arms, wrist, hands, pelvic girdle, arms, ankles and feet

·      Joints-junction of the bones

o   Types (structure)-fibrous (no movement), cartilaginous (slight movement), synovial (full movement)

o   Types (rotation)- uniaxial(rotates on one axis), biaxial (rotates around two perpendicular axis), multiaxial (rotates around three axis)

o   Hyaline cartilage- on articulating bone surfaces

o   Synovial fluid- occurs in a joint capsule

·      Skeletal musculature

o   Each skeletal muscle is an organ with muscle, tissue, connective tissue, nerves, blood vessels

o   Tendon- connects muscle to bone, all connective tissue is contiguous (next to or together in sequence) with tendon

o   Epimysium, perimysium and endomysium

o   Fasciculus- bundles of muscle fibers-perimysium

o   Muscle fibers- muscle cells-endomysium

o   Neuromuscular junction- site at which a motor neuron innervates the muscle fiber 

o   Motor unit-motor neuron and all the muscles it innervates

·      Motor unit

o   Several hundred muscle fibers innervated

o   Motor unit innervates based on muscle fiber type- type I, type IIa, and type IIb

o   All muscle fibers within a unit contract when stimulated and are the same type of fiber

·      Muscle fiber

o   Sarcoplasm contains contractile components-protein filaments, stored glycogen and fat particles, enzymes, and special organelles (mitochondria and sarcoplasmic reticulum), myofibrils (contain myofilaments-myosin and actin), sarcomere (aligns myosin and actin longitudinally) (~4,500 per cm of muscle length) (z line to z line)

o   As muscles contract, z-lines get closer together

o   I band- a space where there are only actin filaments-has two adjacent sarcomeres

o   Z line- in the middle of the I band

o   H zone- only myosin filaments

o   Sarcoplasmic reticulum- surrounds each myofibril (stores calcium ions)

o   T-tubule-runs perpendicular to sarcoplasmic reticulum and terminates near Z line

·      Muscle contraction

o   Action potential from motor neuron

o   Calcium released from sarcoplasmic reticulum into myofibril

o   Tension develops in the muscles due to sliding filament theory

sliding filament theory

  • myosin cross bridges pull on actin

  • actin filaments slide inward on the myosin filaments

  • z lines are pulled towards the center of the sarcomere

  • h zone and i band shrink

  • all of this causes the muscle to shorten

  • phases

    • resting phase- occurs when there’s very few cross bridges between myosin and actin

    • excitation-contraction coupling phase- CA2+ is released from the sarcoplasmic reticulum; calcium binds to troponin→causes tropomyosin to shift; myosin attaches to actin (defined as cross bridge)

      • force is dictated by the number of cross bridges

    • contraction phase- when the power stroke occurs

      • hydrolysis oxidizes ATP→ADP + phosphate

      • myosin pulls actin

    • recharge phase- ATP binds with myosin

    • relaxation- stimulation of motor neuron stops; calcium is pumped into sarcoplasmic reticulum

  • events are repeated as long as calcium is available, ATP is available for uncoupling myosin from actin, and there’s sufficient myosin ATPase (breaks down ATP, dictates type I and II) is available

  • low force potential- reduction in cross bridging alignment

  • high force potential- increased when muscle is partially shortened, which shows optimal cross bridge actin alignment

  • when a muscle is shortened, there is low force potential because the muscle is already fully contracted, low cross bridge actin alignment

Neuromuscular System

  • action potential causes all innervated muscle fibers to activate

    • action potential causes acetylcholine is released (what diffuses across the neuromuscular junction)

    • ample ACH→ excitation of the sarcolemma→ CA2+ is released→ muscle contraction occurs

  • twitch-brief contraction that occurs from each action potential

    • in order for contraction to occur, we need to decrease time between twitches

    • greater cross bridge binding→ greater cross bridge binding and force

  • tetanus-allows for the motor unit to produce maximum force

    • review in textbook

  • motor neuron-all the muscle in a motor unit contract and provide force at the same time (all or nothing)

  • precision of movement is determined by fewer muscle fibers innervated

Muscle fiber

  • can be categorized by twitch time

  • slow twitch fibers (type I)-long twitch time, contract and relax slowly, fatigue resistant, low anaerobic power, high aerobic energy supply, low force development

  • fast twitch fibers (type II)- short twitch time, develop force quickly and relax quickly, highly fatiguable, high anaerobic power, low aerobic energy supply, high force development

    • type IIa- greater aerobic capacity, more capillaries around it (more oxygen available)

    • type IIx- less aerobic capacity, more fatiguable

  • motor unit recruitment- force output is varied via…

    • the frequency of activation of individual motor units

    • number of activated motor units

  • proprioceptors- specialized sensory receptors

    • located in joints muscles and tendons (muscle spindles and golgi tendon organs)

    • provide the central nervous system with information needed to maintain muscle tone and perform complex coordinated movements

  • muscle spindles- run parallel to extrafusal muscle fibers

    • sense stretch-detect change in muscle length

    • when it’s activated, it will activate the motor neuron and that muscle will contract

  • golgi tendon organ

    • located in tendon

    • senses tension

    • inhibits motor neurons to relax the same muscle

Cardiovascular System

  • 2 interconnected, but separate pumps

    • right-sends blood through the lungs

    • left-sends blood through the body

  • ventricles-provides main force for blood being circulated

    • muscle actions-systole (ventricular contraction), diastole (ventricular relaxation)

  • heart

    • valves to prevent back flow- tricuspid and bicuspid

    • conduction system-controls mechanical contraction of heart, begins in SA node, ECG records electric conduction

  • blood vessels

    • closed circuit system-arterial system

    • arterial system-arteries carry blood away from the heart, under high pressure, have muscular walls; capillaries have thin permeable walls;

    • venous system-veins bring deoxygenated blood to the heart, under low pressure so they’re thinner

  • hemoglobin

    • iron-protein molecule carried by red blood cells

    • transports O2

    • red blood cells deliver O2 and facilitate CO2 removal

Respiratory System

  • exchange O2 and CO2

  • exchange of air-expansion and recoil of our lungs