MUSCULAR SYSTEM Structure and Function

Skeletal Muscle Properties 1. Excitability = ability to receive and respond to a stimulus  Also called irritability or responsiveness 2. Contractility = ability to shorten when stimulated 3. Extensibility = ability to be stretched 4. Elasticity = ability to recoil and resume resting length

 muscle cell = muscle fiber  A “muscle” is composed of many muscle fibers arranged in bundles called fascicles  Fascia = connective tissue that separates muscles; also forms tendons Organization of Skeletal Muscle

 Three layers of connective tissue: epimysium perimysium endomysium Muscle fiber

Organization of Skeletal Muscle  Three layers of connective tissue: 1. Epimysium outermost layer; surrounds the entire muscle 2. Perimysium surrounds a bundle of muscle fibers (fascicle) 3. Endomysium surrounds individual muscle fibers  All three layers of connective tissue come together to form a tendon which attaches the muscle to bone

epimysium perimysium endomysium Organization of Skeletal Muscle

 Hierarchy of organization: Muscle  fascicle  fiber  myofibrils  myofilaments (actin/myosin)

Skeletal Muscle Anatomy  Sarcolemma = plasma membrane of muscle cell  Sarcoplasm = cytoplasm of muscle cell  Sarcoplasmic reticulum = stores calcium for contraction  Transverse tubules = network of narrow tubes that form passageways through a muscle fiber  Myofibrils = protein filaments (myofilaments)  Actin and myosin

Skeletal Muscle Anatomy sarcolemma mitochondria sarcoplasm myofibril nucleus

Skeletal Muscle Anatomy  Sarcomere = functional unit of skeletal muscle  There may be thousands of sarcomeres in one fiber aligned end to end

Myofilaments  Form dark and light bands which gives skeletal muscle a striped appearance  Interaction between these myofilaments within each sarcomere causes the sarcomere to shorten during contraction

Actin (thin filament)  The most abundant protein in eukaryotic cells  Forms the light band of the sarcomere  Each actin molecule has an active site capable of interacting with myosin  At rest, active sites are blocked

Myosin (thick filament)  Motor protein  Forms the dark band of the sarcomere  Each myosin molecule has a globular head and a long fibrous tail  Myosin head interacts with actin active site during contraction

LABEL THIS DIAGRAM IN YOUR NOTES!!!

Sarcomere Structure  DArk band = A band = Myosin (thick)  LIght band = I band = actin (thin)

Sarcomere structure  Z line = marks the boundaries of each sarcomere  M line = middle of the sarcomere

Sarcomere structure  H zone = region of sarcomere where thick filaments do not overlap with thin filaments  Zone of overlap = region of sarcomere where thick filaments overlap with thin filaments

Sarcomere Structure sarcomere Z line Thick filament Thin filament I band A band M line H zone Animation

Microscopes…Skeletal Muscle  In your lab notebook, create a new Lab titled “Skeletal Muscle”  Part I. Microscopic observations  Draw a picture of skeletal muscle under HIGH POWER  Identify and label multiple nuclei, muscle fibers, banding patterns

Label…A, I, M, Z, H animation

 Changes to the sarcomere  I band gets smaller  Z lines move closer together  H zone decrease  Zones of overlap get larger  Width of A bands doesn’t change Label…A, I, M, Z, H animation

Sliding Filament Theory (model)  The theory of how muscle contracts  Contraction occurs as the thin filament “slides” past the thick filament  Involves 5 different molecules and calcium  Myosin  Actin  Tropomyosin  Troponin  ATP

Role of Nervous System in Muscle Contraction  Skeletal muscles contract only under nervous system control  Neuromuscular junction = association of nerve fiber with muscle fiber  Acetylcholine = the neurotransmitter that acts on skeletal muscle by triggering calcium release  Calcium is the “key” to allowing actin and myosin to interact during contraction

Before contraction begins…  Muscle at rest  Troponin is bound to actin filaments  Tropomyosin binds to the troponin Blocks active site preventing contraction from occuring  Myosin heads (cross bridges) are “primed” for contraction  Signal from nervous system (impulse) triggers the release of calcium

animation

What do you know about a muscle at rest?

The contraction cycle 1. Calcium binds to troponin, shifting tropomysoin, exposing actin active site

The contraction cycle 2. Myosin head binds to actin active site (cross-bridge forms)

The contraction cycle 3. Attached myosin head pivots toward center of sarcomere  This step requires energy that was stored in myosin head prior to contraction

The contraction cycle 4. Cross-bridges detach; myosin binds another ATP

The contraction cycle 5. Myosin is reactivated. The entire cycle can now be repeated

The contraction cycle  Ends when calcium ion concentration returns to normal Accomplished by active transport of calcium into sarcoplasmic reticulum Contraction animation

Sketches in your lab book 1. Skeletal muscle  If you do not, get a microscope and do this first! 2. Actin Filament  Label actin molecules, troponin, tropomyosin and an active site 3. Myosin Filament  Should contain multiple myosin molecules  Label head and tail 4. Sarcomere (at rest vs. contracted)  Without detail of individual filaments  Label A band, I band, M line, Z line, H zone  Describe changes to these structures during contraction

Sketches

The Contraction Cycle BIG PICTURE  Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs  The trigger for contraction is calcium  Calcium ions are released when the muscle fiber is stimulated by an impulse from a motor neuron  Contraction is an active process  Relaxation and return to normal are passive processes

Rigor Mortis  Upon death, circulation stops and skeletal muscles are deprived of nutrients  Muscle fibers run out of ATP  Calcium ions cannot be cleared  Sustained contraction occurs  All body muscles are involved  Individual becomes “stiff”