Exercise 14 Microscopic Anatomy, Organization, and Classification of Skeletal Muscle

SKELETAL MUSCLE Voluntary or involuntary? Striated or not? Multinucleate or uninucleate?

Terminology “Myo-” or “-Mys-” = muscle “Sarco-” = flesh

Microscopic Anatomy Muscle Fiber = long, slender, cylindrical, MULTInucleate cell Nuclei located just under sarcolemma (cell membrane) Fig. 10-2

Muscle Fiber Anatomy Sarcolemma Sarcoplasm Plasma membrane of muscle fiber Trans-membrane potential: charge differences on either side Sarcoplasm Cytoplasm of muscle fiber Stored glycogen, mitochondria, myoglobin, actin, myosin

Fig. 10-3

Transverse tubules Myofibrils T tubules Narrow tubes, continuous with sarcolemma Extend into sarcoplasm, fluid-filled Myofibrils Parallel, extend length of muscle fiber Each muscle fiber contains 100s-1000s Composed of myofilaments Actin (thin filaments) & myosin (thick filaments)

Fig. 10-3

Sarcoplasmic Reticulum (SR) Smooth ER Tubular network around each myofibril Terminal cisternae: enlarged portion of SR on either side of T tubule Storage/release of Ca Triad = T tubule + 2 terminal cisternae Fig. 10-3

Muscle contraction begins when Calcium ions are released from the terminal cisternae Into the sarcoplasm ADAM cd-rom

Sarcomeres Organized group of myofilaments Contractile units, smallest functional unit of muscle fiber Z-line to Z-line Fig. 10-4

Sarcomeres A band: DARK I band: LIGHT Thick and thin filaments M line is in the middle of the sarcomere/A band I band: LIGHT midline is Z line/disc Thin filaments only Fig. 10-4

Myofilaments Thick filaments: myosin Rod-like tail bound to other myosin Head projects outward has binding sites for actin (thin filaments), ATP “cross-bridges” Contraction: The myosin head changes shape & will grab onto the actin…and pull it toward the M-line Fig. 10-7

Myofilaments Thin filaments: 3 types of protein molecules actin Double helix (2 strands) coil around each other has binding sites for myosin Site is blocked by troponin-tropomyosin complex when muscle’s NOT contracting Tropomyosin Rod-like, coils around actin to stiffen it Covers binding sites Troponin 3 protein-complex One of each binds to: actin, tropomyosin, calcium ions (this one’s empty in a resting cell…see ADAM cd)

Fig. 10-7

Connective Tissue Wrappings: 3 Layers Epimysium = outer Perimysium = central Endomysium = inner

Connective Tissue Wrappings Epimysium Epi = on (outside layer) Surrounds entire muscle Dense layer of collagen fibers, connected to deep fascia Separates muscle from other tissues/organs Fig. 10-1

Connective Tissue Wrappings Perimysium Peri = around (central layer) Divides muscle into compartments: Fascicle = bundle of muscle fibers (cells) Collagen, elastic fibers Blood vessels, nerves to each fascicle Connective Tissue Wrappings Fig. 10-1

Connective Tissue Wrappings Endomysium Endo = inside (inner layer) Surrounds individual skeletal muscle cells (fibers) Interconnects adjacent muscle fibers Capillary networks to supply blood to cells, nerve fibers to control muscle Fig. 10-1

Connective Tissues, cont. Layers are continuous & interwoven – blend into one another Muscle’s ends: layers unite to form Bundle: TENDON Attach skeletal muscles to bones Contraction pulls the bone(s) Broad sheet: APONEUROSIS Attach skeletal muscles to bones or other muscles

Muscle—covered by epimysium Fascicles—covered by perimysium Connective tissue layers! Muscle—covered by epimysium Fascicles—covered by perimysium Fiber (cell)—covered by endomysium Myofibril—covered by sarcoplasmic reticulum Sarcomere—contains thick and thin filaments

Fig. 10-6

Blood & Nerve Supply Muscles need lots of energy, lots of oxygen Blood vessels supply these Each fiber (cell) has capillary blood supply Contraction stimulated by nerve impulses Axons (nerve fibers) penetrate connective tissue layers, & innervate individual muscle fibers (cells)

How Muscles Contract: ADAM cd-rom

Neuromuscular Junction Junction of motor neuron & a muscle fiber Motor neuron sends a signal to cause a movement of a skeletal muscle Neuron—elongated portion = axon Ends in expanded branches (synaptic terminals) synaptic cleft: space between synaptic terminal & muscle fiber’s sarcolemma

Neuromuscular Junction Synaptic terminals Contain synaptic vesicles Filled with acetylcholine (ACh) Neurotransmitter: chemical released by neuron Release of ACh triggers muscle contraction Sarcolemma of motor end plate has receptors to bind ACh

Neuromuscular Junction “Motor end plate” Sarcolemma highly folded Increases surface area Sites of ACh binding “Action Potential” Fig. 10-10

Neuromuscular Junction Motor unit = A single motor neuron & ALL the muscle fibers it controls Fig. 10-17

Fig. 10-10

Fig. 10-10