Chapter 10.2: Micro Anat of SKM, Innervation of SKM Fibers, SKM Fiber Rest

Parts of cell (fiber)

• sarcoplasm (cytoplasm)

• sarcolemma (plasms membrane)

• myofibrils

• sarcoplasmic reticulum

• myofilaments (thick and thin)

Sarcoplasm

• cytoplasm

• organelles, contractile proteins, other specializations

• multiple nuclei

• formed in embryo where myoblasts fuse

• some myoblasts become undifferentiated satellite cells to support and repair muscle fibers

Sarcolemma

• plasma membrane

• voltage-gated ion channels

• T-tubules (transverse tubules)

• voltage-sensitive calcium channels

Myofibrils

• hundred-thousand per cell

• bundles of myofilaments enclosed in sarcoplasmic reticulum

Sarcoplasmic reticulum

• internal membrane complex similar to smooth ER

• terminal cisternae: bind sacs of sarcoplasmic reticulum

• reservior for calcium ions

  • calcium pumps import calcium

• triad: two cisternae T-tubule inbetween

Myofilaments

• contractile proteins in myofibrils

• thick: bundles of myosin

  • myosin heads point to end of filaments

• thin: twsited strands of actin (F-actin, G-actin)

  • G-actin = myosin binding site

• tropomyosin and troponin

Organization of sarcomere

• repeating units of myofilaments

• overlapping thick and thin filaments

• delineated both ends by Z discs

I bands

• light-appearing regions contain only thin filaments

• bisected by Z disc

• get smaller when muscle contracts (disappear maximal contraction)

A bands

• dark appearing region contains thick filaments and overlapping thin filaments

• contains H zone and M lines (central part of sarcomere)

H zone

• central portion of A band

• only thick filaments

• disappear maximal muscle contraction

M line

• middle of H zone

• protein meshwork structure

• attachment site for thick filaments

Connectin

• Z disc to M line

• stabilizes thick filaments and “springlike” properties (passive tension)

Dystrophin

• anchors myofibrils to sarcolemma proteins

• abnormalities of these cause muscular dystrophy

Mitochondria and other structures associated with energy production

• muscle fibers abdunant mitochondrai for aerobic ATP

• myoglobin in cells store oxygen

• glycogen stores fuel when quick energy needed

• CP gives phosphate to ADP to replenish ATP

Axons from spinal cord (pr brain)…

• innervate numerous muscle fibers

• number of fibers neuron innervates varies

Small motor units

• less than 5 muscle fibers

• prescise control of force output

Large motor units

• thousand muscle fibers

• production of large force (not precise control)

Neuromuscular Junction

• location motor neuron innervates muscle (mid-region)

• has synaptic knob, synaptic cleft, motor end plate

Neuromuscular Junction: Synaptic Knob

• expanded tip of motor neuron axon

• houses synaptic vesicles

  • small sacs filled with NT acetylcholine (ACh)

• has Ca²⁺ pumps in plasma membrane (estabilishes calcium gradient)

• voltage gated Ca²⁺pumps

Neuromuscular Junction: Motor End Plate

• specialized regions of sarcolemma with numerous folds

• many ACh receptors

• plasma membrane protein channels, opened bonding ACh, Allow Na⁺ entry and K⁺ exit

Neuromuscular Junction: Synaptic Cleft

• narrow fluid-filled space

• separated synaptic knob from motor end plate

• Acetylcholinesterase here (breaks down ACh)

Skeletal Muscle Fibers at Rest

• resting membrane potential (RMP) = -90 Mv

• inside negative compared to outside

• established by leak channels and Na/K pumps