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Skeletal muscle structure. striated long multinucleate cells extend from tendon to tendon formed by fusion of myoblasts innervated by somatic nervous system one neuromuscular junction per fiber cardiac & smooth muscle later. fig 9-1a. Skeletal muscle structure. fig 9-2.

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skeletal muscle structure
Skeletal muscle structure


long multinucleate cells

extend from tendon to tendon

formed by fusion of myoblasts

innervated by somatic nervous system

one neuromuscular junction per fiber

cardiac & smooth muscle later

fig 9-1a

skeletal muscle structure3
Skeletal muscle structure

fig 9-3

Don’t bother with: I band, A band, H zone, M line

skeletal muscle structure4
Skeletal muscle structure

6 thin filaments around each thick

3 thick filaments around each thin

fig 9-4

generation of motor end plate potential notes
Generation of motor end plate potential (notes)

action potential in somatic motor neuron

depolarization of axon terminal, opening of voltage gated Ca++ channels

Ca++ enters cell & activates fusion of AcCh vesicles with docking sites

AcCh released into synaptic cleft

AcCh binds to non-specific ligand gated cation channels in motor end plate

opening of channels; Na+ influx greater than K+ efflux

motor end plate potential occurs (EPSP) & spreads to edge of plate

edge of motor end plate acts like initial segment of axon terminal

voltage gated Na+ & K+ channels generate action potential in muscle

note: motor nerve action potential always generates muscle action potential


AcCh release ends; acetylcholinesterase hydrolyses AcCh; choline transported back into axon terminal

ca release from sarcoplasmic reticulum s r
Ca++ release from sarcoplasmic reticulum (s.r.)

fig 9-15 cropped

action potential spreads across muscle membrane and down T tubules

depolarization sensed by dihydropyridine (DHP) receptor in T tubule wall

DHP receptor opens ryanodine receptor & its Ca++ channel in s.r. wall

Ca++ released into cytosol; subsequently returned to s.r. by Ca++ ATPase

interaction of thick and thin filaments
Interaction of thick and thin filaments

fig 9-07a

Myosin cross bridges bind to sites on actin (when exposed)

myosin structure
Myosin structure

fig 9-07b

Heavy chains (paired): tail, hinge & cross bridge

Light chains (2 pairs): involved in ATPase activity & regulation

ca binds to troponin
Ca++ binds to troponin

fig 9-12

Ca++ binds to troponin which causes tropomyosin to move to side

exposed sites on actin bind/release myosin cross bridges

troponin function low ca
Troponin function: low Ca++

fig 9-9a

in absence of Ca++:

troponin holds tropomyosin against cross-bridge binding site on actin

troponin function high ca
Troponin function: high Ca++

fig 9-9b

in presence of Ca++:

troponin moves tropomyosin away from cross-bridge binding site on actin

cross bridge cycling notes
Cross bridge cycling (notes)

Resting state

 [Ca++], X-bridge binding site covered

X-bridge energized (A + *MADPPi)

Ca++ release from s.r.

 [Ca++] exposes X-bridge binding site on actin

energized X-bridge binds to actin, ADP & Pi released (step 1)

X-bridge “uncocks” as thick filament slides past thin filament (AM) (step 2)

ATP gets involved

ATP binds to myosin, releasing actin binding (MATP + A) (step 3)

X-bridge is energized (cocked)

MATP  *MADPPi (step 4)

Cycling continues until [Ca++] falls

muscle relaxation
Muscle relaxation

action potentials in motor nerve cease

AcCh in synaptic cleft hydrolyzed by acetylcholinesterase

action potentials in muscle fiber cease

Ca++ pumped back into sarcoplasmic reticulum

troponin moves tropomyosin to cover X-bridge binding sites

myosin remains in *MADPPi form

antagonistic muscle extends relaxed muscle

muscle fiber contraction
Muscle fiber contraction

fig 9-10

This is the response of a single muscle fiber to a single action potential