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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

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 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

Relaxation:

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