Nerve Stimulus Excites the Muscle Nerve Stimulus Excites the Muscle CellCell• A muscle cell must receive a stimulus to begin the A muscle cell must receive a stimulus to begin the

excitation-contraction couplingexcitation-contraction coupling– Series of events linking electrical signal to muscle Series of events linking electrical signal to muscle

contractioncontraction– Muscle cells can be stimulated by AChMuscle cells can be stimulated by ACh

• ACh- Acetylcholine- neurotransmitterACh- Acetylcholine- neurotransmitter

1.1. Nerve impulse reaches axon terminalNerve impulse reaches axon terminal• Axon- long extension of nerve cell, relays stimulusAxon- long extension of nerve cell, relays stimulus• Neuromuscular Junction- axon branches as it enters muscle, Neuromuscular Junction- axon branches as it enters muscle,

each branch goes to 1 muscle fibereach branch goes to 1 muscle fiber• Synaptic cleft- small space between axon terminal & Synaptic cleft- small space between axon terminal &

muscle fibermuscle fiber

2. Voltage-gated Ca2. Voltage-gated Ca2+2+ channels on axon terminal open channels on axon terminal open CaCa2+2+ goes in goes in synaptic vesicles fuse with membrane synaptic vesicles fuse with membrane• Synaptic vesicles- sacs filled with neurotransmitterSynaptic vesicles- sacs filled with neurotransmitter

3. Exocytosis of ACh 3. Exocytosis of ACh • Motor end plate- folded part of sacrolemma with millions of Motor end plate- folded part of sacrolemma with millions of

ACh receptorsACh receptors Animated Neurotransmission

Resting Potential- PolarizedResting Potential- Polarized• Partial negative charge inside a neuron or Partial negative charge inside a neuron or

muscle cell at rest muscle cell at rest – More KMore K++ inside, more Na inside, more Na++ outside outside– Both KBoth K++ & Na & Na++ diffuse through cell diffuse through cell

membrane, Kmembrane, K++ can get out easier than Na can get out easier than Na++ can get incan get in

– Polarized- difference in charge inside & Polarized- difference in charge inside & outside the celloutside the cell

Membrane

Outside the cell

Cytoplasm

Na+ Na+Na+

Na+Na+

Na+

Na+ Na+ Na+K+

K+ K+

K+K+ K+

K+

K+

K+K+ K+

-

Na+

K+ K+K+ K+-

-- -

-

- - -

-

--

-

Resting membrane potential

Action Potential (AP)- Action Potential (AP)- DepolarizedDepolarized• When muscle cell is stimulated by ACh, chemically When muscle cell is stimulated by ACh, chemically

gated ion (Nagated ion (Na++ & K & K++) channels open) channels open• NaNa++ flows in faster than K flows in faster than K++ flows out flows out DepolarizationDepolarization- change of charge (action - change of charge (action potential)potential)– Causes a ripple effect along sarcolemma, voltage gated Causes a ripple effect along sarcolemma, voltage gated

NaNa++ gates open gates open– Also causes slower KAlso causes slower K++ gate to open, K gate to open, K++ rushes out rushes out RepolarizationRepolarization- return to resting charge- return to resting charge

• Active transport is used to move NaActive transport is used to move Na++ back outside back outside & K& K++ back inside back inside– Refactory period- cell cannot be stimulated again until Refactory period- cell cannot be stimulated again until

repolarization & active transport of ions is completerepolarization & active transport of ions is complete

Membrane

Outside the cell

Cytoplasm

Na+ Na+Na+

Na+Na+

Na+

Na+ Na+ Na+K+

K+ K+

K+K+ K+

K+

K+

K+K+ K+

-

Na+

K+ K+K+ K+

Action Potential

-

-- -

-

- - -

-

--

-

Na+

Na+

Na+ Na+ Na+Na+

Animated Neurotransmission

Excitation-Contraction Excitation-Contraction CouplingCoupling• AP ends before signs of contraction are AP ends before signs of contraction are

obviousobvious

1.1. AP goes along sacrolemma & down T AP goes along sacrolemma & down T tubulestubules

• AP in T tubules causes release of CaAP in T tubules causes release of Ca2+2+ from from adjacent terminal cisternaeadjacent terminal cisternae

2.2. CaCa2+2+ binds to troponin, causing it to move binds to troponin, causing it to move myotroponin away for actin active sitemyotroponin away for actin active site

3.3. Mysosin heads form cross bridges with Mysosin heads form cross bridges with active sites on actin & pull thin filaments active sites on actin & pull thin filaments toward center of sacromere (power toward center of sacromere (power stroke)stroke)

Actin Myosin BridgeExcitation-Contraction CouplingExcitation-Contraction Coupling 2

ATP and the Power StrokeATP and the Power Stroke• Myosin heads have ATP attached to Myosin heads have ATP attached to

them, used for E to “cock” heads backthem, used for E to “cock” heads back– Release ADP & PRelease ADP & P

• Myosin attaches to active sites to form Myosin attaches to active sites to form “cross-bridges”“cross-bridges”

• Myosin head returns to its lower E Myosin head returns to its lower E position once cross bridge is formed, position once cross bridge is formed, moving the thin filament (power stroke)moving the thin filament (power stroke)

• ATP binds to myosin head, actin filament ATP binds to myosin head, actin filament is releasedis released

Actin Myosin Bridge

ContractionContraction• Full contraction of the muscle cell Full contraction of the muscle cell

requires 30+ repeats of power stroke requires 30+ repeats of power stroke actionaction– Process repeats until CaProcess repeats until Ca2+2+ is no longer is no longer

availableavailable

• Acetylcholinesterase Acetylcholinesterase – enzyme that digests acetylcholine to ensure enzyme that digests acetylcholine to ensure

contraction does not persist without nervous contraction does not persist without nervous stimulationstimulation

• No more acetylcholineNo more acetylcholine Ca Ca2+ 2+ is is reabsorbed by SR by active transport reabsorbed by SR by active transport (uses more ATP)(uses more ATP)

Actin Myosin Bridge

Rigor MortisRigor Mortis• When breathing stops, no more OWhen breathing stops, no more O22 can’t can’t

make ATPmake ATP• Dying cells cannot keep extracellular CaDying cells cannot keep extracellular Ca2+2+

out out – CaCa2+2+ goes into muscle cells and promotes goes into muscle cells and promotes

myosin-actin cross-bridgesmyosin-actin cross-bridges– ATP is still being consumed at the cross bridge, ATP is still being consumed at the cross bridge,

when it runs out, detachment becomes when it runs out, detachment becomes impossibleimpossible stiffness stiffness

• Usually starts to set in 3-4hrs postmortem, Usually starts to set in 3-4hrs postmortem, peaks about 12 hrs postmortempeaks about 12 hrs postmortem– As muscle protein begin to break down, rigor As muscle protein begin to break down, rigor

mortis gradually goes awaymortis gradually goes away