cht 9 pt 2

(Excitation)

Each muscle cell is stimulated by a nerve impulse

_____ cell body is in CNS, but sends axons out to
muscles

____ branch so that each cell is in contact with a
neuron

Neurons, Axons

structure of the Neuromotor Junction

Neuron’s axon end
bulb filled with
vesicles of
acetylcholine (ACh)
• Synaptic cleft
• Muscle cell’s motor
end plate with
receptors for ACh

Excitation –

change in Resting
Membrane Potential
Pumps move ions across the membrane
against their concentration gradient (requires
ATP)

Na/K pump

creates and maintains a
resting membrane potential (a voltage
difference across the membrane)

Muscle cells typically have

a -90mV resting
membrane potential

Excitation steps

Nerve impulse reaches axon end bulb
↓
ACh is released into cleft
↓
ACh attaches to receptors (ligand
gated ion channels
↓
Na+ floods more quickly than K+ into the
cell and the sarcolemma
depolarizes
↓
Action Potential

Action Potential
(adjacent Na and K pumps)

1. Membrane potential (Vm )reaches threshold (-55mV)
2. Fast Na+ channels open and Na+ rushes in causing the Vm to depolarize
to +30mV.
3. The depolarization stops when the Na+ channels become inactivated.
4. Slow K+ channels have opened & K+ efflux occurs. This returns Vm
back to its resting level. This is repolarization.

Excitation – Contraction Coupling

Muscle

↓
Neighboring voltage gated Na+ channels open and
depolarization sweeps across sarcolemma and
into T-Tubules
↓
Causes Ca2+ to be released from terminal
cisternae into cytoplasm
↓
Ca2+ attaches to troponin
↓
Troponin changes shape and pulls the
tropomyosin off the myosin binding sites on
the actin
↓
Myosin heads bind to the
exposed binding sites

Muscle Contraction -

Power Stroke
When the cell is resting, Ca2+ levels in the
cell are low and myosin binding sites on
actin are physically blocked by Tropomyosin

When terminal Cisterns
releases
Ca2+ into cytoplasm


Calcium attaches to Troponin

Troponin/Tropomyosin
complex
moves which opens the Myosin
binding sites on the Actin

Power Stokes occur

. ATP hydrolysis

– The ATP binding site on myosin head contains
ATPases
• ATP ADP + P (which are attached to
myosin head)
• This energizes myosin head

. Attachment of myosin to actin to form crossbridges

– Energized myosin head attaches to myosin
binding site on actin and releases P

Power Stroke

– Spot where ADP is bound on myosin head
opens causing the head to rotate and release
ADP
– Force is generated as the head moves near
sarcomere center (actin filament slides past the
myosin filament towards M line)

Detachment of Myosin from Actin

– Myosin head stays attached to actin until ATP binds
to it at ATP binding site
– Myosin head detaches from actin
– Steps are repeated until contraction is complete
– tropomyosin blockage restored; contraction ends

For attachment of myosin to
actin –

P then ADP are
released

For release of myosin from
actin –

ATP must reattach to myosin head

Contraction is the result of

sarcomeres shortening

When relaxed the thin and thick filaments overlap
slightly
• During contraction myosin heads act as little ratchets to
pull the thin filaments to the center of the sarcomere

As a result thin and thick filaments
overlap extensively - H zone disappears
and Z lines are closer to each other

Contraction - Relaxation

Cycle repeats to shorten Sarcomeres
(muscle contracts)

Cycle is broken when calcium is
reabsorbed into Terminal Cisterns

Tropinin/Tropomyosin move back to
cover Myosin binding sites on Actin

Sarcomeres return to original length
(muscle relaxes)

Rigor Mortis

 3 h after death, full rigor about 12 h. After 12 h rigor slowly
ceases, around 72 h rigor disappears

Ca 2+ leaks into cytoplasm & myosin heads
attach to actin –

rigor sets in
no more ATP and heads cant unattach

 tissues decompose -

proteolytic enzymes in the lysosomes of the
muscle cells escape and begin to dissolve the myofilaments – rigor
disappears

Botulism –

blocks release of
Ach......botox