E x citatio n -co n tractio n coupling in sk eletal m uscle • Figure 6.6
Nerve
impulse
Nerve impulse arrives at axon terminal of
motor neuron and triggers release of
acetylcholine (ACh).
Muscle action
potential
ACh diffuses across
synaptic cleft, binds
to its receptors in the
motor end plate, and
triggers a muscle
action potential (AP).
ACh receptor
I
c
Î
Synaptic vesicle
filled with ACh
Acetylcholinesterase in
synaptic cleft destroys
ACh so another muscle
action potential does not
arise unless more ACh is
released from motor neuron
Transverse tubule
"
O
°
u c
Muscle AP traveling along
j
transverse tubule opens Ca2+
release channels in the
sarcoplasmic reticulum (SR)
o membrane, which allows calcium
Muscle relaxes.
A
Troponin-tropomyosin
complex slides back
into position where it
blocks the myosin
binding sites on actin.
Ca2+ active
transport pumps
o
\y/*
^ Y
v J o
o '
; ©
6 0
0
0 0 o ° ô o ° o
°i
Ca2+ release channels in
o
„ o
SR close and Ca 2+ active
°
°Q
f
O
transport pumps use ATP
o
o
o
to restore low level of
°
o 0° \
o
o
o
Ca2+ in sarcoplasm.
0
o
o
«
o M s
k
o
oo
n
o 0
o
/
9
0
o
O
0
o
o o
°
° °
o
O
f
O'
| Ca2+ binds to troponin on the thin filament,
exposing the binding sites for myosin.
Elevated Ca2
'
I Contraction: power strokes
use ATP; myosin heads bind
to actin, swivel, and
release; thin filaments
are pulled toward
center of sarcomere.
Key:
O = Ca2+
) = Ca2+ active
transport pumps
= Ca2+ release
channels
Put It Togethefr~
1. What makes the muscle contract in step 6?
2. What kind of contraction is shown in step 6? (a) a partial contraction (b) a maximum
contraction
3. Why does the muscle relax in step 9?
4. Which numbered steps in this figure are part of the contraction cycle?
Muscles Are Built to Move 161
p r o c e ss
diagram
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