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5.1.5l; 1.1.2a
Muscular contraction
Muscles contract through the interaction of two key proteins, actin and myosin, using ATP as an energy
source.
For contraction to occur, part of the
myosin molecule (head group) attaches
to a binding site on the actin filament;
a cross-bridge is formed.
The head group bends, pulling the actin
filament along, and ADP and Pi are
released – the power stroke.
The attachment of a new ATP molecule
to the myosin head breaks the
cross-bridge.
The head group moves back to its original
confirmation as ATP is hydrolysed to ADP and Pi,
so another cross-bridge can be formed.
ADP
Contraction cycle
ADP continues if ATP
is available and ATP
Pi
Ca2+ level in the
sarcoplasm is high
Motor neurone
1 Impulses arriving at the
neuromuscular junction cause
vesicles to fuse with the
pre-synaptic membrane and
release acetylcholine into
the gap
Vesicles
Muscle fibre
membrane
−
6 Acetylcholinesterase
in the gap rapidly
breaks down
acetylcholine so
that contraction only
occurs when impulses
arrive continuously
+
−
−
−
3 Depolarisation +
wave travels +
2 Acetylcholine binds to
down tubules +
+
receptors on the muscle
(T system)
+
fibre membrane
− −
(sarcolemma)
causing depolarisation
−
−
−
−
+
+
+
+
+
+
Ca2+
Ca2+
Ca2+
Ca2+
5 Ca2+ binds to proteins in the
muscle, which leads to contraction
ADP
ADP
The neuromuscular junction
Myelin sheath
The power stroke
Pi
Mechanism of muscular contraction
4 T system depolarisation leads to
Ca2+ release from stores in
sarcoplasmic reticulum (specialised
endoplasmic reticulum)
The binding of Ca2+ to troponin causes the
myosin binding sites on the actin filament
(hidden by tropomyosin) to be revealed so
the ‘power stroke’ can proceed.
ATP is unstable and only small amounts of
it exist in cells at any one time. Contraction
of muscles uses lots of ATP and the ATP
immediately available is used up after 1−2
seconds. Suggest how more ATP can be
supplied to contracting muscles. (3 marks)
Aerobic respiration in muscle cell
mitochondria produces ATP. Blood
brings oxygen to muscle cells so
glucose can be respired to generate
ATP from the Krebs cycle and oxidative
phosphorylation.
Monitoring muscle contraction
Investigations using the calf muscle removed from a
Single stimulus
frog’s leg with the nerve still attached, show how an increase in stimulus
frequency reduces time between contractions. Repeated large stimuli,
close together, give a sustained and powerful contraction known as
Time
tetanus.
Single stimulus
Time
Repeated stimuli
118
Contraction
recorded
Repeated stimuli
Increasing level
of stimulation
Time
Contraction recorded
Multiple stimuli lead
to tetanus; eventually
muscle fatigue
reduces the level of
contraction
ow would a lack
H
of Ca2+ affect muscle
contraction?
(2 marks)
Pi