Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
348
myosin. The myosin heads bind with these sites, forming
cross-bridges between the two types of filament.
Next, the myosin heads tilt, pulling the actin filaments
along towards the centre of the sarcomere. The heads then
hydrolyse ATP molecules, which provide enough energy
to force the heads to let go of the actin. The heads tip back
to their previous positions and bind again to the exposed
sites on the actin. The thin filaments have moved as a
result of the previous power stroke, so myosin heads now
bind to actin further along the thin filaments closer to the
Z disc. They tilt again, pulling the actin filaments even
further along, then hydrolyse more ATP molecules so that
they can let go again. This goes on and on, so long as the
troponin and tropomyosin molecules are not blocking
the binding sites, and so long as the muscle has a supply
of ATP.
Stimulating muscle to contract
Skeletal muscle contracts when it receives an impulse from
a neurone. An impulse moves along the axon of a motor
neurone and arrives at the presynaptic membrane
(Figure 15.29). A neurotransmitter, generally acetylcholine,
diffuses across the neuromuscular junction and binds
to receptor proteins on the postsynaptic membrane –
Events at the neuromuscular junction
which is the sarcolemma (the cell surface membrane of
the muscle fibre). The binding of acetylcholine stimulates
the ion channels to open, so that sodium ions enter to
depolarise the membrane and generate an action potential
in the sarcolemma.
Impulses pass along the sarcolemma and along the
T-tubules towards the centre of the muscle fibre. The
membranes of the sarcoplasmic reticulum are very close to
the T-tubules. The arrival of the impulses causes calcium
ion channels in the membranes to open. Calcium ions
diffuse out, down a very steep concentration gradient, into
the sarcoplasm surrounding the myofibrils.
The calcium ions bind with the troponin molecules
that are part of the thin filaments. This changes the shape
of the troponin molecules, which causes the troponin
and tropomyosin to move away and expose the binding
sites for the myosin heads. The myosin heads attach to
the binding sites on the thin filaments and form crossbridges
(Figures 15.28 and 15.29). When there is no longer
any stimulation from the motor neurone, there are no
impulses conducted along the T-tubules. Released from
stimulation, the calcium ion channels in the SR close and
the calcium pumps move calcium ions back into stores in
the sarcoplasmic reticulum. As calcium ions leave their
Events in muscle fibre
1 An action potential arrives.
2 The action potential causes the diffusion of calcium
ions into the neurone.
3 The calcium ions cause vesicles containing
acetylcholine to fuse with the presynaptic
membrane.
1
3
4
5
Na + 6
Ca 2+
Ca 2+
7 The depolarisation of the sarcolemma
spreads down T-tubules.
8 Channel proteins for calcium ions open
and calcium ions diffuse out of the
sarcoplasmic reticulum.
9 Calcium ions bind to troponin.
Tropomyosin moves to expose
myosin-binding sites on the actin
filaments. Myosin heads form
cross-bridges with thin filaments
and the sarcomere shortens.
2
Ca 2+
7
8
9
4 Acetylcholine is released and diffuses across
the synaptic cleft.
sarcoplasmic
reticulum
5 Acetylcholine molecules bind with receptors in the
sarcolemma, causing them to open channel proteins for
sodium ions.
6 Sodium ions diffuse in through the open channels in the
sarcolemma. This depolarises the membrane and initiates an
action potential which spreads along the membrane.
action potential
ion movements
acetylcholine movements
Figure 15.29 The sequence of events that follows the arrival of an impulse at a motor end plate.
Key