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Slide 1

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Communication and
Homeostasis ­
OCR A2 Biology…read more

Slide 2

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Sensory and Motor Neurones
· Carries action
potential from
CNS to an
· The cell body is
in the CNS and
the long axon · Carries action
potential from
that carries
sensory receptor
action potential to the CNS
to the effector · A long Dendron
carries the action
potential to the cell
body, which lies
outside the CNS
and a short axon
carrying the action
potential to the
CNS.…read more

Slide 3

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Producing Action Potentials
1. The membrane starts in its
resting state. It is polarised with
the inside of the cell being -60
mV. The resting potential in
maintained by pumping 3
sodium ions out the cell and 2
potassium ions into the cell.
2. Sodium channels open and
some sodium ions diffuse in
3. The membrane depolarises and
reaches the threshold potential
4. Voltage gated sodium channels 6. The sodium channels close and the
open and sodium floods in so potassium channels open
the inside becomes more 7. Potassium ions diffuse out of the cell,
positively charged compare to repolarising the membrane
8. The potential difference overshoots slightly
5. The potential across the
making the cell hyperpolarised
membrane reaches +40mV.
9. The potential difference is returned to its
resting state.…read more

Slide 4

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Transmission of the Action
· When an action potential occurs the sodium channels
open at a particular point in the membrane.
· This allows sodium to diffuse into the cell down the
concentration gradient
· The concentration of sodium increases in the area the
sodium channels are open.
· This causes sodium to diffuse sideways and this
movement is called a local current.
· As the sodium moves along it alters the potential
difference of the membrane and so more sodium
gated channels open so sodium further along the
membrane can diffuse through and the action potential
can move along the neurone.…read more

Slide 5

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Myelinated Neurones
· The myelin sheath is a insulating layer made up of
Schwann cells.
· It prevents sodium and potassium diffusing across the
· Gaps between the Schwann cells are called Nodes of
Ranvier. The membrane can only become depolarised at
the nodes.
· This causes the local currents to become elongated and
sodium ions diffuse from one node to another. This is
saltatory conduction.
· Myelinated neurones can transmit an action potential
much quicker than a non-myelinated.
· Myelinated neurones carry signals over long distances.…read more

Slide 6

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Nerve Junctions
A synapse is the junction between two neurones. An
action potential cannot bridge the gap so
neurotransmitter diffuses across the gap to generate a
new action potential on the other side.
Presynaptic Knob ­ It contains many mitochondria to
produce ATP, a large amount of smooth ER, vesicles
containing acetylcholine (the neurotransmitter) and
voltage gated calcium channels.
Postsynaptic Knob ­ contains specialised sodium ion
channels that respond to the neurotransmitter. The
channels consist of 5 polypeptide molecules, 2 of
which are complementary receptors to the
neurotransmitter.…read more

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