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Nervous System
Topic 6.5
What does the NS consist of?
CNS [central nervous system] =
PNS [peripheral nervous system] =
Composed of cells called NEURONS
that carry rapid electrical impulses.
 Diagram
of a
Reflex arc
Nerve impulses are conducted from
receptors to the CNS by sensory
Nerve impulses are conducted
within the CNS by relay neurons.
Nerve impulses are conducted from
the CNS to effectors by motor
Reflex arc diagram
Resting potentials
The resting potential is the electrical
potential across the plasma
membrane of a cell that is NOT
conducting an impulse.
It is equal to approx. -70mV
The inside of the membrane is
negative relative to the outside.
Reasons why cytoplasm is negative
compared to outside the membrane
Higher concentration of anions
(negatively charged ions) on the
Na+/K+ pump actively transports
3Na+ out for every 2K+ it pumps in.
Membrane is more permeable to K+
than Na+ so diffusion more likely.
Na+ channels often closed.
Action potential
An action potential
is the reversal
and restoration
of the electrical
potential across
the plasma
membrane of a
cell as an electrical
impulse passes
along it.
Action potentials
Changes in membrane polarity during
action potentials
Depolarization and repolarization
When a neuron is stimulated, some Na+
channels open and ions enter.
Membrane potential rises (becomes less
If threshold potential of -50mV is
reached, an action potential is triggered.
DEPOLARIZATION occurs when most Na+
channels are open and the ions diffuse
rapidly down their concentration gradient.
Depolarization and repolarization
At peak potential (+35mV), Na+ gates
close and K+ channels open.
K+ diffuse down their concentration
gradient restoring the inside of the
membrane to negative.
K+ gates remain open making the
membrane potential drop below resting
Closing of K+ gates and action of Na+/K+
pump restores resting potential to -70mV.
Transmission along a neuron
An action potential in one part of the
neuron triggers an action potential in an
adjacent area.
Na+ ions diffuse from where the a.p. has
just occurred to the area at rest,
causing the impulse to travel along the
One-way movement is ensured by the
‘hyperpolarization’ or refractory period.
Synaptic Transmission