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Neurones & the Action
Potential
Neurones conduct impulses fromone part of the body to another.
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STRUCTURE
They have three distinctparts:
(1) Cell body,
(2) Dendrites, and(3) the Axon
The particular type ofneuron that stimulates
muscle tissue is called amotor neuron.
Dendrites receive impulsesand conduct themtoward the cell body.
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Myelinated Axons
The axon is a single long,thin extension thatsends impulses to
another neuron.They vary in length and
are surrounded by amany-layered lipid andprotein covering called
the myelin sheath,produced by theschwann cells.
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Resting Potential
In a resting neuron(one that is not
conducting animpulse), there is adifference in
electrical charges on the outside and inside of
the plasma membrane. The outside has apositive charge and the inside has a negativecharge.
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Contribution of ActiveTransport Factor 1
There are different numbers of potassium ions (K+)and sodium ions (Na+) on either side of themembrane. Even when a nerve cell is notconducting an impulse, for each ATP moleculethats hydrolysed, it is
activelytransporting 3molecules Na+ out of
the cell and 2 molecules
of K+ into the cell, at
the same time by
means of the
sodium-potassium pump.
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Contribution of facilitated diffusion
The sodium-potassiumpump creates aconcentration and
electrical gradient forNa+ and K+, whichmeans that K+ tends todiffuse (leak) out of
the cell and Na+
tendsto diffuse in. BUT, the membrane is much morepermeable to K+, so K+ diffuses out along itsconcentration gradient much more slowly.
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RESULTS IN:
a net positive chargeoutside & a net negative charge
inside. Such a membrane isPOLARISED
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Action Potential
When the cell membranesare stimulated, there isa change in thepermeability of the
membrane to sodiumions (Na+).
The membrane becomesmore permeable to Na+and K+, thereforesodium ions diffuse into the cell down a concentrationgradient. The entry of Na+ disturbs the restingpotential and causes the inside of the cell to becomemore positive relative to the outside.
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DEPOLARISATIONAs the outside of the cell
has become morepositive than the insideof the cell, themembrane is now
DEPOLARISED.When enough sodium ions
enter the cell todepolarise the
membrane to a criticallevel (threshold level)an action potentialarises which generatesan impulse.
In order for the neuron togenerate an actionpotential the membranepotential must reach the
threshold of excitation.
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All-or-None Principle
Throughout depolarisation, the Na+ continuesto rush inside until the action potential
reaches its peak and the sodium gates close.If the depolarisation is not great enough toreach threshold, then an action potentialand hence an impulse are not produced.
This is called theAll-or-None Principle.
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Refractory PeriodThere are two types of
refractory period:
Absolute Refractory Period Na+ channels are
inactivated and nomatter what stimulus isapplied they will not re-open to allow Na+ in &depolarise the membrane to the threshold of an
action potential.Relative Refractory Period - Some of the Na+ channels havere-opened but the threshold is higher than normal making itmore difficult for the activated Na+ channels to raise the
membrane potential to the threshold of excitation.
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Speed of Nerve Impulses
Impulses travel very rapidly alongneurones. The presence of a myelin
sheath greatly increases the velocity atwhich impulses are conducted along theaxon of a neuron. In unmyelinated
fibres, the entire axon membrane isexposed and impulse conduction isslower.
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Speed of Nerve Impulses
Impulses travel veryrapidly along neurons.The presence of a
myelin sheath greatlyincreases the velocity atwhich impulses areconducted along theaxon of a neuron. In
unmyelinated fibres, theentire axon membraneis exposed and impulseconduction is slower.