Neurones & the Action Potential

7/30/2019 Neurones & the Action Potential

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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.

Neurones & the Action Potential

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