Nerve Impulse 28770

8/2/2019 Nerve Impulse 28770

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NERVE IMPULSE/ACTION POTENTIAL


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Neurons are specifically designed to conductnerve impulses

Nerve impulses are only conducted when theneuron has recovered from conducting its lastnerve impulse

Must also be sufficiently stimulated to conduct a newone


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Three states of a neuron Resting potential

The state during which no nerve impulse is beingconducted although the neuron is capable of doing so

Action potential The state during which the neuron is actively involved

in conducting a nerve impulse

Recovery/Refractory potential The state during which the neuron is unable to

conduct a nerve impulse since the neuron mustrecover following the last nerve impulse


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1. Resting Potential The state of the neuron when no nerve impulse is being

conducted During resting potential there is an ion displacement

between the inside and the outside of the neuron (i.e. oneither side of the neuron cell membrane) as follows:

There are more Na+ ions on the outside than on the inside There are more K+ ions on the inside than on the outside There are many large organic anions (-ve charged ions) locked

inside since they are too big to pass through the neurons cell

membrane


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Due to this difference in ion displacement there is aNET CHARGE difference across the cell membrane= MEMBRANE POTENTIAL

This membrane potential when the neuron is at restis called the RESTING POTENTIAL=-70mV

This difference in ion displacement and thus theresting potential is largely maintained by a proteinchannel called the Na+/K+ PUMP


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At rest, the inside of aneuron's membrane has anegative charge.

As the figure shows, a Na+ / K+ pump in the cellmembrane pumps sodiumout of the cell andpotassium into it.

However, because the cellmembrane is a bit leakier topotassium than it is tosodium, more potassiumions leak out of the cell.

As a result, the inside of themembrane builds up a netnegative charge relative tothe outside.


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Review Questions Why are sodium ions exposed to a greater driving

force than potassium ions at rest? What is meant by the resting membrane potential? What creates the membrane potential in neurons? What would happen to a neurons ability to

conduct a nerve impulse if the resting potentialdidnt exist?


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Action Potential An action potential occurs when a neuron is conducting a nerve

impulse

In order for an action potential to occur, the neuron must receivesufficient stimulation to open enough Na gates to reach thethreshold level

If sufficient Na gates are opened to reach the threshold level, otherNa and K gates will be stimulated to open

This results in a self-propagating wave of action potentials and Naand K gates opening along the entire length of the neuron and anaction potental and nerve impulse occur

Since an action potential will only occur if the membrane thresholdlevel is reached, an action potential can also be described as an allor none response

Action potential can be divided into 2 phases: depolarization &repolarization


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Depolarization (upswing) If a neuron received sufficient stimulation to reach the

membrane threshold, successive Na gates along theentire neuron membrane will open

The opening of the Na gates allows Na ions to move intothe neuron

The movement of Na ions into the neruon causes themembrane potential to change from -70mV to +40mV

As the membrane potential becomes more positive, Na

gates begin to close. At the end of depolarization, theNa gates are all closed


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Repolarization (Down-swing) At the end of the depolarization phase, K gates

begin to open, allowing K to leave the neuron These K gates are activated at the +ve membrane

potential value of about +40mV

The movement of K ions out of the neuronproduces a change in membrane potential suchthat the potential becomes more ve

Following repolarization, the K gates close slowly


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During the conduction of a nerve impulse,each successive section of a neurons

membrane will undergo an action potentialconsisting of depolarization followed byrepolarization

Thus the nerve impulse is the movement of the action potential along the neuron cell

membrane


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Recovery/Refractory Potential

Immediately following an action potential, a

neuron is unable to conduct a nerve impulse untilit has recovered because its Na gates wont open

A neuron which is undergoing recovery is said to

be refractory since it cannot conduct a nerveimpulse


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During the recovery phase the following eventsare occurring:

1. The K gates are closing2. The Na/K pump is returning the Na ions to the

outside and K ions to the inside of the neuron3. The membrane potential is returning to its resting

value of -70mV

Once the recovery phase is complete, the neuronis no longer in its refractory period and is ready toconduct another nerve impulse


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Salatory Conduction Some neurons have no myelin coating and are described as

unmyelinated

In unmyelinated neurons, an action potential must pass througheach point along the neuron cell membrane which makes theconduction of the nerve impulse relatively slow

Most neurons in humans are (since they are enclosed in a myelin

coat formed by Schwann cell membranes wrapped around theneuron) In myelinated neurons, an action potential does not occur along

sections of the neuron which are wrapped in myelin Ions are unable to cross the nerve cell membrane in these sections


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The gaps in myelin are called Nodes of Ranvier

These gaps are the site of an action potential Thus, in myelinated neurons, the action

potential jumps from one node of Ranvier to

the next in a process called salatoryconduction

Salatory conduction is very rapid, allowingthe nerve impulse to travel very rapidlyalong the neuron


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Review Questions Define the following:

Membrane Threshold Action potential

Explain why the 1 st part of an action potential iscalled depolarization

Explain why the 2 nd part of an action potential iscalled repolarization

Nerve Impulse 28770

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