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