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Summary Neuroanatomy
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This case includes information about brain organisation, function and neuroanatomy.
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Case 2LG1: What is an action potential?
RESTING POTENTIAL
Any chance in the ion concentration and potentials is based on the resting potential
of the axon
It starts at the axon hillock and is translated into a presynaptic potential at the end
plates
Intercellular we have a high concentration of K+ ions
& negative organic anions (A-), and a low
concentration of Na+ ions & Cl- ions.
Extracellular we have a high concentration of Na+ ions
& Cl- ions, and a low concentration of K+ ions &
negative organic ions (A-).
The extracellular and the intercellular fluid are
separated by a selective permeable membrane K+
can pass easily, Na+ not so easy and Cl- & A- can’t
pass at all.
K+ can pass the membrane freely because of the
Brownian molecular movement and their
concentration gradient. More K+ are moving outside
the cell than moving inside less concentration
within the cell.
There is an electrical potential formed outside the
cell membrane it gets positive and inside the cell it
gets negative (-70mV) because more positive ions are
outside than inside the cell.
When -70mV is reached the K+ are passing through the
membrane via both sides electro-chemical
gradient is reached K+ influx = K+ efflux.
-70mV is the preferred electrical value of the action
potential.
Now there is a problem: Na+ can get through the K+
channels, because the two ions are very similar to one
another. There is a low concentration within the cell
and a high concentration outside the cell Na+ passes
through the membrane to get inside the cell the
electrical value would become more positive. The
electro-chemical gradient would be disturbed.
, To compensate that problem, there is a Kalium-
Natrium pump (3K+-2Na+ pump) two Na+ ions are
transported outside the cell against their
concentration and electrical gradient and 3 K+ are
transformed in exchange inside the cell against their
concentration gradient. The pump’s function is to
keep the electro-chemical gradient of K+ which gives
the membrane a potential of -70mV as resting
potential.
20% of our organism’s ATP is used in this active
transport energy required process against the
concentration gradient.
The resting membrane potential can vary between -50mV to -80mV This is due to
a higher resistance of creating action potentials (-80mV) in for example sensory
neurons and a low resistance in creating action potentials (-50mV) in for example
interneurons The actual resting membrane potential depends on the type of
neuron!!
Depolarization = more positive than the resting potential
Hyperpolarization = more negative than the resting potential
ACTION POTENTIAL
A strong depolarization causes an action potential
random or passive changes in the resting membrane potential are called electrotonic
potentials
If a threshold is reached, voltage gated channels open
The action potential is based on the resting membrane potential of the axon. Resting,
non-gated Na+ & K+ channels as well as the Sodium-Potassium pump are providing
the negative milieu.
When the neuronal membrane is depolarized via a stimulus, an EPSP (read below) or
an electrotonic potential some low voltage-gated Na + channels can open. Sodium
passes through the membrane inside the axon along their electrical and chemical
gradient.
When the membrane reaches the threshold potential, all high voltage-gated Na +
channels open and the Na+ influx is bigger than the K+ efflux. The membrane inside
the cell is getting positive.
‘All or nothing’ phenomenon = when the threshold is reached all channels open and if
the threshold is not reached no channels will open. Only if the threshold is reached
there will be an action potential.
The threshold needs to be reached at the axon hillock
where the action potentials are generated. If the threshold
is not reached the initiation of the action potential fails
and the signal won’t be transmitted to the next neuron.
There are six phases of an action potential:
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