Transcript Membrane potential - "G. Galilei" – Pescara

«HUMOUR PROJECT»
Neurons’
electrical
conductivity
Alunno
Davide Brandalise
Classe
III A
Liceo scientifico
«Galilei Galilei»
Prof.ssa Roberta Miscia
The membrane potential
As all living cells, neurons are surrounded by a plasma membrane
impermeable to ions. This allows nerve cells to keep a different ionic
concentration between the inside and the outside of the cell.
The membrane potential
The plasma membrane is made up of a double fatty hydrophobic layer
that makes the membrane impermeable preventing the
ion diffusion from one side to another.
The membrane potential
The only way to pass for ions is through specific transmembrane
channels. These channels, alternating an opening phase
to a closure phase, regulate the selective transport of the ions.
The membrane potential
The ionic concentration gradient
mainly deals with sodium and
potassium ions.
What’s more, the cell houses a high
concentration of proteins with
negative electrical charge.
The concentration gradient
between the inside and the
outside is called
electric potential difference.
The membrane potential
When the neuron is at rest, that is
when it is not conducting a nerve
impulse, the membrane potential is
called resting potential.
The neuron inside is negatively
charged, whereas the outside is
charged positively.
The membrane potential
The concentration gradient
between the two sides is a sort of
potential energy, which is
measurable in volts.
What’s more, it can be measured by
inserting an electrode inside the
cell. The neuron potential is about 70 millivolts at rest.
The membrane potential
The concentration of potassium
ions is higher outside the cell
and this gradient lets these ions
be subject to two different
forces. The first one, that is the
diffusion one, leads the ions
alongside their concentration
gradient, so from the inside
to the outside.
The membrane potential
The second one, that is the electric
one, pushes the ions towards the
inside of the cell, balancing the
positive and negative charges
between the intracellular region to
the extracellular one. When these
two opposite forces balance out, the
ions transiting between the two
regions becomes void.
As a matter of fact, entering ions are
as many as the leaving ones.
The action potential
The action potential can be
divided into five phases:
1.
2.
3.
4.
5.
the resting potential;
threshold;
the rising phase;
the falling phase;
the recovery phase.
The action potential
When the neuron is at rest, only a
small subset of potassium channels are
open, permitting potassium ions to
enter and exit the cell based on
electrochemical forces.
There is no movement of potassium
ions; for each potassium ion that leaves
the cell, another returns, maintaining
the membrane potential constant in
its value.
The action potential
As a depolarizing stimulus arrives at
the segment of the membrane, a few
sodium channels open, permitting
sodium ions to enter the neuron.
The increase in positive ions inside
the cell depolarizes the membrane
potential, thus making it less
negative and brings it closer to the
threshold at which an action
potential is generated.
The action potential
If the depolarization reaches the
threshold potential, additional
voltage-gated sodium channels
open.
As positive sodium ions rush
into the cell, the voltage across
the membrane rapidly reverses
and reaches its most positive
value.
The action potential
At the peak of the action potential,
two processes occur simultaneously.
Firstly, many of the voltage-gated
sodium channels begin to close.
Secondly, many more potassium
channels open, allowing the positive
charges to leave the cell.
This causes the membrane potential
to begin to shift back towards the
resting membrane potential.
The action potential
As the membrane potential
approaches the resting potential,
voltage-gated potassium channels
are maximally activated and open.
The action potential
The membrane actually repolarizes
beyond the resting membrane
voltage.
This undershoot occurs because
more potassium channels are open
at this point than during the
membrane’s resting state, allowing
more positively charged potassium
ions to leave the cell.
The action potential
The return to a steady state
continues as the additional
potassium channels that opened
during the action potential now
close.
The membrane potential is now
determined by the subset of
potassium channels that are
normally open during the
membrane’s resting state.
Glossary
Membrane potential: an electric charge difference between the inside
and the outside of the plasma membrane
Polarized: a cell is polarized when it owns a membrane potential
Ionic channels: ducts that are the only way for ions to cross the
membrane potential
Voltage-gated channels: ducts that open or close in response to
variations in the membrane potential
At rest: neurons that are not conducting nerve impulses
Concentration gradient: the number of the chemical molecules sitting in
a specific area of the cell