Download Electrophysiology

Biology 211
Anatomy & Physiology I
Dr. Thompson
Electrophysiology
Recall: A neuron carries an electrical signal produced by
the movement of ions across its plasma membrane
The mechanism by which it does this should be familiar –
it is very similar to how the plasma membrane of a muscle
cell (its sarcolemma) generates and carries electrical signals
as we have previously discussed …
When resting, the
plasma membrane
of a neuron is
polarized.
Sodium ions are
concentrated on
its outer surface.
Potassium ions and
large negative ions (proteins, phosphate, sulfate, etc)
are concentrated on its inner surface.
Sodium channels and potassium channels are closed so
very few ions are passing across the membrane.
An action potential
begins when the
plasma membrane
begins to depolarize
Sodium gates
(or "gated channels")
open on one section
of the membrane.
(later, we’ll discuss
what causes this to happen)
Large amounts of sodium ions flow into the cell carrying
their positive charges, making the inner surface of the
plasma membrane more positive.
.
A few milliseconds
later, potassium
gates open as
the sodium gates
close.
Potassium ions, with
their positive charges,
flow out of the cell,
again making the
outer surface of the plasma membrane more positive.
The plasma membrane has begun to repolarize.
The potassium
gates then also
close.
The cell quickly
pumps sodium ions
back to the outside
of the membrane
and potassium ions
back to the inside
of the membrane.
The plasma membrane becomes fully repolarized.
This depolarization / repolarization starts at one point on
the membrane, then spreads to nearby regions of the
membrane, causing them
to depolarize / repolarize.
This, in turn, stimulates
regions a little further out
to depolarize / repolarize,
and these events spread
away from the original location
This movement of
depolarization &
repolarization Is an
action potential
which travels along
the plasma membrane
of the neuron.
While some neurons use this continuous type of action
potentials to carry their electrical signals, most neurons
use a more efficient method of carrying action
potentials called saltatory conduction.
This is much more rapid and requires much less
energy.
Saltatory conduction can only occur on myelinated
neuron processes.
The action potential (depolarization then repolarization)
occurs only at nodes of Ranvier, so the action potential
skips from node to node to node .....
Whether the action potential travels along an axon by
continuous or saltatory conduction, it eventually
spreads along telodendria and reaches the axon
terminals.
From here, the signal can be passed to another cell at a
synapse
Two types of synapses:
a) Ions can pass directly from one neuron to another if their
plasma membranes are connected by gap junctions, thus
starting a new
action potential
on the second
cell. This is an
electrical synapse;
it is rare.
b) The action potential
can cause the axon
terminal of the first
neuron to release a neurotransmitter, which binds
to the plasma membrane of the second cell and
stimulates a new action potential on it.
This is a chemical synapse; it is very common
Chemical Synapse
Chemical Synapse
Chemical Synapse
More Definitions
Presynaptic Neuron: The neuron which secretes the
neurotransmitter at a synapse.
Postsynaptic Neuron: The neuron to which this
neurotransmitter binds, thus creating a new action potential
on its plasma membrane.
Notice that the same neuron can be the
postsynaptic neuron at one synapse
and the presynaptic neuron at the
next synapse.
There are dozens of different chemicals which act as
neurotransmitters, some of which are listed in this table
from Saladin.
However: any neuron
can only secrete one
type of neurotransmitter
from all of its axon
terminals
Additionally, at each synapse there
must be a perfect match between
neurotransmitter and receptor:
The postsynaptic cell must have
receptors which are specific for the
neurotransmitter which is secreted
by the presynaptic cell:
For example:
If the presynaptic neuron secretes acetylcholine,
the postsynaptic neuron must have acetylcholine receptors.
If the presynaptic neuron secretes serotonin
the postsynaptic neuron must have serotonin receptors
etc,
Recall: When resting,
the plasma membrane
of a neuron is polarized
because it has more
positively charged ions
on the outside and more
negatively charged ions
on the inside.
A voltage of “0”
means that positive
and negative ions
are mixed together
and not separated
from each other…
Millivolts
This polarization of the membrane is measured as its
voltage. This can be increased or decreased by changing
how many ions are separated.
No separation
of + and - ions
This polarization of the membrane is measured as its
voltage. This can be increased or decreased by changing
how many ions are separated.
+ and – ions
separated
Millivolts
A voltage of “0”
means that positive
and negative ions
are mixed together
and not separated
from each other.
As positive and
negative ions get
separated from
each other across the membrane, the further
away from “0” the voltage will move
No separation
of + and - ions
+ and – ions
separated
The “ - ” means that more
positive ions are on the outside
and more negative ions are on
the inside of the membrane.
Millivolts
A normal resting voltage for a
neuron (that is, how much are
the positive ions and negative
ions separated across its
plasma membrane) is between
-60 and -70 millivolts.
A normal resting voltage for a neuron
is between -60 and -70 millivolts.
As Na+ flows into the cell (the
membrane depolarizes), the voltage
moves closer to “0” and will go past it
(more positive ions on the inside)
As K+ flows out of the cell (the
membrane repolarizes), the voltage
returns toward its resting value
(more positive ions on the outside)
In reality, the plasma membrane of a
neuron does not depolarize (lots of
Na+ flowing in) as quickly as those
earlier diagrams imply since not all
of the sodium gated channels open
at the same time.
Instead, a few Na+ gates open first,
then a few more, and a few more….
Each time this raises the voltage of
the membrane a little bit, until the
voltage reaches a point, called the
threshold voltage, which causes all
of the remaining Na+ gates to open
as well, causing rapid depolarization.
Those small depolarizations are called
Excitatory Postsynaptic Potentials (EPSP)
Each of these raises the
voltage of the neuron’s
plasma membrane closer
to its threshold voltage
If enough EPSPs occur in
a short period of time, the
full action potential begins.
Inhibitory Postsynaptic Potentials (IPSPs) can also
occur, which INCREASE in the separation of positive
and negative ions across the
plasma membrane of the
Neuron (making it even
more polarized.)
These IPSPs make it less
likely that the plasma
membrane of the neuron will
reach threshold voltage.
Remember when we
discussed excitatory synapses
and inhibitory synapses
affecting the axon hillock?
Those synapses produce
EPSPs and IPSPs.
The dendrites (and body) of every neuron are constantly
receiving both stimulatory signals which move its membrane
voltage closer to threshold (EPSPs), and inhibitory signals
which move it membrane voltage further away from
threshold (IPSPs).
Those travel along the membrane until they reach the axon
hillock, and if there are enough more EPSPs than IPSPs to
reach threshold voltage, it will depolarize.
Inhibitory Postsynaptic Potentials can also block the
synapse of a neuron.
If an action potential
(depolarization/repolarization)
moving along the plasma
membrane of that neuron’s
axon and telodendria reaches
a place where another neuron
is creating an IPSP, the
membrane will no longer be able
to reach threshold voltage at that point.
The action potential will not reach the axon terminal, so it
will not be able to release neurotransmitter.