Biology 211
Anatomy & Physiology I
Electrophysiology
Recall: A neuron carries an electrical signal produced by
the movement of ions across its plasma membrane
Any atom or molecule carrying an electrical
charge (+ or -)
Potential energy which exists whenever
opposite electrical charges are separated
The movement or flow of electrical charges
from one place to another
Condition when ions are separated across
a biological membrane (it has a voltage)
Condition when this separation of ions is
lost (it loses its voltage)
Condition when ions are again separated (it
regains its voltage)
Movement along a membrane of a short
segment of depolarization immediately
followed by repolarization (a current)
When resting, the
plasma membrane
of a neuron is
polarized.
Sodium ions are
concentrated on
its outer surface
& potassium ions
are concentrated
on its inner surface.
Large negative ions (proteins, phosphate, sulfate, etc) are
also concentrated on the inner surface.
Sodium channels and potassium channels are closed.
An action potential
begins when the
sodium gates
(or "gated channels")
open on one section
of the membrane.
For now, don't worry
about what causes
this to happen.
Sodium ions, carrying
their positive charges, flow into the cell, making the inner
surface of the plasma membrane more positive.
The plasma membrane has begun to depolarize.
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 begins to
pump 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 at one point on the
membrane spreads to nearby regions of the membrane,
causing them to depolarize then repolarize. This, in turn,
stimulates regions a little further out to depolarize and
repolarize, so these events spread away from the original
location.
This movement of
depolarization and
repolarization is the
action potential
which travels along
the plasma membrane
of the neuron.
While some neurons carry action potentials along their
plasma membranes in this continuous fashion, most of
them 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 depolarization and 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) The current (flow of electric charges carried by ions)
can pass directly from the axon terminal to the second
cell if their plasma membranes are connected by gap
junctions which allow ions to flow between the cells.
This is an electrical synapse; it is rare.
b) The action potential can cause the axon terminal to
release a chemical, called 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
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.
This polarization of the membrane, measured as its
voltage, can be increased or decreased by changing
how many ions are separated.
A greater voltage means that more positive and negative
ions are separated; a lower voltage means that fewer
positive and negative ions are separated
More Electrophysiology Terms to Know:
The level of polarization
(separation of + and ions across the plasma
membrane) at which ion
channels rapidly open
and the membrane
rapidly depolarizes
Excitatory Postsynaptic Potential (EPSP):
A DECREASE in the
separation of ions
across the plasma
membrane of the
postsynaptic cell.
It is less polarized.
Thus, EPSPs raise the
voltage closer to the
threshold voltage
Inhibitory Postsynaptic Potential (IPSP):
An INCREASE in the
separation of ions
across the plasma
membrane of the
postsynaptic cell.
It is more polarized.
Thus, IPSPs make it
less likely that the
membrane voltage will
reach the threshold
voltage
Remember when we discussed excitatory synapses and
inhibitory synapses affecting the axon hillock? Those
are EPSPs and IPSPs
Inhibitory synapses
Excitatory synapses
Axon Hillock
IPSPs can also turn other synapses "off" or "on"