Download The vocabulary of nerve cells

The vocabulary of nerve cells and the
senses
the transduction and coding of stimuli
(resting and action potentials)
…The Bottom Line…
• For any external or internal signal to be recognized by the
brain it must first be transduced into an electrical (neural)
signal.
– Hence, whatever properties of neurons that influence this
transduction also influence the ability to perceive the signal and
its properties.
• Once transduced the signal must be sent to the central
nervous system and processed.
– Hence, any properties of neurons influencing the transmission
and processing of signals also influence the ability to perceive
the signal and its properties.
Site of the action: the nerve membrane
The nerve cell membrane is
much the same as the cell
membrane of any other cell
(phospholipids and proteins). It
differs essentially only in the
specialized proteins present in
it, which determine the function
of the cell.
One such function is the
formation and control of an
electrical potential (voltage)
across the membrane.
Membrane Potential: I
AA- K+
A- K+
K+
AK+
A-
A-
K+
K+
A-
K+
K+
A-
K+
AK+
K+
A-
K+
A-
K+
K+ A-
8 K+, 8 A-
5 K+, 5 A-
A-
A-
At rest, the membrane is
impermeable to charged
particles (ions).
Both sides are electrically
neutral (equal numbers of
anions [A-] and cations
[here, K+] on both sides).
However there is more K+
on the left side.
Membrane Potential: II
AA-
A- K+
K+
K+
AK+
A-
A-
K+
K+
A-
K+
K+
A-
K+
AK+
K+
A-
K+
A-
K+
K+ A-
A-
A-
Next we reveal the
presence of protein pores
(channels) in the
membrane.
These pores are selective
and will only allow K+
through them (A- is
blocked).
At the outset the pores are
closed.
Membrane Potential: III
AA-
A- K+
K+
K+
AK+
A-
A-
K+
K+
A-
K+
K+
A-
K+
AK+
K+
A-
K+
A-
K+
K+ A-
A-
A-
In response to a
stimulus, some pores
open.
Membrane Potential: IV
AAK+
A-
K+
K+ K+ A-
A-
K+
A-
A-
K+
K+
A-
K+
K+
A-
K+
K+
K+
A-
K+
A-
K+ A8 K+, 5 A-
6 K+, 8 A-
+
A-
A-
Potassium flows down its
concentration gradient.
Because the anions (A-)
cannot follow, a potential
forms across the membrane.
This potential opposes and
eventually stops further flow
of K+.
The more channels that
open, the greater the
amplitude of the potential
needed to stop the flow (i.e.,
come to equilibrium).
Thus, an external stimulus has been
turned into a membrane potential, and the
potential is proportional to the strength of
the stimulus.
…the bottom line…
• Since all external signals must be transduced into voltage
in order for the brain to perceive them, and
• Since all changes in electrical signals in the nervous
system are the result of changes in membrane proteins,
then
• For any signal (stimulus) to be perceived by a cell there
must be one or more membrane proteins that can be
influenced by that signal.
Control of permeability: ion channels
Neuron membranes have many
thousands of channels.
Channels differ in the specificity of
what they will transport and what
they will not:
Na+, K+, Cl-, Ca++,etc.
Channels also differ in what causes
them to open and/or close:
Chemicals outside cell
Chemicals inside cell
Mechanical deformation of the
membrane
Etc.
This allows neurons to respond selectively to
stimuli and to encode the stimulus strength as
the amplitude of an electrical potential.
For a signal to get from a peripheral receptor to the
brain, we need a reliable method to transport the
signal over the distance.
This is the job
of nerve axons.
Axons transmit their
information using all-ornone pulses in the
membrane called action
potentials.
The Action Potential:
long-distance carrier of information
The action potential (AP) uses the same mechanisms for
producing a trans-membrane voltage as we just saw.
However, the AP uses sodium (Na+) as well as K+), and the
pores are responsive to the voltage across the membrane.
In the resting neuron there are more Na ions on the outside
and more K ones on the inside. Thus opening Na gates
makes the membrane more positive on the inside, while
opening more K gates makes it more negative.
The Action Potential
The action potential depends on a stimulus
opening up voltage-gated Na+ channels which
depolarizes the membrane which opens up
voltage-gated Na+ channels which…
Eventually the channels spontaneously close and become
inactive for a time before returning to their active state.
Also, slower K+ channels open, repolarizing the membrane.
An action potential at any point on the
axon membrane causes the adjacent
membrane to undergo an action
potential. Thus the signal is transmitted
along the axon.
All Na channels
closed and
inactive.
More and more
Na channels
return to their
active state.
Refractory period and
stimulus coding
• The refractory period is determined by
– The time that Na+ channels are in their inactive
state
– The time that K+ channels are open, opposing
renewed depolarization.
• Time to the next action potential during
this period is determined by stimulus
strength.
– Thus action potential frequency is proportional
to stimulus strength.
…the bottom line…
• Since action potentials are ‘all-or-none’ (i.e., positive feedback)
phenomena, their amplitude is constant and thus conveys no
information.
• All the information then is coded as the frequency of action
potentials, not their amplitude.
– The range of intensity of the stimulus must thus be coded into the
possible range of frequencies of a neuron. The minimum detectable
change in frequency depends on the constancy of firing of the signaling
neuron (most neurons fire constantly). The absolute refractory period
governs the maximum frequency. The range in frequency between these
two factors governs the range of intensity of stimulus which can be
reported.
(Since an action potential lasts about one millisecond you might expect the maximum frequency of action
potentials to be about 1000 per second. Given the absolute refractory period and other considerations, the
practical limit is more like 200-500 per second.)
But there’s no free lunch!
• The both the action and receptor potentials involve the
movement of ions across the cell membrane.
• If this were all there were to it, after a while the ion
gradients would run down and no further potentials could
be developed.
• Instead, cells use an energy-driven pump (Na-K-ATPase)
to pump Na out of the cell in exchange for bringing K in.
• As the name implies, the pump burns ATP to do this. This
constitutes a large portion of your resting energy needs.
And that’s enough neurobiology for
today!!