Topic 4

Biology 463 - Neurobiology
Topic 4
The Action Potential
Action Potential
– Cytosol (cytoplasm) has negative charge relative to extracellular
– Its pusatile nature allows for neural coding
• frequency
• Pattern
– Action potential components to be examined
• Spike
• Nerve impulse
• Discharge
Properties of the Action Potential
– Oscilloscopic movement used to visualize
• Rising phase, overshoot, falling phase, and undershoot
The Generation of an Action Potential
– Caused by depolarization of membrane beyond threshold
– “All-or-none”
For the next series of three slides, please
examine them in rapid sequence to create an
“animation” of action potential movement in
the neuron.
These next two slides show how intensity and frequency of a
stimulus can affect generation of action potentials.
Saltatory conduction is the propagation of an action potential along
myelinated axons from one Node of Ranvier to the next Node. This
process increases the conduction velocity of action potentials without
the need to increase the diameter of an axon.
Artificial injection of current (ions) into a neuron (using a
microelectrode) can be used to experimentally manipulate a
neuron from its interior.
Alan Hodgkin & Andrew Huxley – performed experiments using
the patch clamp method that experimentally proved the
movement inward of sodium ions and the movement outward of
potassium ions during an action potential. This of course, was
essential for our current understanding of the biochemistry of this
The Voltage Clamp
used by electrophysiologists to measure the ion currents
across the membrane of excitable cells, such as neurons
while holding the membrane voltage at a set level
cell membranes of excitable cells contain many different
kinds of ion channels, some of which are voltage gated
allows the membrane voltage to be manipulated
independently of the ionic currents, allowing the currentvoltage relationships of membrane channels to be studied
The voltage clamp technique being used in a segment of giant squid axon.
These are three diagrams of
a voltage-gated sodium
channel protein.
in a) we see four regions or
domains that will be
found in the protein
in b) we see a close up view
of the alpha helix
structures including S4
which is a voltage
sensing alpha helix
also in b) the pore loop is seen
which in effect helps
regulate what can pass
through the pore
in c) we see the entire
channel in the most
common arrangement
In (a) we see the application of a
stimulus resulting in the
generation of an action
In (b) we can see how three
different sodium channels may
respond slightly differently to
the action potential stimulus
In (c) we can see a close up view
of the sodium channel protein,
this time showing the globular
protein (the purple sphere)
that serves as a temporary
gate to occlude the pore until
the membranes themselves
Disorders/Diseases Associated with Voltage-Gated Sodium Channel
– Channelopathies - diseases caused by disturbed function of ion
channels or channel subunits or the proteins that regulate them
• e.g., Generalized epilepsy with febrile seizures
– Toxins as experimental tools
• Toshio Narahashi – ion channel pharmacology
• Puffer fish: Tetrodotoxin (TTX)- Clogs Na+ permeable pore
• Red Tide: Saxitoxin- Na+ Channel-blocking toxin
An example of a startled
Pufferfish which produces
Toshio Narahashi – famous pharmacological neuroscientist who
uncovered the blocking action of tetrodotoxin.