BIOC32H3 Lecture Notes - Lecture 4: Axon Terminal, Depolarization, Multiple Sclerosis
LECTURE 4: THE ACTION POTENTIAL 2
Spontaneous graded potentials and action potentials
are decreased in the striatum in Parkinson`s disease
(PD)
Discovery of the Action Potential
●In 1971 Luigi Galvani discovered that electrical signals from
lightning or primitive batteries could cause contraction of the
leg of an otherwise dead frog
○lead to studies on the heads of hanged criminals
attempting to bring back the dead!
●Using his instrument called the differential rheotome in 1871
Julius Berstein first described an action potential as negative
variation
○in frog nerve muscle preparation
○lasts approx 1ms
○time course independent of stimulus
○action potential almost complete before muscle contraction begins
●In 1949 Hodgkin and Katz rediscovered negative variation / the overshoot
○formulated the sodium hypothesis
○explained transient reversal in resting membrane potential by influx of Na+
●In 1952 Hodgkin and Huxley worked out the ionic basis of the action potential and developed a
mathematical model that successfully predicted the speed of propagation in the squid giant axon
○Beginning of computational neuroscience
○Demonstrates how computer models can reveal biological properties that cannot be examined
directly
○Awarded nobel prize in 1963
The Studies of Hodgkin and Huxley
●Used the squid giant axon
○unmyelinated axons: wires without insulation
○leaky prone, prone to signal loss
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○broad axons to conduct electricity better
●Threaded a fine silver wire inside the axon to measure electrical potential inside a cell
●Delivered current to maintain a particular voltage inside the cell – known as voltage-clamp
●Measured how much current was required to keep the voltage from changing
●Told them amount of current and direction of ion flow
●Performed at many different voltages
●Able to figure out which ions were responsible for currents
●Action potential described as a positive variation: positive inside the cell to
negative outside
Location of Voltage-Gated Na and K Channels in the Neuron
●Na and K channels are located in dendrites and the cell body
●High concentration of Na channels found in the trigger zone and between the
myelin sheaths in the nodes of Ranvier
●No Na+ channels located where the myelin sheath are on the axon
●Do the same types of ion channels cause graded potentials and action
potentials? YES
Na+ Entry Creates a Positive Feedback Loop
●The depolarisation caused by opening of Na
channels causes more voltage-gated Na and K
channels to open
●Action potentials are generated by the opening
of many Na channels
●The positive feedback loop generated by Na
allows conduction of the action potential down
the axon
The Refractory Periods of the AP
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