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Chapter 5

BIO304H5 Chapter 5: 304 9

Department
Biology
Course Code
BIO304H5
Professor
Chapter
5

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A second voltage amplifier (B) receives one input wire from inside the
cell, and the other from a “command voltage”
If Vc-Vmis positive (e.g. -9 mV --65 mV = +56 mV), amplifier Binjects
positivedepolarizing current into the axon, raising Vm
This process continues dynamically until the two inputs into amplifier
Bare equal (i.e. Vm= Vc= -9 mV)
If Vc-Vmis negative (e.g. -85 mV --65 mV = -20 mV), amplifier Binjects
negativehyperpolarizing current
This process continues dynamically until the two inputs into amplifier
Bare equal (i.e. Vm=Vc= -20 mV)
A third voltage amplifier (C) measures the current injected coming from
amplifier B
Has a resistor (R) between its two inputs...
Uses Ohm’s law to read the current exiting amplifier Binto the axon
(V=IR):
There is a known resistance (R)
TheCamplifier can measure the voltage across its two inputs (V)
When amplifier Binjects current into the axon, we can record the current
amplitude over time using ohms' law: I= V/R

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cell, and the other from a “command voltage
Example voltage clamp experiment:
Use Vcto hold voltage at -65 mV
At the onset of the voltage change pulse, Vcis changed to -9 mV, so the
voltage clamp amplifier Bquickly injects current into the axon raising
Vmto -9 mV
Vcholds the Vmat -9 mV for 5 milliseconds (ms)
Vcis then changed back to -65 mV, which returns Vmback down to -65
mV
This is what Hodgkin and Huxley saw in their first voltage-clamp
recordings...
1.A capacitativecurrent
2.An early current
3.A late current
Notice how nice and flat Vmstays during the voltage clamp step,
compared to the AP
Can study GNaandGKat fixed voltages!
Understanding Na+and K+currents recorded with the voltage clamp...
Say we have a holding voltage of -65 mV