Other Methods Notes
Electrode cephalogram*
------------------------Axial resistance!
If there was an AP at A and channel was depolarized, area B will not be depolarized just yet
Axial resistance determines how easy it is to axial current to reach B; AP NOT
INSTANTANEOUS!
Return current! When current comes into region of axon and generates axial currents, that form
will return in the form of an extracellular current
-------------The further down, the less current density there is
If we have two electrodes both in extracellular fluid, one electrode in a “reference” area but not
quite inside the cell and another in a “test”
User specified, arbitrarily calling the reference point to test is downstream
Vref – Vtest
AT rest, extracellular voltage is ZERO not resting; the deflection of the currents do not depict the
sodium channel, only the electrical signal
Let’s consider we were dealing with various axons, a nerve, it possible to apply an electrical
stimulant of shock so that axons generate AP at the same time. One can trigger a compound AP
The current from all axons would sum together; makes it easier to detect. Many experiments can
be done by stimulating nerve.
Not all axons conduct in the same rate [some axons are thinner and have bigger axon resistance,
also conduction would be slower; some axons are myelination which would make conduction
faster] if whole nerve stimulated, axons fire in different times; different waves of signals
Cells connected to each other by gap junctions; cardiomyocytes can conduce AP through gap
junctions
You get cloud of current, densest current is right outside of tissue. If you have strong current [the
heart, thousands of cardiomyocytes firing at the same time], there is enough radiation of return
current that can go up to the skin [that’s what is being detected]; ventricle with stronger surface
signal because it is thicker than the atrium
---------Focus on white trace: recording at someplace downstream, axon was stimulated somewhere
upstream. There was big deflection associated with myelin. In 10m seconds, another transition in
seen; compound AP. That’s associated with conduction with the axons that were not myelinated.
(all axons that have to do with pain are unmyelinated)
Change in temperature then we repeated stimulation, at cooler temps, AP is slower. VG sodium
channels are temperature dependent, at cooler temp channels open slower [SODIUM
CHANNELS ARE RATE LIMITING STEP]
------Electrocardiogram!
Electrodes placed in different places in chest.
You can measure between let’s say 1 to RA (shown in drawing)
For each cycle, you get p wave then qrs interval etc; classic pattern. Each element of this is
associated with the voltage changes within the heart muscle.
P wave is depolarization trend in the atrium
P wave is seeing conduction of AP in the atrium; there is a region of the atrium that gets triggered
Ventricle will become excited with delay, generates a large single current (stronger) QRS
T wave refers to repolarization
We’re not putting electrodes inside heart but on surface, our skin conducts current from one
place to another
Electrocardiogram not measuring contraction; it is looking of AP through the heart and tissues
The AP conducts rapidly through ventricle, entire ventricle is depolarized. The Ap is stopped
where is started first and will create inverse current.
-----Electroencephalogram places several electrodes on the skull; measures brain activity
Cerebral cortex has several layers, some deep some shallow; only measures the electrical activity
closest to the surface
Electrical input in brain is not uniform like previous examples; AP flowing through an axon will
not line up
Certain classes of neurons and seeing synaptic transmission; there are large cells, layer 5
pyramidal neurons, the dendrite had long apical segment that branches like a tree
Structure of dendrite looks like axon
There are synapses that project onto dendrite. The synapse, could be excitatory [when opened,
will allow sodium to coming], if there is transmission it will depolarize membrane. Current will
form. Current will move through axial resistance of dendrite. Unlike AP this does not propagate.
Current loop stays where it is, it is not conductive or regenerative.
---Layer 5 pyramidal neurons line up perfectly.
Don’t worry about pos/neg, there is overlap on how neighboring dendrites will react.
If we consider that when generate these neurons will create loop current, neighboring loops
current will move in same direction and add up
If summed enough, current might radiate to top. Voltages are smaller; exception focal seizures
and/or epilepsy as whole