JENNY LIU wrote in message <372AA4DA.1595 at xtra.co.nz>...
>Could someone help me with these questions please:
>1. Why action potentials recorded with 2 extracellular electrodes from
>a single nerve fibre or whole nerve are usually biphasic? I was
>thinking that is a recording artifact due to the 2 elctrodes,is it
>right?
Well, it depends what you mean by 'artefact'.
Remember, a recording from two electrodes shows the potential of one
electrode relative to the other.
So, imagine electrodes 1 and 2 on the outside of the nerve fibre, some
distance apart.
The wave of depolarisation propagates along the nerve. The region at
electrode 1 first becomes temporarily negative, and a deflection is
recorded. The wave then traverses the distance between the two electrodes,
and then makes electrode 2 temporarily negative, which means electrode 1 is
being made more positive relative to electrode 2, hence you get a second
deflection that is like the first but inverted.
To summarise: over the course of the whole recording, relative to electrode
2, electrode 1 is made first negative and then positive. Hence a biphasic
recording. Simple, no?
>2. What are the advantages in using square-waves electrical stimuli
>ocer other types of stimuli(eg. sinusoidal) for excitation of nerves
>or muscle?
Hmmm, well, I've never thought about it before...but:
1. Square waves are easier to produce (simpler electronics, you just need
simple on/off switching)
2. A square wave involves only two different values for current flow across
the membrane, rather than a constantly varying one as with a sine wave.
Hence its easier to calculate how much current has flowed across the
membrane during a particular stimulus.
3. Nerves are less readily stimulated by slowly rising stimuli; they are
said to 'accomodate' to them.
>3. Why the threshold for nerve or muscle excitation witha square wave
> electrical stimulus depends both on the amplitude and duration of
>the stimulus?
The amplitude of the stimulus is proportional to the quantity of charge
flowing per unit time, i.e. the degree of additional depolarisation caused
per unit time. Excitation requires a certain depolarisation to be achieved.
Hence if the amplitude is too low, charge does not flow quickly enough for
the threshold depolarisation to be reached before the end of the stimulus.
Also, if the duration is too short, there is insufficient time for enough
charge to flow to reach the threshold.
>Thanks
You are most welcome.
>>Jenny