Notes for Experiment #6 -- Bioelectric Measurements

Notes & Hazards

Random helpful notes for your experiment:

  1. This lab is pretty easy but it might still take about 2 hours or so. Most of it is pretty self-explanatory; there are a bunch of new things but since they're almost all software-related, I'll be doing a demo once we get to lab (I'll essentially take you guys through a sample recording).
  2. Still, here are some (hopefully) helpful hints/pointers: remember that the electrode cream helps lower resistance and provides good contact between your skin and the electrodes. But, the stuff tends to dry quickly and you'll be using tape to hold the electrodes on. So make sure that the less hirsute partner takes on the role of guinea pig (trust me, it is not fun pulling off the tape along with a major portion of your body hair. Along these lines, though, make sure you wear shorts, sweats, or other easily-accessible clothing. You'll be taking measurements along your arm and along your leg so you'll need easy access to em.
  3. One of the trickiest things in this lab (maybe the only tricky thing) is to figure out what the signals mean. A good rule of thumb is what the lab manual suggests: the reflex time is the time between the (peak of the) trigger signal and the beginning of the electrical activity. In other words, what you're really measuring is the time of travel for the neural signals associated with the stretch reflexes in the knee and ankle. This time should be essentially constant as the response is mediated via the spinal column and doesn't depend on any secondary processing in the brain whatsoever.

    In (gruesome) detail, the stimulus causes an afferent neuron to fire; this synapses onto an interneuron in the spinal column; which synapses onto an efferent neuron in the muscle; the impulse at the neuro-muscular junction is what we pick up, amplify, and see on the oscilloscope as the signal.

    Since the reflex is completely independent of any other processes, this time should be constant. Whether a small stimulus triggers a small number of afferent neurons or whether a large stimulus triggers a whole mess of sensory neurons, the amount of time required to initiate the reflex should stay constant. The intensity of the response itself (e.g., how high the leg goes), however, might be affected by the strength of the stimulus or may be controlled to a certain extent voluntarily. But the only way to have a variable reflex time is if the subject somehow anticipates the stimulus and starts to react before ever getting hit. Of course, a whole bunch of technical issues also arise (distinguishing artifacts from actual signals, noise in the system from the environment, noise from overlapping, but unbraided, signal wires, etc.).


  1. None.

Required Materials:

  1. Laboratory Manual (SGM 407)
  2. Laboratory Answer Book
  3. Calculator with statistical functions

Some Helpful Links & Miscellaneous Notes

  1. This lab should allow you a bit of a breather and provide some biological applications of all the electrical stuff we've been studying so far. It should be interesting stuff but getting a handle on the computer stuff might end up taking you a couple hours. So if you're comfortable with that and you (or your partner) aren't squeamish about popping on the cream and the electrodes, you might end up getting outta here pretty quickly.
  2. Remember, the 1st midterm is coming up on 2/29/00 from 5-6:30pm in THH 201. I'll be posting more info and hints/tips as we finalize the details.
  3. Feedback on if you're finding these pages helpful (or not) is definitely good. So if you have any strong opinions, or, dare I say it, ideas on how to make this better, drop me a line!

Ricky J. Sethi <>
Last modified: Tue Apr 18 22:51:02 2000