Biology 260 - Human Physiology

Spring 2000

Lab #2 - Nerve Action Potential

As discussed in class, cell communication is a vital component of the body's constant "struggle" to maintain homeostasis. One of the ways muscle and nerve cells (the excitable tissues) receive messages is by changing voltage across their cell membrane (membrane potential). These potentials are generated when ions move from one side of the membrane to the other and back again through ion channels. These action potentials are usually in the millivolt (mV) range and occur in milliseconds (ms).

Action potentials have four phases: depolarization, repolarization, hyperpolarization, and polarization (resting state). All of the phases correspond to movements of sodium (Na⁺) and potassium (K⁺) ions across the membrane.

In this lab, you will be recording and stimulating a nerve preparation with extracellular electrodes. Thus, you will be measuring electrical activity from all the excited fibers of the nerve. Such activity is termed a compound action potential (APs from all the cells). Therefore, when you deliver a small shock, only a few fibers will be excited, with a larger shock more fibers will be excited and so on.

REMEMBER: THE ACTION POTENTIAL OF A SINGLE CELL IS ALWAYS THE SAME SIZE.

For the lab exercises, follow the instructions in the lab manual after completing the frog dissection. Two of the computers will also be set up for use with the Interactive Physiology software. Please remember that you may check these CDs from me at any time.

Lab Assignment

1. Write a description of this lab (2 pages maximum). Please make sure you have included the rationale for the lab, description of the physiological significance, methodology, results. Follow the appropriate guidelines for your printed report. (5 points)

2. Two important components of the action potential (AP) are the intensity and duration of the stimulus. Describe what the relationship between these two components must be in order to generate a specific AP. Graph this relationship between stimulus duration and stimulus voltage necessary to elicit an AP. (2 points)

3. What is the physiological explanation for the above relationship? (3 points)

4. Graph a nerve AP. What is the amplitude? What is the duration? (2 points)

5. Conduction velocity experiment (fill in the numbers):

What was your estimate for the nerve conduction velocity? (2 points)

6. Why is the refractory period important to the function of the neurons? (3 points)