The McGill Physiology Virtual Lab

Compound Action Potential

Experimental preparation

In order to record a CAP successfully, one needs to take special care to dissect out the longest nerve possible and to keep it moist at all times. It is also important to ensure the nerve makes good contact with the stimulating and the recording electrodes in the bath.


Dissecting out the Frog Sciatic Nerve

The schematic view of the frog sciatic nerve to the right shows the path of the nerve from the spinal insertion points down to the ankle.

The frog sciatic nerve dissection is a delicate operation.   During the dissection, the nerve must not be touched with the fingers, cut muscle, frog skin, or with any metal instruments. Care must be taken to keep the preparation moist at all times, and to not stretch the nerve unduly.


Glass probes are used to dissect out the frog sciatic nerve, by gently separating the nerve from the surrounding muscle masses. Great care must be taken when freeing the nerve from tendons at the hip and knee joints. It is desirable to obtain as long a length of nerve as possible, from the vertebral column down to the foot.

A small slip of bone/muscle is left attached to the nerve at the proximal end. A piece of thread is tied to the nerve at the distal end.

The lower reservoir of the nerve bath (above) is filled with frog Ringer's solution, making sure that the Ringer's solution is not touching any of the wires. The nerve is placed in the chamber with the bone end near the two stimulating electrodes (anode and cathode). The lid of the chamber is then placed firmly on top to preserve the humidified air in the bath.


Electronics Setup

Observe the following setup:

The Grass stimulator provides two output pulses. One set of leads, from the stimulator to the pair of stimulating electrodes, carries the Stimulus Pulse that will activate the nerve. Because the CAP is initiated at or near the cathode, and the anodal action slows down or may even suppress the CAP, the cathode should always be closer to the recording electrodes. The nerve will be stimulated repeatedly, and the Amplitude, Duration and Frequency knobs and switches control the corresponding parameters of the Stimulus Pulse. (The value of each parameter is the product of the dial readings on the corresponding range switch and dial.)
A second lead from the stimulator (a two-conductor, co-axial cable) carries a Pre-stimulus Trigger Pulse to the  Trigger connector of the Powerlab recording system. This narrow Trigger Pulse precedes the Stimulus Pulse and is interpreted by the Analogue I/O board in "Powerlab" as a signal to start sampling a new sweep of data.

Satisfactory recording of the electrical response of the nerve depends on a pair of shielded cables leading from the two pairs of Recording Electrodes to the input connectors (CH1 and CH2) of the recording system. The shields of these coaxial cables reduce electrical noise.

The stainless steel electrodes in the bath are equally spaced. Connection to these wires depend on the length of the dissecting nerve. Conduction velocities and refractory measurements are facilitated by the fact that one can record from different sites along the nerve.

Modified from ADinstruments. All Rights Reserved.

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