Action potential of nerve stem is an objective sign of nerve excitation. When a certain intensity of stimulation is given to the nerve stem with excitability, action potential will occur. The extramembrane potential at the excited site is negative compared to the resting site. When the nerve impulse passes through, the extramembrane potential at the excitation site returns to the resting level. This membrane potential change that occurs in the excitation process of the nerve stem is called the nerve stem action potential.

If the two guide electrodes are placed on the surface of a normal and intact nerve trunk, when one end of the trunk is excited, the excitation wave passes through the two guide electrodes successively, and two potential deflection waveforms in opposite directions can be recorded, which is called biphasic action potential. If the nerve tissue between the two guide electrodes is operated and the excitation wave only passes through the first guide electrode and cannot be conducted to the second guide electrode, only a potential deflection waveform in one direction can be recorded, which is called a monophasic action potential .

The nerve stem is composed of many nerve fibers, so the action potential of the nerve stem is different from the action potential of a single nerve fiber. It is a comprehensive potential change synthesized by the action potential of many nerve fibers. Furthermore, this is recording extracellularly, unlike intracellular recordings. Therefore, the amplitude of the action potential of the nerve trunk can change with the change of the stimulation intensity within a certain range.

The conduction of action potentials on nerve fibers has a certain speed. Different types of nerve conduction velocity (υ) vary, the thicker the nerve fiber, the faster the conduction velocity. The sciatic nerve trunk of frogs is dominated by fibrils, and the conduction velocity is about 35~40m/s. The conduction velocity of the nerve impulse can be obtained by measuring the distance (s) that the nerve impulse conducts on the nerve trunk and the time required to pass through these distances (t).

We introduced the ZL-620U medical signal acquisition and processing system into this experiment to learn the method of recording the compound action potential of the nerve stem, master the measurement of the action potential, understand the nerve excitation conduction velocity and its measurement method, and deepen the understanding and the concept of excitation conduction .

【test subject】

toad or frog.

【experiment equipment】

ZL-620U medical signal acquisition and processing system, frog surgical instruments, specimen shielding box, some wiring with electrodes, Ringer's solution.

【Method and steps】

1. Preparation of Toad Sciatica Nerve Trunk Specimens

(1) Method 1: The preparation method of the specimen is basically the same as that of the sciatic nerve gastrocnemius muscle specimen.

It should be noted that:

1) The nerve trunk should be separated as long as possible. It is required to start from the main trunk near the spine, and to separate the common peroneal and tibial nerves from the lower part to the vicinity of the ankle joint.

2) Do not damage the nerve tissue during the separation of the nerve trunk, so as not to affect the experimental effect.

3) Both ends of the nerve should be tied with a thin thread, and then immersed in Ringer's solution for use.

(2) Method 2: Take the lower trunk of the toad after skinning, take the prone position, hold the tail end of the sacrum with pointed forceps and lift it slightly, and cut the sacrum horizontally with tissue scissors. Then lay the specimen supine, separate the sciatic nerve trunks on both sides of the spine with a glass minute needle, thread, ligate the nerves close to the spine respectively, cut the nerves, lift the thread end, and thread the specimen from the sacral incision to the back side, lay the specimen prone, and fix the specimen with a pin. Gently lift the end of the ligated nerve on one side by hand to identify the direction of the sciatic nerve, then place scissors between the nerve and the tissue, close to the femur and the popliteal fossa, and cut the nerve together with the muscle along the direction of the nerve to the Achilles tendon. nerve. Lift the cut neuromuscular specimen, clamp the surface of the gastrocnemius muscle with forceps, and gently pull it along the nerve trunk. After removing the muscle tissue, a sciatic nerve trunk specimen is prepared.

2. Connect the experimental device

(1) Connect the experimental instruments with wires as shown in Figure 1. The stimulation output of the ZL-620U is connected to a pair of stimulating electrodes, and the two pairs of guide electrodes are connected to the CH2 and CH4 channels of the ZL-620U, and the ground wire is grounded. Misconnections or poor contacts must be avoided.

Figure 1 Diagram of the device for observing the action potential of the nerve trunk and measuring the conduction velocity of the nerve impulse

(2) The specimen shielding box is lined with filter paper soaked in Ringer's solution to increase the air humidity in the box and prevent the nerve stem from drying out quickly.

(3) Rotate the nerve stem specimen on the stimulation electrode, ground electrode and guide electrode.

(4) Start the ZL-620U medical signal acquisition and processing system and set the parameters (Table 1). The stimulation mode can also use single stimulation or main cycle stimulation, and gradually change the stimulation amplitude.

3. Experimental observation

1) Observe the effect of different stimulation intensities on the action potential of the nerve trunk: gradually increase the stimulation intensity, and find the stimulation intensity (threshold intensity) that can just cause a tiny nerve trunk action potential. Continue to increase the stimulation intensity, the nerve trunk action potential also increases accordingly. When the action potential increases to the maximum (no longer increases with the stimulus intensity), the stimulus intensity is the maximum stimulus intensity.

(2) Carefully observe the biphasic action potential waveform. The amplitude of the upper and lower phases of the biphasic action potential at the maximal stimulus and the duration of the entire action potential were read out.

(3) Observe the changes in the biphasic action potential waveform after the rotation direction of the nerve stem specimen is reversed.

(4) Determination of action potential conduction velocity

1) When the nerve trunk is stimulated with the maximum stimulation intensity, two biphasic action potential waveforms formed successively can be observed in the sampling windows of channels 2 and 4.

2) Measure the time from the stimulation artifact to the onset of the two action potentials, set channel 2 as t1 and channel 4 as t2 (or directly measure the interval time between the onset of two action potentials), and calculate the time difference.

3) Measure the distance s between the two pairs of guide electrodes CH2 and CH4 in the specimen barrier box (the distance between r1 and r2 should be measured).

(5) Observation and measurement of monophasic action potential waveforms (before injuring nerve specimens, the experiment of "measurement of nerve trunk action potential refractory period" can be done first).

1) Use tweezers to pinch the nerve between the two recording electrodes of CH4 or block it with drugs, and then present a monophasic action potential during re-stimulation.

2) Read out the amplitude of the monophasic action potential and the duration of the entire action potential at the maximum stimulus.

3) Compare the rise time and fall time of the monophasic action potential, and analyze the relationship with the biphasic action potential waveform.

【Experimental Results】

1. Find the range of suprathreshold stimulation when the wave width is a certain value, and record the values ​​of threshold stimulation and maximum stimulation.

2. Print biphasic and monophasic action potential waveforms, and measure their maximum amplitude and duration.

3. Calculate the conduction velocity of the nerve impulse: υ=s/(t2-t1)(m/s).

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