PhysioEX: Nerve Impulse S.J.A. Orense, L.K. Palma, C. Pineda, E.V. Ragodon, K.D. Reyes
Summary Nerve impulse is a wave of action potentials moving along the nerve fiber. It is the propagation of action potentials along a nerve fiber. An action potential is a sequence of depolarization and repolarization. An action potential at the trigger zone causes an electric current to flow a short distance down the nerve fiber, which stimulates the next membrane to its threshold level, triggering another action potential. In this experiment, electrical stimulation, mechanical stimulation, thermal stimulation and chemical stimulation were done unto the sciatic nerve. In the electrical stimulation, the minimum voltage that elicited an action potential was at 3.0V while the maximum voltage that elicited an action potential was at 5.0V. In mechanical stimulation, the glass rod was used and the nerve elicited an action potential was conducted, in thermal stimulation the nerve also responded from the heated glass rod and in chemical stimulation; both HCl and NaCl elicited an action potential. Lidocaine and curare are effective in inhibiting action potentials.
Introduction Various agents affect nerve transmission and different kinds of stimuli trigger an action potential. Unequal distribution of ions in the cell’s inner and outer membranes polarizes a nerve cell. A polarized membrane is stimulated at its threshold or above its threshold. When a membrane overcomes its threshold, a physiological effect begins to be produced. This wave constitutes a nerve impulse, and if it reaches a muscle, the muscle may respond by contracting (Marieb, 125). In this experiment a segment of a nerve has been dissected and suspended over a series of metal bars and reagents that act as electrodes. The electrodes are stimulating electrodes and are connected to an electrical stimulator. The stimulator can be used to apply electrical current to the nerve at different voltages and frequencies to try to elicit an action potential. The other set of electrodes are called recording electrodes, and they are connected to an
oscilloscope. Differences in charge between the two recording electrodes cause the line traced on the oscilloscope screen to deflect. Thus, we can observe any action potentials forming in the nerve (Marieb 130-131). Effects of HCl, NaCl, and heat and glass rod were also taken into consideration in generating nerve impulse. While in inhibiting production of an action potential, lidocaine, curare and ether were used. The objectives of this experiment are to test and identify different stimuli capable of generating an action potential and identify stimuli capable of inhibiting an action potential.
Materials and Methodology The experiment was conducted using a PhysioEx 5.0 software and a Laptop computer. Electrical Stimulation Upon opening the program of PhysioEx, the main
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menu was clicked and the Neurophysiology and Nerve Impulses was selected. The voltage was set to 1.0V by clicking the (+) next to the Voltage display and the Single Stimulus was clicked. If there was no response, the clear button was selected and the voltage was increased together with the clicking of the Single Stimulus button, this step is being repeated until a deflection of the line is seen which indicates an action potential, this is termed the Threshold Potential. The Data was recorded by clicking the Record Data button. If printing is necessary, the tools tab was selected and then Print Graph is clicked. The voltage was increased by 0.5V and Single Stimulus is clicked. The deflection line is compared to the previous line and was recorded by clicking the Record Data button. The voltage was progressively increased by 0.5 V and the Single Stimulus button is clicked until the point beyond which no further increase occurs in the peak of the action potential is found, this is referred to as the Maximal voltage. The data was recorded. Mechanical Stimulation Using the same opened program of PhysioEx, the clear button was clicked. The glass rod located on the bottom shelf on the left side of the screen was dragged over to the nerve. Then the glass rod is over the nerve, the mouse button was released to indicate that the rod is not touching the nerve. Then the oscilloscope screen was observed whether a tracing has occurred or not. The data was recorded by clicking the Record Data. Thermal Stimulation Using the same opened program of PhysioEx, the glass rod was clicked and was dragged on to the heater. The Heat button was clicked and when the rod turns red, this indicates that it has been heated already. The heated-rod was clicked and
dragged over the nerve and the oscilloscope was observed. Chemical Stimulation The dropper from the bottle of sodium chloride was dragged over to the nerve in the chamber and the mouse button was released to dispense the drops. The effect was observed. The threshold voltage setting from the Electrical stimulation was compared. The data was recorded by clicking the Record Data button. The clean button was clicked and together with the clear button. The dropper from the bottle of hydrochloric acid was dragged over to the nerve and the mouse button was released to dispense the drops. The effect was then observed and was compared to the original threshold stimulus. The data was recorded by clicking the Record data button. The clean button was clicked. If wished to print the data, tools tab was selected and the Print data button was selected. Testing the Effects of Ether The dropper from the bottle of ether was clicked and dragged over to the nerve in between the stimulating electrodes and recording electrodes. The mouse button was released to dispense the drops. Using the threshold voltage setting from Electrical stimulation, the Stimulate button was clicked the result was observed. The result was recorded by clicking the Record Data button. The time button on the oscilloscope was clicked until the screen shows a display activity over the course of 10 minutes. The (+) button under the Interval between Stimuli on the stimulator was clicked until the timer is set for 2.0 minutes. The Stimulate button was clicked and the Elapsed Time display is observed. When the nerve returns to normal, the Stop button was clicked to stop the action and return to the Elapsed time to 0.0. The Time (msec) button was clicked to return it to its normal millisecond display. The 2
clear button was clicked and the (-) button under the Interval between Stimuli was clicked until it is reset to 0.00. Testing the Effects of Curare The dropper from the bottle of curare was clicked and dragged on to the position of the nerve in between the stimulating and recording electrodes. The mouse button was released to dispense the drops. The stimulator was set at the threshold voltage and the nerve was stimulated. The effect on the action potential is noted. The data was recorded by clicking the Record Data button. The clean button was selected and the clear button was clicked to clear the oscilloscope screen. Testing the Effects of Lidocaine The dropper from the bottle of lidocaine was clicked and dragged on to the position over the nerve between the stimulating and recording electrodes. The mouse button was released to dispense the drops. The tracing were observed. The data was recorded and if printing is preferred, the Tools tab was selected and Print Data was clicked. The clean button was clicked to remove the lidocaine and return the nerve to its original untouched state. Measuring Nerve Conduction Velocity The Pulse button was clicked and the bioamplifier was turned on by clicking the horizontal bar on the bio-amplifier and dragging it to the On setting. The dropper from the bottle of ethanol was clicked and dragged over the earthworm and the mouse button was released to dispense the drops of ethanol. The earthworm was clicked and dragged into the nerve chamber, using the (+) button next to the Voltage display, the voltage was set to 1.0 V then the stimulate button was clicked. The action potential was observed. The Measure button was clicked and a
vertical yellow line appears on the far left edge of the oscilloscope screen. The (+) button was selected which moved the yellow line to the right. The time elapsed was measured on the graph by positioning the yellow line at the point in the graph where the graph ceases being a flat line and first starts to rise. The data were recorded. The earthworm was clicked and dragged to its original place and the clear button was clicked. The other test organisms were tested using the same procedures and the data were recorded. The data were printed by clicking the tools tab and the Print data button was selected.
Results and Discussion I.
Eliciting a Nerve Impulse
Neurons are cells that correspond by transmitting stimuli to other cells. They have two major physiological properties: irritability and conductivity. Irritability is that capability of neurons to respond to electrical impulses converting them to nerve impulses. On the other hand, conductivity is the neuron’s capacity to transmit nerve impulses. In the experiment, a sciatic nerve dissected from a frog was used. The stimulus travelled from the red and black leads: from the stimulator output to the nerve chamber and also to the oscilloscope. The current produced by the oscilloscope travels to the nerve, the nerve then depolarizes and the electrical current that develops flows continuously to the leads. Starting the experiment, the voltage was set to 1.0V in order to see if an action potential would occur. At 1.0V, there was no action potential that took place. The voltage was increased and the minimum voltage that elicited an action potential was at 3.0V. Once the threshold was 3
established, the voltage was again increased to observe difference in peaks of the action potentials. A slight increase in height of the peaks of the different action potentials are observed until no difference was seen even though there was still a continued increase in the voltage. The maximum voltage was at 5.0V.
oscilloscope screen than the unheated glass bar. This means that the heated glass rod conducted a higher action potential than the unheated glass bar.
Graph 1.2 Comparison of the action potentials elicited in mechanical and thermal stimulation. Red line indicates the heated glass bar while the green line displays the tracings for the unheated glass bar. Graph 1.1 Increasing action potentials corresponding changes in the voltage
Figure 1.2 Comparison on the mechanical and thermal stimulation.
Figure 1.1 Comparison of different voltages whether it will elicit an action potential
The next activity in this experiment was to determine whether the nerve would respond to mechanical, thermal and chemical stimulus. In the mechanical stimulation, a glass bar was placed on the nerve to see if it will draw out a response from the nerve. By placing the glass bar on the nerve, you will observe that there was an action potential coming from the nerve displayed on the oscilloscope screen. The tracing displayed was lower than that of the threshold voltage which was at 3.0V. For thermal stimulation, the glass bar was heated and placed on the nerve. It produced a higher peak on the
Chemical stimulation was also performed in the experiment. NaCl and HCl were added to the nerve to observe if certain action potentials would occur. Based on the Figure 1.3, we can detect that both chemicals triggered an action potential; also, both tracings were the no different than the tracings displayed by the threshold voltage.
Figure 1.3 Comaprison of Nacl and HCl in maxing out an action potential.
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Conclusion Neurons respond to different stimulus may it be electrical stimulation in where you apply electric currents (voltage) in triggering a response; mechanical stimulation in which you elicit a response by use of glass bars; thermal stimulation in where you apply heat in certain objects and placing it on the nerve; and chemical stimulation in where you add different chemicals like NaCl and HCl on the nerve. II.
Inhibiting a Nerve Impulse
Table 1. Summary of Results for PhysioEx experiment on Nerve Impulse Inhibition Figure 2. Summary screenshots of PhysioEx results for Nerve Impulse inhibition using Lidocaine, Curare, and Ether with Curare only showing positive action potential
As several factors could elicit a nerve impulse, a myriad of external causes may also lead to inhibiting action potential. For example, chemicals, particularly local anesthetics do so by inhibiting the depolarization of the nerve membrane through interference of both Na+ and K+ currents. The action potential is not propagated because the threshold level is never attained (Gmyrek et al., 2013). In this experiment, three chemical agents, were tested to inhibit a nerve impulse. Lidocaine
Figure 1. Screen shot of normal nerve impulse at minimum voltage
Lidocaine is a local anesthetic drug whose inhibiting process is both transitory and reversible and is chiefly serving purposes in dental surgery, ventricular heart arrhythmias, and gastroeneterologic matters.
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Curare
Figure 3. Screen shot of Lidocaine trials showing negative action potential despite application of increasing voltage
This anesthetic works by easily binding and blocking the fast voltage gated sodium (Na+) channels to be found both on the heart cardiomyocytes and in the neuronal cell membrane, held responsible for signal propagation. given that it alters signal conduction in neurons. With sufficient blockade, the membrane of the postsynaptic neuron will not depolarize and so fail to transmit an action potential, leading to its anesthetic effects (Vono, 2011). In the simulation, this inhibitory effect was observed when Lidocaine was dropped on the muscle. No action potential was recorded at minimum voltage as well as when the voltage was doubled and reached maximum.
Figure 4. Screen shot of Curare trial showing positive action potential
Curare is a skeletal-muscle– relaxant drug belonging to the alkaloid family of organic compounds (Encyclopaedia Britannica). It was used before for arrow poisons to cause paralysis on the target. Curare’s mechanism involves competing with acetylcholine (which is responsible in stimulating muscles) in binding with neuromuscular junctions thus preventing muscle contraction. This however only occurs postsynaptic and the release of acetylcholine which is happening presynaptic continues, recognized as an action potential and is then still detected and recorded in the simulation as shown in Figure 4.
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Ether
Gymrek, R et al. (2013, Jun 3). Local and Regional Anesthesia. Retrieved from http://emedicine.medscape.com/article/183 1870-overview. Marieb, Elaine N., and Susan J. Mitchell. "Neurophysiology of Nerve Impules: Computer Stimulatiom." Human Anantomy & Physiology. N.p.: n.p., n.d. 125-33. Print. Vono, M. (2011, Jan 17). Lidocaine. Retrieved from http://flipper.diff.org/app/items/info/3239.
Figure 5. Screen shot of Ether trials showing negative action potential despite application of increasing voltage
Ether is an example of general anesthetic. This type of drug decrease brain activity by opening K+ gates; thus allowing these ions out of the cell. The neuron becomes hyperpolarized as in the absolute refractory period, and is unable to discharge. Hyperpolarization prevents the neuron from receiving another stimulus thus no action potential is achieved (Charand, n.d) as shown in figure 5.
Encyclopaedia Britannica (n.d.) Curare. Retrieved from http://global.britannica.com/EBchecked/topi c/146779/curare. (n.d.) Outside Electrode with Action Potential. Retrieved from http://psych.athabascau.ca/html/Psych289/ Biotutorials/2/out_act.shtml.
Conclusion Inhibition of a nerve impulse may occur due to cancellation of depolarization as exhibited in local anesthetics or through hyperpolarization as effected by general anesthetics. Moreover, processes occurring postsynaptic do not affect the release of action potential.
References Charand, K.X. (n.d.). Action Potentials. Retrieved from http://hyperphysics.phyastr.gsu.edu/hbase/ biology/actpot.html.
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