The action potential is an impulse of electrical activity. Action potential occurs when a neuron sends messages electrochemically down an axon called a nerve impulse. When a neuron is not stimulated or inactive, it is at resting potential. When a neuron is at rest, the electrical charge on the inside of the neuron is negative while the electrical charge on the outside of the neuron is positive. The resting membrane potential of a neuron is about -70 mV (mV = millivolts).
At rest, the outside of a neuron contains an excess of sodium ions (Na+) while the inside of a neuron contains an excess of potassium ions (K+). At rest, potassium ions (K+) can cross through the cell membrane easily while sodium ions (Na+) have a more difficult time crossing. When a stimulus reaches a resting potential of a neuron it causes gated ion channels to open allowing the sodium ions (Na+) that was on the outside of the membrane to go into the cell. Sodium has a positive charge, which makes the neuron become more positive and depolarized. As depolarization reaches threshold of -55mV, the action potential is triggered and sodium ions (Na+) rush into the cell. If the neuron does not reach this critical threshold, then no action potential will fire.
It takes longer for potassium channels to open. When they do open, potassium (K+) rushes out of the cell, reversing the depolarization. Also at this time, sodium channels (Na+) is exiting the cell and starting to close. This causes the action potential to go back toward -70 mV, which is called repolarization. When the action potential goes past -70 mV hyperpolarization occurs because the potassium channels stay open. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV. A nerve consists of a bunch of axons clustered together.
An axon is a portion of a nerve cell that carries nerve impulses away from the cell body. The response of the nerve is called the compound action potential. The absolute and relative refractory periods limit action potential frequency. Because the Na channels are closed during the absolute refractory period no new action potentials can be generated. This represents the maximum frequency or maximum threshold at which action potential can be produced. At the maximum threshold all neurons or axons are firing but staying at the same level. Curare is a type of plant poison used by the South American tribes to paralyze their prey.
Paralysis occurs because Curare causes the skeletal muscles to become relax and paralyzed and eventually causes death by asphyxia. Curare is an example of muscle relaxant that causes voluntary muscle paralysis by blocking impulse transmission between nerves and skeletal muscle. Curare binds to the acetylcholine binding sites which will prevent acetylcholine from acting. Lidocaine is known to be a common local anesthetic drug and works by exerting its effects on sodium channels (Na+), preventing depolarization failing to cause an action potential.
In the first experiment Eliciting a Nerve Impulse(as seen above in Table 1) I tested different voltages ranging from 0. 5 volts to 4. 5 volts to see at what voltage will an electrical impulse cause an action potential. The results showed that action potential was not seen from 0. 5 to 2. 5 volts. Under threshold action potential was not seen because the voltage was not high enough to spark an electrical stimulus that causes an action potential. At 3. 0 volts some axons are firing because 3. volts is when threshold is reached (threshold potential is around -55mV) and the minimum voltage needed for action potentials to occur.
When I increased the voltage to 3. 5 volts I can see the line on the Oscilloscope screen went up meaning that even more axons were firing and action potential occured. At 4. 0 volts maximum action potential occurred and all axons were firing. At 4. 5 volts there were action potentials but maximum threshold was reached at 4. volts because all axons were already firing and there was no further increase and stayed the same as when tested at 4. 0 volts. I tested the effects of Curare and Lidocaine on a nerve (as seen above in Table 2). I tested at 3. 0 volts which was the minimum threshold, because the instructions on Activity 6 & 7 in the lab manual had referred me to go back to my first experiment Eliciting a Nerve Impulse for the voltage I determined and 3. 0 volts is where action potential first occured . Action potential is still generated but stops at the first synaptic cleft.
That is because Curare doesn’t prevent an action potential, but actually affects the neurotransmitters so that the action potential cannot be passed on to the next neuron. Curare is an example of muscle relaxant that causes voluntary muscle paralysis by blocking impulse transmission between nerves and skeletal muscle. When I drop Lidocaine on the nerve at 3. 0 volts you do not see any action potential. Lidocaine works binding and blocking the voltage gated Na+ channels which will not open and cannot polarize failing to transmit an action potential.