(A). Sodium ions exit the nerve membrane
(B). Sodium ions enter the nerve membrane
(C). Both sodium ions and potassium ions enter the nerve membrane
(D). Both sodium ions and potassium ions exit the nerve membrane
Nerve conduction (and recovery) follows the steps of depolarization, repolarization, hyperpolarization, and refractory period.
- Depolarization occurs when a stimulus reaches a resting neuron. During the depolarization phase, the gated sodium ion channels on the neuron's membrane suddenly open and allow sodium ions (Na+) present outside the membrane to rush into the cell.
- As the sodium ions quickly enter the cell, the internal charge of the nerve changes from -70 mV to -55 mV.
- When this firing threshold of -55 mV is reached, membrane permeability to sodium increases dramatically and sodium ions enter the axoplasm (the inner portion of the nerve cell) even more rapidly.
- As a result, the inner portion of the nerve cell reaches +40 mV.
- With repolarization, the potassium channels open to allow the potassium ions (K+) to move out of the membrane (efflux). As this happens, the electrical potential gradually becomes more negative inside the nerve cell until the original resting potential of -70 mV is attained again.
To summarize, sodium ions (Na+) enter the nerve membrane during depolarization and potassium ions (K+) leave the nerve membrane during repolarization.
Answer: (B). Sodium ions enter the nerve membrane
Now, what does local anesthesia do in this process? Why do we use local anesthesia in dental hygiene to numb the patient? Local anesthetic molecules work by blocking the sodium channel and interfering with depolarization. As a result, the nerve is NOT “fired” (triggered) and therefore, the patient does NOT feel the pain. This process is explained on the right side of the image above.
If you have any questions regarding local anesthesia, just let me know. By the way, StudentRDH has a fantastic course that gives you everything you need without the pain of reading the entire textbook. The quizzes also give you immediate feedback so you can learn faster. Taking the WREB or CDCA boards? No problem! There is a mock exam that simulates that exact format for the local anesthesia boards. You know where to find me! At clairej@StudentRDH.com
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