What is the mechanism by which acetylcholine from the vagus nerve acts to slow the heart?
Acetylcholine decreases permeability of the sinus node to potassium, effectively hypopolarizing it
Acetylcholine closes the sodium channels of the sinus node, effectively hyperpolarizing it
Acetylcholine opens the calcium channels of the cardiac myocytes, effectively delaying repolarization
Acetylcholine closes the sodium channels of the sinus node, effectively hypopolarizing it
Acetylcholine increases permeability to potassium in of the sinus node, effectively hyperpolarizing it
The Correct Answer is E
Parasympathetic innervation of the heart is primarily mediated through the vagus nerve, which releases acetylcholine (ACh) onto the sinoatrial (SA) node. This chemical signal binds to muscarinic M2 receptors, triggering G-protein mediated changes in membrane potential. The resulting negative chronotropic effect slows the heart rate to maintain resting homeostasis and cardiac output efficiency during periods of low activity.
Rationale:
A. Decreasing permeability to potassium would lead to a buildup of positive charge inside the cell, causing depolarization rather than slowing the heart. This would make the cell more excitable and increase the heart rate. Acetylcholine acts to stabilize the membrane, not to make it more prone to reaching the threshold potential quickly.
B. While sodium channels are involved in the initial "funny" current of the pacemaker potential, closing them is not the primary mechanism of vagal hyperpolarization. The main inhibitory effect of acetylcholine relies on the movement of potassium ions out of the cell. Sodium channel modulation is a secondary effect compared to the direct potassium conductance increase.
C. Opening calcium channels would actually increase the rate of depolarization and strengthen muscular contraction. Acetylcholine actually inhibits the L-type calcium current in the nodal tissue to help slow the rate of firing. This choice incorrectly describes the ion flow and the resulting effect on the cardiac cycle timing.
D. Closing sodium channels would not lead to hypopolarization (becoming less negative). Furthermore, the vagus nerve's primary inhibitory action is not centered on simple sodium channel closure. The heart's response to acetylcholine is characterized by a significant membrane shift toward a more negative, stable state, which is the opposite of hypopolarization or depolarization.
E. Acetylcholine increases permeability to potassium in the sinus node by opening specialized GIRK (G-protein coupled inwardly rectifying potassium) channels. As potassium exits the cell, the membrane potential becomes more negative, a state called hyperpolarization. This moves the resting potential further from the threshold, effectively slowing the rate of pacemaker firing and heart rate.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Step 1 is to identify the definition of percentage weight-in-volume (% w/v)
Percentage w/v = (grams of drug ÷ 100 mL of solution)
Step 2 is to convert the given concentration (mg/mL) into grams per milliliter
10 mg ÷ 1,000 = 0.01
Result at this step = 0.01 g/mL
Step 3 is to calculate the amount of drug in grams per 100 mL
(0.01 × 100) = 1
Result at this step = 1 g per 100 mL
Step 4 is to express this as a percentage
1 g per 100 mL = 1%
Result at this step = 1%
Correct Answer is B
Explanation
Non-parametric tests are statistical tools used when data do not meet the assumptions of a normal distribution. These tests do not rely on parameters such as the mean or standard deviation of a population. They are often used for ordinal or nominal data or when the sample size is too small to assume normality. These methods provide a robust way to analyze data that is skewed or contains significant outliers.
Rationale:
A. The t-test is a classic parametric test used to compare the means of two independent groups. It assumes that the data follow a normal distribution and that the variances between the groups are equal. Because it relies on the calculation of a specific population parameter (the mean), it does not qualify as non-parametric. It is highly sensitive to non-normal data.
B. Chi-square test is the correct answer as it is a widely used non-parametric test for categorical data. it evaluates the difference between observed and expected frequencies rather than comparing means. It makes no assumptions about the underlying distribution of the population. This makes it ideal for analyzing proportions and associations between nominal variables in clinical research.
C. ANOVA (Analysis of Variance) is a parametric method used to compare the means of three or more groups. Like the t-test, it assumes normality and homoscedasticity (equal variance) across all groups being tested. It uses the F-statistic to determine if any group mean is significantly different. It is not suitable for data that violates parametric assumptions.
D. The paired t-test is a parametric procedure designed to compare means from the same group at two different times. It relies on the assumption that the differences between the pairs are normally distributed. Since it is a variation of the standard t-test using mean values, it is classified as parametric. It cannot be used for non-normally distributed dependent samples.
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