Passage
In 1906, the Nobel Prize in Physiology or Medicine was awarded to two biologists for their groundbreaking work in the field of neuroanatomy. Despite using the same technique to visualize neurons, the two biologists formed opposing theories regarding nervous system organization.Hypothesis 1: Reticular TheoryBiologist 1 developed a neural staining technique that was carried out by fixing brain tissue in potassium dichromate (K₂Cr₂O₇) before treating it with silver nitrate (AgNO₃) . A silver chromate (Ag₂CrO₄) precipitate deposited within the neurons, making them appear black. The silver impregnation method was revolutionary because of two features. First, only a random subset of neurons within a given tissue sample (~1% of the total cell population) was subject to silver impregnation. Second, the impregnated cells were stained in their entirety, allowing for visualization of the complete neuron. Biologist 1 concluded that his findings supported the idea that the nervous system is a single reticulum of fused neural processes that transmit information via cytoplasmic continuity.Hypothesis 2: Neuron DoctrineBiologist 2 modified the silver impregnation method by first subjecting the brain tissue to two rounds of staining ("double impregnation") rather than one. Second, where Biologist 1 studied the brain tissue of older animals with myelinated nerve fibers, Biologist 2 employed the silver stain on brain tissue from younger animals, which made it easier for the silver to impregnate the neural processes due to relative lack of myelination. Biologist 2 reasoned that the nervous system is not a single web of interconnected fibers but rather is made up of discrete neural units that communicate via close, but not continuous, contact.The neuron doctrine was later proven correct with the advent of electron microscopy, which allowed scientists to visualize the synapse (the point of communication between two distinct neurons) . Further investigation allowed researchers to ultimately discover that there are two types of synapses: electrical and chemical.
-A neuron is exposed to a compound that prevents the passage of Na⁺ ions through voltage-gated Na⁺ channels. Which of the following graphs best represents the local membrane potential change of this neuron upon injection of a positive current that allows the membrane to reach the threshold value? (Note: The solid line represents the action potential after normal stimulation; the dashed line represents the action potential after exposure to the compound.)

Question

Quizplus · Accepted Answer

11ecba2d_0fb7_2c6d_a3bf_f90cb196d89c_MD0008_00 An action potential (AP) is a rapid electrical impulse that travels from the neuron's cell body to its axon terminal. The AP consists of a series of membrane potential changes as follows: Resting membrane potential (RMP): The membrane potential of the resting neuron is −70 mV (ie, the inside of the neuron is 70 mV more negative than the extracellular space). The RMP is maintained by the Na⁺/K⁺ pump and K⁺ "leak" channels, which always allow passive diffusion of K⁺ ions across the membrane. Ion channels that open or close based on changes in membrane potential (voltage-gated channels) are closed at RMP. Threshold: An excitatory stimulus causes several nearby voltage-gated Na⁺ channels to open, allowing Na⁺ ions to rush into the cell. Consequently, the membrane potential becomes more positive than the RMP (ie, depolarized). If the neuron depolarizes to a certain threshold value, an AP is fired. If the threshold is not reached, no AP is fired and the RMP is restored. Rising phase: At threshold, the remaining voltage-gated Na⁺ channels open, resulting in the rapid depolarization of the local membrane. This rise in membrane potential triggers a positive feedback loop, opening more Na⁺ channels in adjacent segments to propagate the AP down the axon. Overshoot: The ongoing flood of Na⁺ into the cell causes the AP to reach its peak (overshoot), where the membrane potential is most positive. Falling phase: Voltage-gated Na⁺ channels close, and voltage-gated K⁺ channels open. K⁺ rushes out of the cell, causing membrane repolarization and restoring the RMP. Undershoot: Excessive K⁺ efflux causes the membrane potential to fall below the RMP (hyperpolarize), and the local membrane enters a refractory period (absolute or relative). RMP restoration: Both voltage-gated Na⁺ and K⁺ channels are inactive. The membrane potential returns to -70 mV and is maintained _by the Na⁺/K⁺ pump until another action potential is initiated. In this scenario, the compound prevents the voltage-gated Na⁺ channels from opening regardless of membrane depolarization. Therefore, even when the threshold is reached the channels cannot open, the rising phase is not initiated, and no AP is generated. After stimulation, the membrane potential simply returns to the RMP (Choice B). The other choices do not display these characteristics (Choices A, C, and D). Educational objective: An action potential is fired in an all-or-none manner based on the cell's membrane potential. If the threshold is reached, voltage-gated ion channels open and the membrane rapidly depolarizes. The resting membrane potential is restored by the Na⁺/K⁺ pump.

Passage In 1906, the Nobel Prize in Physiology or Medicine Was