Test: Neuron Membrane Potentials - 2 - MCAT MCQ

The resting membrane potential of a neuron is primarily determined by the concentration gradient of potassium ions (K⁺), which tends to move out of the neuron, leaving behind negative charge inside the cell.

A resting potential of -60 mV is typically achieved by having more anions (negatively charged ions) inside the neuron compared to the outside and more cations (positively charged ions) outside the neuron compared to the inside. This creates an electrochemical gradient across the cell membrane.

Option C describes a distribution where there are more anions than cations on the outside of the membrane and more cations than anions on the inside of the membrane. This distribution would result in a net negative charge inside the neuron, contributing to a resting potential of -60 mV.

In the resting state, the concentration of potassium ions (K+) is highest inside a neuron compared to other mineral ions. This is due to the active transport of potassium ions by the sodium-potassium pump, which pumps potassium ions into the cell while simultaneously pumping sodium ions out of the cell. This creates a concentration gradient, with a higher concentration of potassium ions inside the neuron.

The resting membrane potential is primarily determined by the permeability of the cell membrane to potassium ions. The high concentration of potassium ions inside the neuron and the selective permeability of the membrane to potassium ions contribute to the negative resting membrane potential, typically around -70 mV.

Sodium ions (Na⁺), calcium ions (Ca²⁺), and chloride ions (Cl^-) also play important roles in neuronal function, but their concentrations are relatively lower inside the neuron compared to potassium ions in the resting state.

In the resting state, the electrical force vector (positive charge) points out of the neuron for chloride ions (Cl^-). Chloride ions have a negative charge, and as a result, they are attracted to the positively charged exterior of the neuron. This creates an electrical force that tends to drive chloride ions out of the neuron.

The concentration of chloride ions inside the neuron is relatively low compared to the extracellular concentration, and the membrane is permeable to chloride ions. Therefore, chloride ions tend to move out of the neuron, driven by the electrical force and concentration gradient.

On the other hand, potassium ions (K⁺), calcium ions (Ca²⁺), and sodium ions (Na⁺) have positive charges and are found in higher concentrations inside the neuron. The electrical force vectors for these ions point into the neuron, attracting them toward the negatively charged interior.

When the sodium-potassium pump transports 2 sodium cations out of the cell for every 3 potassium cations it brings into the cell, there will be an increased intracellular concentration of potassium ions. This higher intracellular concentration creates a larger concentration gradient, resulting in a larger diffusion force for potassium ions moving out of the cell. The diffusion force acts in the same direction as the concentration gradient, which is from the intracellular space to the extracellular space. Therefore, the diffusion force on potassium cations would be larger and in the same direction.

The primary difference between graded potentials and action potentials is their location within the neuron. Graded potentials occur in the dendrites and soma (cell body) of the neuron, where they are generated in response to incoming signals from other neurons or sensory stimuli. Graded potentials can be either depolarizing (excitatory) or hyperpolarizing (inhibitory) and their magnitude varies depending on the strength of the stimulus.

On the other hand, action potentials occur in the axons of the neuron. They are all-or-nothing events that are initiated when a graded potential reaches a certain threshold. Action potentials are characterized by a rapid and brief depolarization followed by repolarization of the neuron membrane. They are responsible for the long-distance transmission of signals along the axon, allowing information to be communicated between different regions of the nervous system.

In the absence of Schwann cells and myelination, the efficiency of nerve impulse conduction would be reduced. Axons with shorter lengths would experience less attenuation of the electrical signal, allowing for better propagation of the nerve impulses along the axon. Shorter axons would result in a smaller distance for the nerve impulses to travel, leading to faster and more reliable transmission of signals within the nervous system.

Therefore, nematodes with shorter axons would have an adaptive advantage in terms of maintaining efficient neural communication in the absence of Schwann cells and myelination.

Voltage-gated sodium channels are found in greatest concentration in the trigger zones of neurons. The trigger zones, which consist of the axon hillock and the initial segment of the axon, are responsible for generating and initiating action potentials. These areas have a high density of voltage-gated sodium channels, which are crucial for the rapid depolarization phase of an action potential.

When a graded potential reaches the trigger zone and reaches the threshold, voltage-gated sodium channels open, allowing an influx of sodium ions into the neuron. This depolarizes the membrane and generates an action potential that propagates along the axon.

While voltage-gated sodium channels are also present in other parts of the neuron, such as the axon terminals and nodes of Ranvier, their concentration is highest at the trigger zones, where the action potential is initiated. This localization ensures the efficient generation and propagation of action potentials along the axon.

Reduced permeability of potassium leak channels would affect the time to reach maximum repolarization in an action potential.

Potassium leak channels contribute to the resting membrane potential of a neuron by allowing a slow leakage of potassium ions out of the cell. This leakage helps maintain the negative charge inside the cell during the resting state. When an action potential is initiated, there is a rapid depolarization phase followed by repolarization, during which the membrane potential returns to its resting state.

During repolarization, potassium channels play a crucial role by opening and allowing the efflux of potassium ions out of the cell, which restores the negative membrane potential. If the permeability of potassium leak channels is reduced, it would result in a slower efflux of potassium ions, leading to a prolonged repolarization phase. This would affect the time it takes for the membrane potential to reach maximum repolarization.

Therefore, the correct answer is C. The time to reach maximum repolarization would be affected by reduced permeability of potassium leak channels.

The speed of action potential propagation is influenced by factors such as axon diameter, myelination, and temperature. The resting membrane potential, although important for neuronal function, does not directly impact the speed of action potential propagation.