Question

Neuron Anatomy and Resting Membrane

Potential Read the overview and complete the activities that follow.

"A multipolar neuron is composed of a cell body, an axon, and hundreds of dendrites. Each neuron receives numerous inputs at the receptive region, which includes both the dendrites and cell body. Neurons send signals down the axon toward the synaptic knob. Resting neurons require a membrane potential in order to send or receive electrical signals. A typical neuronal resting membrane potential is -70 mV, indicating that the inside of the plasma membrane is more negative relative to the outside. The resting membrane potential is established by the Na+/K+ pump and by the sodium and potassium leakage channels. The leakage channels allow for ions to diffuse down their electrochemical gradients, while the Na+/K+ pump moves ions against their gradients. The Na+/K+ pump establishes the K+ and Na+ gradients, which drive the diffusion of these ions through leakage channels. Although both Na+ and K+ diffusion play a role in establishing the resting membrane potential, K+ diffusion is more important."

Question 1: Normally, Na⁺ and K⁺ leakage channels differ because

A. Na⁺ leakage channels are voltage-gated, while K⁺ leakage channels are ligand-gated.

B. Na⁺ leakage channels require ATP to transport ions, while K⁺ leakage channels do not.

C. Na⁺ leakage channels always remain open, while K⁺ leakage channels must be specifically activated to open.

D. Na⁺ ions diffuse through leakage channels into the cell, while K⁺ ions diffuse through leakage channels out of the cell.

Question 2: A resting membrane potential of -70 mV indicates that the:

A. charges lining the inside of the plasma membrane are negative compared to the charges lining the outside.

B. negative ions are only present within the cytosol and are lacking in the extracellular fluid.

C. positive ions outnumber the negative ions within the cytosol.

D. negative ions will always move into a cell.

Answer 1

1) D. Na+ ion diffuse through leakage channels into the cell, while K+ ions diffuse through leakage channels out of the cell.

In the resting or polarised membrane state, the gradient of Na+ ion is inside the cell, while the diffusion gradient for K+ ion is outside the cell. This is due to high Na+ concentration outside the cell and high K+ inside the cell. So the Na+ ions tend to diffuse into the cell through the leaky channels and the K+ ion diffuses outside the cell to reach the equilibrium. ( But they don't due to difference in the their permeability and constant action Na+K+ ATPase pump)

2) A. Charges lining inside of the plasma membrane are negative compared to the charges lining the outside.

The resting membrane potential is determined mainly by two factors:

1. The differences in ion concentration of the intracellular and extracellular fluids and
2. The relative permeabilities of the plasma membrane to different ion species.

Now a neuron has high concentration of K+ ion and negatively charged proteins and compounds intracellularly. It has high concentration of Na+ ion and Cl- ions in extracellular compartment. But the net negative charge is much more inside the cell as compared to outside. Hence net resting membrane potential is a negative value.

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