Question

Consider the axon hillock of a prototypical neuron at rest.

1) detail the open/closed states of the two relevant voltage-gated ion channels while cell is at rest.

The membrane potential reaches threshold.

2) explain what will happen regarding Na+ at this point.

3) state what will happen to the membrane potential at this point.

Answer 1

A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential.

The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.

In a resting neuron, there are concentration gradients across the membrane for Na+ and K+ . Ions move down their gradients via channels, leading to a separation of charge that creates the resting potential.

The membrane is much more permeable to K+ than Na+ , so the resting potential is close to the equilibrium potential of K+ (the potential that would be generated by K+ if it were the only ion in the system).

In a resting neuron, both Na+ and K+ are permeant, or able to cross the membrane.

Na+ will try to drag the membrane potential toward its (positive) equilibrium potential.

K+ will try to drag the membrane potential toward its (negative) equilibrium potential.

Opening and closing ion channels alters the membrane potential

In a neuron, the resting membrane potential is closer to the potassium equilibrium potential than it is to the sodium equilibrium potential. That's because the resting membrane is much more permeable to K+ than to Na+

If more potassium channels were to open up—making it even easier for K+ to cross the cell membrane—the membrane would hyperpolarize, getting even closer to the potassium equilibrium potential.

If, on the other hand, additional sodium channels were to open up—making it easier for Na+ to cross the membrane—the cell membrane would depolarize toward the sodium equilibrium potential.

Changing the number of open ion channels provides a way to control the cell’s membrane potential and a great way to produce electrical signals.

For a cell’s membrane potential, the reference point is the outside of the cell. In most resting neurons, the potential difference across the membrane is about 30 to 90mV ( a MV is 1/1000 of a volt), with the inside of the cell more negative than the outside. That is, neurons have a resting membrane potential (or simply, resting potential) of about −30 mV to −90 mV.

Because there is a potential difference across the cell membrane, the membrane is said to be polarized.

If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.

If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.

The resting membrane potential is determined by the uneven distribution of ions (charged particles) between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions.

Types of ions found in neurons

In neurons and their surrounding fluid, the most abundant ions are:

Positively charged (cations): Sodium (Na+) and potassium (K+)
Negatively charged (anions): Chloride (Cl-) and organic anions.