Membrane Potential part 1 and part 2
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61 Terms
What is the resting membrane potential (RMP)
The electrical charge difference across the neuronal membrane when the neuron is at rest, usually around –65 to –70 mV
What creates the resting potential
Uneven ion concentrations across the membrane and selective permeability that stores and controls energy like a battery
What ions are concentrated outside the neuron
Sodium (Na⁺) and chloride (Cl⁻)
What ions are concentrated inside the neuron
Potassium (K⁺) and large negatively charged proteins that cannot cross the membrane
Why is the inside of the neuron negative
More positive charges leave (K⁺) than enter, while negatively charged proteins remain trapped inside
What maintains ion gradients across the membrane
The sodium–potassium pump (Na⁺/K⁺ ATPase)
How does the Na⁺/K⁺ pump work
It uses ATP to pump 3 Na⁺ ions out and 2 K⁺ ions in, maintaining the resting potential and ion gradients
What other pump helps regulate ion balance
The calcium pump, which keeps intracellular calcium very low but does not affect the resting potential
What is the function of calcium in neurons
It acts as a signaling ion, especially during neurotransmitter release at the axon terminal
Why can’t ions cross the membrane freely
The phospholipid bilayer’s hydrophobic interior blocks charged particles, requiring ion channels for movement
What is the structure of the neuronal membrane
An amphipathic phospholipid bilayer with hydrophilic heads facing water and hydrophobic tails facing inward
What are ion channels
Proteins embedded in the membrane that allow selective movement of ions across the lipid bilayer
What are leak channels
Channels that are always open, allowing passive ion movement (especially K⁺) to help maintain the resting potential
What are ligand-gated channels
Channels that open when a chemical messenger or neurotransmitter binds to them
What are voltage-gated channels
Channels that open or close in response to changes in membrane potential
How does the Na⁺/K⁺ pump differ from ion channels
It uses ATP to move ions against their gradients, while channels allow passive ion flow
What are the two main forces driving ion movement
The concentration (chemical) gradient and the electrical gradient
What is the concentration gradient
Ions move from areas of high concentration to low concentration, such as Na⁺ entering the cell
What is the electrical gradient
Opposite charges attract and like charges repel, so the negative interior attracts Na⁺ ions
What is current (I)
The movement of ions across the membrane, measured in amperes
What is conductance (g)
The ease of ion flow through open channels, measured in siemens
What is resistance (R)
Opposition to ion flow, measured in ohms, and inversely related to conductance (g = 1/R)
What is voltage (V)
The electrical potential difference across the membrane, measured in volts (mV in neurons)
What does Ohm’s Law state
I = gV, meaning current equals conductance times voltage
Which ion is the membrane most permeable to at rest
Potassium (K⁺)
How much more permeable is the membrane to K⁺ than Na⁺
About 25–40 times more permeable to K⁺
What is the equilibrium potential (Eion)
Voltage at which there is no net movement
What is EK for potassium
About –80 mV
What is ENa for sodium
About +60 mV
Why is the resting potential closer to EK than ENa
The membrane is much more permeable to K⁺ than to Na⁺
What is the actual membrane potential (Vm)
The measured voltage across the membrane, usually around –65 mV
How do ions determine the direction of movement
They compare the actual Vm to their own equilibrium potential (Eion) and move to bring Vm closer to Eion
What happens to K⁺ at rest
It tends to leave the cell due to the concentration gradient but is pulled back in by the electrical gradient
What happens when chemical and electrical forces balance
There is no net ion movement—this defines the equilibrium potential for that ion
What is the equilibrium potential for K⁺ (Ek)
Around –80 mV, close to but slightly lower than the resting potential
Why is the resting potential slightly higher than EK
A small sodium influx raises the potential slightly above EK
What happens to Na⁺ at rest
Both its chemical and electrical gradients drive it into the cell
How does Na⁺ movement influence Vm
It pushes the membrane potential upward toward ENa, countering K⁺ efflux
What does the resting potential represent
A balance between dominant K⁺ efflux and minor Na⁺ influx
Why is the resting potential a steady state and not equilibrium
Because ion leaks occur continuously and must be counteracted by active pumping using ATP
What analogy describes the Na⁺/K⁺ pump system
Like a leaky boat with a bilge pump—stable but requiring constant energy to maintain balance
How much of a neuron’s ATP maintains resting potential
About 20–25%
How many ions need to move to change voltage
Only a tiny fraction, since voltage depends on surface charge, not total ion concentration
How much does K⁺ concentration change during signaling
Practically zero (about 0.00001%), because only a few ions move
What determines voltage across the membrane
The separation of charge right at the membrane surface, not bulk ion concentrations
What remains electrically neutral in neurons
The bulk cytosol and extracellular fluid
How is resting potential measured
Using microelectrodes—one inside the neuron and one outside as a reference
What does a resting potential recording show
A stable baseline around –65 mV, indicating balanced ion movement
What does a negative resting potential mean
The inside of the neuron is negative relative to the outside
What ensures the neuron is ready to fire
The maintained resting potential establishes the electrochemical energy needed for an action potential
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