Source: Comprehensive Study Guide on Neurons, APs, and Synaptic Transmission
Tags: neurons, membrane potential, Vm, resting potential, depolarization, hyperpolarization, repolarization, Na/K pump, sodium-potassium ATPase, ion channels, leak channels, electrochemical gradient, cell membrane, dendrites, axon, cell body, synaptic terminals
Neurons are electrically excitable cells that transmit signals using changes in membrane voltage. The resting membrane potential (around -65 mV) is maintained by the Na+/K+ ATPase pump and selective ion leak channels. Shifts away from or back toward that resting voltage, called depolarization, hyperpolarization, and repolarization, are the basis of all neural signalling.
Neuron
A specialised cell that transmits electrical signals throughout the body. Made up of dendrites, a cell body (soma), an axon, and synaptic terminals.
Membrane potential (Vm)
The difference in electrical charge across the cell membrane, measured relative to the outside of the cell (outside is defined as 0 mV).
Resting membrane potential
The baseline voltage of a neuron when it is not firing, typically around -65 mV. Maintained by the Na/K pump and ion leak channels working together.
Na+/K+ ATPase pump (sodium-potassium pump)
An active transporter that hydrolyses ATP to move 3 Na+ ions out of the cell and 2 K+ ions in. This creates and maintains the chemical gradient across the membrane.
Ion leak channels
Channels that are always open, allowing slow, selective diffusion of ions across the membrane down the electrochemical gradient. These contribute to maintaining the resting potential.
Electrochemical gradient
The combined effect of the concentration gradient (chemical) and the electrical gradient (charge difference) acting on an ion. Determines the direction ions will move when a channel opens.
Depolarization
A shift in membrane potential that makes the inside of the cell more positive (moving toward 0 mV or beyond).
Hyperpolarization
A shift in membrane potential that makes the inside of the cell more negative (moving further from 0 mV, e.g. to -70 mV or -80 mV).
Repolarization
The return of the membrane potential back to its resting value after a depolarization event.
Neurons have four main structural parts:
Dendrites receive incoming signals from other neurons
Cell body (soma) contains the nucleus and integrates incoming signals
Axon carries the electrical signal away from the cell body toward the target
Synaptic terminals (axon terminals) are the endpoints where signals are passed to the next cell
The cell membrane separates the intracellular and extracellular environments, creating a voltage difference
Ions (charged particles like Na+ and K+) cannot freely cross the lipid bilayer on their own; they require transport proteins or channels
The Na+/K+ ATPase pump does most of the heavy lifting:
Uses energy from ATP hydrolysis
Pumps 3 Na+ out for every 2 K+ in
The unequal exchange means a net positive charge leaves the cell each cycle, contributing to the negative resting potential
Ion leak channels provide a passive pathway for ions to trickle across the membrane following the electrochemical gradient
Together, the pump and the leak channels establish and maintain the resting membrane potential at roughly -65 mV
Depolarization occurs when positive ions (typically Na+) flow into the cell, making the interior less negative
Hyperpolarization occurs when positive ions (typically K+) flow out, or negative ions flow in, making the interior more negative than the resting state
Repolarization is the process of returning to -65 mV after the cell has been depolarized
These three shifts are the vocabulary of neural electrical signalling; every signal a neuron sends or receives is built from them
Resting Vm ≈ -65 mV (this value varies slightly by neuron type, but -65 mV is the standard teaching figure)
Na+/K+ ATPase ratio: 3 Na+ out : 2 K+ in per ATP hydrolysed
⚠️ The 3:2 ratio of the Na+/K+ pump is a classic exam question. Remember it is 3 sodium out and 2 potassium in, not the other way round.
⚠️ Ions cannot cross the membrane without channels or transporters. The lipid bilayer is impermeable to charged particles on its own.
⚠️ Depolarization means "more positive inside," not necessarily "positive inside." Moving from -65 mV to -50 mV is depolarization even though the cell is still negative overall.
⚠️ The resting membrane potential is not a static state. It requires continuous energy expenditure (ATP) via the Na+/K+ pump to maintain.
Q: What maintains the resting membrane potential of a neuron?
A: The Na+/K+ ATPase pump (which actively transports 3 Na+ out and 2 K+ in per ATP) and ion leak channels (which allow passive ion flow down the electrochemical gradient). Together they keep the resting potential at approximately -65 mV.
Q: Why can ions not simply diffuse across the cell membrane?
A: The lipid bilayer is hydrophobic and impermeable to charged particles. Ions require specialised transport proteins (pumps for active transport, channels for passive transport) to cross.
Q: What is the difference between depolarization and hyperpolarization?
A: Depolarization makes the inside of the cell more positive (e.g. -65 mV to -40 mV). Hyperpolarization makes the inside more negative (e.g. -65 mV to -80 mV). Both are measured relative to the resting membrane potential.
Q: How does the Na+/K+ ATPase pump contribute to the negative resting potential?
A: It exports 3 positive charges (Na+) for every 2 it imports (K+), resulting in a net loss of one positive charge per cycle. This electrogenic effect helps keep the inside of the cell negative relative to the outside.
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