Neurology | Resting Membrane, Graded, Action Potentials

Main Ideas

1. Resting Membrane Potential

The resting membrane potential is the voltage difference across a cell membrane when the cell is at rest. Although the lecture focuses on neurons, all cells have a resting membrane potential. A neuron’s resting potential is roughly −70 to −90 mV, with −70 mV presented as a common average.

Three factors help establish it:

• Sodium–potassium ATPase pumps move three sodium ions out of the cell for every two potassium ions moved in. This makes the inside slightly more negative and maintains the sodium and potassium concentration gradients.
• Leaky potassium channels allow potassium to move out of the cell down its concentration gradient. Negatively charged molecules, such as proteins and phosphates, remain inside, making the interior more negative.
• Leaky sodium channels allow some sodium to enter, making the interior less negative. However, the membrane is much more permeable to potassium than to sodium, so potassium’s effect predominates.

The Nernst potential is the equilibrium voltage for an individual ion: the point at which movement down the ion’s concentration gradient is balanced by the electrical force pulling it in the opposite direction. The lecture gives the equation:

[ E = \frac{61.5}{z} \times \log_{10}\left(\frac{\text{outside concentration}}{\text{inside concentration}}\right) ]

It illustrates approximate equilibrium potentials of −90 mV for potassium and +70 mV for sodium. The resting voltage depends strongly on the membrane’s relative permeability to each ion.

2. Graded Potentials: Excitation and Inhibition

Graded potentials are local changes in membrane voltage that can move a neuron toward or away from the threshold for an action potential.

• Excitatory postsynaptic potentials (EPSPs) depolarize the postsynaptic cell, usually by allowing positive ions such as sodium or calcium to enter. The lecture gives glutamate as an example of an excitatory neurotransmitter that opens a ligand-gated ion channel.
• Inhibitory postsynaptic potentials (IPSPs) hyperpolarize the cell, moving its voltage farther from threshold. The lecture gives GABA as an example; inhibition can result from chloride entering or potassium leaving the cell.

Graded signals can combine through summation:

• Temporal summation: A presynaptic neuron repeatedly stimulates a postsynaptic neuron, causing successive EPSPs to add together.
• Spatial summation: Several presynaptic neurons stimulate one postsynaptic neuron at the same time, causing their effects to add together.

EPSPs and IPSPs can counteract one another. If the combined input brings the membrane to threshold—approximately −55 mV—an action potential can begin. If threshold is not reached, no action potential occurs.

3. Action Potentials

The axon hillock, also called the trigger zone in the lecture, is where action potentials are initiated. It has a high concentration of voltage-gated sodium channels.

Once threshold is reached:

1. Voltage-gated sodium channels open, allowing sodium to rush into the cell and rapidly depolarize the membrane.
2. The membrane voltage rises from threshold toward approximately +30 mV. At the peak, sodium-channel inactivation gates close, stopping further sodium entry.
3. Voltage-gated potassium channels open, allowing potassium to leave the cell and repolarize the membrane.
4. Potassium channels close relatively slowly, so the membrane may briefly become more negative than its resting voltage. This is called an undershoot or hyperpolarization.
5. Sodium–potassium pumps and leak channels help the membrane return to its resting state.

Depolarization triggers activity in neighboring sections of the axon, allowing the action potential to travel toward the axon terminal.

At the terminal, depolarization opens voltage-gated calcium channels. Calcium entry promotes vesicle fusion with the terminal membrane, involving SNARE proteins, and the release of neurotransmitters into the synaptic space. The neurotransmitters can then bind to receptors on another cell.

4. Refractory Periods

• During the absolute refractory period, sodium channels are inactivated, and the neuron cannot initiate another action potential regardless of stimulus strength.
• During the relative refractory period, which occurs during the hyperpolarized recovery phase, another action potential is possible but requires a stronger-than-usual stimulus.

The lecture connects these periods to the recovery of sodium-channel gates and the membrane’s return toward its resting potential.

Speakers and Sources Featured

• Professor Zach Murphy — instructor and presenter.
• Ninja Nerd — the educational channel hosting the lecture.
• No other speakers are clearly identified in the subtitles.