Neurons’electricalconductivity«HUMOUR PROJECT»

Alunno Davide BrandaliseClasse III A

Liceo scientifico «Galilei Galilei»Prof.ssa Roberta Miscia

The membrane potential

As all living cells, neurons are surrounded by a plasma membrane impermeable to ions. This allows nerve cells to keep a different ionic concentration between the inside and the outside of the cell.

The membrane potential

The plasma membrane is made up of a double fatty hydrophobic layer that makes the membrane impermeable preventing the ion diffusion from one side to another.

The membrane potential

The only way to pass for ions is through specific transmembrane channels. These channels, alternating an opening phase to a closure phase, regulate the selective transport of the ions.

The membrane potentialThe ionic concentration gradient mainly deals with sodium and potassium ions. What’s more, the cell houses a high concentration of proteins with negative electrical charge. The concentration gradient between the inside and the outside is called electric potential difference.

The membrane potentialWhen the neuron is at rest, that is when it is not conducting a nerve impulse, the membrane potential is called resting potential. The neuron inside is negatively charged, whereas the outside is charged positively.

The membrane potentialThe concentration gradient between the two sides is a sort of potential energy, which is measurable in volts. What’s more, it can be measured by inserting an electrode inside the cell. The neuron potential is about -70 millivolts at rest.

The membrane potentialThe concentration of potassium ions is higher outside the cell and this gradient lets these ions be subject to two different forces. The first one, that is the diffusion one, leads the ions alongside their concentration gradient, so from the inside to the outside.

The membrane potentialThe second one, that is the electric one, pushes the ions towards the inside of the cell, balancing the positive and negative charges between the intracellular region to the extracellular one. When these two opposite forces balance out, the ions transiting between the two regions becomes void. As a matter of fact, entering ions are as many as the leaving ones.

The action potentialThe action potential can be divided into five phases: 1. the resting potential;2. threshold;3. the rising phase;4. the falling phase; 5. the recovery phase. 

The action potentialWhen the neuron is at rest, only a small subset of potassium channels are open, permitting potassium ions to enter and exit the cell based on electrochemical forces. There is no movement of potassium ions; for each potassium ion that leaves the cell, another returns, maintaining the membrane potential constant in its value.

The action potentialAs a depolarizing stimulus arrives at the segment of the membrane, a few sodium channels open, permitting sodium ions to enter the neuron. The increase in positive ions inside the cell depolarizes the membrane potential, thus making it less negative and brings it closer to the threshold at which an action potential is generated. 

The action potentialIf the depolarization reaches the threshold potential, additional voltage-gated sodium channels open. As positive sodium ions rush into the cell, the voltage across the membrane rapidly reverses and reaches its most positive value.

The action potentialAt the peak of the action potential, two processes occur simultaneously. Firstly, many of the voltage-gated sodium channels begin to close. Secondly, many more potassium channels open, allowing the positive charges to leave the cell. This causes the membrane potential to begin to shift back towards the resting membrane potential.

The action potentialAs the membrane potential approaches the resting potential, voltage-gated potassium channels are maximally activated and open.

The action potentialThe membrane actually repolarizes beyond the resting membrane voltage. This undershoot occurs because more potassium channels are open at this point than during the membrane’s resting state, allowing more positively charged potassium ions to leave the cell.

The action potentialThe return to a steady state continues as the additional potassium channels that opened during the action potential now close. The membrane potential is now determined by the subset of potassium channels that are normally open during the membrane’s resting state.

GlossaryMembrane potential: an electric charge difference between the inside and the outside of the plasma membranePolarized: a cell is polarized when it owns a membrane potentialIonic channels: ducts that are the only way for ions to cross the membrane potentialVoltage-gated channels: ducts that open or close in response to variations in the membrane potentialAt rest: neurons that are not conducting nerve impulsesConcentration gradient: the number of the chemical molecules sitting in a specific area of the cell