The Electrophysiology of Neuronal Membranes: Potential, Flux, and Transmembrane Transport

Definition of Electricity: Electricity is fundamentally defined as the linear movement of electrons over a specified space. In a biological context, the movement of electrons down the axon of a neuron is comparable to the flow of electricity through a copper wire.
Charge of Cellular Spaces: Both the extracellular space (liquid outside the cell) and the intracellular space (liquid inside the cell) possess an overall charge if they contain ions (charged particles).
Charge Variation: Typically, the electrical charge of the extracellular environment differs from that of the intracellular environment.
Mechanism for Electron Movement: To facilitate the travel of electricity (electrons) down a neuron, the biological system must flip the charge on either side of the cellular membrane.

Membrane Potential: This term describes the separation of charge maintained by the plasma membrane on the extracellular and intracellular sides of the cell.
Mechanism of Charge Inversion:
- To make a space more negative, positive ions ( $Na^+$ , $K^+$ , etc.) are removed from that space.
- To make a space more positive, those ions are moved into it.
- Returning to a "normal" state requires moving the positive ions back to their original side of the membrane (e.g., from the inside back to the outside).
Role of Movement: The movement of ions through transmembrane proteins is the primary method for changing the membrane potential.

Ion Channels:
- Provide a pathway for the "free flow" of ions.
- Movement is driven by diffusion (from high to low concentration) via Brownian motion (bumping particles).
- If channels were always wide open ("open sieves"), the cell would lose control over its membrane potential and electrical capacity.
Transmembrane Pumps:
- Provide a more controlled and specific movement of ions compared to channels.
- The sodium potassium adipase pump is a primary example used in generating action potentials.
- Pumps exhibit specific stoichiometry; for example, a pump may possess precisely $3$ binding sites to move ions in one direction and $2$ binding sites to move them in the opposite direction.
协同作用 (Synergy): Creating an action potential in a neuron (as taught in Anatomy $101$ ) requires the combined effort of both ion channels and pumps to switch charges across the axon segments.

Flux ( $F L U X$ ): This term refers to the movement of ions across the membrane.
- Large Flux: Typically occurs through ion channels.
- Modulated/Controlled Flux: Occurs through pumps.
- Influx: Movement of ions into the cell.
- Outflux: Movement of ions out of the cell.
Electrochemical Gradient: This is the response to the mechanism stimulated by a change in membrane potential. It consists of two components:
- Chemical Gradient: Provided by the matter/atoms themselves (e.g., the number of sodium atoms).
- Electrical Gradient: Provided by the charge difference, resulting from atoms having different numbers of protons and electrons.
Physical Attraction: A fundamental principle of physics is applied here: like charges repel and opposite charges attract. To attract positive ions, a transmembrane protein (channel or pump binding site) must have a negative charge.

Stimulus Initiation: A stimulus (likely from the brain or an interneuron in a reflex) acts on the cell body (nucleus).
The Axon Hillock: This is the specific space located between the cell body and the axon. This is where the movement of ions creates the mechanism that generates the electrical response.
Propagation: The electrical conductivity travels down the axon to the axon terminal.
Synaptic Transmission:
- The axon terminal connects to an effector cell (the target tasked with performing an action, such as a muscle cell).
- Electricity cannot "jump" the space (synapse) between the neuron and the muscle cell because the liquid in that space is not a conductor liquid and air is not a conductor.
- Consequently, the electrical signal must be converted back into a chemical signal to cross the synapse.
Resting Potential Specifics: In a typical resting cell (not stimulated), the extracellular fluid is positive while the intracellular fluid is negative. The internal negativity is relative; it contains negatives but primarily has fewer positive ions than the outside.
Cessation: Every process must have a beginning (stimulation), a mechanism, and an ending (cessation). Stopping the signal can occur by:
- Removing the original stimulus.
- Modulating the pump or channel (e.g., a ligand attaching to the protein to signal it to "knock it off").

Protein Conformation: Transmembrane proteins (like the "purple protein") are folded in at least a tertiary structure with a central pore that can open or close based on shape changes.
Gating Types:
- Ligand-Gated (Chemically Gated): Opened by a ligand attaching to the protein. When the ligand detaches, the channel closes. In pumps, the ions themselves begin to behave as ligands for specific receptor sites.
- Voltage-Gated: Opened or closed based on electrical stimulus and changes in the membrane potential surrounding the protein.
- Mechanically Gated: The slowest response type. It is opened by something physically pushing through, facilitated by opposite charge attraction.
Specificity vs. Mass Movement:
- Channels allow for "bulk movement" where negative charges attract a large quantity of positives.
- Pumps are specific down to exact numbers (e.g., $3$ in, $2$ out).

Question: What would a transmembrane protein be called if we want it to be more controlled (not just a free flow from high to low)?
Answer: A pump.
Question: What charge does an ion channel have to have to move positive ions?
Answer: Negative. Without the negative charge, there would be no affinity.
Question: What is the effector cell?
Answer: It is the "thing you want to do something," such as a muscle cell providing a mechanical response.
Question: How is mechanically gated movement different from ligand or voltage gating?
Answer: It involves something mechanically pushing through the opening, often using opposite charges to work its way into the pore. It is generally the slowest of the three gating types.