Somehow, by the grace of God, I’m still a neuroscience major. But even now, I’m still racked with some pretty basic questions about the nervous system. How does your body alert your brain of the outside world? How do electrical impulses in the brain cause movement, thought, and emotion? Honestly, scientists still don’t really know.
But fear not! You don’t pay astronomical tuition fees for there to be nothing to study.
MACRO STUFF YOU NEED TO KNOW
What is the major function of a cell membrane?
The cell membrane envelops any cell and divides its contents from the contents around it. The cell membrane’s main role is to separate charges so that a driving force can build on either side of it for very fast action potential fires. But the cell membrane by itself doesn’t create a membrane potential by itself. It needs membrane proteins that allow for the passage of ions in and out in the form of channels and pumps.
MACHO Proteins
| Channels | K+ leaky channels: always open
Voltage-gated: Na and K, will not open unless the cell membrane reaches a certain threshold |
| Pumps | Na/K: ATP driven, HUGE energy consumer |
| Exocytosis | Neurotransmitters, controlled by Ca2+ |
The Salty Banana
K+ is more concentrated on the inside, Na+ and Ca+2 are more concentrated on the outside. These ion concentrations are maintained by ion pumps in the neuronal membrane.
The above proteins keep the salty banana afloat in the ocean. What I mean is that K is concentrated inside the cell and wants to leave it desperately (efflux) and the salt, Na+Cl-, wants to desperately get in. Ca++ too! But the peel of the banana, the cell membrane prevents these ions from being where their hearts desire.
If these over-eager ions go to where they wish, we have an action potential!
Voltage gating: when a protein channel is open or closed due to the presence of a chemical or a voltage. Eg NT and ion change can be opened and close by changes in the microenvironment around it.
To get to the other side of the cell membrane, which is impermeable to most charged ions, these ions need to travel through protein channels and pumps.
What is the difference between a channel and a pump? Ion channels are selectively permeable to a particular ion. Ion pumps use ATP the energy taken from hydrolysis.
Action Potential
- Fixed size and duration
- Encoded in frequency differences
- “separation of electrical charge across the membrane (capacitance)”
APs – Action Potenials
MICRO STUFF YOU NEED TO KNOW
The Cast of Chemicals
- Water is a polar solvent that brings them close to the cell
- Ion selectivity permeable to a particular ion. Some are voltage-gated by changes in the extracellular membranes
- Ion pumps use ATP to transport certain ion across the membrane
- Diffusion is the passive, random movement of ions from a place of higher concentration to a place of its lower concentration.
- Mirco changes in the electrochemical and concentration gradients in a cell can lead to huge changes in Vm, membrane potential, and cause an action potential to fire
The Movement of Ions
V=IR
Voltage = Volt (V); I=Current (A); R=Resistance (R) (Ω)
Where:
Electrical current (I) is the movement of electrical charges. SI Units: Amperes
Electrical (potential) voltage (V) is the force exerted on a charged particle. SI Units: Voltage
Electrical resistance (R) is the relative inability of an electric charge to migrate. SI Units: Ohms
This beloved equation is a lie. It doesn’t work as well as we would like for biological matters. This is because this equation supposes that when the voltage, or membrane potential, in this case, is equal to 0, current will also be 0 (since resistance is ‘constant’ in this situation). That simply isn’t the case. So long as there’s a driving force between the inside and outside of the cell, there will always be current even if Vm is 0.
So how do we cover this screw up in math?
We make a new equation!
Ig = g(Vm-Eion)
Ig is the current of a cell.
Vm-Eion is the driving force of the ion we are asking of.
Electrical conductance (g) is the relative ability of an electrical charge to migrate from one point to another. SI Units: Siemens (S)
Eion = the equilibrium potential of a particular ion; *these are estimates*
- resting membrane potential = -65mV
- Potassium (Ek)= -80mV
- Sodium (Ena)= +40mV
- Chloride (Ecl) =-65mV,
Everything I just explained, EVERYTHING, can be explained in two useful mathematical equations that I detail here:
The Nernst and Goldman Equations
Summary of The Big Ideas:
I’m asking this in question form so that you can do a little bit of work. See you!
- Why is the neuron having an excitable membrane important for action potentials?
- How does the cell maintain its resting membrane potential?
- How would we describe this action potential mathematically?
Nico Addai 
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