Neuro 3a – The Resting Potential
Anil Chopra
1. Define - diffusion of an ion
- permeability of a cell membrane
- electrochemical gradient of an ion
Diffusion of an Ion: movement of molecules form a region of higher concentration to an area of lower concentration solely as a result of the molecule’s random thermal motion until the two regions are of a uniform concentration.
Permeability of a Membrane: it is a NUMBER which indicates how easy it is for the ion to cross the membrane. i.e. the flux ( flux= mols crossing surface per second – any unit of time) of the ion through the membrane per unit of concentration gradient.
It is dependent on number of ion channels in membrane.
Electrochemical Gradient of an Ion: the difference in charge across the membrane which prevents a decrease in concentration gradient.
2. Describe how a resting membrane potential can arise from a difference in concentration of an ion across a selectively permeable membrane.
Electrical Concepts:

Electrical forces

move molecules

Opposite charges attract, like charges repel.
 Forces increase with charge quantity.
Membrane Potential Concepts:

Movement of ions not electrons

Potential is the electrical force between the ions, measure in mV.

Current is the flow of ions that influences potential measured in Amps.

Resistance is how hard it is for current to flow through a material, measured in ohms. Lipids have high resistance, salt has low resistance.
The resting membrane potential potential is the potential difference between the outside and inside of the cell.

The zero (0) reference point is always outside the cell

(in resting) the inside is negative

All cells have a membrane potential which is important if the cells are excitable
(neurons & muscle cells).
In order to achieve a membrane potential, the membrane must be selectively permeable (allow some ions through but not others). This membrane separates the charge i.e. the concentration of at least one ion is different on either side of the membrane. e.g. if membrane is permeable to K
+ but not Na
+
K +
K +
Na +

Na
+
ions cannot diffuse across the membrane whereas the K
+
ions can. This means that there are more +ve ions in one side than the other. The membrane therefore has a resting potential.
3. Define the electrochemical equilibrium for an ion.
When the concentration gradient and electrical gradient across a membrane are balanced, the electrochemical equilibrium is reached.
4. What is the equilibrium potential for an ion?
This is the electrical potential (voltage) that prevents diffusion down an ions concentration gradients.
5. The Nernst equation.
E
X+
= (RT/ZF) ln ( C o
/C i
)
R = gas constant F = Faraday’s number, 96,500 coulombs of charge per mol of ion with single charge T = Temp. o Kelvin
Z = charge on ion
-1 for Cl , +2 for Ca +2 ln means log to base e
E
X+
= (61/Z) log
10
( C o
/C i
)
Na
+
conc: in cell: 10mmol/l
K + conc: in cell:150mmol/ in plasma: 150mmol/l in plasma: 5 mol/l
6. Typical values for the concentration for K
+
and Na
+
inside and outside a neuron.
Equilibrium potential of Na
+
= +72mV
Equilibrium potential of K
+
= -90mV
7. Typical value for testing potential of a neuron.
Real membrane potential = -70mV
8. Why does K
+ conc have a stronger effect than Na
+
.
This is because the neuroneal membrane is totally permeable to K + ions but slightly permeable to Na
+ ions. The Na
+
/K
+
exchange pump also works.
Graded potentials are those that:
(1) Can be depolarising or hyperpolarising
(2) Produce a larger potential, the stronger and stimulus is.
(3) Get smaller the further they travel (decremental spread)
(4) Contribute to the initiation or prevention of action potentials at sensory receptors and synapses.