There is a potential difference across the membranes of most
cells, with the inside of the cell negative relative to the outside of the
cell. Called resting membrane potential. Ranges from -9 to -100mV.
Distribution of ions across the membrane and membrane
permeability are responsible for the potential difference across the
membrane. This charge difference is only in the fluid close to the
membrane.
Distribution of some ions in ICF and ECF
Ions
ICF
ECF
K+
400
10
Na+
50
460
Cl-
40-100
540
Polyanions-
345
____
Ionic conc. in squid axon and ECF, (Lowey and Siekevitz)
Distribution is the result of a diffusion gradient and an
electrical gradient for each ion.
Following its concentration gradient, K+
will diffuse out through leakage channels. But the electrical gradient
runs in the opposite direction causing K+ to move in. The two
forces reach an equilibrium with slightly more negative charges inside than
outside.
Following its concentration gradient, Na+
will diffuse in through leakage channels. The electrical gradient runs
in the same direction also causing Na+ to move in. There is
no gradient to force Na+ out. Cl- follows
Na+ but is repelled by the cell's negative charge.
Membrane permeability for K+ is greater than
for Na+, therefore K+ diffuses out faster than
Na+ diffuses in.
Polyanions are too large to leave the neuron in any
quantity.
Na+-K+ pumps maintain the
concentration gradients. Active transport process pumps 3Na out for every
2K transported in.
Figure in class:
Resting membrane potential - a polarization or electrical
difference between the inside and outside of inactive cells caused by
concentration differences of certain ions and selective membrane permeability.
Graded potential - small deviation from the resting membrane
potential caused by a stimulus.
Localized, no refractory periods
Varies in amplitude and is directly related to the
number of voltage-gated Na channels open
Hyperpolarization - more negative than resting
potential
Depolarization - less negative than resting potential