lecture 3- excitability pt 1 Flashcards
define excitability
the generation & conduction of electrical charge by specialized cells in human body
nerves and impulses are ___ cells, can display property of generating and conducting ___
excitable
electrical impulses
define excitable membranes
electrical impulse generated at surface of membrane of excitable cells and travels along membrane
excitable membranes share 2 electrical properties:
capacitance & conductance
___ is the ability of membrane to separate electrical charge one side from the other (positive charge on one side and neg charge on the other)
capacitance
___ is the ability of membrane to transport or move charge one side to the other
conductance
every nerve impulse is an ___
electrical current
name a few charged ions in solution
Na+
K+
Ca2+
Mg2+
Cl-
HCO3-
Pro-
electrical charge- contributes to electrical nature of membranes
describe the distribution of ions across typical nerve cell in mammalian body
sodium higher outside than inside, charge unequally separated across membrane
potassium higher inside than outside, charge unequally distributed across membrane
chlorine higher outside than inside
individual ions and charges separated
(entire total charge- can only tell if membrane has net charge on one side or the other if you measure it)
describe how you measure charge on a membrane
potentiometer (specific voltmeter)- 2 electrodes, put one on outside and one through membrane on inside of cell
- reference electrode sets its side of the membrane to zero
- recording electrode (sticks through membrane to inside) measures the difference in electrical charge b/w reference and recording electrode (inside & outside of membrane)
the inside of the membrane has a net negative charge relative to the outside
what is it? what is it called?
-70 mV
this charge on the membrane called:
potential difference
TMP (transmembrane potential)
resting potential
when you measure the transmembrane potential, how does it differ between nerve cell types?
it is fairly constant (-70 mV) on all nerve cell types –> so it is called resting potential (RP)- electrical charge on a nerve cell at rest, not active
where does the -70 charge exist in a nerve cell?
only at the inside surface of membrane
- relative pos charge only exists on outside surface of membrane at interstitial fluid
- if a fluid is large enough to be measured, going to be relatively neutral (surface of membrane is its own compartment, inside and outside of cell are neutral)
the membrane has the property of ___, unequal distribution of charge - pos on outside and neg on inside
polarity
excitable membrane generates this -70 mV charge by a combination of…
capacitance and conductance
name 3 factors that govern ion movement
concentration gradient
relative speed due to permeability
electrical gradient
describe scenario #1 with beaker of water divided in half (6 Na+ & 6 Cl- on side a)
concentration gradient determines direction– ions begin moving to side b
- have to measure relative permeabilities (the more permeable ion will move faster)- chloride more permeable than sodium, Cl- will diffuse faster
- at an intermediate stage, side a is more positive & side b is more negative because more Cl- has moved
–> sodium will catch up at completion, diffuse until reach equilibrium, no concentration or charge difference across membrane
IF ACTING BY SIMPLE DIFFUSION ALONE, REACHES EQUILIBRIUM, IONS MOVE BACK & FORTH RANDOMLY
real nerve cells are not at equilibrium, must be something more than simple diffusion…
compare distribution of protein at membrane
proteins concentrated high inside the cell, not outside
at physiological pH, protein has a net negative charge
describe scenario #2 with beaker of water
side a: 6 Na+ 6 proteins-
side b: 6 Na+ 6 Cl-
sodium already at equilibrium- no net movement
- protein is very large so net permeability is virtually zero (protein does not diffuse, stays at side a)
- Cl- has large concentration gradient (6:0), so will diffuse from side b to a
intermediate stages: some Cl- has moved to side a (net negative charge at side a and net positive charge at side b)
- so concentration gradient favors movement of Cl- to side a (however Cl is negative and side a is negative so charge is repelled)
–> interaction of 2 opposing forces- conc gradient pushes Cl- from side b to a BUT electrical gradient pushes Cl from side a to side b
- in many cases, concentration gradient and electrical gradient work in opposite directions
- in many cases, electrical gradient may be strong enough to hold an ion outside of equilibrium with respect to its concentration
so in this situation, electrical gradient holds chloride out of equilibrium (2 on side a & 4 on side b)
- opposite for sodium- electrical gradient pulls sodium outside of equil, but conc gradient strong enough to prevent electrical gradient from pulling Na across
Gibbs- Donnan equilibrium
what is Gibbs-Donnan equilibrium
equilibrium established between two forces (concentration and electrical gradient)
- exists when unequal distribution of large impermeable negatively charged ion- (in almost all cases, that is unequal distribution of protein)
- result of Gibbs-Donnan is unequal distribution of permeable particles across membrane
does Gibbs-Donnan explain resting potential of membrane
no
what 2 forces often work in opposite directions across membranes
concentration and electrical gradient
describe scenario #3 with water beaker but put charge across membrane by putting battery and electrodes in solution
equal amounts of sodium & chloride on both sides, side a is positively charged, side b neg
side a is pos charged, side b is neg charged
- start with equal amounts of chlorine and sodium on both sides…electrical gradient creates concentration gradient
- b/c side b is negative, electrical gradient pulls sodium to side b, concentration gradient then wants to pull sodium back to side a because less Na+ now on side a
- same with chlorine, electrical gradient pulls chlorine to side a b/c side a is positive, conc gradient wants to pull Cl- back to side b b/c less there
- at equilibrium, unequal concentration and charges (but have equilibrium b/w concentration gradient and electrical gradient)- conc gradient pushes ions downhill and electrical gradient works to pull them back
how do concentration and electrical gradient even out?
there is a strength of concentration gradient that will equal out the strength of the electrical gradient and vice versa