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C2-23 Ion channels; resting membrane potential
"They be like Smooth"....
11
Biology
Professional
11/19/2010

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Term




All bioelectrical signals are......

Definition

-changes in the underlying resting membrane potential (VM)

 

Action potentials (including EEG, EMG, ECG), synaptic  potentials, receptor potentials, field potentials, , nerve conduction velocity


“Typical” VM: ~ -70 mV (neurons), ~90 mV (muscles)

 

Term




Ion channels are transport proteins 

Definition

- Ion channels facilitate diffusion of ionic species, multiple cations 

  (+H2O)


Must consider concentration (chemical) and charge (electrical)

   gradients = electrochemical gradient  


No binding, so very fast (max 108/s)
Term




Typical plasma membrane is highly permeable 

Definition

-to K+  


High density  of “passive” channels allows K+ influx or efflux ,   (direction depends on electrochemical gradient and VM)

  -Membrane is “freely” permeable: PK = 1

  -Little resistance (R) to  K+ flux


Passive “Leak” flux  refers to all sources of resting fluxes, but is mostly K+

Ionic flux is a type of “conductance” (G)

Term




Gated ion channels 

Definition

Open or close due to physical stimulus, depolarization of VM

   presence/absence of a chemical ligand

   -Key role in generating bioelectrical signals


Typically contribute little to VM, but can help maintain it

. Passive ion channels are largely responsible for the resting membrane potential, but gated channels can help to maintain it

Term




Equilibrium potential 

Definition

-(Eion) also called “Nernst potential" 


-maximum potential due to diffusion of a single ionic

  species or of several ions through a single type of ion channel


If negative z or negative log (ratio is a fraction), Eion  will be   negative

 Also called “reversal” potential

Term




“Typical” mammalian Eion  values

 

Definition

EK = -90 mV, because   [K+]inside : [K+]outside = 40 (not 10)

  ENa = +60 mV; ECl = -75 mV; ECa =   +120 mV; Ecation =  0 mV

Actual Eion will slightly differ due to presence of other types of ionic transport  (e.g., Cl-) and co transporters that use ionic  gradients (e.g., Na+)

 

Term




The  VM is “stored” by......

Definition

-the membrane capacitance (CM)


- Membrane is a capacitor (CM): insulator (lipid bilayer) separating charges (anions and  cations) inside and outside


Membrane resistance (RM): density of open channels permeable to ions that are out of equilibrium (i.e., VM  ¹  Eion)
Term




Extracellular K+ concentration is clinically important

Definition

Increasing [K+]out  depolarizes:

  VM seeks more positive EK

  -Results in hyperexcitability


Low  [K+]out hyperpolarizes

  -Results in hypoexcitability”, 

  cause cause paralysis


Small increases in [Na+]out

  have little effect

  -Pump can handle leakage

Term




The Goldman-Hodgkin-Katz equation 

Definition

-describes VM due to multiple ionic permeabilities


    Note inclusion of P values, absence of z values, and reversed   Cl- ratio

Term




Na+/K+ ATPase (“pump”) 

Definition

-prevents VM from running down due to Na+ leakage


- Pumps 3 Na+ out + 2 K+ in per ATP hydrolysis


Pump is electrogenic: it adds ~-3 mV to VM

Maintains osmotic balance

  -More solute extrusion than uptake, H2O follows solutes ...

Term




Resting VM is a steady-state, not equilibrium, potential

Definition

- Energy (ATP hydrolysis) is required to:

  -Transport Na+ against its electrical + chemical gradients  

  -Transport K+ against its chemical gradient


-Pump is needed to maintain the steady state

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