Electrochemical Gradients

ions move through ion channels in response to chemical and electrical gradients

Chemical Gradients

Electrolyte Concentrations in Plasma (mEq/L)
Na⁺	136-145
K⁺	3.5-5.0
Ca²⁺	1-2
Cl^-	95-105

with respect to neural communication, the critical ions are Na⁺, K⁺, Ca²⁺ and Cl^-
- the other primary electrolytes are Mg²⁺, hydrogen phosphate (HPO₄^2-) and hydrogen carbonate (HCO₃^-)

extracellular (plasma) electrolyte concentrations are determined by the balance between oral intake and renal excretion, with electrolyte homeostasis being under hormonal control
- disturbances in electrolyte concentrations (as occurs during dehydration or overhydration, for example) can cause severe cardiac and neurologic complications, which can be life-threatening

membrane transporters determine the intracellular concentrations of the various ions, and they vary with cell type
- in neurons, the concentration gradients are generally so large that, under normal conditions, no amount of flow through ion channels will change the intra- or extra-cellular concentrations

Intracellular Ion Concentrations (mM)
	Myocyte	Neuron
Na⁺	10	5-15
K⁺	155	140 mM
Ca²⁺	0.0001 mM	0.0001 mM
Cl^-	10-20	4-30 mM

in considering the neuronal membrane potential, the critical concentration gradients are those of Na⁺, K⁺ and Cl^-
in neurons, the Na⁺ and K⁺ gradients are established by Na⁺/K⁺ ATPase
- Na⁺/K⁺ ATPase is an active transporter that exchanges 3 intracellular Na⁺ for 2 K⁺ extracellular ions, expending 1 ATP molecule in the process of moving the ions from one side of the membrane to the other
- the slow speed of the transport, and the fact that there is only a movement of +1 for each pump cycle means that Na⁺/K⁺ ATPase is NOT responsible for generating the large negative resting membrane potential of neurons

The primary function of neuronal Na⁺/K⁺ ATPase is the establishment of the concentration gradients for Na⁺ and K⁺ that are used to establish the resting, graded and action potentials of neurons and myocytes. The actions of Na⁺/K⁺ ATPase are only mildly electrogenic: the net result of the actions of Na⁺/K⁺ ATPase is a V_m of ~ -5 to -12 mV.

decreasing the function of Na⁺/K⁺ ATPase (e.g., by anoxia, which decreases availability of ATP or by excessive administration of antiarrhythmic drugs that block this transporter) can be fatal
- most critical is the resulting depolarization of cardiac myocytes --> arrhythmias (including asystole) --> death

Electrical Gradients

electrical gradients are relatively simple to understand: cations are attracted by negative potentials and repelled by positive (the opposite is true for anions)
- i.e., a negative V_m will tend to cause cations (esp. Na⁺ and K⁺) to move into the cell

2014 Ion Channel Physiology

Electrochemical Gradients

Chemical Gradients

Electrical Gradients