BIOL 4503 Lecture Notes - Lecture 17: Sodium Chloride, Astrocyte, Systematic Chaos

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2 Feb 2013
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The Movement of Ions
Ionic movements through channels are influenced by two factors: diffusion and electricity
Diffusion
o Ions and molecules dissolved in water are in constant motion.
o This temperature-dependent, random movement will tend to distribute the ions
evenly throughout the solution
o There will be a net movement of ions from regions of high concentration to
regions of low concentration diffusion
o Diffusion will cause ions to be pushed through channels in the membrane
For example, NaCl is dissolved in fluid on one side of a permeable
membrane
The Na+ and Cl- ions will cross until they are evenly distributed in the
solutions on both sides
The difference in concentration on both sides is called a concentration
gradient
Thus, it is said that ions flow down a concentration gradient
o Driving ions across the membrane by diffusion happens when:
1) The membrane possess channels permeable to the ions, and
2) There is a concentration gradient across the membrane
Electricity
o Another way to induce a net movement of ions in a solution is to use an electrical
field
o Since opposite charges attract and like charges repel, there will be a net
movement of Na+ toward the negative terminal and of Cl- toward the positive
terminal.
o The movement of electrical charge is called electrical current (I) and is
measured in amperes
o Two important factors determine how much current will flow: electrical potential
and electrical conductance
o Electrical potential (voltage V) is the force exerted on a charged particle and it
reflects the difference in charge between the anode and the cathode
More current will flow as this difference is increased
o Electrical conductance (G) is the relative ability of an electrical charge to
migrate from one point to another
Depends on the number of particles available to carry electrical charge
and the ease with which these particles can travel through space
Electrical resistance (Ω) is simply the inverse of conductance and it is
the relative inability of an electrical charge to migrate
There is a simple relationship known as Ohm’s Law and it is I = gV.
If the conductance is zero, no current will flow even when the
potential difference is very large
The Ionic Basis of the Resting Membrane Potential
The membrane potential (Vm) is the voltage across the neuronal membrane at any
moment.
Vm can be measured by inserting a microelectrode into the cytosol
o A typical microelectrode is a thing glass tube with an extremely fine tip that will
penetrate the membrane of a neuron with minimal damage
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