5. Physiology of Coordination, Dr
Saba Butt
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Effectors Division
CNS Division
Sensory Division
Functional
Divisions of
Nervous
System
6. Physiology of Coordination, Dr
Saba Butt
6
RESTING MEMBRANE POTENTIAL
RMP is “normal
membrane
potential of an
un-stimulated
cell”.
In most cells, it
is – 90 mV (range
is – 70 to – 90
mV).
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Saba Butt
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MECHANISMS RESPONSIBLE FOR RMP
Essentially there are three (3) mechanisms
contribute in RMP, these are:
• The Electrogenic ion Pump (Na+-K+ pump)
• Donnan Equilibrium
• Diffusion potentials
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Saba Butt
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DONNAN EQUILIBRIUM
• Biological membranes vary greatly in their
permeability to different solutes thus they
are complex semipermeable membranes.
• The semipermeability of biological
membrane has far reaching
consequences.
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Saba Butt
10
DONNAN EQULIBRIUM
This property of the cell membrane and
movement of ions across membrane create
equilibrium on their both sides with
distribution of permeable and impermeable
ions, which is called as Donnan equilibrium.
11. Physiology of Coordination, Dr Saba
Butt
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There are three important aspects to the electrical contribution of
ions to membrane potentials:
oIon mobility
oionic concentration difference
oIonic permeability
Diffusion Potentials
12. 1. Ion Mobility: Different ions have varied mobility
due to different sizes and charge density because of
varying hydration layer.
2. Ionic concentration difference: Inside of the cell
has lower Na+& Cl- concentration and higher K+
concentration. There are similar conc. differences of
other ions across membrane.
Physiology of Coordination, Dr
Saba Butt
12
13. Physiology of Coordination, Dr
Saba Butt
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3. Ion permeability:
– There is selective permeabilities of the membranes to
the different ions.
– The ion channels impart this characteristic. Na+, K+,
Ca2+ & Cl- channels are specific for their respective
ions.
– These channels work variedly for the ions
permeabilities at different times.
– Channels action is understood by their channel
blocking agents e.g. Tetrodotoxin from puffer fish
blocks Na+ channels. Saxitoxin blocks Na+ channel in
dinoflagellate, passed to shell fish.
14. Physiology of Coordination, Dr
Saba Butt
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CONCENTRATION OF IMPORTANT IONS
INSIDE AND OUTSIDE THE CELL
The most important ions are
potassium (K+), chloride
(Cl-), sodium (Na+), and
calcium (Ca++).
These are listed in order of
increasing size of the
hydrated ion.
Most pores are large
enough to pass K+, but few
will carry Na+, and almost
none can transport Ca++.
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Saba Butt
15
HYDRATED IONS
Water molecules are slightly
polarized molecule and are
attracted to other polarized
molecules or to ions.
For an ion to move through the
lipid bilayer, it would have to shed
its associated waters, a process
very expensive energetically.
The waters of hydration vs. the
hydrophobic lipid layers prevents
movement of electrolytes across
the cell membrane. Pores and
gates are necessary.
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Saba Butt
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RELATIVE ION SIZE
But in living system, all hydrated ions are in the same solution. The
larger the unhydrated ion in size, the more dispersed is its own charge
and the less strongly it will attract water. This leads to the incongruous
situation of the larger the ion, the less hydrated it is, and the
smaller the relative size of the hydrated whole. Thus, K+ > Na+, but
hydrated K+ < hydrated Na+.
The cloud of water molecules
that associates with an ion is
somewhat indeterminant in
size, depending on energy
dynamics of the solution.
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Saba Butt
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The above figure shows
the situation generally
found in cell membranes.
The result is that most ion
channels allow K+ to pass
inside the cell due to its
small “Hydrated ion size”
but do not allow Na+ due to
its large “hydrated ion
size”.
There is a relative abundance of pores large enough to
pass K+* but too small for Na+. There is a small population
of chloride pores, and a small number of Na+ and Ca++
pores. The effect of this, of course, is a differential
permiability for these 3 ions across the membrane. Quite
simply, there are very many more entryways for K+
than for sodium ion, and very many more Na+
passages than calcium ion tunnels. The intracellular ion
populations reflect these differences.
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Saba Butt
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• The plasma membranes of RBCs with Hereditary
spherocytosis is 3X more permeable than normal. The
level of Na+, K+ ATPase also remains elevated there.
Consequently anemia is caused in these patients.
• Cardiac glycosides such as digitalis increase heart’s
strength of contraction as these inhibit Na+, K+ pump,
thus increase intracellular Na+ and Ca2+ to cause
forceful contraction.
• Changes in intracellular K+ cause serious cardiac
arrhythmias. Hypokalemia by long term use of diuretics
hyperpolarizes a cells and Hyperkalemia in acute renal
failure causes partial depolarization.
In the case of a disease --
