Physiol 04 nervous system 1

Physiology of Coordination, Dr Saba Butt

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PHYSIOLOGY OF COORDINTION

BS LevelBS Level


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VARIOUS TYPES OF NEURONS FOUND IN ANIMALS


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A NEURON SHOWING DIRECTION OF NERVE IMPULSE


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Effectors Division

CNS Division

Sensory Division

Functional Divisions of Nervous System


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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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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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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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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.


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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

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.


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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.


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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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HYDRATED IONSWater 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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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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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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• 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 --


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Education

Physiol 04 nervous system 1