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NERVOUS SYSTEM

PHYSIOLOGY

B.S. Level

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VARIOUS TYPES OF NEURONS FOUND

IN ANIMALS

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STRUCTURE OF A NEURON

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A NEURON SHOWING DIRECTION OF

NERVE IMPULSE

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Effectors Division

CNS Division

Sensory Division

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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.

• These create equilibrium on their both

sides with distribution of permeable and

impermeable ions, which is called as

Donnan equilibrium.

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DONNAN EQULIBRIUM

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RESTING MEMBRANE POTENTIAL

• Three mechanisms contribute to these potentials: The electrogenic pump, Donnan equilibrium (small contribution) and diffusion potentials (most important).

• Diffusion Potentials: There are three important aspects to the electrical contribution of ions to membrane potentials: Ion mobility, Ionic permeability and ionic concentration difference.

• Ion Mobility: Different ions have varied mobility due to different sizes and charge density because of varying hydration layer.

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RESTING MEMBRANE POTENTIAL

CONTD

• 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.

• 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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• 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.