VPT 321: Veterinary Neuropharmacology (2+1)

AXONAL CONDUCTION

• This refers to passage of impulse along a nerve fiber. Click to view the animation...
• It is dependent on selective changes in the permeability of the axonal membrane to the electrolytes.
• At rest the membrane potential is approximately - 85 mV in the axons.
• Electro negativity of the resting membrane potential is due to leaking of potassium ions against the concentration gradient from intracellular to extracellular.  
• An action potential reflects a reversal of the polarization state present at rest and is the result of permeability changes that occur at the axonal surface as an impulse is propagated along a nerve fiber.
• A supra threshold stimulus initiates a localized change in the permeability of the axonal membrane. Suddenly, permeability of the fiber to Na⁺ is greatly increased in relation to K⁺.  Na⁺ moves inward in the direction of its large electrochemical gradient.
• The positively charged Na⁺ increases in concentration within the axon, the membrane potential moves from -85 mV towards zero and then overshoots to the extent that momentarily the inside of the fiber is positive in relation to the exterior of the cell.
• Repolarisation of the membrane occurs rapidly as the selective permeability characteristics of the axonal membrane are quickly re-established.
• The axon once again becomes relatively impermeable to Na⁺ and relatively more permeable to K⁺ and the negativity of the interior is quickly re-established.
• Though the changes associated with an action potential are extremely short lived, they elicit similar alterations in membrane function in immediately adjacent quiescent areas of the axon.
• Thus the action potential is self propagating and in this manner an action potential is conducted along the axonal fiber. Click to view the animation...
• The axonal membrane is refractory for a brief interval after the passage of an action potential, thereby preventing antidromic and excess impulse traffic.
• Axonal conduction is insensitive to most drugs.