VPB 121: Veterinary Physiology - II (2+1)

PROPERTIES

All or nothing principle

• The principle operates in the membrane excitation of the fibers i.e. once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane. This is called the all or nothing principle of membrane excitation. 
• It applies to all normal excitable tissues.  However, when the fiber is in an abnormal state the action potential may reach a point in the membrane at which it does not generate sufficient voltage to stimulate the adjacent resting point of the membrane and therefore the spread of depolarization may stop. 
• But under normal conditions, such stoppage will never occur and the impulse generated stops only when it reaches the fiber end. 
• Therefore, for normal propagation of impulse, the ratio of action potential to the threshold for excitation of the fiber called “Safety factor” at all times be greater than one.

Conduction (Signal transmission) in nerve fibers

• A typical small nerve trunk is constituted by both the large nerve fibers and small nerve fibers. 
• The large fibers are myelinated and the small fibers are unmyelinated. 
• The ratio of distribution of these myelinated and unmyelinated fibers in such a trunk is 1:2 respectively.