Properties of Impulse and stimulus
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.
Classification of Qualitative stimulus
- Chemical stimuli: Strong acid or alkali or any chemical irritant inducing excitability.
- Thermal stimuli: Heat applied to the skin causing the excitability.
- Mechanical stimuli: Pinching, pin prick causing the excitability.
- Osmotic stimuli: Tissue when subjected to hypo or hypertonic solutions will be stimulated.
- Electrical stimuli: Exposed to shock.
Electrical stimulus
The electrical stimulus is more or less similar to the natural stimulus present in the body. Moreover, the strength of the electrical stimulus can be regulated as required. So it is often preferred for conducting experiment. Based on the strength( Quantitative) of the electrical stimulus it is classified as follows-
- Sub-minimal or sub-threshold strength
- Minimal or threshold strength
- Sub-maximal or above threshold strength
- Maximal strength
- Above maximal strength (destructible to the tissues)