The "All or Nothing Law" pertains to how action potentials are triggered within neurons. It states that an action potential is either generated at a consistent magnitude, or not generated at all, depending on whether the stimulus reaches a threshold value of approximately -50 to -55 millivolts.
When a stimulus is too weak and fails to open sufficient sodium channels, the increase in membrane potential does not reach the threshold, and no action potential is generated.
Regardless of the stimulus's strength, once the threshold is crossed and an action potential is generated, its magnitude remains the same. A stronger stimulus does not result in a stronger action potential, highlighting the binary nature of this law.
Generator Potential
The generator potential is a concept linked to sensory receptors, where a stimulus can excite or inhibit the generation of an action potential.
The potential is considered 'generated' when the stimulus is enough to exceed the threshold value, leading to an action potential.
The frequency of action potentials is influenced by the stimulus's strength; a stronger stimulus results in more frequent action potentials, though their magnitude remains unchanged between weak and strong stimuli.
Myelination and Impulse Transmission
Myelination significantly enhances the rate of impulse transmission across neurons. This process involves the insulation of axons by Schwann cells, creating an insulating barrier that prevents ion movement across the membrane at myelinated segments.
The only places where ions can move and depolarization can occur are at the Nodes of Ranvier, intervals between Schwann cells. This setup facilitates saltatory conduction, where the impulse effectively 'jumps' from node to node, though it is essential to note that the impulse does not literally jump, but the depolarization is localized to these nodes.
This arrangement increases the efficiency of impulse transmission via longer localized circuits compared to continuous depolarization along an unmyelinated axon.
Exam Preparation Tips:
Avoid stating that impulses 'jump' from node to node. Instead, focus on describing the process as depolarization localized to the Nodes of Ranvier.
When discussing the nervous system, prioritize using the term "membrane" over "axon" or "nerve" for precision.