Introduction
Excitability and Excitation: General Characteristics
- Excitability is the ability of a cell, particularly nerve or muscle cells, to respond to a stimulus, typically an electrical one.
- Excitation refers to the process of stimulating the cell, leading to an action potential if the stimulus reaches the threshold level.
- Excitability can be quantified as the inverse of the threshold stimulus intensity
- , where a lower threshold indicates higher excitability.
- Factors Affecting Excitability:
- The difference between the resting membrane potential and the threshold potential significantly influences excitability.
- Variability in threshold potential can be caused by Na+ channel inactivation, increased K+ permeability, or external factors like hypoxia or elevated potassium levels (hyperkalemia).
- Steady depolarization (e.g., at the cathode) tends to decrease excitability by inactivating Na\+channels, a phenomenon known as cathodic depression. Conversely, steady hyperpolarization (e.g., at the anode) increases excitability by reducing Na+ channel inactivation.
- Clinical Relevance:
- Clinically, similar effects to cathodic depression are observed in conditions like hypoxia. Anodic currents are often utilized therapeutically to restore or enhance nerve excitability.
- Action potentials are nerve signals. Neurons generate and conduct these signals along their processes in order to transmit them to the target tissues. Upon stimulation, they will either be stimulated, inhibited, or modulated in some way.
- Propagation of action potential An action potential is generated in the body of the neuron and propagated through its axon. Propagation doesn’t decrease or affect the quality of the action potential in any way, so that the target tissue gets the same impulse no matter how far they are from neuronal body
- The action potential generates at one spot of the cell membrane. It propagates along the membrane with every next part of the membrane being sequentially depolarized. This means that the action potential doesn’t move but rather causes a new action potential of the adjacent segment of the neuronal membrane.
- The action potential always propagates forward, never backwards. This is due to the refractoriness of the parts of the membrane that were already depolarized, so that the only possible direction of propagation is forward. Because of this, an action potential always propagates from the neuronal body, through the axon to the target tissue.