Overview
Nerve, skeletal muscle and heart muscle are excitable tissues: their cells keep a resting potential and answer a sufficient stimulus with an action potential. This topic covers the resting potential, the excitable tissues, the action potential and the ion channels behind it.
Resting potential
The resting potential is the electrical potential difference across the membrane of an unstimulated cell, with the inside negative: about −70 mV in neurons and −90 mV in skeletal and ventricular muscle cells. It results from the unequal distribution of ions (K+ and organic anions high inside, Na+ and Cl− high outside), the much higher permeability of the resting membrane to K+ than to Na+ (K+ leak channels), and the electrogenic Na+/K+ pump, which moves 3 Na+ out for 2 K+ in. It is close to the potassium equilibrium potential.
The action potential
The action potential - nature, formation, propagation. The Hodgkin-Huxley equations. Ion channels - model structure, effects of toxins and drugs.
An action potential is a short, rapid rise and fall of the membrane potential that follows a fixed pattern. It occurs in excitable cells: neurons, muscle cells and some endocrine cells. In neurons it carries signals along the axon to the synapses, and in muscle cells it is the first step towards contraction.
Action potentials depend on voltage-gated ion channels and run in stages:
- Threshold: a stimulus depolarizes the membrane to its threshold, and voltage-gated sodium channels open.
- Depolarization: sodium flows in, which depolarizes the membrane further and opens more channels. This positive feedback reverses the polarity of the membrane.
- Repolarization: sodium channels inactivate and potassium channels open, so potassium flows out and the potential falls.
- Hyperpolarization: potassium channels close slowly, so the potential briefly drops below the resting level.
During the refractory period the membrane cannot readily fire again, which stops the signal from traveling backward. The sodium-potassium pump restores the ion balance.
Propagation: local currents depolarize the neighboring membrane, so the action potential spreads along the axon. In myelinated axons it jumps between the nodes of Ranvier (saltatory conduction), which is much faster.