Unmyelinated and Myelinated Axons
Ion Channel Proteins
The phospholipid bilayer forms a barrier that limits the passage of ions through the neuron cell membrane and separates the surrounding water environment into two compartments:
- the extracellular fluid (ECF) and
- intracellular fluid (ICF).
- Ion channels are transmembrane proteins that allow small ions to move between the ECF and ICF without binding or using cellular energy.
- The size, shape, and charge of each channel’s pore selectively allows only certain types of ions to pass.
- Some types of channels are functionally open most of the time and continually allow ions to diffuse through the selective pore.
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- K+ leak channels (in nerve and muscle cells) are examples. They allow K+ to flow out of the cell, which plays a major role in establishing resting membrane potentials.
- Other types of membrane channels have gates that usually block the flow of ions unless certain stimuli (chemical, voltage, mechanical, etc) are present.
- When stimulated, the channel gates open and ions diffuse through the selective pore.
- Na+ and K+ voltage-gated channels (also in nerve and muscle cells) are examples. A change in voltage causes these channels to open and the exchange of ions produces an action potential (or impulse).