B2.1.14 Gated ion channels in neurons (HL) Notes

Voltage-Gated Sodium and Potassium Channels Are Essential Components of Neural Activity

1. Ion channels are specialized proteins embedded in cell membranes, allowing specific ions to pass across the membrane.
2. This process enables ion diffusion along the concentration gradient, from a region of higher to lower concentration, without requiring energy.

Voltage-Gated Sodium and Potassium Channels

1. Nerve Impulses and Ion Movement
   1. Sodium and potassium ions move rapidly across the neuron’s membrane through facilitated diffusion.
   2. These ions pass through voltage-gated channels, which open or close in response to changes in membrane voltage.
2. Voltage Dependency
   1. A negative voltage (below -50 mV) inside the neuron keeps the channels closed.
   2. When the voltage increases to -50 mV or higher, sodium channels open, allowing sodium ions (Na⁺) to diffuse into the neuron.
   3. This influx causes the voltage to rise further, and when it reaches +40 mV, potassium channels open, allowing potassium ions (K⁺) to diffuse out.

How Do Voltage-Gated Channels Work?

1. Conformational Changes
   1. Voltage-gated channels switch between open and closed conformations, allowing or preventing ion passage.
   2. This change involves subunits rearranging to form or close a central pore.
2. Potassium Channel Mechanism
   1. Potassium channels consist of four subunits forming a narrow pore.
   2. A "ball-and-chain" subunit temporarily blocks the pore after potassium ion diffusion, ensuring unidirectional ion flow.

Ion Specificity in Voltage-Gated Channels

1. Selective Passage
   1. Sodium channels permit Na⁺ ions but block K⁺ ions, while potassium channels allow K⁺ ions but block Na⁺.
   2. The narrowest part of the potassium channel measures 0.3 nm, allowing only dehydrated potassium ions to pass.
2. How Potassium Channels Ensure Specificity
   1. Potassium ions are slightly smaller than the channel pore but surrounded by a shell of water molecules.
   2. To pass through, the channel breaks the bonds between potassium ions and water, forming new temporary bonds with amino acids lining the pore.
   3. Sodium ions are too small to form these temporary bonds, making them unable to pass.

Nicotinic Acetylcholine Receptors

1. Nicotinic acetylcholine receptors are proteins in synapses that bind acetylcholine and nicotine.
2. Each receptor consists of five transmembrane subunits with a binding site for acetylcholine.
3. Binding causes a conformational change, opening a pore that allows positively charged ions (e.g., sodium) to enter the postsynaptic neuron.
4. This changes the neuron’s voltage and can activate voltage-gated sodium channels.

Neural Signal Transmission

1. By controlling the flow of sodium ions, these receptors ensure precise neural communication.
2. Their reversible binding allows signals to stop once acetylcholine dissociates, maintaining regulated synaptic transmission.