IB Biology Topic B2.1.14: Gated Ion Channels in Neurons Notes

Voltage-Gated Sodium and Potassium Channels Drive Neural Activity

1. Ion channels are specialized proteins embedded in cell membranes that let specific ions cross.
2. They allow ions to diffuse down the concentration gradient, from high to low concentration, without using energy.
3. Voltage-gated sodium and potassium channels are critical in neurons, where fast ion movement carries nerve impulses.

How Voltage Controls the Channels

1. Nerve impulses and ion movement
   1. Sodium and potassium ions move rapidly across the neuron membrane by facilitated diffusion.
   2. They pass through voltage-gated channels that open or close in response to changes in membrane voltage.
2. Voltage dependency
   1. At rest, the inside of the neuron sits at about -70 mV, which keeps the channels closed.
   2. When the voltage rises to the threshold of about -50 mV, sodium channels open and Na⁺ diffuses into the neuron.
   3. This influx pushes the voltage higher, and near the peak of about +40 mV potassium channels open and K⁺ diffuses out.

How Voltage-Gated Channels Work

1. Conformational changes
   1. Channels switch between open and closed shapes, which allow or block ion passage.
   2. Subunits rearrange to form or close a central pore.
2. Potassium channel mechanism
   1. A potassium channel is built from four subunits forming a narrow pore.
   2. A ball-and-chain subunit briefly blocks the pore after potassium passes, keeping ion flow one-way.

Why Channels Are Selective for One Ion

1. Selective passage
   1. Sodium channels let Na⁺ through but block K⁺.
   2. Potassium channels let K⁺ through but block Na⁺.
   3. The narrowest part of the potassium channel is about 0.3 nm, so only dehydrated potassium ions fit.
2. How potassium channels ensure specificity
   1. In solution, each potassium ion is surrounded by a shell of water molecules.
   2. To pass, the channel strips away the water and forms temporary bonds with amino acids lining the pore.
   3. Sodium ions are too small to form these bonds, so they cannot pass.

Nicotinic Acetylcholine Receptors Are Ligand-Gated Channels

1. Nicotinic acetylcholine receptors are proteins in synapses that bind acetylcholine and nicotine.
2. Each receptor has five transmembrane subunits with a binding site for acetylcholine.
3. Binding causes a conformational change that opens a pore, letting positive ions such as sodium enter the postsynaptic neuron.
4. This changes the neuron voltage and can trigger voltage-gated sodium channels.

How the Receptor Controls Neural Signals

1. By controlling the flow of sodium ions, these receptors ensure precise neural communication.
2. Their reversible binding stops the signal once acetylcholine dissociates, keeping synaptic transmission regulated.