Ion channel · Lobeworks/17

An ion channel is a protein that crosses the cell membrane and opens a water-filled pore through it, letting one kind of ion flow down its gradient at up to millions of ions per second, and it is the part that turns the chemistry of a neuron into electricity.

Ion channel. An ion channel is a protein that crosses the cell membrane and opens a water-filled pore through it, letting one kind of ion flow down its gradient at up to millions of ions per second, and it is the part that turns the chemistry of a neuron into electricity.

The membrane itself is a double layer of lipids whose oily core charged ions cannot cross, so every current in the brain passes through proteins. A channel is folded so that its stretches of hydrophobic amino acids sit inside the lipid layer and its polar ones line the pore, and that lining decides what passes. In the potassium channel whose structure Roderick MacKinnon's group solved in 1998 (KcsA, from a bacterium), the narrowest part, the selectivity filter, is lined by backbone oxygens spaced exactly where the water around a potassium ion would be: potassium sheds its water and fits, while the smaller sodium ion cannot be held as well by the same ring and stays out. The work earned the 2003 Nobel Prize in Chemistry.

A channel is a gate, a pump is a motor. A channel spends no energy and only lets ions run downhill; the sodium-potassium pump spends ATP to push them back up. The pump charges the battery, the channels discharge it.

What opens it names the family. Voltage opens the sodium, potassium and calcium channels of the spike, a transmitter opens ionotropic receptors such as AMPA and GABA-A, and leak channels stay open and set the resting potential.

A channel is the product of a gene, so a mutation can change excitability. Diseases of channels are called channelopathies, and several epilepsies are among them. In Dravet syndrome one copy of SCN1A, the gene of the sodium channel NaV1.1, is lost; that channel matters most in inhibitory interneurons, so losing it weakens the brake and the brain becomes more excitable, even though a sodium channel was lost.

A channel is a pore with a shape, and the shape is the selectivity.

Which ion passes is decided by how the amino acids of the pore fit that ion's size and its water.

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