Ion Channels
Objective 1
Classify the different categories of ion channels.
In previous units, we’ve examined the wiring and function of the nervous system. Now, in Unit 13, we take a detailed look at the operation of neurons.
How Substances Cross the Neuronal Membrane (Permeability)
Gases can cross the membrane without assistance through simple diffusion. In Unit 6, and later in Unit 20, we discuss a gas, nitric oxide (NO), that does this as it carries a neural signal. In Units 16 and 17, we will see how oxygen and CO2 diffuse during the process of internal and external respiration.
Small, lipid-soluble molecules like steroids (modified cholesterol) can cross the membrane without assistance through simple diffusion. In this unit, we’ll see how neurosteroids do that. In Unit 14, we’ll see how the steroid hormones cross the cell membrane and act on intracellular receptors.
Water gets across the cell membrane without assistance, but that’s a miracle. No one has figured that one out yet. Although water is capable of moving through a membrane by diffusion, most of the water moves due to a protein called aquaporin, as we’ll discuss in Unit 19.
Everything else needs some sort of carrier or channel to get across.
Ion Channels
In order to understand the working of nerve cells, we will focus on the movement of four ions:
- sodium (Na+)
- potassium (K+)
- calcium (Ca2+)
- chloride (Cl–)
Here, we expand on the color code for atoms that we learned about in Unit 2.
In general, ion channels are selective. That means they prefer to pass just one ion on the above list. Some ion channels will pass more than one but they always have a preference for one ion over another.
We’ve already encountered the mechanically-gated ion channel as part of transduction in the somatosensory system, auditory system, and vestibular system. The mechanically-gated channel responds to pressure on the cell membrane by opening and letting sodium ions pass.
The remaining classes of ion channels that we haven’t discussed yet are shown in this diagram, and each will be discussed in detail.
- Leakage channels
- Voltage-gated channels
- Ligand-gated channels
- Signal-gated channels
Leakage Channels
The leakage channel is like a hole in a sieve that is specific for a certain ion. The most prominent kinds are potassium (K+) leakage channels and sodium (Na+) leakage channels. In a typical neuron, there are about a dozen times more potassium leakage channels than sodium leakage channels; we’ll see why this is important a bit later on. Even though they were named “leakage channel” because we thought they were always open, more recent evidence indicates their permeability can be regulated by intracellular messengers. Another name for this type of channel is an ungated channel to distinguish it from the gated channels we’ll look at later.
The concentration of potassium (K+) is almost always higher inside the neuron than outside. If we open a potassium channel, potassium goes from where it’s at high