The Neuronal Membrane is Selectively Permeable
Objective 1: Explain which substances can cross the neuronal cell membrane by simple diffusion, and which need channel or pump proteins.
How Substances Cross the Neuronal Membrane (Permeability)
Gases can cross the membrane without assistance through simple diffusion. We’ll see a gas, nitric oxide (NO), that does this as it carries a neural signal.
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.
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–)
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 higher concentration to where it’s at lower concentration: it goes from inside to outside the neuron.
The concentration of sodium (Na+) is almost always higher outside the neuron than inside. We’ve already seen, in Unit 3 and Unit 4 and in the clinic, that 0.9% (0.15 M) NaCl is a good blood substitute. If we open a sodium channel, sodium goes from where it’s at higher concentration to where it’s at lower concentration: it goes