Following the discussion of macro-level structure of the nervous system, we now turn to a discussion of micro-level structure. The most numerous cells in the nervous system are not neurons, but glia. Glia serve many functions:

· provide myelin—wrap around axons in patches, leaving nodes in between

· provide structural support-a matrix in which neurons are placed

· serve an immune function

· regulate flow of substances through the BBB—located in blood vessel walls that supply the brain—tightly packed endothelial cells prevent dissolved substances from leaving the blood vessels. Consequence is that necessary substances need to be pumped through the barrier using energy. There are some areas of the brain that need access to blood contents to do their job properly—these areas have a weak blood brain barrier.

· Maintain ion balance

This last function of glial cells is particularly important because the correct ion distribution surrounding the signaling cells of the nervous system (the neurons) is essential for proper neuron functioning.

Ions are charged particles dissolved in the body’s water compartments—extracellularly (between cells) and intracellularly (within cells). There is an uneven charge distribution between the inside and outside of cells—thus the resting neuron is said to be POLARIZED—more positive charges located outside than inside.

Reductions in polarization (making inside more positive, outside more negative) is a DEPOLARIZATION

Increases in polarization (making outside more positive, inside more negative) is a HYPERPOLARIZATION

Normally, ions will move in response to two forces—electrostatic attraction (opposites attract, likes repel) and concentration gradients (move from areas of high concentration to low concentration—the way sugar molecules disperse through a cup of tea with time).

It is movement of ions between inside and outside of cells, made possible by the polarized state of the resting neuron that accomplishes electrical communication.

Communication in the nervous system takes place by electrical and chemical means. Electrical communication takes place within one neuron, from the dendrite/cell body end (axon hillock) to the axon terminal. This type of communication is very fast, and it involves the exchange of ions between the inside of the cell and outside of the cell at a local sites on the axon. The ion exchange takes place repeatedly down the length of the axon, in this sense, the signal "travels" along the length of the axon. We will discuss this process in more detail later.

Chemical communication is the means by which signals get transmitted from one neuron to another-the electrical signal, when it reaches the end of an axon, causes the neuron to release a chemical substance (a neurotransmitter) which signals an adjacent neuron to generate an electrical signal.

In order to understand how cells of the body can create an electrical signal, we need to examine the microstructure of the nervous system more closely.

Neuronal membrane: phospholipid bilayer with embedded proteins. Ion channels, pumps. Ion channels are pores through the membrane—allow the passage of ions—they are specific and can have several states (open, closed, inactive). Pumps use energy to transfer ions from one side to the other.

The distribution of ions surrounding the neuron is highly uneven, with more positive charges outside, and more negative charges inside. Among the ions that are in high concentration outside the cell are Sodium ions (Na+, a positively charged ion). K+ ions are in high concentration inside the cell. There are a variety of reasons that Na is in such high concentration outside, but their distribution is far more uneven that that of K+. In other words, there are more Na+ outside than there are K+ inside. Note that these are both positively charged ions, but due to their uneven distribution, it is possible to observe a charge differential (a polarization) due solely to these two ions. These are not the only ions involved, however. There are large proteins—gene products—synthesized inside the cell that carry a negative charge. They are too large to leave the cell through any of the protein pores, therefore they also contribute to the charge differential between inside and outside. Thus, the state of the neuron when it is not signaling…when it is ‘at rest’, is a polarized state due solely to charge differentials between inside and outside. We’ll talk about why this polarization arises, and what happens when it changes next time.

Note that the signaling cycle is just that—a cycle—electrical events (in presynaptic events) cause chemical events to occur in synaptically adjacent cells (postsynaptic cells), which results in electrical events in subsequent cells. To discuss this, we’ll have to describe one event first (in this case, it will be the electrical events) but it will be vague how this arises in an actual neural circuit (because we will have not yet discussed the chemical event that initiates the electrical event). But we have to break in somewhere…just trust me that if it isn’t clear how the electrical event arises at first, don’t let this interfere with your understanding of the electrical event in general. We’ll get to the initiating event when we discuss chemical communication across the synapse.