Signaling between Nerve Cells

by Ulrika Kahl, PhD

The human brain is without doubt one of nature's most perfect works. It is built of over a hundred billion nerve cells, which work together to help us do anything from walk, stand, sit, lie down, eat, and do our daily chores, to talk, read, understand, remember and solve complicated problems. Every organ in our body is directly or indirectly connected to the brain, and it is also the target for most medications available.

Nerve Cells

In order for this to work the nerve cells have to cooperate, and for this purpose the nerve cells are equipped with features that make them different from the other, "regular" cells in the body. The nerve cells are capable of transferring signals between each other. While other cells normally are round or oval, and relatively symmetrical in shape, typical nerve cells have a quite special appearance (see schematic drawing below).

Figure 1 Schematic drawing of a nerve cell with the cell body with dendrites, the axon, and the cell terminal (not to scale). Larger image

The Cell Body

In the cell body, with its characteristic extending so-called dendrites, there are the nucleus and other so called organelles. Organelles are essential, functional components of the cell. In these, proteins, carbohydrates, fibers, fats and other so called macromolecules, which are necessary for the structure and function of the cell, are synthesized. The energy maintenance and various metabolic processes also take place here. In the nucleus there are nucleic acids (DNA and RNA), or more commonly: the genome.

The Cell Terminal

In the cell terminal, some smaller molecules are made, which in one way or the other are involved in the signaling process. The terminal is also the part that is in contact with other nerve cells. We will return to this later. You can also read more about the terminal under "What Happens at the Postsynapse?".

The Axon

Like a connection between the cell body and the terminal there is the axon. This looks like a narrow string and gives the nerve cell its characteristic shape. Through the axon molecules are normally transported from, but sometimes also to, the cell body by means of a complex transport molecule machinery.
The axon is constructed in order to function optimally as a conductor in the signaling process. In the surface - or cell membrane - there are different types of channels, which all work by letting charged particles, so called ions, through. These ions are either positively or negatively charged, and every channel is selective for one, or sometimes several, types of ions. This way the axon acts as an electric circuit. The signal, which is transported along the axon as a result from the passage of ions back and forth across the cell membrane, is necessary to stimulate various processes in the terminal, where the actual signal transmission step to the next nerve cell takes place.

The Synapse

The area where the terminal is in contact with other cells is called the synapse. The side of the terminal from where the signal comes is called the presynapse, whereas the receiving side is called the postsynapse. The receiving side can either be a cell body, another terminal, or some other part of a cell, but we will not get into this here.

There are in principal two types of synapses – electrical and chemical. In the electrical synapses the cells are in direct connection with each other and the signal transmission is due to the movement of charged ions from one cell to the other through pores, which are built up by the two cells together. We have decided not to deal with the electrical synapses here, but will focus on the chemical ones.

Chemical Synapses and Neurotransmitters

At the chemical synapses the signaling happens in steps.
First the presynapse, or terminal, releases something that is called "neurotransmitters". Neurotransmitters are molecules, which can be of many different kinds.
Some are relatively small, and these are often called classical transmitters.
Others are larger and are composed of a number of units. These larger molecules are the neuropeptides, which in a very simple way can be described as small proteins.
In any case the transmitter molecules are transported across the synaptic cleft to the postsynapse.

Receptors

A the postsynapse there are receiver stations, or receptors. Receptors are proteins, which are inserted into the membrane. Each type of receptor is designed to catch a certain kind of transmitters, and when a transmitter finds its receptor it binds to this like a key in a lock.

Once the transmitter is bound to its receptor a number of events can take place. The actual contact between the two acts as a signal to the receiver cell. This signal will, depending on what transmitter and receptor are involved, activate different biochemical processes in the receiver cell. Here we could get into very complicated discussions, but for the sake of simplicity, we will stay away from that. Instead we will just briefly mention that the processes that are activated aim to affect the receiver cell machinery, so that the cell can forward the signal.
You can read a somewhat more detailed text about this under "What Happens at the Postsynapse?".

Effects of Signal Transmission

The signals that move from cell to cell in the brain will on their way exert a number of effects. What happens exactly depends on what transmitter and receptors involved, and what processes are activated along the way. There are also many types of brain cells, and the connections that are seen between the various parts of the brain constitute an enormously complex system, which still to date is far from completely investigated.

If you want to read more about neurotransmitters or the brain in general, click on any of the links below

Neurotransmitters - The Messengers of the Brain
What Happens at the Postsynapse?
The Human Nervous System
The Neuron