The Secret to Success: Communication

At the heart of neural circuits are individual nerve cells. They are the brain’s smallest unit and yet much more than mere building blocks ▸ Neurons: Building Blocks of Thought (video). Even their external shape hints at their complexity ▸ The Neuron: Form and Function (interactive). Neurons are extremely polarized, more so than almost any other cell type. With their three iconic components, they are vaguely reminiscent of a tree: a densely branched crown of dendrites surrounding the cell body, a central trunk – the Axon – and numerous further branches in the root system of the synapses. A closer look reveals that the function and interaction of these components are what give the Neuron its special abilities – as a data octopus and a supercomputer. Neurons are specialized in gathering, processing, and distributing information. They are masters of communication. For both the exchange and the processing of information, the “outer qualities” of the nerve cell – namely, subtle differences in chemistry and electricity at the cell surface – play a crucial role.

An astonishing number of communication tools are therefore located directly in the cell membrane. It is much more than a shell that separates individual neurons from their environment. Rather, it houses channels, pumps, and receptors – the tools the nerve cell uses to receive, transmit, and process information.

The interface for the exchange of information is the Synapse ▸ What Connects Nerves. Nerve cells are involved in up to ten thousand of these contact structures. The basic idea is simple: there is a presynaptic part that sends out signals, a synaptic cleft that the signals cross, and a postsynaptic part that receives them. The presynaptic part consists of the terminal buttons of the axons at the root region of a nerve cell, while the postsynaptic part consists of the dendrites and the cell body. Neurons thus interconnect end-to-end and usually make a strict distinction between information intake (via the dendrites and cell body) and information output (via the axon).

In a typical synapse, the Terminal button releases chemical signals (neurotransmitters) that bind to specialized Receptor proteins in the opposing postsynaptic membrane, triggering a reaction there. In addition, there are the less common electrical synapses. Here, the presynaptic and postsynaptic cells are linked via jointly formed ion channels, technically known as gap junctions. Through these, they exchange electrically charged particles (ions).

The primary language of nerve cells is electricity. The recipient cell also immediately converts the information received at a chemical Synapse back into bioelectric signals and transports these via its Axon to its own presynaptic regions. This conversion is carried out by ion channels in the cell membrane ▸ Communication channels: ion channels, which are directly or indirectly coupled to the Neurotransmitter receptors. When a transmitter molecule binds to the receptor, the Ion channel opens (or closes) and – depending on the configuration of the transmitters and channels – allows more (or fewer) positively charged sodium, calcium, or potassium ions, or negatively charged chloride ions, to cross the cell membrane. This alters the electrical charges inside the cell and in the surrounding fluid – and thus also the electrical potential between these two regions: the Membrane potential The resulting change in potential is called the postsynaptic potential.

Depending on which neurotransmitters and which channels are involved at a synapse and how the ions are distributed within the specific neuron, a postsynaptic potential can be excitatory or inhibitory and vary in strength. With its unique qualitative and quantitative signature, each postsynaptic potential functions as a piece of information in the bioelectrical communication of nerve cells.

Since a typical Neuron receives signals from other cells at many thousands of sites on its dendrites, it must integrate this abundance of signals – with their temporal and spatial patterns – into a meaningful overall message – an extremely complex computational task. The result of this computational process influences how the neuron behaves in the next step: Will it send an Action potential to its own presynaptic regions – and thus signals to other cells – or not?

At rest, a negative membrane potential of approximately -70 mV prevails, at which there are fewer positively charged particles inside the cell than outside. If the membrane potential rises above a certain threshold (approximately -55 mV) as a result of the overall assessment of the received signals, additional voltage-gated ion channels open, allowing a sudden influx of positively charged sodium ions into the cell interior. This causes the membrane potential in this region to rise even further. An action potential is generated, which then propagates in a chain reaction along the axon until it reaches the presynaptic terminals and, via the synapses located there, transmits the signal to the next neurons in the neural network. The neuron has “fired.” 

Unlike postsynaptic potentials, action potentials in vertebrates cannot be stronger or weaker. Instead, their informational content lies in whether they occur at all – and how often. The speed at which they reach their targets downstream in the information cascade is also crucial. Here, the finest differences matter – for example, a few microseconds when distinguishing between different tones in the auditory system (see box). 

Action potentials sometimes have to travel enormous distances – axons can grow up to many meters long, depending on the organism. How fast the signals travel depends on the thickness of the axon, or rather, on how well it is electrically insulated ▸ High Speed Thanks to Myelin. In vertebrates, specialized Glial cells called Oligodendrocytes wrap around the Axon and form a myelin sheath, comparable to the insulation around an electrical cable. Only at occasional gaps – known as Nodes of Ranvier – is the axon’s cell membrane exposed. And only here does the sudden ion exchange and the associated change in electrical voltage, which the Action potential needs to continue its journey, occur effectively. Within the electrically insulated segments of the axon, however, the voltage difference is transmitted almost instantaneously by an electric field. This field also propels ions further away within the cell sufficiently to create the conditions at the next node of Ranvier for the voltage-gated ion channels there to open. The action potential thus “jumps” from node to node.