| The Neuronal Processor |
| Sunday, 01 October 2006 | |
Tom Baden shows how we study ions in neuronesThe nervous system consists of billions of inter-connected nerve cells. Over one
hundred years ago, on the basis of morphological studies, Ramon y Cajal reported
nerve cells to be the fundamental individual unit of the nervous system. Indeed,
neurones are the cellular unit of the brain, however, they are not necessarily
the smallest unit of computation. In fact, each nerve cell is a complex information
processor in itself.
A major contributing factor to neuronal processing is neuronal morphology or three-dimensional shape. Inputs are not received at one point only, but are received throughout a complex tree of cellular extensions, or tube-like compartments, called dendrites. Neighbouring neurones communicate through these dendrites by releasing chemical signals called neurotransmitters at specialized sites described as synapses. This results in small local perturbations in the electrical potential of the receiving neurone which can either be excitatory or inhibitory.  Information processing in a typical neurone. Signals from neighbouring neurones (red or green) generate signals or spikes at the spike generating zone of the receiving neurone (yellow). The spikes travel along the axon to the axon terminals where neurotransmitter is released and signals to other neurones (purple). The processing power of a single neurone is exemplified in figure 1. Excitatory input A reaches the neurone at dendrite 1, while excitatory input B is received at dendrite 2. Activating dendrite 1 or 2 alone elicits a small response at the spike generating zone, producing only a few spikes. Coincident excitement at dendrite 1 and 2, however, results in many spikes at the spike generating zone—more than twice as many as before. Further, there can be an input of opposite sign, such as inhibitory input C at dendrite 1. Due to their spatial proximity, input C will inhibit the effect of A. The outcome of input B, on the other hand, will be unaffected. Understanding the processing in neurones requires an understanding of the local processing at dendrites. Neural function and processing is based on the movement of ions along the cell and across the cell membrane. Excitation signals and spikes are generally carried by sodium ions and terminated by potassium ions, while chloride ions often oppose the effects of sodium ions and mediate inhibition. A further important ion, calcium, is critical in the release of neurotransmitters and often acts as a regulator of neuronal excitability. How does one study these ions in neurones? The main method of analyzing the electrical activity of neurones involves inserting sharp, glass microelectrodes through the neurone’s membrane into the cytoplasm. This allows the measurement of local electrical activity within the neurone. This technique, however, carries one distinct disadvantage: it can only record changes in electrical potential at one site at a time. It is therefore extremely difficult to gain insight into how specific parts of a neurone may contribute to its functionality. A different approach that overcomes this limitation is optical imaging. The neurone is injected with fluorescent indicator molecules that reveal the presence of particular ions. The optical properties of these molecules change when bound to their respective ligands. For example, a fluorescent calcium ion indicator, upon binding a free calcium ion within the neurone, may appear brighter, or of a different colour—the neurone will glow when calcium is present. These fluorescent indicator molecules can be used to measure local calcium ion dynamics within different branches of the same neurone simultaneously. However, just like with the microelectrode technique, there is a major drawback. The highly light-sensitive cameras required are slow, often much slower than needed to resolve particularly fast events occurring within neurones. Advances in the field have meant that these two approaches are not mutually exclusive. In several model systems it has recently been possible to maintain a microelectrode within a neurone while simultaneously applying optical imaging techniques. In this way electrical activity within a neurone can be directly related to local ion dynamics.  The cricket Omega-1-Neurone, a model system for neuronal signal integration and processing. The neurone has been microinjected with a fluorescent calcium indicator. In video 1, a single sound was presented as the stimulus. It is evident that calcium ion levels have risen to different degrees in different cellular compartments. For example, particularly large and long-lasting fluorescent changes are evident at the spike-generating zone, while the axon hardly lights up at all. This may be related to the simultaneously recorded spike activity, which temporarily ceases immediately after the acoustic stimulus. Spiking then recovers towards resting levels as calcium ion levels at the spike-generating zone decay. One possible interpretation is that calcium ions at the spike-generating zone control a temporary desensitization of the neurone to further stimulation. Video 2 is rather unscientific, but somewhat entertaining. Tom Baden is a PhD student in the Department of Zoology |
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