The Cortex Machina project has its origins in a deeply rooted fascination with neurobiology in general, and the biological origins of consciousness in particular. Unfortunately, it can often be very challenging to discuss and share these fascinating concepts, simply because they generally require a lot of prior knowledge which is, to say the least, quite unfamiliar to most people. That’s why we have started this series of short columns: to kindle interest in the fields of neuroscience and neurobiology and to provide my readers with enough general knowledge on the human brain and cognition to start exploring and sharing these fascinating concepts themselves. We will start with a series of articles on brain anatomy, to provide the reader with an understanding of brain architecture and nomenclature, which will be helpful later on when we discuss other topics. So without further ado, let’s enter now into your Neurons!
Nerve cells (= Neurons)
Neurons are electrically excitable cells that form the base unit on which the electrical wiring of the brain is built. You could think about it as the basic electrical component of the brain that takes in signals through input cables (dendrites), performs a basic electrical operation on these incoming signals and sends the result to its output cable (axon). Neurons come in all shapes and sizes, but all consist of three main parts:
- The cell body of the neuron: also called the “Soma”, this is where the nucleus of the neuron cell is located and where most protein synthesis takes place.
- Dendrites: these are the ‘input cables’ (or antenna) of the neuron and consist of cellular extensions of the soma with many branches. Due to its appearance, they are collectively known as the ‘dendritic tree’ of the neuron. An average neuron will have 5 – 7 of dendrites, each with an average length of 2 micrometers.
- At least one axon: this is the ‘output cable’ of the neuron. Typically, a neuron has only one axon, but this axon undergoes extensive branching, enabling communication with many target cells.
Dendrites and axons are very fine threads, typically one micrometer in diameter, and generally reaching lengths of less than a millimeter but sometimes up to a meter long! Since each neuron can have up to 10000 connections with other neurons through extensive branching of the axon, even with such a fine diameter these ‘threads’ end-up forming a significant part of the bulk mass of the brain, recognizable as the ‘white matter’ in the brain (the ‘grey matter’ in the brain consists of the neuron cell bodies themselves).
Dendrites act as antennas, picking up signals from other neurons (sometimes even on the other side of the brain!) and transmitting that signal to the cell body. The cell body determines if the signal is passed along or not. If the electrical excitation induced by the incoming signals passes a certain threshold, the cell body “fires” and a signal is sent along its axon. The axon itself usually consists of a long unbranched and myelinated (the blue “pearls” on the figure below, see also “Glia cells”) fiber stretching out of the cell body, followed by an unmyelinated and extensively branched part. These branches end up in a “synapse”, where the axon ‘connects’ to a dendrite of another neuron.
Figure 1: Neuron, dendrites, axon, and synapse.
A big thanks for your reading from the entire Cortex Machina team!
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