It’s not too typically that you see handkerchiefs around anymore. Today, they’re largely viewed as unsanitary and well… just plain gross. You’ll be quite disappointed to learn that they have absolutely nothing to do with this article other than a couple of similarities they share when compared to your neocortex. If you were to pull the neocortex from your brain and stretch it out on a table, you most likely wouldn’t be able to see that not only is it roughly the size of a large handkerchief; it also shares the same thickness.
The neocortex, or cortex for short, is Latin for “new rind”, or “new bark”, and represents the most recent evolutionary change to the mammalian brain. It envelopes the “old brain” and has several ridges and valleys (called sulci and gyri) that formed from evolution’s mostly successful attempt to stuff as much cortex as possible into our skulls. It has taken on the duties of processing sensory inputs and storing memories, and rightfully so. draw a one millimeter square on your handkerchief cortex, and it would contain around 100,000 neurons. It has been estimated that the typical human cortex contains some 30 billion total neurons. If we make the conservative guess that each neuron has 1,000 synapses, that would put the total synaptic connections in your cortex at 30 trillion — a number so large that it is literally beyond our ability to comprehend. and apparently enough to store all the memories of a lifetime.
In the theater of your mind, think of a stretched-out handkerchief lying in front of you. Sei tu. It contains everything about you. Every memory that you have is in there. Your best friend’s voice, the smell of your favorite food, the song you heard on the radio this morning, that feeling you get when your kids tell you they love you is all in there. Your cortex, that little insignificant looking handkerchief in front of you, is reading this article at this very moment.
What an fantastic machine; a machine that is made possible with a special type of cell – a cell we call a neuron. In this article, we’re going to explore how a neuron works from an electrical vantage point. That is, how electrical signals move from neuron to neuron and create who we are.
A basic Neuron
Neuron diagram via Enchanted Learning
Despite the fantastic feats a human brain performs, the neuron is comparatively simple when observed by itself. Neurons are living cells, however, and have many of the same complexities as other cells – such as a nucleus, mitochondria, ribosomes, and so on. Each one of these cellular parts could be the subject of an entire book. Its simplicity arises from the basic job it does – which is outputting a voltage when the sum of its inputs reaches a certain threshold, which is roughly 55 mV.
Using the image above, let’s examine the three major components of a neuron.
Soma
The soma is the cell body and contains the nucleus and other components of a typical cell. There are different types of neurons whose differing characteristics come from the soma. Its size can range from 4 to over 100 micrometers.
Dendrites
Dendrites protrude from the soma and act as the inputs of the neuron. A typical neuron will have thousands of dendrites, with each connecting to an axon of another neuron. The connection is called a synapse but is not a physical one. There is a gap between the ends of the dendrite and axon called a synaptic cleft. information is relayed through the gap via neural transmitters, which are chemicals such as dopamine and serotonin.
Axon
Each neuron has only a single axon that extends from the soma, and acts similar to an electrical wire. Each axon will terminate with terminal fibers, forming synapses with as many as 1,000 other neurons. Axons vary in length and can reach a few meters long. The longest axons in the human body run from the bottom of the foot to the spinal cord.
The basic electrical operation of a neuron is to output a voltage spike from its axon when the sum of its input voltages (via its dendrites) crosses a specific threshold. and since axons are connected to dendrites of other neurons, you end up with this vastly complicated neural network.
Since we’re all a bunch of electronic types here, you might be thinking of these ‘voltage spikes’ as a difference of potential. but that’s not how it works. Not in the brain anyway. Let’s take a closer look at how electricity flows from neuron to neuron.
Action Potentials – The communication Protocol of the Brain
The axon is covered in a myelin sheet which acts as an insulator. There are small breaks in the sheet along the length of the axon which are named after its discoverer, called Nodes of Ranvier. It’s important to note that these nodes are ion channels. In the spaces just outside and inside of the axon membrane exists a concentration of potassium and sodium ions. The ion channels will open and close, creating a local difference in the concentration of sodium andioni di potassio.
Diagramma tramite Washington U.
Dovremmo tutti sapere che un ione è un atomo con una carica. In uno stato di riposo, la concentrazione di ioni di sodio / potassio crea una differenza negativa di 70 mV di potenziale tra l’esterno e all’interno della membrana assone, con là è stata una maggiore concentrazione di ioni di sodio all’esterno e una maggiore concentrazione di ioni di potassio all’interno. SOMA creerà un potenziale d’azione quando è raggiunto -55 mV. Quando ciò accade, si aprirà un canale ionico di sodio. Ciò consente agli ioni di sodio positivi dall’esterno della membrana Axon a perdita all’interno, cambiando la concentrazione di ioni di sodio / potassio all’interno dell’asse, che a sua volta cambia la differenza di potenziale da -55 mV a circa +40 mV. Questo processo è noto come depolarizzazione.
Grafico tramite Washington U.
Uno per uno, i canali ioni di sodio si aprono lungo l’intera lunghezza dell’assone. Ognuno si apre solo per un breve periodo, e subito in seguito, i canali ionici di potassio si aprono, consentendo agli ioni di potassio positivi di passare dall’interno della membrana Axon all’esterno. Ciò cambia la concentrazione di ioni di sodio / potassio e porta la differenza di potenziale al suo luogo di riposo di -70 mV in un processo noto come ripolarizzazione. Fro inizia a finire, il processo richiede circa cinque millisecondi da completare. Il processo causa un picco di tensione di 110 mV di guidare la lunghezza dell’intero assone ed è chiamato un potenziale d’azione. Questo picco di tensione finirà nel Soma di un altro neurone. Se quel particolare neurone ha abbastanza di questi picchi, anche lui creerà un potenziale d’azione. Questo è il processo di base del modo in cui i modelli elettrici si propagano in tutta la corteccia.
Il cervello dei mammiferi, in particolare la corteccia, è una macchina incredibile e capace di gran lunga più che anche i nostri computer più potenti. Capire come funziona ci darà una migliore conoscenza della costruzione di macchine intelligenti. E ora che conosci le proprietà elettriche di base di un neurone, sei in una posizione migliore per capire le reti neurali artificiali.
Fonti
Potenziale d’azione nei neuroni, tramite Youtube
Su Intelligence, di Jeff Hawkins, ISDN 978-0805078534
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