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. Eres tú. 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 andiones de potasio.
Diagrama a través de Washington U.
Todos deberíamos saber que un ion es un átomo con una carga. En un estado de reposo, la concentración de iones de sodio / potasio crea una diferencia negativa de 70 mV de potencial entre el exterior y el interior de la membrana axón, con la mayor concentración de iones de sodio fuera y una mayor concentración de iones de potasio dentro. El SOMA creará un potencial de acción cuando se alcanza -55 MV. Cuando esto sucede, se abrirá un canal de iones de sodio. Esto permite que los iones de sodio positivos fuera de la membrana axón para filtrar dentro, cambiando la concentración de iones de sodio / potasio dentro del axón, lo que a su vez cambia la diferencia de potencial de -55 MV a alrededor de +40 MV. Este proceso conocido como despolarización.
Gráfico a través de Washington U.
Uno por uno, canales de iones de sodio abiertos a lo largo de toda la longitud del axón. Cada uno se abre solo por un corto tiempo, y de inmediato después, los canales de iones de potasio se abren, permitiendo que los iones de potasio positivos se muevan desde dentro de la membrana del axón hasta el exterior. Esto cambia la concentración de iones de sodio / potasio y trae la diferencia de posibilidad de nuevo a su lugar de descanso de -70 MV en un proceso conocido como la repolarización. Para empezar a terminar, el proceso toma alrededor de cinco milisegundos para completar. El proceso hace que una espiga de voltaje de 110 mV se monte a la longitud de todo el axón, y se denomina potencial de acción. Esta espiga de voltaje terminará en el soma de otra neurona. Si esa neurona en particular recibe suficiente de estas espigas, también creará un potencial de acción. Este es el proceso básico de cómo se propagan los patrones eléctricos en toda la corteza.
El cerebro de mamífero, específicamente la corteza, es una máquina increíble y es capaz de mucho más que incluso nuestras computadoras más poderosas. Comprender cómo funciona, nos dará una mejor idea de construir máquinas inteligentes. Y ahora que conoce las propiedades eléctricas básicas de una neurona, está en una mejor posición para entender las redes neuronales artificiales.
Fuentes
Potencial de acción en las neuronas, a través de YouTube.
En inteligencia, por Jeff Hawkins, ISDN 978-0805078534