Inside our heads is the most complex network ever known: 86 billion neuronsconnected by trillions of called connections synapses. THE neurons they are the fundamental cells of the nervous system, they can be considered as the basic units of the brain that coordinate the functioning of the organism. They are highly specialized cells with a specific purpose: receive, process and transmit signals (nerve impulses) throughout the body. They allow the brain to communicate with the muscles, the sense organs to send information and thoughts to form. Discovering how they work, how they communicate and how they organize themselves into real information highways means understanding the very essence of the human brain.
What are neurons and what are they for: the units of the nervous system
To understand this extraordinary network we must start from the basic unit: the neuronthe cell specialized in receiving, processing and transmitting nerve signals. Each neuron is made up of three main regions:
- The somaor cell body, which contains the nucleus and organelles responsible for the metabolic functions of the cell.
- THE dendritesthin branches that receive signals from other neurons.
- THE’axona long extension that transmits outgoing electrical impulses to other nerve, muscle or glandular cells.
At the end of the axon are the synaptic terminalswhich approach but do not touch the dendrites of the following neuron. The space that separates them is the synapsesa microscopic area where the passage of signals from one neuron to another occurs.
How neurons in the brain communicate
When a neuron receives a sufficient stimulus, its membrane generates a transient change in electrical potential: the action potential. The inside of the cell, normally negative (about –70 mV), becomes positive for an instant (depolarization) due to the entry of sodium ions (Na⁺); subsequently it returns negative (repolarization) thanks to the release of potassium ions (K⁺). A brief follows hyperpolarizationwhich prevents the neuron from immediately reactivating and ensures the directionality of the signal along the axon.
In the 1950s, Alan Hodgkin And Andrew Huxley described this phenomenon mathematically, developing a model that explains the behavior of membrane ion channels. Their equations — which won the Nobel Prize in 1963 — are still the basis of modern computational neuroscience.
When the electrical impulse reaches the end of the axon, the vesicles present in the synaptic terminals release neurotransmitters in the synaptic space. These chemical molecules bind to specific receptors on the membrane of the next neuron, triggering a new action potential. It’s a continuous alternation of electricity and chemistry which happens billions of times per second and which allows the brain to process information, generate emotions and control movements.
Thanks to the insulating coating of the myelinmade up of glial cells such as oligodendrocytes, the electrical impulse travels more rapidly, “jumping” from one node to another (i Ranvier nodes) and reaching speeds exceeding 100 meters per second.
How many neurons and connections do we have: from cells to networks
The human brain contains approximately 86 billion neurons, each with thousands of connections. Observed under a microscope, it appears like a intricate network of dendrites, axons and synapses. However, it is not chaos: neurons organize themselves into local circuits and in functional areas specialists, who collaborate in a coordinated manner.
The large structures — cerebral cortex, cerebellum and brainstem — cooperate with each other, and the cortex is divided into frontal, parietal, temporal and occipital lobeseach associated with specific functions such as language, movement, perception or memory.
At depth lie key structures such as the thalamuswhich sorts sensory information, and thehippocampuscrucial for memory and learning.
The streets of the brain: tractography
Brain areas do not work in isolation: they are connected by long bundles of white mattercomposed of myelinated axons, which constitute the vera highways of the brain. Even an apparently simple gesture, such as wearing earphones, involves a chain of signals that cross motor, sensory and visual areas in a few thousandths of a second.
These maps were obtained thanks to tractographyan MRI technique that reconstructs the path of nerve fibers based on the diffusion of water in the tissues.
Despite its limitations, it gives us one of the most fascinating images of the brain by showing us its network of connections.
