The Neuron: How a Thought Is Born
The neuron is the structural and functional unit of the nervous system. That’s how Wikipedia defines it. In general, all basic cells are similar in structure: a membrane, cytoplasm, and a nucleus with a nucleolus. Kind of like an amoeba, minus a few details. By the way, the neuron’s membrane is made of lipids (essentially, fat). The neuron receives its nutrients through this membrane, which allows fat-soluble substances like oxygen and glucose to pass through. Naturally, the neuron’s membrane is constantly being renewed, just like everything else in the body. So, for those who love low-fat diets, think twice—your slimmed-down brain might be left without its “sweets.”
There are different types of neurons: anaxonic, unipolar, pseudounipolar, bipolar, and multipolar neurons. They are distinguished by their structure and function, except for the anaxonic neuron. It just sits in the spinal cord, looking sad and doing nothing—or maybe it does something, but no one cares. Maybe that’s why it’s sad. 🙂 We’re not interested in that one, though. What matters to us is the multipolar neuron, which is the structural unit of the cerebral cortex.
Our neuron is different from the others in many ways, but what we need to focus on are its processes. Our neuron has one axon and many branched dendrites.
Dendrites: Building the Neural Network
In our context, dendrites are the bullseye on our target. They’re what we need to grow and develop. Dendrites form our neural network by connecting with other neurons. As a dendrite (or an axon, for that matter) grows, a small swelling appears at its tip, called a “growth cone.” This cone is not static—it’s always moving, like a team of workers constantly laying new bricks. The building materials are transported to the growth cone in membrane vesicles along the neuron’s cytoskeleton microtubules, which are made of the protein tubulin. This growth happens at about one millimeter per day. It may seem slow, but in the scale of a neuron, it’s not that bad. So, what does a dendrite do? Why is it important?
A dendrite receives signals from the axon of a neighboring neuron through a synaptic gap. But let’s get to that in a moment. First, let’s talk about the axon.
The Axon: Transmitting the Signal
The axon is another process of the neuron, and unlike dendrites, there’s only one. Like dendrites, it has a tubular structure. At its base, near the neuron’s body, there’s an axon hillock, which is also the neuron’s trigger zone (the area of highest excitability). The axon is covered with a myelin sheath (dendrites don’t have this). At its end, the axon branches into nerve terminals (endings). These terminals connect the axon to the dendrites of neighboring neurons. Sometimes, the axon connects to the bodies of other neurons or even to other axons, forming axo-somatic and axo-axonal synapses. The latter are involved in inhibition processes.
The main purpose of the axon is to transmit nerve impulses from the neuron’s body to the dendrites of neighboring neurons. The axon also transports neurotransmitters (like dopamine, adrenaline, serotonin, etc.), which it uses to affect the dendrites of other neurons, along with a whole host of other biomolecules that we won’t get into here.
The Synapse: Where Neurons Connect
Next comes the synapse. In reality, neurons don’t touch each other directly—they interact through the synapse. The synapse, or synaptic gap, is the junction between dendrites and an axon. They communicate using neurotransmitters (hormones) that are released into the synaptic gap from the axon’s terminals. After crossing the synapse, neurotransmitters reach the receptor zone of the neighboring neuron’s dendrites. The dendrite’s receptor zone is selective—each neurotransmitter has its own specific receptor.
How Is a Thought Born?
Hopefully, you remember from school physics that electric current is the directed movement of charged particles. In a neuron, these charged particles are ions of potassium, sodium, chlorine, and so on. I forgot to mention that on top of the neuron’s lipid membrane is a layer of proteins that forms potassium and sodium channels leading into the neuron. When at rest, these channels are closed. Positively charged ions are outside the neuron, and negatively charged ions are inside. This creates a voltage difference across the neuron’s membrane, known as the resting potential.
When neurotransmitters enter the neuron through dendrite receptors, they trigger chemical changes that open the ion channels, allowing positively charged ions to flow into the neuron. This process is called depolarization and is accompanied by a change in voltage, resulting in a discharge. This discharge is called the action potential or nerve impulse. This is, in essence, a “fragment of thought.” The potassium-sodium balance in the cell is then restored by potassium-sodium pumps (specialized proteins in the neuron), and the nerve impulse travels further along the axon to other neurons, changing and multiplying as it goes. That’s how it all works, dear readers. I hope you found it interesting—or at least understandable.
Normal mental activity is ensured not only by excitation processes but also by inhibition processes.