As you can see, the image shows a tree 🌳Why a tree? Because technically, all types of paralysis that a person can experience—when we are talking about the motor system—are structured in exactly this way.

There is the corticospinal system, which begins at the top of the human brain — essentially, at the crown of the tree.

This corticospinal system gradually descends downward and transitions into the roots. Along the way, part of this system becomes the upper motor neuron, while in the roots it is predominantly represented by lower motor neurons and motor reflex arcs.

Why does it look like this? 🤔

Because the human body is, in fact, a structure containing a vast number of highly diverse muscles. These muscles can be activated in countless combinations. The crown of the tree — the human brain — is capable of iterating through these combinations and achieving specific postures.

The trunk of the tree mainly represents motor neurons that transmit information along the spinal cord, partially including the corticospinal tract, which forms systems of complex reflexes. The roots, in turn, represent the lower motor neurons.

How does all of this work? ⚙️

The corticospinal system represents a default movement pattern located in the cerebral cortex.

Motor neurons—especially the upper motor neuron—are a combination of systems that transform corticospinal tract signals into specific signals directed at muscles.

The upper motor neuron is able to determine which muscles are activated and where exactly the signal is sent. It can select muscles, but it does not operate in isolation. It is partially connected to consciousness and partially to the corticospinal tract, functioning as a movement control system.

And here’s what is especially interesting 👀 The lower motor neuron is already a concrete “call address” for a muscle. It can activate not just a muscle, but a fragment of a muscle, down to individual muscle fibers, which contract with a specific amplitude and at a specific point in space.

Recovery After Injury 🔄

The structures that recover best are the roots, meaning the lower motor neurons. These are the nerves that run through the body, and they regenerate most effectively after neural injury.

The cerebral cortex recovers much worse. Yes, it has some capacity for compensation, and certain cortical areas can take over damaged functions. However, in most cases, full recovery does not occur, and the quality of movement generalization declines significantly.

Nevertheless, a certain degree of compensation is still possible, since some neurons remain intact.

Cortical injuries include:

neurodegenerative diseases (e.g., multiple sclerosis);
cerebral injuries, cerebral palsy;
traumatic brain injuries;
strokes 🩸.

Why is the prognosis worst in spinal cord injuries? 🚨

Pyramidal neurons and motor neurons are extremely long—up to 1.5 meters—and possess an enormous number of synapses.

When such a neuron is damaged, recovery is often impossible, because the body simply does not “know” how to restore a structure with such an immense number of connections.

As a result, damage to large pyramidal or motor neurons can lead to a complete loss of either postural innervation or muscle innervation. This leads to the formation of plegia, which is, frankly, terrifying.

Plegia is the absence of innervation of a specific muscle.

Where does the core problem of CNS recovery lie? 🧩

Surprisingly, it lies primarily in the cerebral cortex (the crown of the tree 🌳) and even more so in the trunk.

The real “magic” of the cortex lies in its ability to generalize. It can isolate functions with extreme precision—down to individual movement angles, fine adjustments, and muscle fragments. The cortex distinguishes differences between these micro-movements and transmits them into the corticospinal system.

Within the cerebral cortex exists the concept of the motor homunculus.

This makes sense, because the human body is a massive system of muscles operating in three-dimensional space. We rarely realize how complex it is to move an 80-kilogram body in 3D 🤯

All of this is made possible by corticospinal systems, which essentially function as generalization mechanisms—or, in modern terms, vector embedding mechanisms.

Why are spinal injuries the biggest problem? ⚠️

The spinal cord has very limited capacity for compensation. When it is injured, the damage is often complete: motor neurons die, and innervation of muscles, sensation, and postural control disappears.

This leads to the development of:

cerebral paralysis;
spastic paresis patterns;
plegias.

The more complete the spinal injury, the more severe the plegia becomes.

So what actually recovers? 🔧

Out of the three systems, only the lowest one—the root system—recovers fully.

The lower motor neuron is essentially a conductor that carries signals from the upper motor neuron and corticospinal tract directly to muscle fibers. It can isolate muscle elements and innervate them.

The growth rate of a lower motor neuron is about 1 mm per day 📏 This was measured quite literally with rulers. Thanks to this, procedures such as limb transplantation became possible—and yes, technically, the limbs did take root.

In a more expanded form, we’ll discuss all of this in the next article ✨ We’ll talk about n-dimensional space, generalization, grid controllers, and why a healthy human can move so effectively in three-dimensional space—often without any explicit training at all.

HappyBai - eng

On Paralysis 🧠 How It Works. Part 1