Motor unit: the concept behind the toxin
Quick facts
- Definition
- One motor neuron and all the muscle fibers it commands.
- Components
- Neuron cell body, axon, nerve terminals and muscle fibers.
- Size
- Varies: muscles that make fine movements have few fibers per neuron.
- Relation to the toxin
- The toxin acts at the junction of each terminal with its fiber, reducing contraction.
- Relevance in HOF
- Explains the gradient of effect, the latency and the gradual recovery.
A motor unit is the set formed by one motor neuron and all the muscle fibers it innervates. When the neuron fires, all the fibers in its unit contract together. A muscle is made up of many motor units, and the strength and precision of the movement depend on how many of them are recruited.
What is the motor unit like in facial muscles?
Muscles that make fine movements, such as those of facial expression, tend to have small motor units: each neuron commands few fibers, which allows delicate adjustments. Muscles of great strength, such as those of the leg, have large units. This difference helps explain why facial expression is so subtle and why toxin doses are smaller in the face than in the large muscles of the body.
What does the toxin do to motor units?
Botulinum toxin acts at the neuromuscular junction, the point where each nerve terminal touches the muscle fiber, cleaving the protein SNAP-25 and reducing the release of acetylcholine. As a result, the fibers of that unit are no longer activated with the same intensity. Because the effect happens unit by unit, it is not "all or nothing": contraction decreases gradually, and whether the paralysis is partial or complete depends on the amount and distribution of the product.
This gradual nature lies behind the toxin latency (the interval until the effect appears) and the slow recovery, as the nerve terminal rebuilds its connections.
Why does this matter in practice?
Understanding the motor unit helps explain to the patient why the result is progressive, why neighboring muscles may be affected if the toxin spreads, as seen in toxin diffusion, and why the effect is not the same in everyone. It also justifies the need to assess the dynamics of each muscle, such as the corrugator and the frontalis, before deciding on any approach.
In Harmy
Harmy's projection is an illustrative image and does not simulate muscle contraction or the action of the toxin. The app shows volume changes in the marked areas, not muscle dynamics.
Also called: motor unit.
Frequently asked questions
Why do facial muscles respond to small doses?
Because the muscles of expression have small motor units, with few fibers per neuron, which allows fine movements. This is part of the dosing reasoning, which belongs to the professional.