How does the botulinum toxin act in humans?

The botulinum toxin acts in humans by binding to specific receptors on the surface of nerve cells. Once bound, the toxin is internalized and cleaves a protein called SNAP-25, which is essential for the release of neurotransmitters. This blocks the transmission of signals from the nerve cells to the muscles, leading to temporary paralysis.

Specifically, here's how the botulinum toxin works in humans:

Toxins Entry: The botulinum toxin enters the body either through ingestion of contaminated food (foodborne botulism) or through open wounds (wound botulism).

Binding to Nerve Cells: The toxin circulates in the bloodstream and eventually reaches the peripheral nerves. It specifically binds to motor neurons, which are responsible for controlling muscle movements.

Internalization: Once bound to the nerve cells, the toxin is internalized through a process called endocytosis. It enters the cell's cytoplasm where it can exert its effects.

Cleavage of SNAP-25: Inside the nerve cell, the botulinum toxin cleaves a protein called synaptosomal-associated protein 25 (SNAP-25). SNAP-25 is a crucial component of the SNARE complex, which is involved in the release of neurotransmitters from the nerve cells.

Inhibition of Neurotransmitter Release: By cleaving SNAP-25, the botulinum toxin disrupts the normal function of the SNARE complex. This prevents the release of neurotransmitters, such as acetylcholine, which are responsible for stimulating muscle contractions.

Muscle Paralysis: The absence of neurotransmitter release prevents signals from reaching the muscles, resulting in temporary muscle paralysis or weakness. Affected muscles become relaxed and immobile.

Depending on the dose and the site of injection (in the case of cosmetic or medical uses), the botulinum toxin's effects can be localized or widespread, leading to various symptoms and therapeutic applications.