vesicle docking 

Synaptic Vesicle Fusion and Calcium's Role

• SNAREs (T and V): Essential for vesicle fusion with the membrane, enabling the release of contents.

• Synaptotagmin: A protein on vesicles that interacts with calcium.

  • Without Calcium: Synaptotagmin prevents T and V SNAREs from interacting.

  • With Calcium: Synaptotagmin binds to calcium, causing a conformational change that allows V and T SNAREs to interact, leading to exocytosis.

Neurotoxins and Their Effects on Exocytosis

• Neurotoxins often interfere with exocytosis, disrupting neurotransmitter release.

• The specific side effects depend on the affected neurons and neurotransmitters.

Tetanus Toxin

• Effect: Causes muscle contraction and inability to relax (lockjaw).

• Mechanism: Acts on inhibitory neurons in the spinal cord.

  • These inhibitory neurons usually release neurotransmitters that prevent motor neurons from signaling muscle contraction.

  • Tetanus toxin blocks the release of these inhibitory neurotransmitters.

  • As a result, motor neurons constantly signal muscles to contract, leading to sustained muscle contraction.

Botulinum Toxin (Botox)

• Effect: Causes muscle weakness or paralysis.

• Mechanism: Acts on motor neurons.

  • Prevents motor neurons from releasing neurotransmitters.

  • Muscles do not receive the signal to contract, resulting in weakness and paralysis.

Key Difference

• Tetanus toxin inhibits inhibitory neurons, leading to overstimulation of motor neurons.

• Botulinum toxin inhibits motor neurons directly, preventing muscle contraction.

• The different effects of neurotoxins depend on which neurons they affect.