Module 1 Lesson 7 - 4

Neuromuscular Junctions

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Now you know how messages are communicated from neuron to neuron through the nervous system. However, when you try to talk, how do the muscles in your mouth "get the message" from the neurons?

The tiny gap located between the axon terminal and the muscle cell is called a neuromuscular junction (NMJ). As the action potential travels along the motor neuron, the axon terminal releases the neurotransmitter acetylcholine by fusing the synaptic vesicles to the presynaptic membrane. Acetylcholine is released into the synaptic cleft through exocytosis and interacts with the receptor protein located on the sarcolemma, the muscle cell membrane. The muscle cell membrane depolarizes, and contraction of the muscle fibre occurs. The acetylcholine neurotransmitters are released from the receptor proteins and an enzyme called cholinesterase is released into the synaptic cleft to break down the acetylcholine.

( Note:  The figure in the textbook incorrectly labels the sarcolemma as the neural membrane. In fact, the sarcolemma is a muscle cell membrane.)

Neurotoxins – Historical Uses

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Interruption of neuromuscular transmission can have deadly effects on the body, particularly if the breathing muscles such as the diaphragm are affected.

The native tribes of South America apply a plant extract called curare to the tips of their arrows. Curare affects specifically the neuromuscular junctions by blocking a type of acetylcholine receptor, thus preventing muscle contractions. In high dosage, it results in asphyxiation through paralysis of the diaphragm. Although curare is harmless when ingested, it was a highly effective hunting method for the tribesmen because it transferred to the blood stream from the arrow points.

Botulism is a fatal food poisoning that can be contracted by eating improperly canned (spoiled) foods. Interestingly, the neurotoxin involved in botulism is also the main ingredient in wrinkle reduction injections called Botox.

Botulinum toxin prevents the release of acetylcholine neurotransmitters into the neuromuscular junction. It results in respiratory failure due to paralysis of the respiratory muscles. Initially, botulinum toxins were modified for use with patients suffering from muscle disorders; later, it became popular as the Botox injection to smooth facial wrinkles by preventing muscle contraction in the localized area. Although effective, many adverse reactions, including death, have occurred because the botulinum toxin migrated to other parts of the body.