Question

spotlight figure 10.10: neuromuscular junction (nmj)
anatomy of the nmj
which of the following statements is true?
choose the best answer.

• the sarcolemma is the presynaptic membrane.
• acetylcholinesterase (ache) is stored in vesicles within the presynaptic motor neuron.
• the acetylcholine (ach) receptors are located on the myofibril.
• the neurotransmitter is stored in the presynaptic motor neuron.

Explanation:

🆕 New Concept Discovered: Neuromuscular Junction Anatomy
How motor neurons communicate with muscle fibers.

Step 1: Analyze the components of the Neuromuscular Junction (NMJ)

The neuromuscular junction (NMJ) is a specialized synapse where a motor neuron communicates with a skeletal muscle fiber. Let's define the key structures involved:

• Presynaptic element: The axon terminal of the motor neuron, which contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).
• Synaptic cleft: The narrow space between the neuron's membrane and the muscle cell's membrane. It contains the enzyme acetylcholinesterase (AChE), which breaks down ACh.
• Postsynaptic element: The sarcolemma (specifically the motor end plate) of the muscle fiber, which contains ACh receptors.

Step 2: Evaluate each statement

• Statement 1: "The sarcolemma is the presynaptic membrane."
• Evaluation: Incorrect. The sarcolemma is the cell membrane of the muscle fiber, making it the postsynaptic membrane.
• Statement 2: "Acetylcholinesterase (AChE) is stored in vesicles within the presynaptic motor neuron."
• Evaluation: Incorrect. The neurotransmitter acetylcholine (ACh) is stored in presynaptic vesicles, whereas the enzyme acetylcholinesterase (AChE) is found in the synaptic cleft to break down ACh after signaling.
• Statement 3: "The acetylcholine (ACh) receptors are located on the myofibril."
• Evaluation: Incorrect. ACh receptors are transmembrane proteins located on the sarcolemma (specifically at the motor end plate), not on the internal contractile myofibrils.
• Statement 4: "The neurotransmitter is stored in the presynaptic motor neuron."
• Evaluation: Correct. The neurotransmitter acetylcholine (ACh) is synthesized and stored inside synaptic vesicles within the axon terminal of the presynaptic motor neuron until an action potential triggers its release.

Answer:

The neurotransmitter is stored in the presynaptic motor neuron.