Once a synaptic junction between neurons has allowed transmission of a nerve impulse, it is made ready to transmit next impulse by the action of

Options

A. Secretin
B. Acetylcholine
C. Cholecystokinin
D. Acetylcholinesterase

Answer

To solve the question regarding how a synaptic junction between neurons is made ready to transmit the next impulse, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Synaptic Junction**:
- A synaptic junction is the gap between two neurons where the transmission of nerve impulses occurs. This junction consists of a presynaptic neuron (sending the signal) and a postsynaptic neuron (receiving the signal).

2. **Role of Neurotransmitters**:
- Neurotransmitters are chemical substances that facilitate communication between neurons. When a nerve impulse reaches the presynaptic neuron, neurotransmitters are released into the synaptic cleft.

3. **Transmission of Impulse**:
- The neurotransmitters bind to receptors on the postsynaptic neuron, allowing the impulse to be transmitted. In this case, acetylcholine is a common neurotransmitter involved in this process.

4. **Preparation for Next Impulse**:
- After the transmission of the nerve impulse, it is essential to clear the synaptic cleft of the neurotransmitter to prepare for the next signal. This is done by breaking down the neurotransmitter.

5. **Role of Acetylcholinesterase**:
- The enzyme responsible for breaking down acetylcholine in the synaptic cleft is called acetylcholinesterase. This enzyme hydrolyzes acetylcholine into acetate and choline, effectively terminating the signal and allowing the synapse to reset.

6. **Conclusion**:
- Therefore, the action that makes the synaptic junction ready to transmit the next impulse is the action of acetylcholinesterase, which breaks down the neurotransmitter acetylcholine.

### Final Answer:
The correct answer is **acetylcholinesterase** (Option 4).

---