The correct answer is: O Sending cell nucleus.
Explanation:
1. The three parts of a synapse are:
- Pre-synaptic membrane of the sending neuron: This is the membrane of the neuron that releases neurotransmitters into the synaptic cleft.
- Synaptic cleft: This is the small gap between the pre-synaptic membrane and the post-synaptic membrane where neurotransmitters travel across.
- Pre-synaptic membrane of the receiving neuron: This is the membrane of the neuron that receives the neurotransmitters released by the sending neuron.
2. The sending cell nucleus is not one of the three parts of a synapse. The nucleus is located within the cell body of the neuron and is responsible for controlling the cell's activities and functions, but it is not directly involved in the transmission of signals at the synapse.
Therefore, the sending cell nucleus is not considered one of the three main parts of a synapse.
