The pre-synaptic part is:

Most insecticides affect which system of an insect?

$A$ : Transmission of the nerve impulse across a synapse is accomplished by neurotransmitters.
$R$ : Transmission across a synapse usually requires neurotransmitters because there is a small space,$i.e.$,synaptic cleft,that separates one neuron from another.

How is the synaptic cleft formed?

Transmission of nerve impulse across the synapse is accomplished by:

Identify the location and function of the following structures:
$(1)$ Synaptic Knob
$(2)$ Bipolar nerve cell