Most insecticides affect which system of an insect?

Which chemical substance is responsible for the transmission of nerve impulses across the synapse from one neuron to another?

In the following diagram showing axon terminal and synapse,$A, B, C, D$ and $E$ respectively represent:

Which of the following is secreted at the ends of nerve fibres?

Arrange the following events in proper sequence during impulse transmission across a chemical synapse:
$(a)$ Fusion of synaptic vesicles with the cell membrane of the presynaptic neuron.
$(b)$ $Ca^{++}$ channels open and $Ca^{++}$ diffuses into the synaptic knob from the extracellular fluid.
$(c)$ Neurotransmitter binds with receptors of the postsynaptic cell.
$(d)$ Release of neurotransmitter by exocytosis.

What is responsible for the unidirectional transmission of nerve impulses in a neuron?