Why is the flow of signals in a synapse from axonal end of one neuron to dendritic end of another neuron but not the reverse?

When an electrical signal reaches the axonal end of a neuron, it releases a chemical substance. This chemical diffuses towards the dendrite end of next neuron where it generates an electrical impulse or signal. Hence, the electrical signal is converted into a chemical signal at the axonal end. Since these chemicals are absent at the dendrite end of the neuron the electrical signal, cannot be converted into chemical signal.

(i) When an electrical signal reaches the axonal end of a neuron, it releases a chemical substance. This chemical diffuses towards the dendrite end of next neuron where it generates an electrical impulse or signal. Hence, the electrical signal is converted into a chemical signal at the axonal end. Since these chemicals are absent at the dendritic end of the neuron, the electrical signal cannot be converted into chemical signal. 

(ii) Sameer’s action was voluntary because rushing out of the room was under his conscious control. The smoke and smell were perceived by the receptors in the sense organs and signals are sent to the brain. The brain, then sent signals to the effector organs, i.e., the muscles, to move out of the room.

A nerve impulse travels in a form of electrical charge from a neuronal axon end to the synapse. Here, the electrical impulse is converted to the chemical impulse in the synaptic cleft. Further this nerve impulse flows from synaptic cleft to dendritic tips, which converts the chemical impulse to electrical impulse. The neurotransmitters required is synthesised only by axon terminals and the receptors required to detect these chemical transmitters are only present on the dendritic ends. Thus, the impulse always flows from one axonal end of a neuron to another dendron head of a neuron.