Final answer:
Signals in synapses flow unidirectionally from axonal ends to dendritic tips due to the structural and functional polarization of neurons, specialized release and receptor mechanisms, and the integrating function of the axon hillock.
Step-by-step explanation:
The flow of signals in a synapse from the axonal end of one neuron to the dendritic tip of another neuron is unidirectional and not reversible. This is because neurons are structurally and functionally polarized. Neurons have specialized structures called axon terminals, which release chemicals known as neurotransmitters that cross the synaptic gap and bind to receptors on the postsynaptic dendrites. These dendritic receptors are part of the downstream neuron that receives the signal, which then travels to the soma and may eventually lead to an action potential that propagates down its axon.
Moreover, the axon hillock, a specialized structure at the junction between the soma and the axon, integrates signals from multiple synapses. It is specially equipped to initiate the action potential down the axon if the cumulative signals exceed a certain threshold. The unidirectional flow is further supported by the design of neurotransmitter receptors and the release mechanisms which are not conducive to signal flow in the reverse direction.