The synapse actually act like a one-way valve because the chemical substance is present only on one side of the gap. This chemical diffuses towards the dendrite end of next neuron where it generates an electrical signal. Since, the chemicals are absent at the dendritic end of neuron, the nerve impulse can go across only from one side (which contains the chemical substance). In this way, it is ensured that nerve impulses travel in only one direction (through particular set of neurons).

The Flow of Signals in a Synapse: From Axonal End to Dendritic End

The flow of signals in a synapse is unidirectional, meaning it goes from the axonal end of one neuron to the dendritic end of another neuron. This unidirectional flow is crucial for the proper functioning of the nervous system and efficient transmission of information. Let's explore the reasons behind this unidirectional flow in detail.

1. Anatomy of a Synapse
A synapse is a specialized junction between two neurons, where information is transmitted from one neuron to another. It consists of three main components:
- Presynaptic Neuron: The neuron sending the signal.
- Synaptic Cleft: A small gap separating the presynaptic and postsynaptic neurons.
- Postsynaptic Neuron: The neuron receiving the signal.

2. Neurotransmitter Release and Reception
The flow of signals in a synapse is accomplished through the release and reception of neurotransmitters, which are chemical messengers. When an action potential reaches the axonal end of the presynaptic neuron, it triggers the release of neurotransmitters into the synaptic cleft. These neurotransmitters then diffuse across the cleft and bind to specific receptor molecules on the dendritic end of the postsynaptic neuron.

3. Receptor Specificity
The specific receptors present on the postsynaptic neuron's dendritic end determine the direction of signal flow. These receptors are designed to recognize and bind with specific neurotransmitters. The receptor specificity ensures that the signals are received and processed in the correct direction.

4. Postsynaptic Density
The dendritic end of the postsynaptic neuron contains a specialized region called the postsynaptic density (PSD). The PSD is enriched with various proteins, including receptors, ion channels, and signaling molecules. These proteins are arranged in a specific manner to optimize signal reception and processing. The organization of the PSD is unidirectional, further enforcing the flow of signals in a specific direction.

5. Molecular Machinery
The molecular machinery involved in neurotransmitter release and reception is also directed towards a specific flow of signals. The machinery responsible for neurotransmitter release is located in the presynaptic neuron, while the machinery for neurotransmitter reception is present in the postsynaptic neuron. This spatial segregation of molecular components ensures that signals are transmitted in a unidirectional manner.

Conclusion
In summary, the flow of signals in a synapse is unidirectional, going from the axonal end of one neuron to the dendritic end of another neuron. This unidirectional flow is primarily determined by the anatomy of the synapse, receptor specificity, the organization of the postsynaptic density, and the spatial segregation of molecular machinery. These mechanisms ensure efficient and accurate transmission of information within the nervous system.

Because the synapse acts as a valve which ensure that the signals will flow only in one direction.