what's synapse?with the help of diagram explain reflex action on knee jerk?

A synapse is a small gap between the two adjacent neurons. Signal is transferred from one neuron to another through the synapse. The signal passing neuron is known as the presynaptic neuron and thee signal receiving neuron is termed as post synaptic neuron. So, a synapse consists of three things,

1) Presynaptic neuron : it releases various neurotransmitters to transfer the the signal to the next neuron.

2) Postsynaptic neuron : It has receptors for the neurotransmitters to receive the signal.

3) Synaptic cleft : The area between two neurons.

A knee jerk reflex can be presented in the following way;

It is a simple monosynaptic reflex, where, only one synapse is involved at the spinal cord.