The main activity that occurs at the transmissive segment is the release of neurotransmitter from synaptic vesicles . Prior to the arrival of the action potential, Ca2+ pumps embedded in the plasma membrane of a synaptic knob establish a calcium concentration gradient by pumping it out to the IF. Consequently, there is more calcium inside of the synaptic knob than outside it.

Further Explanation:

At synaptic junctions:

The action potential travels along the membrane until the synapse where it’s electrical depolarization leads to the opening of channels allowing Ca2+ to rush into the terminal due to higher extracellular concentrations
these flow through a presynaptic membrane until the concentration is built up, activating ion sensitive proteins attached to vesicles containing neurotransmitters
this leads to changes in the proteins leading to the fusion with the membrane of the presynaptic cell, so vesicles are open and neurotransmitter is released. The neurotransmitter diffuses across to chemical receptors on the presynaptic cell where they bind temporarily
This leads to activation of specific complexes, enabling the transmission of information. Thus, the chemical signal is transferred through this neuron as an electrical impulse

Learn more about the autonomic nervous system at brainly.com/question/10386413