**1. Introduction**

The brain is the control center that transmits the information of neurons to other groups of neurons. A single neuron can be affected simultaneously by excitatory and inhibitory stimuli from one axon or many other axons. Billions of neurons have different functions and constantly check the internal environment and external universe: light, touch, pressure, sound, balance, images, pain, emotion, consciousness. The nervous system from different external stimuli produces a continuous flow of information, memory, learning, and this conscious state throughout life seems infinite.

The nervous system of vertebrate is complex to be studied in individual or small groups of neurons. The cephalopods Coleoidea group (squid, sepia, and octopus), in addition to sea snails and nematodes, which contain few neurons and large structures, are relatively easy to manipulate experimentally and contributed to most of the current knowledge about the nervous system.

In general, these studies can be applicable to nervous system complexes, such as the human system. For example, mammals have a complexity of the Neuropil, an area composed of mostly unmyelinated axons, dendrites, and glial cell processes.

For few (μ<sup>2</sup> ) at Neuropil mammals, we find hundreds of bud's terminals with their corresponding dendritic spines and pre-postsynaptic terminals, where pointing them out individually would be a difficult task.

It was noticed that the Neuropil from squid nervous system also has many prepostsynaptic terminals such as a mammal. However, these are much larger in the squid nervous system and allow different types of experimental manipulations. In fact, light microscopy cannot resolve synaptic densities and synaptic vesicles, but the active zones in synapses can be observed easily from squid nervous system using low-power electron microscopy.

Thus, the squid nervous system with giant synapses and giant axon system have contributed to the current knowledge of Electrophysiology, Molecular Biology, and Biochemistry and allowed to verify the synaptic transmission and synaptic plasticity.
