Understanding the human nervous system is one of the hardest challenges for humanity. Despite all of the energy we use, there are many unexplored parts. This is the reason why our brain is so fascinating. There are 86 billion neurons in a human brain and it represents the center of the human being.
You, your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules.
~ Francis Crick
This quote of one of the two discoverers of the DNA structure gives the right perspective of what the nervous system represents.
Neurons: a brief story
Neurons are a relatively recent discovery. In 1836-38, J.Purkinje, who at that time was a physiology professor at Braslow University (now Wroclaw, in Poland), with his learner G. Valentin, gave, for the first time, indications about these cells.
Forty years later, Camillo Golgi, in 1873, discovered a coloring which was able to point out neurons under the microscope: the “black reaction”. This coloring is a two-step preparation: firstly cells are fixed with Potassium Bichromate and then filled with Silver Nitrate. It was a revolution. Golgi’s coloring gave other scientists, such as Santiago Ramon y Cajal, the opportunity to understand better these particular cells.
In 1897 Charles Sherrington coined the term “synapse” and described the first neural circuit. For this, he received Nobel prize for Medicine or Physiology in 1932.
Since then a large variety of neurons were discovered, each one with its own particular shape, localization, and functions.
A Little Closer Look at Neurons
Neurons are extremely particular cells. Each neuron is composed of a cell body, numerous dendrites, and an axon. Through the axon the neuron sends information, while it receives information through the dendrites.
One of the features of neurons is the ability to communicate with other cells. Neurons “speak” to each other through action potentials, or in other words, electrical pulses. In basal conditions, cell membranes create an electrical difference between the cytoplasmic environment and the extracellular one. This difference is equal to -70 millivolts, with the internal environment more negative than the outside.
Synapses and Signal Transmission
A synapse is a structure that allows a neuron to pass a signal to another neuron. A space of 40 nanometers separates a presynaptic neuron from a postsynaptic one. The axon of the presynaptic neuron, nears the synapse, has particular structures called “active areas.” In those areas there are neurotransmitter vesicles, which are essential in signal transmission.
When a pulse comes to the synapse’s zone, it opens ionic channels and positive ions flow into the cell. This event enables vesicular exocytosis.
On postsynaptic membranes, there are receptors for a specific neurotransmitter which have the function of transmitting the signal.
One single synapse is not strong enough to transmit the message. Neurons respond is an “all or nothing” manner. Each neuron has its own potential threshold. It is necessary for the membrane potential to exceed this threshold in order to activate the postsynaptic neuron. To make this possible, many synapses have to be active at the same moment on the same neuron.