Brain: Nerve cells or neurons
The brain of mammals contains, depending on the species, between 100 million and 1 billion neurons. The human brain contains an average of 85 billion of these neurons. They specialize in transmitting information to other nerve cells, but also to muscle cells or exocrine and endocrine cells.
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Typically, a nerve cell or neuron consists of a cell body that contains the cell nucleus and the various cell organelles. One end of the neuron has tree-shaped dendrites that are extensions of the cell body and receive information from other neurons. On the other side of the neuron, the cell body extends along an axon that can measure a few tens of millimeters to a meter in certain cases. At its extremity, the axon branches into many nerve endings to transmit nerve impulses.
In the nervous system, neurons receive and send messages constantly in the form of electrical impulses flowing from the dendrites to the axon. Most axons are covered by a myelin sheath whose function is to accelerate the circulation of nerve impulses. This myelin sheath is made up of specialized cells present in large numbers in the nervous system, the glial cells. These cells are important because, in addition to making the myelin sheath, they carry nutrients to the neurons and remove cell debris.
The passage of nerve impulses is based on the opening and closing of ion channels that cross the plasma membrane of neurons, allowing the entry and exit of ions selectively. This ion flow is what creates an electric current. The passage of nerve impulses from the dendrites to the axon creates an inversion of the electrical potential of the cell membrane. This is the potential for nerve action or impulse to travel at high speed from one nerve cell to another.
When this nerve impulse reaches the end of the axon, called a synapse, it triggers the release of a chemical molecule called a neurotransmitter that flows into the synaptic cleft and then binds to the receptors on the other neuron or a muscle cell or gland. Once fixed, this neurotransmitter alters the action potential of the recipient cell by allowing the nerve impulse to continue on its way.
Synthetic nerve cells that release neurotransmitters
This is not the first time scientists have tried to develop synthetic neurons. So far, and even with very conclusive results, all attempts have led to the creation of electronic circuit-based systems that make them look like little computer chips.
Researchers from the Department of Chemistry and the Department of Biochemistry at Oxford University have achieved the feat of making synthetic neurons that look almost like real nerve cells thanks to a hydrogel, designed by a process internal, to the research team.
They are actually water droplets with a volume corresponding to nanoliter and hydrogel fibers. The set is held together by a lipid bilayer like the plasma membranes of real cells. These soft, flexible synthetic neurons are 0.7 millimeters in diameter and 25 millimeters long. They are still a bit large, because a human neuron is about 700 times smaller in diameter.
These synthetic neurons have the ability to release neurotransmitters to allow the passage of a nerve impulse from one synthetic cell to another. The circulation of the electrical signal is ensured by proton pumps formed by protein pores installed in the lipid bilayer and sensitive to light.
As soon as one of these artificial neurons is subjected to light, the proteins in the proton pump begin to pump hydrogen ions that move through the water drop. This ion displacement creates an electrical signal that propagates to the end of the artificial neuron. The transition from one neuron to another is ensured by adenosine triphosphate (ATP), which “plays” the role of neuromediator.
Like real nerve cells, these synthetic neurons are capable of both excitability and conductivity!
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The development of new more efficient neural implants
A synthetic nerve cell in which an electric current flows is good, but several is much better, as it would create a real synthetic nervous system. To do so, researchers at Oxford University were able to bring together seven synthetic neurons in parallel to create an artificial nerve.
They then tried to circulate several different signals simultaneously on this synthetic nerve. They noticed that the signals propagated, thus allowing the transmission of space-time information.
The results of this research are really very encouraging and suggest many applications. For example, researchers would like to be able to use these light-activated synthetic neurons to transport and deliver various types of drugs.
But that’s not all, because synthetic nerves could be used as next-generation neural implants, for example in the case of cochlear implants or in the case of artificial retinal transplants.
Finally, these artificial nerve cells and especially these artificial nerves may be able to allow the development of non-invasive brain-machine interfaces to stabilize and treat patients with neurodegenerative diseases such as Parkinson’s or Alzheimer’s.
This is still the case in the future, as these synthetic neurons have yet to undergo many improvements such as the permanent and continuous supply of neurotransmitters as in a real nervous system.
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