Here are the first synthetic neurons capable of transmitting a nerve impulse!

Brain: nerve cells or neurons

The mammalian brain contains, depending on the species, from 100 million to 1000 billion neurons. The human brain contains an average of 85 billion of these neurons. They specialize in transmitting information to other nerve cells as well as muscle cells or exocrine and endocrine cells.

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Typically, a nerve cell or neuron consists of a cell body containing a cell nucleus and various cell organelles. One end of the neuron has dendrites in the form of a tree, which are an extension of the cell body and receive information from other neurons. On the other side of the neuron, the cell body expands with an axon, which in some cases can range in size from a few tens of millimeters to a meter. At its end, the axon branches into many nerve endings for the transmission of nerve impulses.

In the nervous system, neurons constantly receive and send messages in the form of electrical impulses that travel from the dendrites to the axon. Most axons are covered with a myelin sheath, the role of which is to speed up the passage of nerve impulses. This myelin sheath is formed by specialized cells that are abundant in the nervous system, the glial cells. These cells are important because, in addition to building the myelin sheath, they transport nutrients to neurons and remove cellular debris.

The passage of nerve impulses is based on the opening and closing of ion channels that cross the plasma membrane of neurons, which allows selective entry and exit of ions. It is this flow of ions that creates the electric current. The passage of nerve impulses from the dendrites to the axon causes a change in the electrical potential of the cell membrane. This is an action potential or nerve impulse that is transmitted at high speed from one nerve cell to another.

When this nerve impulse reaches the end of an 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 receptors on another neuron or on a muscle or gland cell. Once fixed, this neurotransmitter changes the action potential on the receptor cell, allowing the nerve impulse to continue its journey.

Synthetic nerve cells that release neurotransmitters

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Illustration of neuronal activity and the release of neurotransmitters at the level of neuronal synapses to ensure the continuity of the circulation of nerve impulses.

This is not the first time scientists have tried to develop synthetic neurons. So far, and even with very convincing results, all attempts have resulted in systems based on electronic circuits that make them look like tiny computer chips.

Researchers from the Department of Chemistry and the Department of Biochemistry at the University of Oxford, for their part, have been successful in creating synthetic neurons that almost resemble real nerve cells, thanks to a hydrogel developed through an in-house process for the research team.

In fact, they are water droplets with a volume corresponding to nanoliters and hydrogel fibers. Everything is held together by lipid bilayers, much like the plasma membranes of real cells. These soft and flexible synthetic neurons are 0.7 millimeters in diameter and 25 millimeters long. They are still a little big because a human neuron is about 700 times smaller in diameter.

These synthetic neurons have the ability to release neurotransmitters to allow the nerve impulse to travel from one synthetic cell to another. The circulation of the electrical signal is provided by proton pumps, which consist of protein pores installed in the lipid bilayer and sensitive to light.

As soon as one of these artificial neurons is exposed to light, proton pump proteins start pumping hydrogen ions that move through the water drop. This displacement of ions creates an electrical signal that propagates to the end of the artificial neuron. The transition from one neuron to another is provided by adenosine triphosphate (ATP), which “plays” the role of a neurotransmitter.

Just like real nerve cells, these synthetic neurons are both excitable and conductive!

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Development of new, more efficient neural implants

An artificial nerve cell in which an electric current circulates is good, but a few is much better, as this will allow you to create a real synthetic nervous system. To do this, researchers at the University of Oxford managed to connect seven synthetic neurons in parallel to create an artificial nerve.

Then they tried to transmit several different signals simultaneously in this synthetic nerve. They realized that signals propagate, which allows the transmission of space-time information.

The results of this study are indeed very encouraging and offer many applications. For example, researchers would like to be able to use these light-activated synthetic neurons to transport and deliver several types of drugs.

But that’s not all, because synthetic nerves can be used as next-generation neural implants, for example in the case of cochlear implants or in the case of artificial retina transplants.

Finally, these artificial nerve cells, and especially these artificial nerves, may enable the development of non-invasive brain-machine interfaces for the stabilization and treatment of patients suffering from neurodegenerative diseases such as Parkinson’s disease or Alzheimer’s disease.

At the moment, this is still in the future, as these synthetic neurons have yet to undergo many improvements, such as a constant and continuous supply of neurotransmitters, as in a real nervous system.

>> Read also: Alzheimer’s, Parkinson’s disease: regenerating neurons, a promising direction

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