Synapses
In the nervous system, a synapse is a structure that permits a nerve cell to pass an electrical or chemical signal to another nerve cell. The word comes from the Greek synapsis, which means “conjunction”, and was first employed by the English neurophysiologist Charles Sherrington in 1897.
When I arrived in Saint Louis in 1958 and began to spend some afternoons at the Central Institute for the Deaf, Dr. Donald H. Eldredge – a neurophysiologist and also a good friend – told me a story concerning the synapses of the cochlear hair cells.
With the advent of the electron microscope in the early 1950s, nerve endings were found to contain a large number of small vesicles. The term synaptic vesicles was given to these structures. He also told me that one of the researchers involved in electron microscopy had a special interest for quantum theory and made some calculations on the size of the vesicles. As predicted by Max Planck, the quantum mechanics are absolutely essential to deal with small structures.
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. The calculations that were performed suggested that each vesicle had exactly one quantum of energy to be transported across the synapse. Many years later it was proved that the vesicles contain a neurotransmitter called acetylcholine, approximately 1,000 molecules per vesicle. The stimulation of the nerve cell causes a certain number of vesicles to send their neurotransmitters across the membranes to reach the other side of the synapse, as seen in the schematic illustration.
It was only in 1953, however, that the studies of the organ of Corti by means of electron microscopy began to be performed, pioneered by Engström and Wersäll in Sweden, and later by Catherine Smith in Saint Louis.
And then there was a problem. In the cochlear hair cells the synaptic vesicles were mainly on the wrong side. There were some inside the cells, but the majority was inside the nerve endings.
How could this thing be explained? Was the whole theory wrong?
No, the theory was correct. It was then realized that the cochlear cells are extensively controlled by afferent nerve fibers, that is, fibers that originate in the Central Nervous System. Data related to an afferent system actually existed at that time, but the number of afferent fibers and the size of their nerve endings was a surprise.
In fact your brain is all the time working to improve your hearing capacity and for this reason there are synaptic vesicles for the efferent system, that go from the cells to the nerve fibers, to transmit the cochlear information to higher centers, and there are synaptic vesicles for the afferent system, that go from the Central Nervous System to the hair cells, to make them work as well as possible.
Do not forget that the ear is our most sensitive organ; when we hear a very low sound the excursion of the basilar membrane is just about the size of a hydrogen molecule. If our sound thresholds were 10 decibels better than the best normal human hearing we would be able to hear the Brownian motions of endolymph molecules inside the inner ear. And besides that, we have good discrimination for different sounds – the special neurophysiological improvement that allowed the human species to develop language.
When I arrived in Saint Louis in 1958 and began to spend some afternoons at the Central Institute for the Deaf, Dr. Donald H. Eldredge – a neurophysiologist and also a good friend – told me a story concerning the synapses of the cochlear hair cells.
With the advent of the electron microscope in the early 1950s, nerve endings were found to contain a large number of small vesicles. The term synaptic vesicles was given to these structures. He also told me that one of the researchers involved in electron microscopy had a special interest for quantum theory and made some calculations on the size of the vesicles. As predicted by Max Planck, the quantum mechanics are absolutely essential to deal with small structures.
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. The calculations that were performed suggested that each vesicle had exactly one quantum of energy to be transported across the synapse. Many years later it was proved that the vesicles contain a neurotransmitter called acetylcholine, approximately 1,000 molecules per vesicle. The stimulation of the nerve cell causes a certain number of vesicles to send their neurotransmitters across the membranes to reach the other side of the synapse, as seen in the schematic illustration.
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| Schematical drawing of a synapsis (Wikimedia Commons) |
And then there was a problem. In the cochlear hair cells the synaptic vesicles were mainly on the wrong side. There were some inside the cells, but the majority was inside the nerve endings.
How could this thing be explained? Was the whole theory wrong?
No, the theory was correct. It was then realized that the cochlear cells are extensively controlled by afferent nerve fibers, that is, fibers that originate in the Central Nervous System. Data related to an afferent system actually existed at that time, but the number of afferent fibers and the size of their nerve endings was a surprise.
In fact your brain is all the time working to improve your hearing capacity and for this reason there are synaptic vesicles for the efferent system, that go from the cells to the nerve fibers, to transmit the cochlear information to higher centers, and there are synaptic vesicles for the afferent system, that go from the Central Nervous System to the hair cells, to make them work as well as possible.
Do not forget that the ear is our most sensitive organ; when we hear a very low sound the excursion of the basilar membrane is just about the size of a hydrogen molecule. If our sound thresholds were 10 decibels better than the best normal human hearing we would be able to hear the Brownian motions of endolymph molecules inside the inner ear. And besides that, we have good discrimination for different sounds – the special neurophysiological improvement that allowed the human species to develop language.
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