Information transfer by the way
Neurons only talk to each other via their button-like ends, the synapses - this was previously believed. But there is another way.
Obviously, a chicken pecks under a tree for seeds. Suddenly, a light breeze blows the shady leaves aside - a ray of sunshine falls on the bird's eyes. The nerves kick into action at lightning speed and report to the brain: "Something is dazzling here! Please take the necessary measures immediately!" The reaction comes promptly: The corresponding command races back to the eye via the nerve tracts, and the pupil narrows – the blinding incidence of light is reduced.
Such extremely practical reflexes, but also every movement, no matter how simple, only work if the nerves communicate smoothly with each other or with the muscles. They have special connections for this, the synapses. There, the nerve endings thicken slightly and make contact with other cells.
According to the conventional view, small sacs in the presynaptic membrane of the sending neuron, the synaptic vesicles, release a messenger substance, the neurotransmitter, in the synapse for information transfer. This diffuses through the synaptic cleft that separates both cells from each other, binds to special receptors in the postsynaptic membrane of the receiving cell and thus triggers the excitation. That's the doctrine.
Recent observations do not quite fit this view. Thus, vesicles with neurtransmitters were found on electron micrographs, which are not in the synaptic cleft, but next to it. Should an exchange of information possibly take place outside the synapse? This is also supported by quantitative measurements of the release of neurotransmitters, which suggest that the nerves do not only communicate with each other directly at the synapses. So far, however, there has been no proof of such a release of neurotransmitters, referred to as "ectopic".
Jay Coggan from the Salk Institute in La Jolla and an interdisciplinary team led by Terrence Sejnowski have now tackled the problem of ectopic release using computer simulation. The scientists worked with electron tomographic data from the chicken ciliary ganglion. Here, information from cranial nerves is switched to neurons, which in turn control the shape of the eye lenses and the size of the pupil opening. Researchers like to study synaptic processes in the ciliary ganglion because of its relatively simple structure and easy access. In addition, nerve impulses can be derived from either the presynaptic or the postsynaptic membrane, or from both.
There are two different receptors for the neurotransmitter acetylcholine (nAChR) in the postsynaptic membranes of the ciliary ganglion: The alpha3-nAChRs are mainly found in the postsynaptic membrane, whereas the alpha7-nAChRs are rarely found there, but are more likely to be found next to them are. Nevertheless, according to physiological measurements, the Alpha7-nAChRs lying outside the synaptic cleft are responsible for the excitation of the transmitting cell. Coggan and his team therefore suspected that in this case the information is not conveyed via the synaptic cleft, but alongside it: via an ectopic release of neurotransmitter.
Now the researchers modeled the structure of a synapse using the available electron tomographic data, distributed the two receptor types according to the physiological model and fed the computer with information about the amount of neurotransmitter released as well as its binding constant and rate of degradation. Then they allowed their virtual vesicles to fuse with the membrane and release their contents. They then observed excitation at the postsynaptic membrane.
The result was clear: the observations from physiological experiments on living cells could only be reproduced if a large number of ectopic vesicles were released.
Thus, in the chicken ciliary ganglion, much if not all communication between neurons occurs outside of the synapses. However, that does not necessarily mean that there are no synapses that correspond to the traditional doctrine of information transmission. But apparently there are several ways for cells to communicate with each other - and one of them leads past the synapse.
