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Analyzing Emergence in Biological Neural Networks Using Graph Signal Processing
Published in Larry B. Rainey, O. Thomas Holland, Emergent Behavior in System of Systems Engineering, 2022
Kevin Schultz, Marisel Villafañe-Delgado, Elizabeth P. Reilly, Anshu Saksena, Grace M. Hwang
This chapter will also discuss typical ANN models, which assume dendrites to be passive linear receivers of neural activity, and thus all synaptic inputs are homogeneously summed within a point-like neural unit. This total neural input is then transformed by a non-linear threshold that represents the net effect of computation in biological neurons. Therefore, ANN models also exclude dendrites and axons. The weighted connection between neural units, however, is referred to as a “synapse,” despite the lack of dendrites or any other aspect of a biological synapses besides its strength. Another difference between real neurons and ANNs is that while artificial neural units can have positive and negative weights onto their targets, biological neurons can only make one type of connection to downstream cells, either positive (excitatory neurons) or negative (inhibitory neurons). This division of connection valence according to cell type is known as Dale’s law (Strata and Harvey 1999). Finally, it should be noted that synapses in ANN models are inspired by chemical synapses as described above. The biological brain also has electrical synapses, based on a physical connection called a gap junction, that allow neurons to communicate directly by sharing membrane voltage. While electrical synapses have also been largely ignored by ANN models, further discussion is out of scope for this chapter. Again, a comprehensive review is beyond the scope of this text, and we will instead focus at a more abstract level.
Synapses
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
Electrical synapses, as commonly understood, have a direct connection between the cytoplasms of two cells by means of gap junctions consisting of at least several hundreds of channels (Figure 6.18). In vertebrates, these channels are made up of isoforms of the protein connexin having a molecular mass in the range of 26–57 kD, where a protein isoform is one of the different forms of the same protein. Six connexin molecules, not necessarily identical, form a hexameric hemichannel, or connexon, about 5–7.5 nm long and with an external diameter of about 7 nm. A connexon spans across the cell membrane and extends for 1–1.5 nm into the extracellular space. Each connexon has six protrusions, one from each of the connexin molecules, that fit into the depressions between the protrusions of the other connexon of the channel. A channel having a pore of 1.2–2 nm diameter is thus formed, with tight interlocking in the extracellular gap of 2–3.5 nm separating the membranes of the two cells. The tight interlocking is necessary to prevent leakage of ions between the channel and the extracellular space. The two connexons are held together noncovalently by hydrogen, hydrophobic, and ionic bonds between the extracellular loops of the connexin molecules.
Pseudo-transistors for emerging neuromorphic electronics
Published in Science and Technology of Advanced Materials, 2023
Jingwei Fu, Jie Wang, Xiang He, Jianyu Ming, Le Wang, Yiru Wang, He Shao, Chaoyue Zheng, Linghai Xie, Haifeng Ling
As the junction between pre- and postsynaptic neurons (Figure 3), synapses are the basic units of neural networks and undertake the key tasks of material interaction and information transfer between neurons [78,79]. Synapses are mainly divided into two categories: chemical synapses and electrical synapses [80]. Electrical synapses transmit information with the help of electrical signals. Chemical synapses transfer information via neurotransmitter, which are found primarily in the human body. The transmission and handling of information is a complex process: firstly, action potentials control the opening of Ca2+ channels at presynaptic neuron, releasing excitatory or inhibitory neurotransmitters in the synaptic cleft. At the postsynaptic neuron, the neurotransmitters bind to a specific protein receptor. This receptor converts the chemical signal back into an electrical signal. In this way, the neurotransmitters can initiate an electrical response that either excite or inhibit the postsynaptic neuron.