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Propagation of the Action Potential
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The solution, in the form of myelinated axons, is ingeniously simple and highly effective. The axon is surrounded by a myelin sheath consisting of up to 200 layers or so of passive cell membrane interrupted at regular intervals in what are referred to as the nodes of Ranvier (Figure 4.7). The region between adjacent nodes is the internode, whose length is roughly 100–150 times the axon diameter and ranges in length between about 200 µm and 2.5 mm, depending on axon diameter. The sheath is wrapped around the axon during embryonic development by specialized satellite cells of the nervous system – the glial cells (Section 1.2.3). In the central nervous system, the glial cells that form the myelin sheath are referred to as oligodendrocytes, with each oligodendrocyte forming one internode of myelin for up to about 50 adjacent axons. In the peripheral nervous system, a glial cell referred to as a Schwann cell forms one internode of only a single axon.
Use of Artificial Neural Networks in
Published in Surajbhan Sevda, Anoop Singh, Mathematical and Statistical Applications in Food Engineering, 2020
RA Conde-Gutiérrez, U Cruz-Jacobo, JA Hernández
The brain is the main organ of the human body and is very to analyse, in comparison to other organs, and this is basically due to its unique capacity to carry out certain functions. According to the first researches conducted on the human brain, there were about 100 billion cells, called neurons, but in the investigation of Herculano-Houzel (2009), an average of 14 billion was counted. Neurons perform the function of communicating with each other accurately and rapidly over long distances and with other cells. Electrical signals, known as nerve impulses, are transmitted through the neurons. The body of a neuron is the source of synthesis of an organic molecule and these molecules are transported as a nerve impulse through the axon until reaching the terminals, where they are stored to be applied in the stimulation, as described by Williams (2003). Nodes of Ranvier are interruptions between the axon that enable the nervous impulse to travel at a higher speed. The nucleus of the neuron is related to the synthesis of ribonucleic acid (RNA) and dendrites are the extensions of neurons that appear to be branches or points that extend from the cell body with the function of receiving chemical messages from other neurons (Fig. 1).
Signals, Noise, and Thresholds
Published in Ben Greenebaum, Frank Barnes, Bioengineering and Biophysical Aspects of Electromagnetic Fields, 2018
The node of Ranvier is where the action potential for myelinated nerve cells is generated.78 There is a high concentration of ion channels in the node of Ranvier and, in the days before patch clamp, it was a good place to record membrane electrical activity. In the mid-1960s, Verveen and Derksen measured 5 to 10 minutes of cell membrane voltage noise at a Ranvier node of an unstimulated nerve cell.79,80 The resulting power spectrum showed two decades, between 10 Hz and 1,000 Hz, of 1/f noise (see Figure 8.5). This 1/f noise, they found, was much larger in magnitude than what Nyquist’s 4kBTR formula (cf. Equation (8.5)) would predict. Following the explanation for 1/f noise in ordinary resistors (cf. Equations (8.10) and (8.11)), Verveen and Derksen suggested that an ion channel could, from time to time, get “clogged up.” The wide distribution of waiting times (i.e., the λ’s in Equations (8.8–8.11)) before getting unclogged would then give rise to the 1/f spectrum.81
A Functional BCI Model by the P2731 working group: Physiology
Published in Brain-Computer Interfaces, 2021
Ali Hossaini, Davide Valeriani, Chang S. Nam, Raffaele Ferrante, Mufti Mahmud
Alongside an idealized neuron, Figure 5 depicts an oligodendrocyte cell whose branches enclose the axon of a cerebral neuron in a myelin sheath. While myelin is not essential for neural function, it is used in many classes of neurons within and outside the central nervous system to enhance electrical conduction. In the brain, unmyelinated neurons constitute ‘gray matter’ while myelinated neurons are called ‘white matter’ because a mass of fatty myelin has a white appearance [35,36]. Myelin sheaths are punctuated by breaks called Nodes of Ranvier that enable an action potential to propagate efficiently through the length of the axon through a process known as saltatory conduction [37].
Biological function simulation in neuromorphic devices: from synapse and neuron to behavior
Published in Science and Technology of Advanced Materials, 2023
Hui Chen, Huilin Li, Ting Ma, Shuangshuang Han, Qiuping Zhao
Neuron is the signaling unit of nervous system. A typical neuron has four morphologically defined parts: (1) soma (or cell body), (2) dendrites, (3) axon, and (4) presynaptic terminal (Figure 1(a)). Therein, soma can synthesize the neurotransmitters and integrate the electrical signals from the dendrites to control the transmission of action potential, which is the metabolic center of a neuron that contains the nucleus (provides the genes) and endoplasmic reticulum (synthesizes the proteins). The soma usually gives rise to two types of cytoplasmic protrusions: several short dendrites and one long tubular axon. Dendrites that branch out in tree-like fashion are the main apparatus to receive the incoming signals from other nerve cells and play a critical role in filtering and integrating these signals to determine whether to fire a signal or not, but they cannot amplify these input signals. Dendrites can assist self-neurons to monitor instructions from neighboring neurons [17]. Compared with dendrites, axons have a larger length, and some can extend over 2 m within the body. In addition, most axons in the central nervous system are very thinner (0.2–20 μm in diameter) compared with the diameter of the soma (≥50 μm). For typical axons, many of them are insulated by some sheathes of fatty myelin that are regularly interrupted at gaps called the nodes of Ranvier. The axons are considered as the transmitting element of a neuron that can deliver electrical signals to other neurons through presynaptic terminal, where the synapses are formed with the postsynaptic dendrites. Usually, a neuron has one soma, one axon, many dendrites and presynaptic terminals, in which the basic function of one neuron is to exchange information by receiving, integrating, conducting and exporting information (electrical signals).