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Neurons
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
The simplest case to consider is that illustrated in Figure 7.2a, which shows a neuron having some representative synapses A, B, and C, the former two being excitatory, the latter inhibitory. Conventionally, an excitatory synapse is shown as an unfilled triangular shape, whereas an inhibitory synapse is shown as a filled one. The part of the cell body that connects to the axon is the axon hillock. At the time of early investigations on the electrophysiology of neurons, it was assumed that the dendrites and cell bodies were passive structures, so that generated excitatory postsynaptic potentials (epsps) and inhibitory postsynaptic potentials (ipsps) propagate passively as electrotonic spread governed by the cable (RC) properties of the membrane. These postsynaptic voltages summate in the region of the axon hillock, and when the net sum exceeds threshold, a neuronal AP, or spike, is generated in the initial, unmyelinated segment of the axon as the output AP of the neuron. There is evidence that in some cases, the output AP may be initiated at the first node of Ranvier. It is well established that the density of voltage-gated Na+ channels at the nodes of Ranvier is high, which lowers the threshold for the AP. Once initiated, the AP propagates in the forward direction along the axon and its depolarization spreads backwards into the cell body and dendrites.
The Emergence of Order in Space
Published in Pier Luigi Gentili, Untangling Complex Systems, 2018
Every excitable portion of the neural membrane has two critical values of electric potential: one is the resting potential and the other is the threshold potential. The resting potential represents the value that the membrane maintains as long as it is not perturbed; usually, in the axon hillock, it is −70 mV. The inputs, which a neuron receives, induce either a hyperpolarization or depolarization of the membrane. In other words, the inputs either decrease or increase its electric potential. When a sufficiently strong excitatory input depolarizes the membrane, and its potential overcomes the threshold value (that is ≈ −55 mV in the axon hillock), the neuron triggers an action potential (Figure 9.22). The electric potential of the axon hillock soars quickly, in roughly 1 ms or less, up to reach a positive value, as large as +50 mV. Then, it drops to values that are more negative than the resting potential. Finally, it recovers its initial state, and the neuron is ready to discharge another action potential.
Electronic and Ionic Conductivities of Microtubules and Actin Filaments, Their Consequences for Cell Signaling and Applications to Bioelectronics
Published in Sergey Edward Lyshevski, Nano and Molecular Electronics Handbook, 2018
Jack A. Tuszynski, Avner Priel, J.A. Brown, Horacio F. Cantiello, John M. Dixon
We now explain how this integrative view may serve as a regulatory (adaptive) mechanism. The input level denoted by A in Figure 18.22 is associated with external electric perturbations passing through membranes, mainly synaptic inputs arriving from other neurons. These signals arrive at the cytoskeleton and directly affect MTNs, and/or actin filaments (bundles), in spiny synapses. However, the actin cytoskeleton is responsible for the propagation of signals to the MTN (see level B). These propagated signals (level C) are in turn used as inputs to the MTN, viewed as a dynamic system. We further propose that the MTN generates diverse phase space trajectories in response to different input vectors. The requirements from such a system are not too restrictive since the output-state is not an attractor of the system. In other words, information processing at this level is not necessarily based on attractor dynamics but rather on real-time computations. This proposition relies on the observed ability of the MTN to propagate signals and on the specific topological features of MTNs in dendrites, in which shorter MTs of mixed polarity are interconnected by MAP2s. The output from the MTN would be a function of the evolved state vector in certain areas accessed by actin filaments and/or directly linked to ion channels (see both possibilities at level D). These output signals may modify the temporal channel activity, either by directly arriving from the MTN to the channels, or mediated by actin filaments. Hence, the channel-based synaptic membrane conductance is regulated, in particular in the axon hillock region, which is, in most cases, responsible for the generation of action potentials.
Emerging memristive neurons for neuromorphic computing and sensing
Published in Science and Technology of Advanced Materials, 2023
Zhiyuan Li, Wei Tang, Beining Zhang, Rui Yang, Xiangshui Miao
Despite this great variety, they all have a typical neuronal structure, comprising of three main parts: (i) Dendrites, which are the tree-root-shaped part of the neuron which are usually extending from the soma. Dendrites receive neural signals from pre-synaptic neurons and then transmit them to soma. It has been found that many complex calculations can be done in dendrites before the signal reaches soma, including Boolean operating, coincidence detecting, etc [44,45]. (ii) The soma, also called cell body, which is the essential center of the neuron. The soma synthetizes neurotransmitters, generates an action potential and then sends it to the axon. (iii) The axon, also referred to as nerve fiber, is a tail-like structure of the neuron which joins the soma at a junction called the axon hillock, acting as the output channel of neuron signals in biological system. The function of the axon is to transmit neuron signals away from the soma to the post-synaptic neurons.