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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 cell
Published in Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella, Essentials of Human Physiology and Pathophysiology for Pharmacy and Allied Health, 2019
Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella
The action potential is initiated at the axon hillock (see Figure 1.5). This region is particularly excitable due to the presence of an abundance of voltage-gated Na+ channels. As the axon hillock is stimulated by excitatory inputs, there is a marked influx of Na+ ions and this region of the cell membrane becomes positive inside, resulting in an action potential. The rest of the axon is still at its resting membrane potential and is negative inside. As with graded potentials, this electrical signal also travels by local current flow (see Figure 1.6). The (+) charges in the region of the action potential are attracted to the negative charges in the immediately adjacent region of the axonal membrane. This current flow depolarizes the new region, causing an increase in the permeability of the cell membrane to Na+ ions through the voltage-gated ion channels. The subsequent influx of Na+ ions further depolarizes the membrane so that it reaches threshold and a new action potential is generated in this region. At the same time, the original site of action potential generation at the axon hillock repolarizes due to the efflux of K+ ions. This process of generating new action potentials sequentially along the membrane enables the signal to maintain its strength as it travels the distance to the axon terminal.
The control systems: nervous and endocrine
Published in Nick Draper, Helen Marshall, Exercise Physiology, 2014
Neurons are normally made up of three main components, dendrites, a cell body and a single axon. Dendrites receive information (sensory or neurotransmitter stimuli) which is conveyed toward the cell body for processing. The axon then generates nerve impulses and conducts them away from the cell body, along the axolemma (axon plasma membrane), to the secretory axon terminals. The axon is a specialised cellular filament that arises from the cell body at a site known as the axon hillock. There are three main structural classifications of neurons related to the number of processes extending from the cell body. These are shown in Figure 4.2 with the direction of impulse travel indicated by an arrow. Although all neurons have only one axon leaving the cell body, multipolar neurons have many dendrites connecting to the cell body, bipolar neurons have a single dendrite coming to the cell body, and unipolar neurons have a joint dendrite axon process connecting with the cell body. Generally, motor neurons are multipolar in structure and sensory neurons are mainly unipolar although some specialist neurons, such as those of the rods and cones of the eye, are bipolar. Figure 4.3 shows a multipolar neuron in more detail.
The osmotic demyelination syndrome: the resilience of thalamic neurons is verified with transmission electron microscopy
Published in Ultrastructural Pathology, 2020
Jacques Gilloteaux, Joanna Bouchat, Jean-Pierre Brion, Charles Nicaise
Further away from the deteriorated region, ODS12h neurons preserved cell bodies with axon hillock extensions, even if some were difficult to outline (Figure 9a–d). However, most ODS12h cell bodies maintained axo-somatic synaptic zones along the neurolemmal perimeter as exemplified in Figure 9d. The nerve cell bodies were still the largest cells amongst the neuropil showing round to ovoid nuclei appeared euchromatic and kept only small indents while the nucleoli condensed in large accumulated-like and elongated complex spheroids of granular ribonucleoproteins that seemed separated by narrow splits from the associated contrasted, chromatin fibrillar region aggregated, detected as one or more neighboring patches. Most of these features revealed stoppage of transcriptional activities but not enduring damages found in oligodendrocytes and astrocytes. Simultaneously, a loosen, marbled aspect of the nucleoplasm exposed its euchromatic features with displays of innumerable freckles of heterochromatin whose groupings can be revealed throughout the nucleoplasm and outwardly enhanced the inner membrane of the envelope. The perikaryon also contained a few erratically long, winded RER cisterns accompanied by lots of small polyribosomes where several small stacks or elongated Golgi apparatus saccules were viewed as circumscribing dilated parts of the perikaryon. In the same areas, the surrounding neuropil contained parts of intermingled oligodendrocytes (Figure 9b and Figure 9d), conspicuous with their condensed nucleus and cytoplasm, indicating that general acidification and distal, regional damages these cells had undergone while ODS recovery was apparently happened in neurons, as seen in this study. These nerve cell bodies were noted having preserved many axosomatic synaptic zones (Figure 9d).