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Introduction to botulinum toxin
Published in Michael Parker, Charlie James, Fundamentals for Cosmetic Practice, 2022
No matter which movement we are trying to initiate, the precentral gyrus is where it all begins. Within the precentral gyrus are neurons known as Betz cells that have long axons which course through the cerebral white matter. They progressively become closer to one another until they form part of the posterior limb of the internal capsule. Some of these fibres then travel to the opposite side of the brain where they synapse with the motor nuclei of the cranial nerves. These synapses can be found within the midbrain, pons and medulla oblongata. Those primary motor neurones which do not synapse with cranial nerves will continue inferiorly to the medulla oblongata, where the majority of them will cross the midline to the contralateral side. This is known as pyramidal decussation.
Spinal Cord Disease
Published in Philip B. Gorelick, Fernando D. Testai, Graeme J. Hankey, Joanna M. Wardlaw, Hankey's Clinical Neurology, 2020
Macroscopically, brain and spinal cord often appear normal; the precentral gyrus and corticospinal tracts may show atrophy (Figures 23.28–23.32). Microscopic findings (Figures 23.33, 23.34) are: Loss of Betz’ cells of the motor cortex.Degeneration and gliosis of the corticospinal tracts.Degeneration of lower brainstem motor nuclei (not oculomotor nuclei) in most cases.Cytoplasmic eosinophilic inclusions (Bunina's bodies) and ubiquitin immunoreactive inclusion bodies (containing TAR DNA binding protein 43 [TDP-43]) in degenerating cranial motor nuclei, anterior horn cells, and Betz’ cells.Muscle shows features of denervation.Nonmotor pathways also demonstrate pathologic changes, including sensory pathways and peripheral sensory nerves.
Neurons
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
A single axon emerges from the pyramidal base and branches quite extensively. Betz cells, a special type of pyramidal cells found in the primary motor cortex, have the largest cell bodies of neurons in humans, up to 100 µm wide, and a length of up to about 120 µm. Pyramidal cells are the basic computational unit of the cerebral cortex.
The Role of Primary Motor Cortex: More Than Movement Execution
Published in Journal of Motor Behavior, 2021
Sagarika Bhattacharjee, Rajan Kashyap, Turki Abualait, Shen-Hsing Annabel Chen, Woo-Kyoung Yoo, Shahid Bashir
Early investigation of the motor cortex in humans (Penfield & Boldrey, 1937; Woolsey, 1952) had functionally divided the motor cortex into two major areas: the primary motor cortex (M1) and premotor area (PMA; Fulton, 1935). M1 is located in the precentral gyrus of the frontal lobe that plays a crucial role in the execution of voluntary movements (Pearson, 2000). Histological examination of M1 has revealed the presence of giant pyramidal neurons called Betz cells. Betz cells are also known as upper motor neurons because they send axons to the lower motor neurons situated in the gray column of the spinal cord. The upper motor neuron contributes to the corticospinal pathway, whereas the lower motor neurons innervate the skeletal muscle fibers situated at the periphery (Porter & Lemon, 1993). With this structural construct, M1 is predominantly considered to only have a role in motor execution.
Better understanding the neurobiology of primary lateral sclerosis
Published in Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration, 2020
P. Hande Ozdinler, Mukesh Gautam, Oge Gozutok, Csaba Konrad, Giovanni Manfredi, Estela Area Gomez, Hiroshi Mitsumoto, Marcella L. Erb, Zheng Tian, Georg Haase
In an effort to better understand the mechanisms of neurodegeneration in PLS, numerous mouse models were generated based on PLS-linked mutations, such as ALS2/Alsin, C9Orf72, DCTN/Dynactin 1, FIG4/Phosphoinositide 5-phosphatase, OPTN/Optineurin, SETX/Senataxin, SPG7/Paraplegin or UBQLN2/Ubiquilin 2 (17). However, generation and characterization of a mouse model for an UMN disease is challenging. Humans heavily depend on their Betz cells for the initiation and modulation of voluntary movement and there are direct projections from Betz cells to the spinal motor neurons. In mice, in addition to the corticospinal tract, the rubrospinal tract also plays an important role and the circuitry within the spinal cord includes an interneuron component (24). Therefore, when humans have defects in their corticospinal tract, they may be paralyzed, whereas mice will be able to move, albeit with loss of their ability for fine movement. During evolution, humans have become more specialized in fine movement in the expense of making themselves vulnerable to significant spinal cord injuries. Rodents, however, have better capabilities to recover from an injury, but they are not as skilled as humans when it comes to dexterity (24,44). Moreover, there are no good outcome measures, which can quantitatively assess the timing and the extent of UMN degeneration in mouse models of PLS. Despite these two limitations the UMNs in mice and human share many common cell biological features and display similar pathologies at a cellular level (26,27,45). Hence, many different labs generated mouse models for genes that had been found mutated in PLS patients. Albeit most mouse lines did not have a prominent phenotype at a species level (46), detailed cellular investigations of their UMNs began to reveal the underlying problems.
Neurophysiological features of primary lateral sclerosis
Published in Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration, 2020
Mamede de Carvalho, Matthew C. Kiernan, Seth L Pullman, Kourosh Rezania, MR Turner, Zachary Simmons
Overall, markers of cortical hyperexcitability can be observed early in the disease course of PLS, but with disease progression, the motor cortex becomes progressively inexcitable, corresponding to a severe loss of motor neurons, typically with a greater level of dysfunction involving excitatory pathways compared to inhibitory synapses. In support, neuropathological and imaging studies have identified severe atrophy of the cortical motor strip and selective depletion of Betz cells in layer V, changes which likely contribute to the neurophysiological abnormalities (21).