1. Chapter 6 Diseases of Motor Neurons and Neuromuscular Junctions
From Diseases of the Nervous System. Copyright © 2015 Elsevier Inc. All rights reserved.

2. Figure 1 Schematic of the human motor system
Figure 1 Schematic of the human motor system. The corticospinal tract carries motor commands from upper motor neurons to the lower motor neurons in the anterior horn of the spinal cord. These project their axons to the skeletal muscles that are involved in voluntary movement. The lower motor neurons that innervate the bulbar muscles of the face and throat are in the medulla.
Head drawing from Patrick J. Lynch, Medical Illustrator, Creative Commons.
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3. Figure 2 Simplified schematic of the neuromuscular junction
Figure 2 Simplified schematic of the neuromuscular junction. Action potentials (APs) arriving from lower motor neurons depolarize the presynaptic terminal, resulting in a Ca2+ influx via voltage-gated Ca2+ channels (VGCCs). These are the target of autoantibodies in Lambert Eaton myotonia. The released acetylcholine (AcH) transmitter binds to postsynaptic AcH receptors (AcH-Rs) on the motor endplate. Autoantibodies to these receptors cause myasthenia gravis. AcH receptors mediate the influx of Na+, which gives rise to the postsynaptic muscle AP. This in turn causes muscle contraction via release of Ca2+ from the sarcoplasmic reticulum.
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4. Courtesy of Dr Peter King, University of Alabama Birmingham.
Figure 3 Electromyography in a patient with myasthenia gravis shows a characteristic decrease in the amplitude of the compound muscle action potential of the abductor digiti quinti, with repeated ulnar nerve stimulation, reaching the lowest amplitude after three or four stimuli, when the action potentials plateaus.
Courtesy of Dr Peter King, University of Alabama Birmingham.
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5. Figure 4 Schematized sites of pathology comparing Lambert Eaton myotonia (LEM) and myasthenia gravis (MG) with the neuromuscular junction in an unaffected normal individual. In LEM presynaptic (pre) Ca2+ channels are lost because of an immune attack, causing fewer transmitter-containing vesicles to be released, whereas in MG it is the postsynaptic acetylcholine (AcH) receptors that are reduced in number by the immune attack. The compound muscle action potential normally has a constant amplitude yet shows a marked reduction in LEM that increases gradually as more transmitter is released. In MG the action potential decreases over time since not enough AcH receptors (AcH-Rs) are available for activation with repeated stimulation.
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6. Figure 5 Topographic organization of the motor neurons in the anterior horn of the spinal cord.
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7. Figure 6 Innervation of multiple muscle fibers by a single motor neuron. These multiple fibers constitute a motor unit.
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8. Figure 7 Spinal cord pathology in amyotrophic lateral sclerosis showing mild (A) and severe atrophy (B) of corticospinal tracts (arrows).
From Ref. 9.
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9. Figure 8 Muscle pathology in amyotrophic lateral sclerosis (ALS)
Figure 8 Muscle pathology in amyotrophic lateral sclerosis (ALS). (A) Biopsy sample of the left vastus lateralis muscle from a patient with ALS, stained with ATPase (pH 9.4). The biopsy sample highlights grouped atrophic fibers with both type I and type II fibers (mixed-type fibers, encompassed by the red box). Pathophysiology of motor unit degeneration and reinnervation (B), with superimposition of 10 traces (C) demonstrating the typically large, polyphasic, unstable (complex) motor units observed in established ALS (sweep duration, 50 ms), with late components, indicating some reinnervation.
From Ref. 10.
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10. Figure 9 Examples of inclusion bodies in three representative examples of motor neurons. The arrows point to compact basophilic neuronal cytoplasmic inclusions. (A) and (B) are upper motor neurons in layer V of the primary motor cortex from two different patients. (C) is an example of a lower motorneuron in the nucleus hypoglossus of yet another ALS patient.
From Ref. 9.
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11. Figure 10 Physiological roles for TDP-43 and FUS
Figure 10 Physiological roles for TDP-43 and FUS. Proposed roles for FUS/TLS include general transcriptional regulation (1) and cotranscriptional deposition (2). Both TDP-43 and FUS/TLS associate with promoter regions (3). TDP-43 binds single-stranded, TG-rich elements in promoter regions, thereby blocking transcription of the downstream gene. In response to DNA damage FUS/TLS is recruited in the promoter region of cyclin D1 (CCND1) by sense and antisense noncoding RNAs (ncRNAs) and represses CCND1 transcription. Both TDP-43 and FUS/TLS bind long intron-containing RNAs (4), thereby sustaining their levels. TDP-43 and FUS/TLS control the splicing of >950 or >370 RNAs, respectively, either via direct binding or indirectly (5). TDP-43 and FUS/TLS bind long ncRNAs (6), complex with (7) Drosha (consistent with an involvement in micro RNA [miRNA] processing), and bind 3′ untranslated regions (UTRs) of a large number of messenger RNAs (8). Both TDP-43 and FUS/TLS shuttle between the nucleus and the cytosol and are incorporated into transporting RNA granules (9) and stress granules (10), in which they form complexes with messenger RNAs and other RNA-binding proteins.
Figure and legend reproduced with permission from Ref. 12.
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12. Reproduced with permission from Ref. 17.
Figure 11 Pathophysiology of amyotrophic lateral sclerosis seems to be multifactorial and involves multiple cell types. Mitochondrial dysfunction, aggregation of RNA regulatory proteins, and neurofilament accumulations lead to neuronal and axonal dysfunction. The loss of astrocytic glutamate transporters causes excitotoxic increases in glutamate. The release of inflammatory molecules by microglial cells causes damage to the neuron and axon.
Reproduced with permission from Ref. 17.
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13. Reproduced with permission from Ref. 20.
Figure 12 Mitochondrial dysfunction in amyotrophic lateral sclerosis. The aggregation of mutant SOD1 causes a failure in energy production, a breakdown of the mitochondrial membrane potential, and a loss in Ca2+ buffering by mitochondria. The release of cytochrome c initiates apoptotic death in the affected neuron. ADP, adenosine diphosphate; ROS, reactive oxygen species.
Reproduced with permission from Ref. 20.
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14. Reproduced with permission from Ref. 25.
Figure 13 Non-cell-autonomous toxicity in amyotrophic lateral sclerosis (ALS). SOD1 mutations in different cell types contribute in different ways to ALS. Mutations in motor neurons initiate disease, yet microglial and astrocytic changes accelerate disease onset and increase severity.
Reproduced with permission from Ref. 25.
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15. Reproduced with permission from Ref. 29.
Figure 14 Tract-based imaging using diffusion tensor magnetic resonance imaging, an imaging approach that highlight axonal fiber tracts, revealing a thinning in the corticospinal tracts in a patient with ALS (left) compared with a healthy control (right).
Reproduced with permission from Ref. 29.
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16. Reproduced with permission from Ref. 36.
Figure 15 Neuropathological findings in a patient after stem cell implantation. (A) Gross image of the cervical spinal cord at the time of autopsy. Serial sections through the region of transplantation did not demonstrate regions of cystic change, hemorrhage, or significant tissue disruption. (B) Representative cross section showing intact cord morphology using hematoxylin and eosin (H&E) staining. There is a nest of cells (circled) that are not intrinsic to the spinal cord and do not stain with glial or neuronal markers (not shown). (C) Magnification of the circled region in B showing the morphology of these cells, which is reminiscent of the morphology of the stem cells before transplantation (inset, H&E stain).
Reproduced with permission from Ref. 36.
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