Authored by Sergio Gonzalez-Gonzalez
Myelination is essential for the rapid propagation of action potentials along axons in both the central and peripheral nervous systems, and the Schwann cells are the responsible of myelin sheath production in the peripheral nervous system. In the cochlea, sensory hair cells and neurons are in close association with several types of glial cells. Whereas hair cells are surrounded by supporting cells, spiral ganglion neuron axons are myelinated by Schwann cells. This myelin contributes to axonal protection and allows for efficient action potential transmission along the auditory nerve. Because in the last decade, myelin research has been notably focusing on the molecular and cellular mechanism of demyelination process and the associated axonal loss, in this review we summarize the role of the myelin sheath in auditory nerve and how Schwann cell demyelination results in a reduction in the velocity of action potential propagation, and an increase in nerve conduction vulnerability.
Myelination is essential for the rapid propagation of action potentials along axons in both the central (CNS) and peripheral (PNS) nervous systems. In the PNS, the myelin sheath is formed by Schwann cells (SCs) [1]. Myelinating Schwann cells wrap around axons so that the molecular machinery required to propagate action potentials is concentrated at regular sites, known as nodes of Ranvier. The scarcity of data regarding axon-glia physiology in peripheral nerves underscores the complexity of this cell-cell relationship and the relative inadequacy of the current approaches used in peripheral nerve research. Recent data have highlighted the role of subcellular organelles and molecular factors in the interaction between SC and axons [2]. This is consistent with the emerging opinion in the field that the interdependence between the axon and the myelinating glia is so deep that CNS and PNS neuropathies cannot be correctly addressed and treated without first understanding the extent and complexity of the relationship between neurons and glial cells [3]. The PNS shows a surprising capacity of regeneration compared to the CNS. This ability of peripheral nerves to recover quickly following damage is to a large extent due to the remarkable plasticity of Schwann cells [4].
SCs are derived from neural crest cells that differentiate into SC precursors and then into immature SCs between E12 and E15 in mice. Around birth, axonal sorting and myelination start in the peripheral nerves. Some SCs establish a 1:1 relationship with large-diameter axons, wrap them multiple times to form a thick and compact myelin sheath. Nerve homeostasis, trophic support and myelin maintenance are other important functions of SCs in adulthood [5]. The myelin geometry, diameter and internodal length, is therefore a critical parameter of the nerve conduction velocity and changes in this geometry underlie numerous functional properties of the brain and nerves.
Moreover, reduced internodal length, which strongly impairs nerve conduction velocity [6,7], is linked to human neuropathies such as multiple sclerosis in the central nervous system [8] and Charcot-Marie-Tooth (CMT) diseases [9-11] and congenital neuromuscular dystrophy 1A [12] in the peripheral nervous system. While the molecular mechanisms responsible for the homogeneity of myelin thickness in Schwann cells along the same axon have been uncovered [13-15], the mechanisms responsible for myelin elongation and the formation of regular myelin lengths remain unknown. Besides their major roles in normal nerve physiology, SCs play a key function for repair in many pathological conditions thanks to their striking plasticity [16]. For example, after a peripheral nerve injury, they are capable of switching into a SC immature-like phenotype that drives nerve repair. Over the last decades, major progress has been made in unraveling molecular mechanisms and signaling pathways that drive SC dedifferentiation and regulate their plasticity. This high regenerative capacity opens the door to the development of new therapeutic approaches for the regeneration of peripheral axons after injury.
Hearing Loss
Hearing loss is the most common form of sensory impairment in humans, affecting 360 million persons worldwide, with a prevalence of 183 million adult males and 145 million adult females. In no syndromic deafness, only hearing function is noticeably altered, whereas syndromic deafness is accompanied by other physiological defects. Hearing loss can be caused by environmental factors, such as exposure to noise or ototoxic chemicals, or by aged related senescence. Traumatic injury, such as injury caused by exposure to an explosion or to the firing of a gun, can lead to sudden hearing loss. Sometimes this hearing loss is accompanied by the perception of a constant ringing noise called tinnitus [17]. Moreover, genetic factors as mutations in MT-TS1, MYO7A or ACTG1 genes [18,19], between many others, have already been linked to nonsyndromic hearing loss. Noise exposure is responsible for approximately 10% of hearing loss in adults, in particular military veterans [20]. Short impulses of high intensity noise such as a gunshot or explosion can trigger sudden hearing loss, which is generally irreversible and associated with structural tissue damage of cochlea and auditory nerve. Susceptibility to damaging effects of noise differs remarkably ampled to the ribbon synapses, whereas the myelinated axons extend from the habenula perforate to the SGN stomata. Once the central SGN process enters the CNS, it becomes myelinated by oligodendrocytes.Two subpopulations of SGNs exist: type I, which account for 90%-95% of total SGNs, and type II, which comprise the remaining 5%-10% of SGNs. The cell bodies of both neuron types are housed within Rosenthal’s canal and extend peripheral axons through the osseous spiral lamina to form connections with hair cells in the organ of Corte. Type I SGNs extend a single axon to a single inner hair cell (IHC) and type II SGNs innervate multiple outer hair cells (OHC) with numerous axonal projections [21]. Type I SGN somas are covered by myelinating satellite cells and their axons are unsheathed by myelinating Schwann cells. Conversely, type II SGNs are enveloped by non-myelinating satellite and Schwann cells. Recent studies have demonstrated that migration, maturity and survival of SGNs is largely dependent on surrounding glial cells.
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