Age-related reductions in number and size of anterior horn cells at C6 level of the human spinal cord

Common synaptic inputs and persistent inward currents of vastus lateralis motor units are reduced in older male adults

Although muscle atrophy may partially account for age-related strength decline, it is further influenced by alterations of neural input to muscle. Persistent inward currents (PIC) and the level of common synaptic inputs to motoneurons influence neuromuscular function. However, these have not yet been described in the aged human quadriceps. High-density surface electromyography (HDsEMG) signals were collected from the vastus lateralis of 15 young (mean ± SD, 23 ± 5 y) and 15 older (67 ± 9 y) men during submaximal sustained and 20-s ramped contractions. HDsEMG signals were decomposed to identify individual motor unit discharges, from which PIC amplitude and intramuscular coherence were estimated. Older participants produced significantly lower knee extensor torque (p < 0.001) and poorer force tracking ability (p < 0.001) than young. Older participants also had lower PIC amplitude (p = 0.001) and coherence estimates in the alpha frequency band (p < 0.001) during ramp contractions when compared to young. Persistent inward currents and common synaptic inputs are lower in the vastus lateralis of older males when compared to young. These data highlight altered neural input to the clinically and functionally important quadriceps, further underpinning age-related loss of function which may occur independently of the loss of muscle mass.

Clinical outcomes of peripheral nerve interfaces for rehabilitation in paralysis and amputation: a literature review

Peripheral nerve interfaces (PNIs) are electrical systems designed to integrate with peripheral nerves in patients, such as following central nervous system (CNS) injuries to augment or replace CNS control and restore function. We review the literature for clinical trials and studies containing clinical outcome measures to explore the utility of human applications of PNIs. We discuss the various types of electrodes currently used for PNI systems and their functionalities and limitations. We discuss important design characteristics of PNI systems, including biocompatibility, resolution and specificity, efficacy, and longevity, to highlight their importance in the current and future development of PNIs. The clinical outcomes of PNI systems are also discussed. Finally, we review relevant PNI clinical trials that were conducted, up to the present date, to restore the sensory and motor function of upper or lower limbs in amputees, spinal cord injury patients, or intact individuals and describe their significant findings. This review highlights the current progress in the field of PNIs and serves as a foundation for future development and application of PNI systems.

Sarcopenia of the longitudinal tongue muscles in rats

The tongue is a muscular hydrostat, with lingual movements occurring during breathing, chewing, swallowing, vocalization, vomiting, coughing and grooming/sexual activities. In the elderly, reduced lingual dysfunction and weakness contribute to increased risks of obstructive sleep apnea and aspiration pneumonia. In Fischer 344 (F344) rats, a validated model of aging, hypoglossal motor neuron death is apparent, although there is no information regarding tongue strength. The intrinsic tongue muscles, the superior and inferior longitudinal, transversalis and verticalis exist in an interdigitated state. Recently, we established a method to measure the specific force of individual intrinsic tongue muscle, accounting for the tissue bulk that is not in the direction of uniaxial force. In the longitudinal muscles of 6- (n = 10), 18- (n = 9) and 24-month-old (n = 12) female and male F344 rats, we assessed specific force, fatigability, fiber type dependent cross-sectional area (CSA) and overall CSA. Muscle force and fatigue was assessed ex vivo using platinum plate simulation electrodes. Tongue muscles were frozen in melting isopentane, and transverse sections cut at 10 µm. Muscle fiber type was classified based on immunoreactivity to myosin heavy chain (MyHC) isoform antibodies. In H&E stained muscle, CSA and uniaxial muscle contributions to total tongue bulk was assessed. We observed a robust ∼30% loss of longitudinal specific force, with reductions in overall longitudinal muscle fiber CSA and specific atrophy of type IIx/IIb fibers. It will be important to investigate the mechanistic underpinnings of hypoglossal motor neuron death and tongue muscle weakness to eventually provide therapies for age-associated lingual dysfunctions.

CHIT1-positive microglia drive motor neuron ageing in the primate spinal cord

Ageing is a critical factor in spinal-cord-associated disorders1, yet the ageing-specific mechanisms underlying this relationship remain poorly understood. Here, to address this knowledge gap, we combined single-nucleus RNA-sequencing analysis with behavioural and neurophysiological analysis in non-human primates (NHPs). We identified motor neuron senescence and neuroinflammation with microglial hyperactivation as intertwined hallmarks of spinal cord ageing. As an underlying mechanism, we identified a neurotoxic microglial state demarcated by elevated expression of CHIT1 (a secreted mammalian chitinase) specific to the aged spinal cords in NHP and human biopsies. In the aged spinal cord, CHIT1-positive microglia preferentially localize around motor neurons, and they have the ability to trigger senescence, partly by activating SMAD signalling. We further validated the driving role of secreted CHIT1 on MN senescence using multimodal experiments both in vivo, using the NHP spinal cord as a model, and in vitro, using a sophisticated system modelling the human motor-neuron-microenvironment interplay. Moreover, we demonstrated that ascorbic acid, a geroprotective compound, counteracted the pro-senescent effect of CHIT1 and mitigated motor neuron senescence in aged monkeys. Our findings provide the single-cell resolution cellular and molecular landscape of the aged primate spinal cord and identify a new biomarker and intervention target for spinal cord degeneration.

Loss of larger hypoglossal motor neurons in aged Fischer 344 rats

The intrinsic (longitudinal, transversalis and verticalis) and extrinsic (genioglossus, styloglossus, hyoglossus and geniohyoid) tongue muscles are innervated by hypoglossal motor neurons (MNs). Tongue muscle activations occur during many behaviors: maintaining upper airway patency, chewing, swallowing, vocalization, vomiting, coughing, sneezing and grooming/sexual activities. In the tongues of the elderly, reduced oral motor function and strength contribute to increased risk of obstructive sleep apnoea. Tongue muscle atrophy and weakness is also described in rats, yet hypoglossal MN numbers are unknown. In young (6-months, n=10) and old (24-months, n=8) female and male Fischer 344 (F344) rats, stereological assessment of hypoglossal MN numbers and surface areas were performed on 16µm Nissl-stained brainstem cryosections. We observed a robust loss of ~15% of hypoglossal MNs and a modest ~8% reduction in their surface areas with age. In the larger size tertile of hypoglossal MNs, age-associated loss of hypoglossal MNs approached ~30% These findings uncover a potential neurogenic locus of pathology for age-associated tongue dysfunctions.

Chemogenetic inhibition of TrkB signalling reduces phrenic motor neuron survival and size

Brain derived neurotrophic factor (BDNF) signalling through its high-affinity tropomyosin receptor kinase B (TrkB) is known to have potent effects on motor neuron survival and morphology during development and in neurodegenerative diseases. Here, we employed a novel 1NMPP1 sensitive TrkBF616 rat model to evaluate the effect of 14 days inhibition of TrkB signalling on phrenic motor neurons (PhMNs). Adult female and male TrkBF616 rats were divided into 1NMPP1 or vehicle treated groups. Three days prior to treatment, PhMNs in both groups were initially labeled via intrapleural injection of Alexa-Fluor-647 cholera toxin B (CTB). After 11 days of treatment, retrograde axonal uptake/transport was assessed by secondary labeling of PhMNs by intrapleural injection of Alexa-Fluor-488 CTB. After 14 days of treatment, the spinal cord was excised 100 μm thick spinal sections containing PhMNs were imaged using two-channel confocal microscopy. TrkB inhibition reduced the total number of PhMNs by ∼16 %, reduced the mean PhMN somal surface areas by ∼25 %, impaired CTB uptake 2.5-fold and reduced the estimated PhMN dendritic surface area by ∼38 %. We conclude that inhibition of TrkB signalling alone in adult TrkBF616 rats is sufficient to lead to PhMN loss, morphological degeneration and deficits in retrograde axonal uptake/transport.

Aging alters mechanisms underlying voluntary movements in spinal motor neurons of mice, primates, and humans

Spinal motor neurons have been implicated in the loss of motor function that occurs with advancing age. However, the cellular and molecular mechanisms that impair the function of these neurons during aging remain unknown. Here, we show that motor neurons do not die in old female and male mice, rhesus monkeys, and humans. Instead, these neurons selectively and progressively shed excitatory synaptic inputs throughout the soma and dendritic arbor during aging. Thus, aged motor neurons contain a motor circuitry with a reduced ratio of excitatory to inhibitory synapses that may be responsible for the diminished ability to activate motor neurons to commence movements. An examination of the motor neuron translatome (ribosomal transcripts) in male and female mice reveals genes and molecular pathways with roles in glia-mediated synaptic pruning, inflammation, axonal regeneration, and oxidative stress that are upregulated in aged motor neurons. Some of these genes and pathways are also found altered in motor neurons affected with amyotrophic lateral sclerosis (ALS) and responding to axotomy, demonstrating that aged motor neurons are under significant stress. Our findings show mechanisms altered in aged motor neurons that could serve as therapeutic targets to preserve motor function during aging.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

Clinical Indicators of Surgical Outcomes After Laminoplasty for Patients With Cervical Ossification of the Posterior Longitudinal Ligament: A Prospective Multicenter Study

STUDY DESIGN

A prospective multicenter study.

OBJECTIVE

This study aims to evaluate patient-reported outcomes using the Japanese Orthopedic Association Cervical Myelopathy Evaluation Questionnaire (JOACMEQ) and clarify clinical factors that affect the therapeutic effects for patients with cervical ossification of the posterior longitudinal ligament (OPLL).

SUMMARY OF BACKGROUND DATA

Although previous studies identified factors that affected the surgical outcomes, their assessment was mainly based on the Japanese Orthopedic Association score, which only includes neurological function. Investigating this pathology through multiple functions and quality of life (QOL) is pivotal to understanding the comprehensive clinical pictures of the cervical OPLL and its therapeutic outcomes.

MATERIALS AND METHODS

This study was performed by the Japanese Multicenter Research Organization for Ossification of the Spinal Ligament. A total of 478 patients with myelopathy caused by cervical OPLL from 28 institutions were prospectively registered from 2014 to 2017 and followed up for 2 years. Of the patients, 168 received laminoplasties and fully completed questionnaires. Demographic information, imaging findings, and clinical outcomes were collected. Patients were grouped according to effective or ineffective surgical outcomes as defined by the JOACMEQ using logistic regression analyses.

RESULTS

Laminoplasty resulted in functional improvement in the cervical spine and upper extremity around 40% of the patients, while QOL showed only 21.4% ( P <0.01). Multivariable analyses revealed that younger age and a postoperative decrease in arm or hand pain were correlated with significantly improved function of the upper extremities. A reduction in lower limb pain favorably affected the postoperative lower extremity function. A postoperative reduction in upper extremity pain enhanced the QOL recovery.

CONCLUSIONS

Surgeons should recognize the diversity of surgical outcomes after laminoplasty and understand the necessity of pain management even after the surgery to enhance bodily functions and QOL in patients with cervical OPLL.

Changes in neuromuscular function in elders: Novel techniques for assessment of motor unit loss and motor unit remodeling with aging

Functional muscle fiber denervation is a major contributor to the decline in physical function observed with aging and is now a recognized cause of sarcopenia, a muscle disorder characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength. There is an interrelationship between muscle strength, motor unit (MU) number, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functional MUs. During normal aging, there is a time-related progression of MU loss, an adaptive sprouting followed by a maladaptive sprouting, and continuing recession of terminal Schwann cells leading to a reduced capacity for compensatory reinnervation in elders. In amyotrophic lateral sclerosis, increasing age at onset predicts worse survival ALS and it is possible that age-related depletion of the motor neuron pool may worsen motor neuron disease. MUNE methods are used to estimate the number of functional MU, data from MUNIX arguing for motor neuron loss with aging will be reviewed. Recently, a new MRI technique MU-MRI could be used to assess the MU recruitment or explore the activity of a single MU. This review presents published studies on the changes of neuromuscular function with aging, then focusing on these two novel techniques for assessment of MU loss and MU remodeling.

Pharmacologic treatment with OKN-007 reduces alpha-motor neuron loss in spinal cord of aging mice

Aging is associated with molecular and functional declines in multiple physiologic systems. We have previously reported age-related changes in spinal cord that included a decline in α-motor neuron numbers, axonal loss, and demyelination associated with increased inflammation and blood-spinal cord barrier (BSCB) permeability. These changes may influence other pathologies associated with aging, in particular loss of muscle mass and function (sarcopenia), which we and others have shown is accompanied by neuromuscular junction disruption and loss of innervation. Interventions to protect and maintain motor neuron viability and function in aging are currently lacking and could have a significant impact on improving healthspan. Here we tested a promising compound, OKN-007, that has known antioxidant, anti-inflammatory and neuroprotective properties, as a potential intervention in age-related changes in the spinal cord. OKN-007 is a low molecular weight disulfonyl derivative of (N-tert Butyl-α-phenylnitrone) (PBN) that can easily cross the blood-brain barrier. We treated middle age (16 month) wild-type male mice with OKN-007 in drinking water at a dose of 150 mg/kg/day until 25 months of age. OKN-007 treatment exerted a number of beneficial effects in the aging spinal cord, including a 35% increase in the number of lumbar α-motor neurons in OKN-treated old mice compared to age-matched controls. Brain spinal cord barrier permeability, which is increased in aging spinal cord, was also blunted by OKN-007 treatment. Age-related changes in microglia proliferation and activation are blunted by OKN-007, while we found no effect on astrocyte proliferation. Transcriptome analysis identified expression changes in a number of genes that are involved in neuronal structure and function and revealed a subset of genes whose changes in response to aging are reversed by OKN-007 treatment. Overall, our findings suggest that OKN-007 exerts neuroprotective and anti-inflammatory effects on the aging spinal cord and support OKN-007 as a potential therapeutic to improve α-motor neuron health.

The emerging role of the sympathetic nervous system in skeletal muscle motor innervation and sarcopenia

Examining neural etiologic factors'role in the decline of neuromuscular function with aging is essential to our understanding of the mechanisms underlying sarcopenia, the age-dependent decline in muscle mass, force and power. Innervation of the skeletal muscle by both motor and sympathetic axons has been established, igniting interest in determining how the sympathetic nervous system (SNS) affect skeletal muscle composition and function throughout the lifetime. Selective expression of the heart and neural crest derivative 2 gene in peripheral SNs increases muscle mass and force regulating skeletal muscle sympathetic and motor innervation; improving acetylcholine receptor stability and NMJ transmission; preventing inflammation and myofibrillar protein degradation; increasing autophagy; and probably enhancing protein synthesis. Elucidating the role of central SNs will help to define the coordinated response of the visceral and neuromuscular system to physiological and pathological challenges across ages. This review discusses the following questions: (1) Does the SNS regulate skeletal muscle motor innervation? (2) Does the SNS regulate presynaptic and postsynaptic neuromuscular junction (NMJ) structure and function? (3) Does sympathetic neuron (SN) regulation of NMJ transmission decline with aging? (4) Does maintenance of SNs attenuate aging sarcopenia? and (5) Do central SN group relays influence sympathetic and motor muscle innervation?

TrkB signaling contributes to transdiaphragmatic pressure generation in aged mice

Ventilatory deficits are common in old age and may result from neuromuscular dysfunction. Signaling via the tropomyosin-related kinase receptor B (TrkB) regulates neuromuscular transmission and, in young mice, is important for the generation of transdiaphragmatic pressure (Pdi). Loss of TrkB signaling worsened neuromuscular transmission failure and reduced maximal Pdi, and these effects are similar to those observed in old age. Administration of TrkB agonists such as 7,8-dihydroxyflavone (7,8-DHF) improves neuromuscular transmission in young and old mice (18 mo; 75% survival). We hypothesized that TrkB signaling contributes to Pdi generation in old mice, particularly during maximal force behaviors. Old male and female TrkBF616A mice, with a mutation that induces 1NMPP1-mediated TrkB kinase inhibition, were randomly assigned to systemic treatment with vehicle, 7,8-DHF, or 1NMPP1 1 h before experiments. Pdi was measured during eupneic breathing (room air), hypoxia-hypercapnia (10% O2/5% CO2), tracheal occlusion, spontaneous deep breaths ("sighs"), and bilateral phrenic nerve stimulation (Pdimax). There were no differences in the Pdi amplitude across treatments during ventilatory behaviors (eupnea, hypoxia-hypercapnia, occlusion, or sigh). As expected, Pdi increased from eupnea and hypoxia-hypercapnia (∼7 cm H2O) to occlusion and sighs (∼25 cm H2O), with no differences across treatments. Pdimax was ∼50 cm H2O in the vehicle and 7,8-DHF groups and ∼40 cm H2O in the 1NMPP1 group (F8,74 = 2; P = 0.02). Our results indicate that TrkB signaling is necessary for generating maximal forces by the diaphragm muscle in old mice and are consistent with aging effects of TrkB signaling on neuromuscular transmission.NEW & NOTEWORTHY TrkB signaling is necessary for generating maximal forces by the diaphragm muscle. In 19- to 21-mo-old TrkBF616A mice susceptible to 1NMPP1-induced inhibition of TrkB kinase activity, maximal Pdi generated by bilateral phrenic nerve stimulation was ∼20% lower after 1NMPP1 compared with vehicle-treated mice. Treatment with the TrkB agonist 7,8-dihydroxyflavone did not affect Pdi generation when compared with age-matched mice. Inhibition of TrkB kinase activity did not affect the forces generated during lower force behaviors in old age.

Impairment Mechanisms and Intervention Approaches for Aged Human Neuromuscular Junctions

The neuromuscular junction (NMJ) is a chemical synapse formed between a presynaptic motor neuron and a postsynaptic muscle cell. NMJs in most vertebrate species share many essential features; however, some differences distinguish human NMJs from others. This review will describe the pre- and postsynaptic structures of human NMJs and compare them to NMJs of laboratory animals. We will focus on age-dependent declines in function and changes in the structure of human NMJs. Furthermore, we will describe insights into the aging process revealed from mouse models of accelerated aging. In addition, we will compare aging phenotypes to other human pathologies that cause impairments of pre- and postsynaptic structures at NMJs. Finally, we will discuss potential intervention approaches for attenuating age-related NMJ dysfunction and sarcopenia in humans.

Influence of age on the frequency characteristics of the soleus muscle response to Achilles tendon vibration during standing

KEY POINTS

Proprioceptive sensory information from the ankle joint is critical for the control of upright posture and balance. We examined the influence of age (n = 54 healthy adults, 20-82 years old) on lower limb muscle responses to proprioceptive perturbations evoked by Achilles tendon vibration during standing. The frequency bandwidth of the muscle response became narrower, and the gain (the muscle response relative to the stimulus) and scaling (increases in response amplitude with increases in stimulus amplitude) decreased with age. Mechanics of the muscle-tendon unit (mechanical admittance) did not differ with age during standing, and thus probably did not mediate the age-related changes observed in soleus muscle responses to vibration. These findings add to our understanding of how altered proprioceptive responses may contribute to impaired mobility and falls with ageing.

ABSTRACT

Proprioceptive information from the ankle joint plays an important role in the control of upright posture and balance. Ageing influences many components of the sensorimotor system, which leads to poor mobility and falls. However, little is known about the influence of age on the characteristics of short latency muscle responses to proprioceptive stimuli during standing across frequencies that are encoded by muscle spindles. We examined the frequency characteristics of the soleus muscle response to noisy (10-115 Hz) Achilles tendon vibration during standing in 54 healthy adults across a broad age range (20-82 years). The results showed the frequency bandwidth of the soleus response (vibration-electromyography coherence) became progressively narrower with ageing. Coherence was significantly lower in middle-aged relative to young adults between ∼7-11 and 28-62 Hz, lower in older relative to middle-aged adults between ∼30-50 Hz and lower in older relative to young adults between ∼7-64 Hz. Muscle response gain was similar between age groups at low frequencies, although gain was lower in older relative to young adults between ∼28-54 Hz. Across the age range, the response amplitude (peak-to-peak cross-covariance) and the scaling of the response with stimulus amplitude were both negatively correlated with age. Muscle-tendon mechanics (admittance) did not differ with age, suggesting this did not mediate differences in soleus responses. Our findings suggest there is a progressive change in the soleus response to proprioceptive stimuli with ageing during standing, which could contribute to poorer mobility and falls.

Diaphragm neuromuscular transmission failure in aged rats

In aging Fischer 344 rats, phrenic motor neuron loss, neuromuscular junction abnormalities, and diaphragm muscle (DIAm) sarcopenia are present by 24 mo of age, with larger fast-twitch fatigue-intermediate (type FInt) and fast-twitch fatigable (type FF) motor units particularly vulnerable. We hypothesize that in old rats, DIAm neuromuscular transmission deficits are specific to type FInt and/or FF units. In phrenic nerve/DIAm preparations from rats at 6 and 24 mo of age, the phrenic nerve was supramaximally stimulated at 10, 40, or 75 Hz. Every 15 s, the DIAm was directly stimulated, and the difference in forces evoked by nerve and muscle stimulation was used to estimate neuromuscular transmission failure. Neuromuscular transmission failure in the DIAm was observed at each stimulation frequency. In the initial stimulus trains, the forces evoked by phrenic nerve stimulation at 40 and 75 Hz were significantly less than those evoked by direct muscle stimulation, and this difference was markedly greater in 24-mo-old rats. During repetitive nerve stimulation, neuromuscular transmission failure at 40 and 75 Hz worsened to a greater extent in 24-mo-old rats compared with younger animals. Because type IIx and/or IIb DIAm fibers (type FInt and/or FF motor units) display greater susceptibility to neuromuscular transmission failure at higher frequencies of stimulation, these data suggest that the age-related loss of larger phrenic motor neurons impacts nerve conduction to muscle at higher frequencies and may contribute to DIAm sarcopenia in old rats. NEW & NOTEWORTHY Diaphragm muscle (DIAm) sarcopenia, phrenic motor neuron loss, and perturbations of neuromuscular junctions (NMJs) are well described in aged rodents and selectively affect FInt and FF motor units. Less attention has been paid to the motor unit-specific aspects of nerve-muscle conduction. In old rats, increased neuromuscular transmission failure occurred at stimulation frequencies where FInt and FF motor units exhibit conduction failures, along with decreased apposition of pre- and postsynaptic domains of DIAm NMJs of these units.

Spinal motor neurons and motor function in older adults

This study examined the relation between lumbar spinal motor neuron (SMN) indices and motor function proximate to death in community-dwelling older adults. Older adults (N = 145) participating in the Rush Memory and Aging Project underwent structured clinical testing proximate to death and brain and spinal cord autopsy at time of death. Ten motor performances were summarized by a composite global motor score. Choline acetyltransferase immunostaining was used to identify spinal motor neurons of the L4/5 segment. SMN counts and area and ventral horn area were collected. Linear regression modeling showed that the association of SMN counts and density with global motor scores proximate to death varied with sex. Separate models in men and women showed that this significant interaction was due to the association of higher SMN counts and density with higher global motor scores proximate to death in men but not women. These associations were unchanged when we controlled for indices of brain pathologies or chronic health conditions. In 38 cases with counts of activated microglia available, higher counts of activated microglia were associated with lower SMN counts. Activated spinal microglia and loss of spinal motor neurons may contribute to motor impairments in older men.

Decomposing sensorimotor variability changes in ageing and their connection to falls in older people

The relationship between sensorimotor variability and falls in older people has not been well investigated. We developed a novel task having shared biomechanics of obstacle negotiation to quantify sensorimotor variability related to locomotion across age. We found that sensorimotor variability in foot placement increases continuously with age. We then applied sensory psychophysics to pinpoint the visual and somatosensory systems associated with sensorimotor variability. We showed increased sensory variability, specifically increased proprioceptive variability, the vital cause of more variable foot placement in older people (greater than 65 years). Notably, older participants relied more on the vision to judge their own foot’s height compared to the young, suggesting a shift in multisensory integration strategy to compensate for degenerated proprioception. We further modelled the probability of tripping-over based on the relationship between sensorimotor variability and age and found a correspondence between model prediction and community-based data. We reveal increased sensorimotor variability, modulated by sensation precision, a potentially vital mechanism of raised tripping-over and thus fall events in older people. Analysis of sensorimotor variability and its specific components may have the utility of fall risk and rehabilitation target evaluation.

α-Motor neurons are spared from aging while their synaptic inputs degenerate in monkeys and mice

Motor function deteriorates with advancing age, increasing the risk of adverse health outcomes. While it is well established that skeletal muscles and neuromuscular junctions (NMJs) degenerate with increasing age, the effect of aging on α‐motor neurons and their innervating synaptic inputs remains largely unknown. In this study, we examined the soma of α‐motor neurons and innervating synaptic inputs in the spinal cord of aged rhesus monkeys and mice, two species with vastly different lifespans. We found that, in both species, α‐motor neurons retain their soma size despite an accumulation of large amounts of cellular waste or lipofuscin. Interestingly, the lipofuscin profile varied considerably, indicating that α‐motor neurons age at different rates. Although the rate of aging varies, α‐motor neurons do not atrophy in old age. In fact, there is no difference in the number of motor axons populating ventral roots in old mice compared to adult mice. Moreover, the transcripts and proteins associated with α‐motor neurons do not decrease in the spinal cord of old mice. However, in aged rhesus monkeys and mice, there were fewer cholinergic and glutamatergic synaptic inputs directly abutting α‐motor neurons, evidence that aging causes α‐motor neurons to shed synaptic inputs. Thus, the loss of synaptic inputs may contribute to age‐related dysfunction of α‐motor neurons. These findings broaden our understanding of the degeneration of the somatic motor system that precipitates motor dysfunction with advancing age.

Breathing: Motor Control of Diaphragm Muscle

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

Phrenic motor neuron loss in aged rats

Sarcopenia is the age-related reduction of muscle mass and specific force. In previous studies, we found that sarcopenia of the diaphragm muscle (DIAm) is evident by 24 mo of age in both rats and mice and is associated with selective atrophy of type IIx and IIb muscle fibers and a decrease in maximum specific force. These fiber type-specific effects of sarcopenia resemble those induced by DIAm denervation, leading us to hypothesize that sarcopenia is due to an age-related loss of phrenic motor neurons (PhMNs). To address this hypothesis, we determined the number of PhMNs in young (6 mo old) and old (24 mo old) Fischer 344 rats. Moreover, we determined age-related changes in the size of PhMNs, since larger PhMNs innervate type IIx and IIb DIAm fibers. The PhMN pool was retrogradely labeled and imaged with confocal microscopy to assess the number of PhMNs and the morphometry of PhMN soma and proximal dendrites. In older animals, there were 22% fewer PhMNs, a 19% decrease in somal surface area, and a 21% decrease in dendritic surface area compared with young Fischer 344 rats. The age-associated loss of PhMNs involved predominantly larger PhMNs. These results are consistent with an age-related denervation of larger, more fatigable DIAm motor units, which are required primarily for high-force airway clearance behaviors. NEW & NOTEWORTHY Diaphragm muscle sarcopenia in rodent models is well described in the literature; however, the relationship between sarcopenia and frank phrenic motor neuron (MN) loss is unexplored in these models. We quantify a 22% loss of phrenic MNs in old (24 mo) compared with young (6 mo) Fischer 344 rats. We also report reductions in phrenic MN somal and proximal dendritic morphology that relate to decreased MN heterogeneity in old compared with young Fischer 344 rats.

Sarcopenia: Neurological Point of View

Sarcopenia is an age-related geriatric syndrome which is characterized by the gradual loss of muscle mass, muscle strength, and muscle quality. There are a lot of neurologic insults on sarcopenia at various levels from the brain to the neuromuscular junctions (NMJs) to generate a volitional task. Dopaminergic downregulation, inadequate motor programming and motor coordination impairment lead to decline of supraspinal drive. Motor unit reorganization and inflammatory changes in motor neuron decrease conduction velocity and amplitude of compound muscle action potential. Furthermore, NMJ remodeling and age related neurophysiological alterations may contribute to neuromuscular impairment. Sarcopenia is an age-associated, lifelong process which links to multiple etiological factors. Although not all the causes are completely understood, we suggest that compromised nervous system function may be one of the important contributors to the sarcopenia.

Age-related surgical outcomes of laminoplasty for cervical spondylotic myelopathy

Study Design Retrospective clinical study. Objective To investigate the age-related surgical outcomes of laminoplasty. Methods One hundred patients who underwent an en bloc laminoplasty for cervical spondylotic myelopathy from 2004 to 2008 and were followed for at least 1 year were included in this study. The clinical outcomes were assessed with the Japanese Orthopaedic Association (JOA) score. Acquired points (postoperative JOA score minus preoperative JOA score) were also calculated. To investigate the age-related effect for laminoplasty, two analyses were conducted: (1) the correlation between age and clinical outcome; and (2) the clinical outcomes by decade. Patients were divided into four groups according to their age at the time of operation as follows: group 50s, 50 to 59 years old; group 60s, 60 to 69 years; group 70s, 70 to 79 years; and group 80s, 80 to 89 years. The pre- and postoperative JOA scores, acquired points, preoperative comorbidities, and postoperative complications were then compared among the groups. Results Significant correlations were detected between age and JOA scores at the preoperative (p = 0.03), postoperative maximum (p < 0.0001), and final assessments (p < 0.0001). An age-related decline of JOA scores was observed over all periods. The analysis by decades showed the same results. On the other hand, the significant differences were not found for acquired points over all periods by either method. The preoperative comorbidities of hypertension and diabetes mellitus increased with age. Delirium was more common postoperatively in elderly patients. Conclusions Although an age-related decline of JOA scores was found over all periods, there were no severe sequelae and no differences in the acquired points that were age-related.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Relationships between motor unit size and recruitment threshold in older adults: implications for size principle

As a part of the aging process, motor unit reorganization occurs in which small motoneurons reinnervate predominantly fast-twitch muscle fibers that have lost their innervation. We examined the relationship between motor unit size and the threshold force for recruitment in two muscles to determine whether older individuals might develop an alternative pattern of motor unit activation. Young and older adults performed isometric contractions ranging from 0 to 50% of maximal voluntary contraction in both the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles. Muscle fiber action potentials were recorded with an intramuscular needle electrode and motor unit size was computed using spike-triggered averaging of the global EMG signal (macro EMG), which was also obtained from the intramuscular needle electrode. As expected, older individuals exhibited larger motor units than young subjects in both the FDI and the TA. However, moderately strong correlations were obtained for the macro EMG amplitude versus recruitment threshold relationship in both the young and older adults within both muscles, suggesting that the size principle of motor unit recruitment seems to be preserved in older adults.

The effects of aging on hypoglossal motoneurons in rats

Aging can result in a loss of neuronal cell bodies and a decrease in neuronal size in some regions of the brain and spinal cord. Motoneuron loss in the spinal cord is thought to contribute to the progressive decline in muscle mass and strength that occurs with age (sarcopenia). Swallowing disorders represent a large clinical problem in elderly persons; however, age-related alterations in cranial motoneurons that innervate muscles involved in swallowing have been understudied. We aimed to determine if age-related alterations occurred in the hypoglossal nucleus in the brainstem. If present, these changes might help explain alterations at the neuromuscular junction and changes in the contractile properties of tongue muscle that have been reported in older rats. We hypothesized that with increasing age there would be a loss of motoneurons and a reduction in neuronal size and the number of primary dendrites associated with each hypoglossal motoneuron. Neurons in the hypoglossal nucleus were visualized with the neuronal marker NeuN in young (9-10 months), middle-aged (24-25 months), and old (32-33 months) male F344/BN rats. Hypoglossal motoneurons were retrograde-labeled with injections of Cholera Toxin beta into the genioglossus muscle of the tongue and visualized using immunocytochemistry. Results indicated that the number of primary dendrites of hypoglossal motoneurons decreased significantly with age, while no age-associated changes were found in the number or size of hypoglossal motoneurons. Loss of primary dendrites could reduce the number of synaptic inputs and thereby impair function.

Role of sustained overexpression of central nervous system IGF-I in the age-dependent decline of mouse excitation-contraction coupling

We investigated the effects of exclusive and sustained transgenic overexpression of insulin-like growth factor (IGF)-I in the central nervous system (CNS) on the age-dependent decline in muscle strength, excitation-contraction coupling, muscle innervation and neuromuscular junction postterminal architecture. We found that (1) transgenic IGF-I overexpression in the CNS does not modify the decline in extensor digitorum longus (EDL) and soleus muscle weight with aging and (2) strength significantly decreases in transgenic (Tg) compared to wild-type mice. The latter finding is consistent with (3) the decreased absolute and specific force measured in the EDL muscle in vitro and (4) the decreased charge movement and peak intracellular Ca(2+) mobilization in individual muscle fibers from old IGF-I Tg mice compared to young wild-type mice, which also is associated with (5) decreased dihydropyridine receptor alpha(1)-subunit expression in old compared to young IGF-I Tg mice. (6) Tg IGF-I prevents a change in muscle fiber type that is associated with (7) improved muscle innervation and postterminal neuromuscular structure. (8) IGF-I is expressed extensively across the spinal cord gray matter and the lateral motor column. Our results raise questions about the timing and cell location of CNS IGF-I overexpression necessary to prevent or to ameliorate age-dependent alterations in the structure and function of skeletal muscle.

Age-related changes in histogram pattern of anterior horn cells in human cervical spinal cord

The purpose of the present study was to clarify age-related changes in histograms of spinal anterior horn cells. The study examined Rexed lamina IX of the C7 spinal cord segment in 22 men who had died of non-spinal disease (age range, 0-85 years). First, we confirmed that the size of nucleoli exhibited a linear relationship to the diameter of spinal anterior horn cells by preparing histograms of nucleoli. Second, this formula was used to create histograms of cervical anterior horn cells. Results were as follows: (i) diameter of nucleoli ranged from 2.0 microm to 6.0 microm; (ii) in each subject, no changes were seen in histogram patterns among ventral, intermediate, dorsal and overall sections; (iii) at < or =20 years of age, histograms displayed a single peak for the diameter of nucleoli at about 4.0-4.5 microm; (iv) at 21-60 years of age, histograms displayed two peaks, at about 3.5-4.0 microm and 5.0-5.5 microm; and (v) at 61-85 years of age, histograms displayed a single peak at about 5.0-5.5 microm.

Morphometric evaluations of the human nervous system

It is very important to evaluate and accurately understand the various conditions of the human nervous system. In this review article, we introduce several morphometric reports that are proven to be accurate from the view point of various errors (range of tissue shrinkage ratios, microscopic multiple counting, artifacts of microscopic structures, etc.). We review the following aspects of the selected reports: methodology, developmental research, neuronal differences, gender differences, aging process and miscellaneous (nerve fibers, unmyelinated fibers, in relation to neuropathology, clinical image-analysis and immunohistochemistry).

Evidence of self-sustained motoneuron firing in young and older adults

Motoneurons demonstrate a type of self-sustained firing behavior that seems to be produced by a prolonged period of depolarization caused by intrinsic long-term changes in the motoneuron. Such self-sustained firing behavior has previously been reported in human motor units. The purpose of the present study was to investigate the occurrence of self-sustained firing behavior in older adults. Eight young (mean age 24 yrs) and eight older (mean age 73 yrs) individuals participated in the investigation. While subjects produced light dorsiflexion contractions, a brief vibration stimulus was applied to the tibialis anterior muscle. Motor unit recordings were also obtained from the tibialis anterior muscle. Self-sustained firing behavior was evidenced by the appearance of new motor unit recruitment following vibration, even as the motor units that fired before the vibratory stimulus maintained a steady firing rate. The proportion of motor units exhibiting self-sustained firing activity was similar in both young and older adults (approx. 23% of trials). We conclude that self-sustained firing behavior is a ubiquitous phenomenon that does not seem to be affected by the aging process.

Morphometric comparison of human nerve cells: pyramidal motor system

We compared morphological and morphometric data on various motor neurons in the human pyramidal system using the modified Klüver-Barrera staining method with extremely minimized shrinkage ratio and an image-analyzer. We classified motor neurons in the human pyramidal system into three groups according to the measurement data. This report may be of interest to better understand the process of nerve conduction in the human pyramidal system.

Morphometric comparison of human spinal cord anterior horn cells between the cervical and lumbar levels

Morphometric comparisons of anterior horn cells (AHCs) were conducted between cervical and lumbar spinal cords in 29 cadavers. The sizes of the AHCs were always larger at the lumbar level than at cervical level irrespective of the age of the spicemens, but a decrease in size with age was observed at both levels. The correlation coefficient shows a larger decrease in numbers and volumes at lumbar level than at cervical level.

Morphometric analyses of axons in the human lateral corticospinal tract: cervical/lumbar level comparison and relation to the ageing process

Myelinated axons in the human lateral corticospinal tract (LCST) at the C6 and L4 levels were examined on 16 male cadavers, with age ranging from 41 to 88 years. The average area of axons in the LCST was measured using a microscopic image-analyzing system. Our data show that the average area of axons at the C6 and L4 levels decreased with age. We also noticed that the average area of axons was larger at the C6 level than at the L4 level in all cases. This decrease may be related to a parallel decrease of conduction velocity in the LCST during the ageing process.

Surgical treatment for cervical myelopathy in patients aged > 80 years

The surgical outcomes of 13 patients who were diagnosed with cervical spondylotic myelopathy were reviewed retrospectively. Mean patient age at surgery was 83 years. The severity of cervical spondylotic myelopathy was evaluated using the Japanese Orthopaedic Association score. Daily activities were evaluated using the Barthel index. The preoperative JOA score and Barthel index were 7.8 and 63.5, respectively. The mean JOA score and Barthel index maximum recovery rate were 35% and 24%, respectively. The results of this study imply that surgery for patients with cervical spondylotic myelopathy aged > 80 years is warranted.

Target-derived trophic effect on skeletal muscle innervation in senescent mice

In the present work, we tested the hypothesis that target-derived insulin-like growth factor-1 (IGF-1) prevents alterations in neuromuscular innervation in aging mammals. To explore this hypothesis, we studied senescent wild-type mice as a model of deficient IGF-1 secretion and signaling and S1S2 transgenic mice as a tool to investigate the role of sustained overexpression of IGF-1 in striated muscle in neuromuscular innervation. The analysis of the nerve terminal in extensor digitorum longus muscles from senescent mice showed that the decrease in the percentage of cholinesterase-stained zones (CSZ) exhibiting nerve terminal branching, number of nerve branches at the CSZ, and nerve branch points was partially or completely reversed by sustained overexpression of IGF-1 in skeletal muscle. Target-derived IGF-1 also prevented age-related decreases in the postterminal alpha-bungarotoxin immunostained area, as well as the reduction in the number and length of postsynaptic folds, and area and density of postsynaptic folds studied with electron microscopy. Overexpression of IGF-1 in skeletal muscle may account for the lack of age-dependent switch in muscle fiber type composition recorded in senescent mice. In summary, the use of the S1S2 IGF-1 transgenic mouse model allowed us to provide morphological evidence for the role of target-derived IGF-1 in spinal cord motor neurons in senescent mice.

Neural control of aging skeletal muscle

Functional and structural decline in the neuromuscular system with aging has been recognized as a cause of impairment in physical performance and loss of independence in the elderly. Alterations in spinal cord motor neurones and at the neuromuscular junction have been identified as evidence of denervation in skeletal muscles from aging mammals, including humans. However, the reciprocal influences of neurones on gene expression in muscle and of muscle on age-related neurodegeneration are poorly understood, and, as a result, interventions aimed at delaying or preventing degeneration of the neural component in aging muscle have been largely unsuccessful. The present article discusses the evidence for neural influence on age-related impairments of skeletal muscle, including a role in excitation-contraction uncoupling. The role of nerves in regulating the trophic actions of insulin-like growth factor-1 (IGF-1) and other neurotrophic factors is considered as a novel influence on the effects of aging on the neuromuscular junction. A better understanding of nerve-muscle interactions will allow for more rational interventions in the aging neuromuscular system.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Effects of Age on the Spinal Stretch Reflex

The spinal stretch reflex consists of a relatively simple neuronal network. The Ia afferent fiber of the muscle spindle communicates to the alpha motoneuron via a single synapse. This basic pathway has been studied extensively over the past century, yet considerable information continues to emerge concerning the manner in which this pathway adapts to aging. It is well accepted that the amplitude of the spinal stretch reflex declines with normal aging, and it is intuitively agreed that these changes have a detrimental impact on the motor output of aging individuals. Understanding the changes observed in the spinal stretch reflex pathway due to aging requires a recognition of the changes that can occur in each component of this spinal network. This review will address these changes by following the spinal stretch reflex from initiation to completion. The components that result in the sensory input to the motoneuron will be covered first, followed by a review of the physiological changes that can occur to the motoneuron soma that can affect the processing of the sensory input. The output of the motoneuron encompasses the remaining components from the motor axon itself, to the neuromuscular junction, and then to the characteristic changes in the muscle. Finally, the functional effect that these changes have on the reflex as a fundamental motor behavior will be addressed, especially in terms of its impact on posture and balance.

Age-related changes in fastest and slowest conducting axons of thenar motor units

Single thenar motor unit F waves (FMUPs) were collected from 23 healthy volunteers (age range 21-91 years, mean 46 +/- 20 SD). In each subject, 10 distinct FMUPs were recorded, using surface stimulating and recording electrodes, and the conduction velocity (CV) of each motor unit was calculated. The distribution of CVs (overall range 42-66 m/s; individual FMUP CV dispersion range 6-27% of the maximal FMUP CV) was close to those previously reported whatever the technique used. With age, a progressive CV reduction was observed, and maximal FMUP CV was significantly correlated with age (r = -0.58, P < 0.01), whereas no statistically significant correlation was found between minimal FMUP CV and age (r = -0.27, ns). Individual FMUP CV dispersion presented a statistically significant decrease with age (r = -0.46, P < 0.05). Furthermore, thenar motor unit number (MUNE), estimated by the adapted multiple point stimulation method, decreased progressively with age and was statistically correlated with maximal FMUP CV (r = 0.59, P < 0.01), whereas there was no correlation with minimal FMUP CV (r = 0.34, ns). Thus, we propose that motor unit loss is progressive with age throughout life, affecting particularly the largest and fastest conducting motor units. Preferential involvement of these fibers could be responsible for the age-related changes in motor nerve CV.

Outcomes of surgical treatment for cervical myelopathy in patients more than 75 years of age

STUDY DESIGN

Retrospective study on the results of surgical treatment of compressive cervical myelopathy in patients more than 75 years of age.

OBJECTIVES

To investigate clinical features and surgical outcomes of compressive cervical myelopathy in aged patients and to discuss the role of surgical treatment.

SUMMARY OF BACKGROUND DATA

There are few data focused on the outcomes of surgery in patients with cervical myelopathy who are more than 75 years of age.

METHODS

Seventeen patients with compressive cervical myelopathy who underwent surgery were reviewed. The average age at the time of surgery was 77.2 years. Posterior decompression in 15 patients and anterior decompression in 2 patients were performed. Neurologic deficits before and after surgery were assessed using a scoring system proposed by the Japanese Orthopaedic Association (JOA score). Independence of daily living was evaluated. Radiologic features were examined with radiographs and magnetic resonance imaging. Clinical results were compared with those of patients less than 65 years old as a control.

RESULTS

The preoperative mean JOA score was 6.1, the postoperative maximum JOA scores averaged 11.4, and the recovery rate was 48.4%. These were significantly inferior to scores in those less than 65 years of age. All seven of the patients who could not walk even with aids before surgery became independent in daily activities after surgery. At the final follow-up, the mean JOA score had decreased to 10.7 and the recovery rate to 39.1%. Five of nine patients whose follow-up periods were more than 5 years showed decreases in JOA score, although all patients were still ambulatory.

CONCLUSIONS

Surgical decompression for cervical myelopathy appears to be beneficial, even in patients more than 75 years of age, in improving neurologic function and ability to engage in activities of daily living.