Motor Facilitation Research Articles

Dynamical entrainment of corticospinal excitability during rhythmic movement observation: a Transcranial Magnetic Stimulation study.

Spontaneous modulations of corticospinal excitability during action observation have been interpreted as evidence for the activation of internal motor representations equivalent to the observed action. Alternatively or complementary to this perspective, growing evidence shows that motor activity during observation of rhythmic movements can be modulated by direct visuomotor couplings and dynamical entrainment. In-phase and anti-phase entrainment spontaneously occur, characterized by cyclic movements proceeding simultaneously in the same (in-phase) or opposite (anti-phase) direction. Here we investigate corticospinal excitability during the observation of vertical oscillations of an index finger using Transcranial Magnetic Stimulation (TMS). Motor-evoked potentials (MEPs) were recorded from participants' flexor and extensor muscles of the right index finger, placed in either a maximal steady flexion or extension position, with stimulations delivered at maximal flexion, maximal extension or mid-trajectory of the observed finger oscillations. Consistent with the occurrence of dynamical motor entrainment, increased and decreased MEP responses - suggesting the facilitation of stable in-phase and anti-phase relations but not an unstable 90° phase relation - were found in participants' flexors. Anti-phase motor facilitation contrasts with the activation of internal motor representation as it involves activity in the motor system opposite from activity required for the execution of the observed movement. These findings demonstrate the relevance of dynamical entrainment theories and methods for understanding spontaneous motor activity in the brain during action observation and the mechanisms underpinning coordinated movements during social interaction.

Chronic intermittent hypoxia induces a recoverable impairment in the capacity to elicit inactivity‐induced phrenic motor facilitation (iPMF) following intermittent neural apnea

PKCζ dependent increase in phrenic burst amplitude, a form of compensatory plasticity known as inactivity‐induced phrenic motor facilitation (iPMF). We previously suggested that iPMF and related forms of plasticity in other respiratory neurons represent a form of homeostatic plasticity that stabilizes inspiratory motor output and prevents recurrent cessations in breathing. Here, we tested the hypotheses that: 1) chronic intermittent hypoxia (CIH), which is characteristic of sleep apnea, impairs the capacity to elicit iPMF and 2) the capacity to elicit iPMF is restored within days after termination of CIH. Envigo Sprague Dawley rats were exposed to 7 days of CIH (8 hrs/day, 2 min 10.5% O2 separated by 2 min 21% O2) or an equivalent duration of normoxia. Rats were then urethane‐anesthetized and mechanically ventilated; intermittent reductions in respiratory neural activity were induced by briefly hyperventilating below the CO2 apneic threshold for breathing (5, ~1 min neural apnea episodes, separated by 5 min). In rats exposed to 7 days of normoxia, intermittent neural apnea triggered a significant increase in phrenic burst amplitude for up to 60min post‐neural apnea, when compared to time control rats that did not experience reduced respiratory neural activity (49%±6 versus 9±4, %baseline; p<0.05), indicating iPMF. Consistent with our hypotheses, rats exposed to CIH did not express increased phrenic burst amplitude following intermittent neural apnea (0±7, %baseline, p>0.05), indicating CIH impairs iPMF. However, following 7 days recovery from CIH, increased phrenic burst amplitude following intermittent neural apnea was again apparent (56±18, %baseline, p<0.05), suggesting that CIH‐induced impairments in iPMF fully recover when hypoxia is mitigated. To confirm that mechanisms giving rise to iPMF following recovery of CIH are similar to normal rats, subgroups of rats received intrapleural injections of small interfering RNAs directed against PKCζ (100 pg/side) to reduce PKCζ in phrenic motor neurons. As expected, preliminary data indicate that rats receiving non‐targeting siRNA did not express increases in phrenic burst amplitude following intermittent neural apnea immediately following CIH exposure (−10 %baseline), but fully recovered the capacity for intermittent neural apnea to elicit increased phrenic burst amplitude by 7 days post‐CIH exposure (63 %baseline). By contrast, rats treated with intrapleural siPKCζ did not recover the capacity to increase phrenic amplitude following intermittent apnea 7 days following CIH exposure (18±6, %baseline), confirming an essential role for spinal PKCζ in iPMF. Together, these data demonstrate that CIH restrains the expression of iPMF, but that mechanisms of iPMF spontaneously recover over time once hypoxia is resolved. As intermittent reductions in respiratory neural activity and CIH are characteristic of central sleep apnea, we suggest that the occlusion of iPMF by intermittent hypoxia may contribute to pathological sleep apnea.Support or Funding InformationSupported by NIH‐HL105511.

Mechanisms of cross‐talk inhibition between phrenic motor facilitation elicited by 5‐HT2B and 5‐HT7 receptors

Intermittent spinal serotonin receptor activation elicits long‐lasting phrenic motor facilitation (pMF), a model of respiratory motor plasticity. When activated alone, spinal Gq protein‐coupled serotonin type‐2 receptors (5‐HT2) initiate pMF by a mechanism that requires ERK MAP kinase signaling and new synthesis of BDNF (i.e. the Q pathway to pMF). Activation of spinal Gs protein‐coupled serotonin type‐7 receptors (5‐HT7) also elicits pMF, but by a distinct mechanism that requires Akt (not ERK) signaling and new TrkB (not BDNF) synthesis. When both receptor subtypes are activated simultaneously, pMF is attenuated or even abolished, demonstrating powerful cross‐talk inhibition between these competing mechanisms. However, we know little concerning the mechanisms of cross‐talk inhibition between these competing pathways to pMF. Here, we tested the hypotheses that: 1) concurrent 5‐HT2 subtype‐B (5‐HT2B) and 5‐HT7 receptors undermine pMF; 2) 5‐HT7 receptors inhibit the 5‐HT2B receptor induced Q pathway to pMF via a protein kinase A dependent mechanism; and 3) 5‐HT2B receptors inhibit the 5‐HT7 receptor induced S pathway to pMF via a NADPH oxidase dependent mechanism. Selective agonists for 5‐HT2B (BW 723C86 hydrochloride) and 5HT7 receptors (AS‐19) were administered intrathecally at C4 (3×, 5‐min injections) in anesthetized, vagotomized and ventilated rats. After spinal 5‐HT2B or 5‐HT7 receptor activation alone, integrated phrenic nerve burst amplitude increased progressively (i.e. pMF). In contrast, concurrent 5‐HT2B and 5‐HT7 activation failed to elicit pMF. With concurrent receptor activation, pMF was restored by inhibiting NADPH oxidase activity (apocynin) or protein kinase A activity (Rp‐8‐Br‐cAMPS), consistent with our hypotheses. We suggest an elaborated model accounting for inter‐receptor cross‐talk interactions regulating serotonin‐induced phrenic motor facilitation.Support or Funding InformationSupported by NIH HL69064 and the McKnight Brain Institute.

Differential expression of VEGF isoforms and receptors in the phrenic motor nucleus of end‐stage SOD‐1G93A mutant rats

Amyotrophic lateral sclerosis (ALS) is a devastating motor neuron disease causing paralysis and death from respiratory failure. Despite major loss of phrenic motor neurons (PMN) In a rodent model of familial ALS (SOD1G93A rats), breathing capacity is maintained until late in disease progression (Nichols, et al., 2013). Thus, compensatory respiratory plasticity preserves breathing capacity despite progressive respiratory motor neuron cell death. Vascular endothelial growth factor (VEGF), often regarded as an angiogenic factor, also plays a pivotal role in neuroprotection and neuroplasticity. VEGF‐A and VEGF receptor 2 (VEGFR‐2) are expressed in PMN of naïve rats, and intrathecal VEGF‐A injections induce long‐lasting phrenic motor facilitation (Dale‐Nagle et al., 2011). Little is known concerning in the expression of different VEGF isoforms or their corresponding receptors in PMN, or changes in their expression with motor neuron disease. Since VEGF may play a key role in compensating for respiratory motor neuron loss, we investigated expression of multiple VEGF isoforms (A, B, C, D, E) and receptor subtypes (VEGFR‐1/Flt‐1, VEGFR‐2/KDR/Flk‐1, VEGFR‐3) in the phrenic motor nucleus of end‐stage SOD1G93A rats (20% decrease from peak body mass). We utilized immunohistochemistry to label different VEGF isoforms and receptors within neurons (NeuN positive), astrocytes (GFAP positive) and microglia (OX‐42 positive). Presumptive phrenic motor neurons expressed VEGF‐A, with minimal or no expression of VEGF‐B, ‐C, ‐D, and ‐E. These neurons expressed all VEGF receptors (1, 2 and 3). On the other hand, reactive astrocytes expressed all VEGF isoforms and VEGF receptors 1 and 2, but not VEGFR‐3. Interestingly, activated microglia expressed only VEGF‐B, ‐C and ‐D, but not VEGF‐A or ‐E; microglia expressed VEGFR‐1 and VEGFR‐2, but not VEGFR‐3. In conclusion, multiple VEGF isoforms and receptors are differentially expressed among different cells types of the phrenic motor nucleus of SOD‐1 mutant rats. Increased VEGF expression in glial cells neighboring motor neurons may confer neuroprotection, or help surviving motor neurons compensate for the loss of their neighbors during diseases progression. However, knowledge of the specific role(s) played by different VEGF isoforms/receptors in ALS is lacking.Support or Funding InformationSupported by McKnight Brain Institute

Phrenic Motor Neuron Expression of Receptors Known to Elicit Phrenic Motor Facilitation After Daily Acute Intermittent Hypoxia Preconditioning

Spinal serotonin receptor activation elicits a form of respiratory motor plasticity known as phrenic motor facilitation (pMF). One specific form of pMF is phrenic long‐term facilitation (pLTF) following acute intermittent hypoxia (AIH) exposure. pMF exhibits metaplasticity, defined as a future change in the capacity to express plasticity based on experience. For example, repetitive AIH preconditioning enhances AIH‐induced pLTF.Distinct cellular mechanisms give rise to pMF, including the Q and S pathways, which are induced by 5HT2A/5HT2B and 5HT7 or adenosine 2A signaling, respectively. Although Q and S pathways each elicit pMF when acting alone, they are mutually inhibitory; thus, pLTF following moderate AIH results predominantly via the Q pathway, and is partially restrained by concurrent S pathway activation. Our working hypothesis is that repetitive AIH preconditioning causes pLTF metaplasticity by: 1) reducing inhibitory interactions between the S and Q pathways, and 2) increasing the relative contribution of the S pathway to pLTF. To begin exploring this global hypothesis, we began by investigating the impact of repetitive AIH on the phrenic motor neuron expression of receptors initiating the Q (5HT2A,5HT2B) versus S (5HT7, A2A) pathways to pMF. Four days after intrapleural injections of Cholera toxin B fragment (CTB) to identify phrenic motor neurons, rats were exposed to normoxia or daily AIH (10, 5 min episodes per day, 5 min intervals; 14 days). The rats were perfused and tissue sections were analyzed for 5HT2A, 5HT2B, 5HT7 and A2A receptors, and a downstream mediator of the S pathway (EPAC2) via immunofluorescence. Following dAIH: 1) 5HT2A, 5HT2B and A2A receptor expression were unchanged; but 2) 5HT7 and EPAC2 expression were increased versus controls. Increased 5HT7 and EPAC2 protein expression may support enhanced contributions of the S pathway to pMF, but do not provide information concerning the possibility that cross‐talk inhibition between the Q and S pathways was reduced. Priming a system with dAIH may increase the capacity for respiratory plasticity, and may be a useful therapeutic intervention to treat disorders that cause respiratory motor deficits, including spinal injury, sleep apnea or motor neuron disease.Support or Funding InformationSupported by: T32 Neuromuscular Plasticity Training Program NICHD: T32‐HD043730, NIH HL69064, and the McKnight Brain Institute. We acknowledge Dr. Mark Brownfield for providing us with the serotonin receptor antibodies.

Motor and verbal perspective taking in children with Autism Spectrum Disorder: Changes in social interaction with people and tools

BackgroundChildren with Autism Spectrum Disorder (ASD) have difficulty communicating with others nonverbally, via mechanisms such as hand gestures, eye contact and facial expression. Individuals with ASD also have marked deficits in planning future actions (Hughes, 1996), which might contribute to impairments in non-verbal communication. Perspective taking is typically assessed using verbal scenarios whereby the participant imagines how an actor would interact in a social situation (e.g., Sally Anne task; Baron-Cohen, Leslie, & Frith, 1985). MethodThe current project evaluated motor perspective taking in five children with ASD (8–11 years old) as they participated in a narrative intervention program over the course of about 16 weeks. The goal of the motor perspective-taking task was to facilitate the action of an experimenter either hammering with a tool or putting it away. ResultsInitially, children with ASD facilitated the experimenter’s action less than neurotypical control children. As the narrative intervention progressed, children with ASD exhibited increased motor facilitation that paralleled their increased use of mental state and causal language, indicating a link between verbal and motor perspective taking. ConclusionsMotoric perspective taking provides an additional way to assess understanding and communication in children with ASD and may be a valuable tool for both early assessment and diagnosis of children with ASD.

Changes in Cortical Plasticity in Relation to a History of Concussion during Adolescence.

Adolescence and early adulthood is a critical period for neurophysiological development potentially characterized by an increased susceptibility to the long-term effects of traumatic brain injury. The current study investigated differences in motor cortical physiology and neuroplastic potential across a cohort of young adults with adolescent concussion history and those without. Transcranial magnetic stimulation (TMS) was used to assess motor evoked potential (MEP) amplitude, short-interval cortical inhibition (SICI) and intracortical facilitation (ICF) before and after intermittent theta burst stimulation (iTBS). Pre-iTBS, MEP amplitude, but not SICI or ICF, was greater in the concussion history group. Post-iTBS, the expected increase in MEP amplitude and ICF was tempered in the concussion history group. Change in SICI was variable within the concussion history group. Post hoc assessment revealed that SICI was significantly lower in individuals whose concussion was not diagnosed at the time of injury compared to both those without a concussion history or whose concussion was medically diagnosed. Concussive impacts during adolescence appear to result in a persistent reduction of the ability to modulate facilitatory motor networks. Failure to report/identify concussive impacts close to injury during adolescence also appears to produce persistent change in inhibitory networks. These findings highlight the potential long-term impact of adolescent concussion upon motor cortical physiology.

Direct eye contact enhances mirroring of others’ movements: A transcranial magnetic stimulation study

Direct eye contact is a powerful social cue to regulate interpersonal interactions. Previous behavioral studies showed a link between eye contact and motor mimicry, indicating that the automatic mimicry of observed hand movements is significantly enhanced when direct eye contact exists between the observer and the observed model. In the present study, we aim to investigate the neurophysiological basis of the previously reported behavioral enhancements. Here, transcranial magnetic stimulation (TMS) was applied to assess changes in cortico-motor excitability at the level of the primary motor cortex (M1) to explore whether and how the motor system is facilitated from observing others’ hand movements and, in particular, how this process is modulated by eye contact. To do so, motor evoked potentials (MEPs) were collected from two hand muscles while participants received single-pulse TMS and naturally observed video clips of an actor showing hand opening movements or static hands. During the observation, either direct or averted eye gaze was established between the subject and the observed actor. Our findings show a clear effect of eye gaze on observation-induced motor facilitation. This indicates that the mapping or ‘mirroring’ of others' movements is significantly enhanced when movement observation is accompanied by direct eye gaze compared to averted eye gaze. Our results support the notion that eye contact is a powerful social signal with the ability to direct human non-verbal social behavior. Furthermore, our findings are important for understanding the role of the mirror motor system in the mapping of socially relevant actions.

Adenosine-dependent phrenic motor facilitation is inflammation resistant.

Phrenic motor facilitation (pMF), a form of respiratory plasticity, can be elicited by acute intermittent hypoxia (i.e., phrenic long-term facilitation, pLTF) or direct application of drugs to the cervical spinal cord. Moderate acute intermittent hypoxia (mAIH; 3 × 5-min episodes of 35-50 mmHg arterial Po2, 5-min normoxic intervals) induces pLTF by a serotonin-dependent mechanism; mAIH-induced pLTF is abolished by mild systemic inflammation induced by a low dose of lipopolysaccharide (LPS; 100 μg/kg ip). In contrast, severe acute intermittent hypoxia (sAIH; 3 × 5-min episodes of 25-30 mmHg arterial Po2, 5-min normoxic intervals) elicits pLTF by a distinct, adenosine-dependent mechanism. Since it is not known if systemic LPS blocks the mechanism giving rise to sAIH-induced pLTF, we tested the hypothesis that sAIH-induced pLTF and adenosine 2A (A2A) receptor-induced pMF are insensitive to mild systemic inflammation elicited by the same low dose of LPS. In agreement with our hypothesis, neither sAIH-induced pLTF nor cervical intrathecal A2A receptor agonist (CGS-21680; 200 μM, 10 μl × 3)-induced pMF were affected 24 h post-LPS. Pretreatment with intrathecal A2A receptor antagonist injections (MSX-3; 10 μM, 12 μl) blocked sAIH-induced pLTF 24 h post LPS, confirming that pLTF was adenosine dependent. Our results give insights concerning the differential impact of systemic inflammation and the functional significance of multiple cascades capable of giving rise to phrenic motor plasticity. The relative resistance of adenosine-dependent pMF to inflammation suggests that it provides a "backup" system in animals lacking serotonin-dependent pMF due to ongoing inflammation associated with systemic infections and/or neural injury.NEW & NOTEWORTHY This study gives novel insights concerning how a mild systemic inflammation impacts phrenic motor plasticity (pMF), particularly adenosine-dependent pMF. We suggest that since this adenosine-dependent pathway is insensitive to systemic inflammation, it represents an alternative or "backup" mechanism of pMF when other mechanisms are suppressed.

The role of hearing ability and speech distortion in the facilitation of articulatory motor cortex

Excitability of articulatory motor cortex is facilitated when listening to speech in challenging conditions. Beyond this, however, we have little knowledge of what listener-specific and speech-specific factors engage articulatory facilitation during speech perception. For example, it is unknown whether speech motor activity is independent or dependent on the form of distortion in the speech signal. It is also unknown if speech motor facilitation is moderated by hearing ability. We investigated these questions in two experiments. We applied transcranial magnetic stimulation (TMS) to the lip area of primary motor cortex (M1) in young, normally hearing participants to test if lip M1 is sensitive to the quality (Experiment 1) or quantity (Experiment 2) of distortion in the speech signal, and if lip M1 facilitation relates to the hearing ability of the listener. Experiment 1 found that lip motor evoked potentials (MEPs) were larger during perception of motor-distorted speech that had been produced using a tongue depressor, and during perception of speech presented in background noise, relative to natural speech in quiet. Experiment 2 did not find evidence of motor system facilitation when speech was presented in noise at signal-to-noise ratios where speech intelligibility was at 50% or 75%, which were significantly less severe noise levels than used in Experiment 1. However, there was a significant interaction between noise condition and hearing ability, which indicated that when speech stimuli were correctly classified at 50%, speech motor facilitation was observed in individuals with better hearing, whereas individuals with relatively worse but still normal hearing showed more activation during perception of clear speech. These findings indicate that the motor system may be sensitive to the quantity, but not quality, of degradation in the speech signal. Data support the notion that motor cortex complements auditory cortex during speech perception, and point to a role for the motor cortex in compensating for differences in hearing ability.

BDNF Val66Met Polymorphism Influences Visuomotor Associative Learning and the Sensitivity to Action Observation.

Motor representations in the human mirror neuron system are tuned to respond to specific observed actions. This ability is widely believed to be influenced by genetic factors, but no study has reported a genetic variant affecting this system so far. One possibility is that genetic variants might interact with visuomotor associative learning to configure the system to respond to novel observed actions. In this perspective, we conducted a candidate gene study on the Brain-derived neurotrophic factor (BDNF) Val66Met polymorphism, a genetic variant linked to motor learning in regions of the mirror neuron system, and tested the effect of this polymorphism on motor facilitation and visuomotor associative learning. In a single-pulse TMS study carried on 16 Met (Val/Met and Met/Met) and 16 Val/Val participants selected from a large pool of healthy volunteers, Met participants showed significantly less muscle-specific corticospinal sensitivity during action observation, as well as reduced visuomotor associative learning, compared to Val homozygotes. These results are the first evidence of a genetic variant tuning sensitivity to action observation and bring to light the importance of considering the intricate relation between genetics and associative learning in order to further understand the origin and function of the human mirror neuron system.

Ownership Status Influences the Degree of Joint Facilitatory Behavior

When engaging in joint activities, humans tend to sacrifice some of their own sensorimotor comfort and efficiency to facilitate a partner’s performance. In the two experiments reported here, we investigated whether ownership—a socioculturally based nonphysical feature ascribed to objects—influenced facilitatory motor behavior in joint action. Participants passed mugs that differed in ownership status across a table to a partner. We found that participants oriented handles less toward their partners when passing their own mugs than when passing mugs owned by their partners (Experiment 1) and mugs owned by the experimenter (Experiment 2). These findings indicate that individuals plan and execute actions that assist their partners but do so to a smaller degree if it is the individuals’ own property that the partners intend to manipulate. We discuss these findings in terms of underlying variables associated with ownership and conclude that a self-other distinction can be found in the human sensorimotor system.

Divergent cAMP signaling differentially regulates serotonin-induced spinal motor plasticity

Spinal metabotropic serotonin receptors encode transient experiences into long-lasting changes in motor behavior (i.e. motor plasticity). While interactions between serotonin receptor subtypes are known to regulate plasticity, the significance of molecular divergence in downstream G protein coupled receptor signaling is not well understood. Here we tested the hypothesis that distinct cAMP dependent signaling pathways differentially regulate serotonin-induced phrenic motor facilitation (pMF); a well-studied model of spinal motor plasticity. Specifically, we studied the capacity of cAMP-dependent protein kinase A (PKA) and exchange protein activated by cAMP (EPAC) to regulate 5-HT2A receptor-induced pMF within adult male rats. Although spinal PKA, EPAC and 5-HT2A each elicit pMF when activated alone, concurrent PKA and 5-HT2A activation interact via mutual inhibition thereby blocking pMF expression. Conversely, concurrent EPAC and 5-HT2A activation enhance pMF expression reflecting additive contributions from both mechanisms. Thus, we demonstrate that distinct downstream cAMP signaling pathways enable differential regulation of 5-HT2A-induced pMF. Conditional activation of independent signaling mechanisms may explain experience amendable changes in plasticity expression (i.e. metaplasticity), an emerging concept thought to enable flexible motor control within the adult central nervous system.

Sustained Hypoxia Elicits Competing Spinal Mechanisms of Phrenic Motor Facilitation.

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a form of spinal motor plasticity. Competing mechanisms give rise to phrenic motor facilitation (pMF; a general term including pLTF) depending on the severity of hypoxia within episodes. In contrast, moderate acute sustained hypoxia (mASH) does not elicit pMF. By varying the severity of ASH and targeting competing mechanisms of pMF, we sought to illustrate why moderate AIH (mAIH) elicits pMF but mASH does not. Although mAIH elicits serotonin-dependent pLTF, mASH does not; thus, mAIH-induced pLTF is pattern sensitive. In contrast, severe AIH (sAIH) elicits pLTF through adenosine-dependent mechanisms, likely from greater extracellular adenosine accumulation. Because serotonin- and adenosine-dependent pMF interact via cross talk inhibition, we hypothesized that pMF is obscured because the competing mechanisms of pMF are balanced and offsetting during mASH. Here, we demonstrate the following: (1) blocking spinal A2A receptors with MSX-3 reveals mASH-induced pMF; and (2) sASH elicits A2A-dependent pMF. In anesthetized rats pretreated with intrathecal A2A receptor antagonist injections before mASH (PaO2 = 40-54 mmHg) or sASH (PaO2 = 25-36 mmHg), (1) mASH induced a serotonin-dependent pMF and (2) sASH induced an adenosine-dependent pMF, which was enhanced by spinal serotonin receptor inhibition. Thus, competing adenosine- and serotonin-dependent mechanisms contribute differentially to pMF depending on the pattern/severity of hypoxia. Understanding interactions between these mechanisms has clinical relevance as we develop therapies to treat severe neuromuscular disorders that compromise somatic motor behaviors, including breathing. Moreover, these results demonstrate how competing mechanisms of plasticity can give rise to pattern sensitivity in pLTF. Intermittent hypoxia elicits pattern-sensitive spinal plasticity and improves motor function after spinal injury or during neuromuscular disease. Specific mechanisms of pattern sensitivity in this form of plasticity are unknown. We provide evidence that competing mechanisms of phrenic motor facilitation mediated by adenosine 2A and serotonin 2 receptors are differentially expressed, depending on the pattern/severity of hypoxia. Understanding how these distinct mechanisms interact during hypoxic exposures differing in severity and duration will help explain interesting properties of plasticity, such as pattern sensitivity, and may help optimize therapies to restore motor function in patients with neuromuscular disorders that compromise movement.

Reduced respiratory neural activity elicits a long-lasting decrease in the CO2 threshold for apnea in anesthetized rats

Two critical parameters that influence breathing stability are the levels of arterial pCO2 at which breathing ceases and subsequently resumes – termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO2 were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief “massed” neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.

The superior colliculus is sensitive to gestalt-like stimulus configuration in hemispherectomy patients

Patients with cortical blindness following a lesion to the primary visual cortex (V1) may retain nonconscious visual abilities (blindsight). One intriguing, though largely unexplored question, is whether nonconscious vision in the blind hemifield of hemianopic patients can be sensitive to higher-order perceptual organization, and which V1-independent structure underlies such effect. To answer this question, we tested two rare hemianopic patients who had undergone hemispherectomy, and in whom the only post-chiasmatic visual structure left intact in the same side of the otherwise damaged hemisphere was the superior colliculus (SC). By using a variant of the redundant target effect (RTE), we presented single dots, patterns composed by the same dots organized in quadruple gestalt-like configurations, or patterns of four dots arranged in random configurations, either singly to the intact visual hemifield or bilaterally to both hemifields. As reported in a number of prior studies on blindsight patients, we found that bilateral stimulation yielded faster reaction times (RTs) than single stimulation of the intact field for all conditions (i.e., there was an implicit RTE). In addition to this effect, both patients showed a further speeding up of RTs when the gestalt-like, but not the random shape, quadruple patterns were projected to their blind hemifield during bilateral stimulation. Because other retino-recipient subcortical and cortical structures in the damaged hemisphere are absent, the SC on the lesioned side seems solely responsible for such an effect. The present results provide initial support to the notion that nonconscious vision might be sensitive to perceptual organization and stimulus configuration through the pivotal contribution of the SC, which can enhance the processing of gestalt-like or structured stimuli over meaningless or randomly assembled ones and translate them into facilitatory motor outputs.

Cross‐talk interactions between spinal 5‐HT7 and 5‐HT2A receptors regulate serotonin‐induced phrenic motor plasticity

Spinal serotonin receptors elicit respiratory motor plasticity, expressed as a long‐lasting enhancement in respiratory motor output; i.e. phrenic motor facilitation (pMF). While serotonin type 2 (5‐HT2A) and 7 (5‐HT7) receptors elicit phenotypically similar pMF, they do so via distinct cellular mechanisms. We previously demonstrated that low‐doses of serotonin give rise to pMF via 5‐HT2A receptors, but cross‐talk inhibition from 5‐HT7 receptors abolish pMF at higher doses indicating a “bell shaped” dose‐curve response for serotonin‐induced pMF. Thus, 5‐HT7 receptors both give rise to and constrain pMF. We have recently identified cAMP as a critical point of divergence for these contrasting effects. Specifically, 5‐HT7 receptors constrain 5‐HT2A receptor‐induced pMF via protein kinase A (PKA), but elicit pMF via exchange protein activated by cAMP (EPAC). Our working hypothesis is that selective activation of these divergent cAMP signaling pathways enables refined regulation of serotonin‐induced pMF. Further, we hypothesized that coincident 5‐HT2A receptor and EPAC activation would enhance pMF, whereas 5‐HT2A and PKA activation would constrain/eliminate pMF expression. In anesthesized, paralyzed and mechanically ventilated rats, intermittent intrathecal injections of a 5HT2A receptor agonist (DOI; 50.6 ± 3.1%; p ≤ 0.05) or EPAC activator (8pCPT; 6.2 ± 16.7%; p ≤ 0.05) alone elicited pMF; concurrent EPAC and 5‐HT2A receptor activation gave rise to additive pMF (110.9 ± 10.0%; p ≤ 0.05). In contrast, while PKA activator (6Bnz) alone gave rise to pMF (58.9 ± 8.6%; p ≤ 0.05), concurrent PKA and 5‐HT2A receptor activation abolished the capacity for either to elicit pMF (18.6 ± 6.5%; p ≥ 0.05). Thus, divergent cAMP signaling exerts contrasting effects on serotonin‐induced respiratory motor plasticity. Targeting of these distinct signaling pathways may be useful for treating diseases causing motor deficits, including spinal injury or motor neuron diseases.Support or Funding InformationSupported by NIH HL080209 and HL69024. DPF was supported by an Advanced Opportunity Fellowship at the University of Wisconsin, NIH fellowship (1F30HL126351‐010) and the Medical Scientist Training Program (NIH T32 GM008692).

Inactivity‐induced phrenic motor facilitation is initiated though TNF receptor 2 and impaired by TNF receptor 1

Prolonged reductions in synaptic inputs to phrenic motor neurons results in the release of spinal, soluble TNFα (sTNFα) to initiate a long lasting increase in phrenic burst amplitude termed inactivity‐induced phrenic motor facilitation (iPMF). TNFα can activate TNF‐R1 or TNF‐R2, which are two distinct cell surface receptors that tend to initiate discrete signaling cascades. TNF‐R1 mediates the majority of TNFα activity; therefore, we hypothesized sTNFα released within or near the phrenic motor pool in response to reduced activity binds to TNF‐R1 on phrenic motor neurons to initiate iPMF. To test this hypothesis, Harlan Sprague Dawley rats were injected with small interfering RNAs directed against TNF‐R1 or TNF‐R2 in the interpleural space (100pg/side) to selectively reduce TNF‐R1 or TNF‐R2 in phrenic motor neurons. After 3 daily injections of siRNAs, iPMF was measured in anesthetized and ventilated rats for 60min following a prolonged (30min) neural apnea. As expected, control rats receiving injections of non‐targeting siRNA expressed an increase in phrenic burst amplitude for up to 60min following neural apnea (48±15,%baseline), indicating iPMF. Contrary to our hypothesis, knockdown of TNF‐R1 in phrenic motor neurons led to an enhanced iPMF following prolonged neural apnea (80±18,%baseline), whereas knockdown of TNF‐R2 completely abolished the expression of iPMF 60min post neural apnea (−8±5,%baseline,). Collectively, these data suggest that TNF‐R2 signaling in phrenic motor neurons is necessary for iPMF and that TNF‐R1 signaling may initiate mechanisms within phrenic motor neurons that oppose iPMF. Thus, both TNFα receptors may contribute to sculpting phrenic burst amplitude in response to a reduction in respiratory‐related neural inputs.Support or Funding InformationNIH‐HL105501 and DoD SC120226

Inactivity‐Induced Phrenic Motor Facilitation underlies Spontaneous Recovery of Ventilation and Diaphragm EMG activity Following Cervical Spinal Injury

Cervical spinal injury disrupts neural pathways to phrenic motor neurons, thereby causing diaphragm paresis and paralysis and diminished breathing capacity. Some spontaneous functional recovery in the capacity to generate phrenic nerve or diaphragm activity is observed days‐to‐months post‐injury via mechanisms that not understood. We have previously found that prolonged reductions in spinal synaptic inputs to phrenic motor neurons in uninjured rats results in a PKCz‐dependent, rebound enhancement of phrenic burst amplitude, a form of spinal plasticity known as inactivity‐induced phrenic motor facilitation (iPMF). Little is known regarding the role for iPMF in the control of breathing. Here, we tested the hypothesis that mechanisms giving rise to iPMF underlie spontaneous recovery of ventilation and diaphragm EMG activity following a high cervical spinal injury. Sprague Dawley rats (Harlan colony 217, 12–16 weeks old) chronically instrumented with radiotelemetry for bilateral diaphragm electromyogram (EMG) recordings received small interfering RNAs directed against PKCz or non‐targeting siRNAs in the interpleural space (100pg/side) three days prior to a C3 midline contusion. Three days following contusion, EMG and plethysmography were recorded in unanesthetized rats breathing room air (FIO2/FICO2=0.21/0.0, NX) or with maximal chemoreceptor stimulation (FIO2/FICO2= 0.105/0.07, MCS). In injured rats receiving control injections, tidal volume under both inspired gas concentrations and diaphragm EMG during MCS showed minimal change from baseline conditions (TV NX: 99%, TV MCS: 90%, diaphragm EMG right: 95%, left: 100%). By contrast, in injured rats receiving siPKCz, tidal volume during normoxia and MCS (63% NX, 90% MCS) and electrical activity of the diaphragm during MCS (right: 85%, left: 80%) were reduced from pre‐injury levels, suggesting that impairment of iPMF prevented spontaneous recovery of phrenic motor output (and hence, diaphragm EMG activity) following spinal injury. Thus, we speculate that iPMF is a form of compensatory plasticity that senses and responds to reductions in spinal synaptic inputs to phrenic motor neurons to maintain stable phrenic motor output following spinal injury.Support or Funding InformationSupported by NIH R01‐HL105511, DoD SC120226 and NIH T32‐OD010423‐09

Temporal Prediction in lieu of Periodic Stimulation.

Predicting not only what will happen, but also when it will happen is extremely helpful for optimizing perception and action. Temporal predictions driven by periodic stimulation increase perceptual sensitivity and reduce response latencies. At the neurophysiological level, a single mechanism has been proposed to mediate this twofold behavioral improvement: the rhythmic entrainment of slow cortical oscillations to the stimulation rate. However, temporal regularities can occur in aperiodic contexts, suggesting that temporal predictions per se may be dissociable from entrainment to periodic sensory streams. We investigated this possibility in two behavioral experiments, asking human participants to detect near-threshold auditory tones embedded in streams whose temporal and spectral properties were manipulated. While our findings confirm that periodic stimulation reduces response latencies, in agreement with the hypothesis of a stimulus-driven entrainment of neural excitability, they further reveal that this motor facilitation can be dissociated from the enhancement of auditory sensitivity. Perceptual sensitivity improvement is unaffected by the nature of temporal regularities (periodic vs aperiodic), but contingent on the co-occurrence of a fulfilled spectral prediction. Altogether, the dissociation between predictability and periodicity demonstrates that distinct mechanisms flexibly and synergistically operate to facilitate perception and action.