For decades, the dominant explanation was simple: muscles shrink with age, a condition known as sarcopenia, and smaller muscles produce less force. But population data have long revealed a puzzle. Strength declines at a substantially faster rate than muscle mass itself, which means that atrophy alone cannot account for the full loss of power. This discrepancy has pushed researchers to look beyond the muscle and toward the nervous system, which is required to generate and sustain muscular force. Most of that work has focused on the spinal cord and the periphery, where scientists have well characterized changes in spinal circuit excitability, motor neuron intrinsic properties, synaptic balance, and the transmission and remodeling of the neuromuscular junction, the synapse where nerves talk to muscle fibers.

Far less attention has been paid to the motor cortex, the region of the brain’s outer layer that serves as the cortical source of voluntary movement. Human studies have pointed to decreased voluntary activation of limb muscles in older adults, along with altered cortical output to muscle and imbalances between excitatory and inhibitory circuits in the motor cortex. But human imaging and stimulation techniques lack the resolution to identify which specific cortical circuits, neuronal populations, or molecular mechanisms change with aging, or to tie those changes directly to motor dysfunction. A new study in aged mice, published in the journal Aging Cell, set out to close that gap by measuring brain, nerve, and muscle function in the very same animals.

The research team, based at the University of Missouri, built on their own earlier discovery that layer V pyramidal neurons of the primary motor cortex become hyperexcitable in aged mice. These neurons are a heterogeneous population of deep-layer projection cells that includes the neurons responsible for transmitting and shaping cortical motor commands to the spinal cord. The finding was striking because cortical hyperexcitability mirrors what is seen in neurodegenerative diseases such as amyotrophic lateral sclerosis, where it has been implicated in motor decline, and Alzheimer’s disease, where it has been linked to cognitive deterioration. The question was whether this hyperexcitability in aging is merely a curiosity or is genuinely coupled to weakness.

To answer it, the researchers ran an unusually comprehensive battery of tests on young mice aged three months and aged mice aged 24 months, with equal numbers of males and females in each group. Behavioral testing showed that aged mice were dramatically weaker: grip strength, rotarod coordination, and a demanding weighted cart pull task were all significantly reduced. In vivo electrophysiology revealed that the compound muscle action potential, a measure of overall neuromuscular excitability, was smaller in aged animals, as was the estimated number of functioning motor units, while individual motor units were larger and repetitive nerve stimulation revealed greater transmission failure at the neuromuscular junction. Direct measurements of muscle contractility confirmed that both twitch and tetanic plantar flexion torque were substantially reduced in the aged animals.

The most provocative result came from motor evoked potential recordings, which measure the brain’s output to muscle. When the researchers stimulated across the motor cortex, the evoked response in the gastrocnemius muscle was larger in aged mice, indicating enhanced cortical output. In contrast, stimulation at the cervical spinal cord produced smaller responses in aged animals, indicating reduced spinal output. In other words, the aged nervous system showed a paradoxical signature: a weakening spinal and neuromuscular apparatus accompanied by a louder signal coming down from the cortex. This pattern suggested that the hyperexcitable cortical neurons the team had identified previously might be actively reshaping the motor system rather than passively reflecting its decline.

Patch-clamp recordings from 160 individual layer V pyramidal neurons across the same animals confirmed the intrinsic hyperexcitability. Aged neurons fired more vigorously at every depolarizing current step tested, from 50 to 300 picoamperes, roughly doubling their firing frequency at both low and high stimulation. Their membranes were also more resistive and had longer time constants, properties that reduce the current needed to reach the threshold for an action potential. Single-cell molecular analysis using digital PCR revealed that aged neurons carried elevated transcript levels of Nav1.6 and Nav1.1, sodium channel subunits that carry the persistent inward current, a powerful amplifier of repetitive firing, as well as increased levels of the serotonin receptor 5-HT2C, a known modulator of that current. These transcriptional changes provide candidate molecular markers of the hyperexcitable phenotype.

Synaptic recordings added another layer of mechanism. When the researchers stimulated layer II/III of the motor cortex, the canonical source of feedforward drive onto layer V neurons, they found that excitatory inputs onto aged neurons showed an increased paired-pulse ratio, indicating reduced initial release probability but a capacity to sustain transmission during high-frequency activity. Inhibitory inputs showed the opposite pattern: smaller evoked responses and a decreased paired-pulse ratio, suggesting that inhibitory synapses release strongly on the first pulse but deplete rapidly during repeated activation. This combination could allow excitatory drive to dominate during the sustained, high-frequency firing that characterizes voluntary movement, potentially feeding and maintaining the hyperexcitable state.

The statistical centerpiece of the study came from correlating all of these measurements within individual animals. Across 250 pairwise relationships between cortical variables and neuromuscular or behavioral outcomes, 109 were statistically significant after correction for multiple comparisons, and the large majority were negative, meaning that animals with higher cortical excitability tended to have worse motor outcomes. Firing frequency at 50 picoamperes emerged as the single strongest cortical correlate of a composite neuromuscular dysfunction index, explaining 90 percent of its variance. A mediation analysis then showed that this firing frequency statistically accounted for 75 percent of the effect of aging on neuromuscular function, although a reverse model, in which the mediator and outcome were swapped, accounted for a smaller 42.2 percent, underscoring that observational data of this kind cannot establish causal direction.

The authors are careful on this point: the mediation models represent a statistical decomposition of variance, not proof that cortical hyperexcitability causes weakness. It remains possible that the brain’s overactive output is a compensatory response to a failing periphery, or that both phenomena flow from a common aging process. Still, recent evidence that chemogenetically inducing layer V hyperexcitability in the motor cortex reduces muscular force and coordination in mice raises the possibility that the phenotype is maladaptive. The parallels with ALS, where cortical hyperexcitability is a well-established feature associated with downstream motor deficits, suggest it may be a shared mechanism across distinct contexts of motor dysfunction. If future experiments show that dampening layer V hyperexcitability in aged animals restores strength, the motor cortex could emerge as a genuine therapeutic target for one of aging’s most burdensome consequences, transforming weakness from an inevitable decline into a treatable disorder of brain circuitry.

Subject of Research: Motor cortex hyperexcitability and its relationship to neuromuscular dysfunction in aged mice

Article Title: Motor Cortex Hyperexcitability Is Coupled to Neuromuscular Dysfunction in Aged Mice

Article References: Viteri, J. A., Kerr, N. R., Darvishi, F. B., Dashtmian, A. R., Brennan, C. D., Ayyagari, S. N., Moore, P. J., Wang, M., Snyder, H., Yu, B., Santin, J. M., & Arnold, W. D. (2026). Motor Cortex Hyperexcitability Is Coupled to Neuromuscular Dysfunction in Aged Mice. Aging Cell, 25(10), Article e70731. https://doi.org/10.1111/acel.70731

Image Credits: AI Generated

DOI: 10.1111/acel.70731

Keywords: aging, motor cortex, muscle weakness, sarcopenia, layer V pyramidal neurons, hyperexcitability, neuromuscular junction, patch clamp, persistent inward current, ALS, synaptic transmission, electrophysiology

For decades, scientists have understood that psychedelics—including classic compounds like LSD and psilocybin—primarily interact with serotonin receptors, notably the 5-HT2A subtype. However, exactly how these compounds influence prefrontal cortical circuits at a cellular level has remained elusive. The new findings from Schmitz, Chiu, Foglesong, and colleagues identify that psychedelics evoke direct excitation of layer V pyramidal neurons in the mPFC by harnessing intracellular signaling cascades downstream of 5-HT2A receptors, a G protein-coupled receptor subtype predominantly coupling to the Gq/11 family.

Layer V neurons of the medial prefrontal cortex represent a critical nexus integrating cortical and subcortical inputs implicated in cognition, mood regulation, and executive function. Dysregulation of these circuits features prominently in neuropsychiatric disorders such as depression, anxiety, and schizophrenia. By directly demonstrating that psychedelics produce excitation in this specific cortical population via 5-HT2A Gq activation, the study provides a mechanistic explanation for both the profound subjective and therapeutic effects observed in humans.

Using a combination of advanced in vitro electrophysiology, pharmacological manipulations, and molecular interventions, the researchers demonstrated that psychedelic compounds produce a marked increase in action potential firing specifically in mPFC layer V neurons. This increase was abolished when 5-HT2A receptors or their downstream Gq signaling pathways were pharmacologically inhibited, conclusively linking the receptor’s Gq pathway activation to neuronal excitation. Notably, this excitation was intrinsic to the neurons themselves rather than a result of altered network activity, underscoring a direct postsynaptic effect.

These discoveries hold tremendous implications for the development of antidepressant and anxiolytic treatments. Psychedelic therapies are currently undergoing clinical trials, with early reports indicating rapid and sustained symptom relief in patients with treatment-resistant depression and PTSD. Understanding that layer V mPFC neurons serve as a direct psychedelic target provides a cellular blueprint for drug development aimed at maximizing therapeutic efficacy while minimizing side effects. Such precision medicine approaches may include designing agonists or modulators that selectively engage 5-HT2A Gq signaling in specific cortical layers.

Further elucidating the intracellular pathways activated by psychedelics, the study details how Gq protein stimulation leads to activation of phospholipase C (PLC), subsequent generation of inositol trisphosphate (IP3), and release of intracellular calcium stores. This cascade ultimately modulates ion channels, including non-selective cation channels, to promote membrane depolarization and increased excitability. This refined understanding elucidates the molecular underpinnings of the heightened cortical responsiveness and network plasticity attributed to psychedelic states.

The medial prefrontal cortex’s layered structure and heterogeneous neuronal populations present a complex organizational challenge. However, the present work’s layer-specific identification of 5-HT2A-mediated excitation advances prior findings that often lacked cellular precision. Layer V pyramidal neurons project extensively to subcortical regions such as the thalamus and striatum, implicating this excitation as a critical node for widespread brain network modulation observed in psychedelic experiences. This challenges the classical notion that psychedelics’ actions were diffuse and nonspecific.

Additionally, the research addresses conflicting reports regarding the role of Gq versus other G protein pathways, such as Gi/o or Gs, in mediating 5-HT2A receptor function. The selective involvement of Gq-related signaling in the direct excitation of layer V neurons clarifies receptor signaling bias and provides a framework for dissecting receptor pharmacology in greater detail. This specificity is crucial for avoiding untoward effects linked to recruitment of alternate pathways during therapeutic drug design.

The methodology utilized in this study combined whole-cell patch-clamp recordings from acute brain slices of murine mPFC with selective application of psychedelic compounds and specific antagonists. Genetic knockdown models targeting 5-HT2A receptors and Gq alpha subunits further confirmed causality. These rigorous approaches ensured that observed neuronal responses were not artifactual but highly physiologically relevant. The translational value is enhanced by parallel gene-expression analyses confirming receptor localization in human prefrontal tissue.

From a clinical perspective, understanding that psychedelics promote excitability in layer V mPFC neurons through 5-HT2A Gq activation could also inspire future neuromodulatory interventions. Techniques such as transcranial magnetic stimulation (TMS) might be optimized to target these neuronal populations synergistically with pharmacotherapy for enhanced outcomes. Moreover, the direct excitation of layer V neurons could underlie reported improvements in cognitive flexibility and emotional regulation in psychedelic-assisted psychotherapy.

This research also provides a foundational framework for addressing challenges in psychedelic research related to tolerance, side effects, and individual variability. Elucidating the intracellular pathways responsible for the primary neuronal excitation opens avenues to selectively modulate or desensitize specific signaling components, potentially mitigating adverse effects like anxiety or perceptual distortions. Pharmacogenomic approaches could harness these molecular insights to stratify patient populations most likely to benefit from therapy.

While the study offers transformative insights, the authors note limitations such as species differences between rodent models and human neuroanatomy. Future research will be necessary to confirm whether identical molecular and cellular processes occur in the human brain in vivo. Nonetheless, the convergence of pharmacology, neurophysiology, and molecular biology showcased here sets a new benchmark for psychedelic neuroscience.

In conclusion, the work by Schmitz and colleagues marks a major advance in decoding the neural actions of psychedelics by identifying a direct excitatory effect on medial prefrontal cortex layer V neurons mediated through 5-HT2A receptor Gq activation. This discovery elucidates fundamental neurobiological mechanisms driving psychedelic states and highlights new targets for psychiatric drug development. As the psychedelic renaissance continues to accelerate, such mechanistic insights will be integral to translating ancient mind-altering compounds into precise, effective, and safe mental health treatments.

Subject of Research:
Neurophysiological mechanisms of psychedelic compounds targeting 5-HT2A receptors in layer V medial prefrontal cortex neurons.

Article Title:
Psychedelic compounds directly excite 5-HT2A layer V medial prefrontal cortex neurons through 5-HT2A Gq activation.

Article References:
Schmitz, G.P., Chiu, YT., Foglesong, M.L. et al. Psychedelic compounds directly excite 5-HT2A layer V medial prefrontal cortex neurons through 5-HT2A Gq activation. Transl Psychiatry 15, 381 (2025). https://doi.org/10.1038/s41398-025-03611-0

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41398-025-03611-0