Study Of Human Physiology Research Articles

Set Up and Utilization of a Three-Dimensional In Vitro Bioreactor System for Human Intestinal Studies and Microbial Co-Cultures.

The study of human intestinal physiology and host-microbe interactions is crucial for understanding gastrointestinal health and disease. Traditional two-dimensional cell culture models lack the complexity of the native intestinal environment, limiting their utility in studying intestinal biology. Here, we present a detailed protocol for the set up and utilization of a three-dimensional (3D) in vitro bioreactor system for human intestinal studies and bacterial co-culture. This article outlines the design and assembly of the bioreactor system, scaffold fabrication, bacterial culture techniques, analysis methods, and troubleshooting tips. By providing step-by-step instructions, the goal is to enable other laboratories to utilize physiologically relevant tissue models of the human intestine, incorporating key features, such as nutrient flow, multiple human cell types, 3D architecture, and microbial communities. The incorporation of commensal bacteria into the bioreactor system allows for the investigation of complex host-microbe interactions, providing insight into gastrointestinal health and pathology. This article serves as a comprehensive resource for scientists seeking to advance their understanding of intestinal biology toward the development of novel therapeutic strategies for gastrointestinal disorders. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Scaffold design Basic Protocol 2: Intestinal cell culture: Caco2 cells Basic Protocol 3: Intestinal cell culture: organoids Basic Protocol 4: Bioreactor design and set up Basic Protocol 5: Bacteria in 3D bioreactor set up Basic Protocol 6: Bacteria and drug dosing.

Electrophysiology of the <i>Danio rerio</i> Heart

Tropical teleost fish Danio rerio is increasingly used as a model object for electrophysiological studies of human cardiac physiology and pathology. D. rerio is characterized by the similarity with humans in such functional parameters of the electrical activity of the heart as heart rate, action potential morphology, as well as in a set of ion currents depolarizing and repolarizing the cell membrane. D. rerio is easy to breed, easy to handle experimentally, and easy to genetically modify. This overview presents current data on the structural and functional organization of ion channels in D. rerio heart myocytes.

Generation and characterization of mature hepatocyte organoids for liver metabolic studies.

Hepatocyte organoids (HOs) generated in vitro are powerful tools for liver regeneration. However, previously reported HOs have mostly been fetal in nature with low expression levels of metabolic genes characteristic of adult liver functions, hampering their application in studies of metabolic regulation and therapeutic testing for liver disorders. Here, we report development of novel culture conditions that combine optimized levels of triiodothyronine (T3) with the removal of growth factors to enable successful generation of mature hepatocyte organoids (MHOs) of both mouse and human origin with metabolic functions characteristic of adult livers. We show that the MHOs can be used to study various metabolic functions including bile and urea production, zonal metabolic gene expression, and metabolic alterations in both alcoholic liver disease and non-alcoholic fatty liver disease, as well as hepatocyte proliferation, injury and cell fate changes. Notably, MHOs derived from human fetal hepatocytes also show improved hepatitis B virus infection. Therefore, these MHOs provide a powerful in vitro model for studies of human liver physiology and diseases. The human MHOs are potentially also a robust research tool for therapeutic development.

A novel TASK channel antagonist nasal spray reduces sleep apnea severity in physiological responders: a randomized, blinded, trial.

Preclinical and human physiological studies indicate that topical, selective TASK 1/3 K+ channel antagonism increases upper airway dilator muscle activity and reduces pharyngeal collapsibility during anesthesia and nasal breathing during sleep. The primary aim of this study was to determine the effects of BAY2586116 nasal spray on obstructive sleep apnea (OSA) severity and whether individual responses vary according to differences in physiological responses and route of breathing. Ten people (5 females) with OSA [apnea-hypopnea index (AHI) = 47 ± 26 events/h (means ± SD)] who completed previous sleep physiology studies with BAY2586116 were invited to return for three polysomnography studies to quantify OSA severity. In random order, participants received either placebo nasal spray (saline), BAY2586116 nasal spray (160 µg), or BAY2586116 nasal spray (160 µg) restricted to nasal breathing (chinstrap or mouth tape). Physiological responders were defined a priori as those who had improved upper airway collapsibility (critical closing pressure ≥2 cmH2O) with BAY2586116 nasal spray (NCT04236440). There was no systematic change in apnea-hypopnea index (AHI3) from placebo versus BAY2586116 with either unrestricted or nasal-only breathing versus placebo (47 ± 26 vs. 43 ± 27 vs. 53 ± 33 events/h, P = 0.15). However, BAY2586116 (unrestricted breathing) reduced OSA severity in physiological responders compared with placebo (e.g., AHI3 = 28 ± 11 vs. 36 ± 12 events/h, P = 0.03 and ODI3 = 18 ± 10 vs. 28 ± 12 events/h, P = 0.02). Morning blood pressure was also lower in physiological responders after BAY2586116 versus placebo (e.g., systolic blood pressure = 137 ± 24 vs. 147 ± 21 mmHg, P < 0.01). In conclusion, BAY2586116 reduces OSA severity during sleep in people who demonstrate physiological improvement in upper airway collapsibility. These findings highlight the therapeutic potential of this novel pharmacotherapy target in selected individuals.NEW & NOTEWORTHY Preclinical findings in pigs and humans indicate that blocking potassium channels in the upper airway with topical nasal application increases pharyngeal dilator muscle activity and reduces upper airway collapsibility. In this study, BAY2586116 nasal spray (potassium channel blocker) reduced sleep apnea severity in those who had physiological improvement in upper airway collapsibility. BAY2586116 lowered the next morning's blood pressure. These findings highlight the potential for this novel therapeutic approach to improve sleep apnea in certain people.

Application of biomaterial-based three-dimensional bioprinting for organ-on-a-chip fabrication

An organ-on-a-chip is a microfluidic device that simulates the microenvironment of organs, facilitating the study of human physiology and disease mechanisms. Through the integration of tissue engineering and micromachining technologies, it effectively manages the cellular microenvironment and implements tissue-specific functions and physiological responses with high fidelity. Several factors must be appropriately considered in the fabrication of an organ-on-a-chip, including the choice of biomaterials to simulate the extracellular matrix (ECM), selection of cells constituting the target organ, incorporation of humanized design to realize the primary function and structure of the organ, and the use of appropriate biofabrication methods to build a tissue-specific environment. Notably, three-dimensional (3D) bioprinting has emerged as a promising method for biofabricating organ-on-a-chip. Three-dimensional bioprinting offers versatility in adapting to various biomaterials with different physical properties, allowing precise control of 3D cell arrays and facilitating cyclic movements of fluidic flow within microfluidic platforms. These capabilities enable the precise fabrication of organ-on-a-chip that reflects tissue-specific functions and microenvironments. Additionally, 3D-bioprinted organ-on-a-chip can serve as a disease-on-a-chip platform, achieved through the implementation of pathophysiological environments and integration with devices such as bioreactors. Their significance in pharmacology research lies in their exceptional resemblance to the 3D microenvironment structure of actual organs, which are conducive for the validation of sequential mechanism of drug action. This review describes recent examples of organ-on-a-chip applications for various organs and state-of-the-art 3D bioprinting techniques employed in organ-on-a-chip fabrication. The discussion extends to the future prospects of this technology, encompassing aspects such as commercialization through mass production and its potential application in personalized medicine or drug-screening platforms. Serving as a relevant guide, this review offers insights for future research and developments in in vitro micromodel fabrication.

Optimized and Scalable Precoating-Free Reprogramming of Human Peripheral Blood Mononuclear Cells into iPSCs.

Human disease modeling has been profoundly transformed by the introduction of human induced pluripotent stem cells (iPSCs), marking the onset of a new era. This ground-breaking development offers a tailored framework for generating pluripotent cells from any individual, effectively enabling the development of cellular models for the study of human physiology and diseases on an unprecedented scale. Although technologies for iPSCs generation have advanced rapidly over the past two decades, protocols for reprogramming patient-derived somatic cells into stem cells still pose a major challenge for the development of automated pipelines capable of generating iPSCs at scales that are cost-effective, reproducible, and easy to implement. Most methods commonly rely on extracellular matrix protein mixtures or synthetic substrates to promote efficient proliferation of iPSCs. Nonetheless, employing these substances entails a laborious and time-consuming process, as the culture surface requires coating treatments before cell seeding. Here we describe a method for reprogramming blood-derived mononucleated cells that eliminates the need to precoat culture surfaces for the entire experimental flow. This procedure is suitable for fresh or frozen purified peripheral blood mononuclear cells (PBMCs) and allows seeding of reprogrammed cells in a culture medium containing a fragment of laminin-511, regardless of the method of reprogramming employed. Our protocol incorporates a streamlined workflow that optimizes key factors, including cell density, culture medium composition, and iPSC culture propagation techniques. Using a precoating-free approach, we eliminate the time-consuming steps, while our optimized subcloning method improves the scalability of the protocol, making it suitable for large-scale applications. Additionally, the automation-friendly nature of our protocol allows for high-throughput processing, reducing the labor and costs associated with manual handling. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Miniaturized and time efficient precoating-free reprogramming of fresh or frozen PBMCs Alternate Protocol: Erythroid progenitor cells (EPCs) enrichment and reprogramming into iPSCs using Sendai viral vectors Basic Protocol 2: Picking and precoating-free optimized expansion of iPSC clones.

Opinião dos estudantes da área da saúde sobre a utilização de metodologias ativas no ensino remoto de fisiologia humana

The Covid-19 pandemic was a challenge for education systems. Educational institutions have had to adapt to the new correlations between teaching and learning. The aim of this study was to understand the perception of higher education students regarding the use of active methodologies in the remote study of Human Physiology. This is a cross-sectional descriptive study with a mixed methods approach. Thirty-five responses were obtained and 25 of the students considered the use of active methodologies in the teaching of Human Physiology to be positive. In recent decades, teaching in the health area has been debated, and the curriculum of active methodologies has stood out, as it seeks training with a broad view of man, forming an ethical, critical, reflective, humanized subject capable of transforming his surroundings. The use of active methodologies during the remote teaching period led to greater gain in the knowledge of human physiology

Assessment of doxorubicin toxicity using human cardiac organoids: A novel model for evaluating drug cardiotoxicity

Cardiovascular diseases pose a huge threat to global human health and are also a major obstacle to drug development and disease treatment. Drug-induced cardiotoxicity remains an important clinical issue. Both traditional two-dimensional (2D) monolayer cell models and animal models have their own limitations and are not fully suitable for the study of human heart physiology or pathology. Cardiac organoids are three-dimensional (3D) and self-organized structures that accurately retain the biological characteristics and functions of heart tissue. In this study, we successfully established a human cardiac organoid model by inducing the directed differentiation of human embryonic stem cells, which recapitulates the patterns of early myocardial development. Moreover, this model accurately characterized the cardiotoxic damage caused by the anticancer drug doxorubicin, including clinical cardiac injury and cardiac function indicators, cell apoptosis, inflammation, fibrosis, as well as mitochondrial damage. In general, the cardiac organoid model can be used to evaluate the cardiotoxicity of drugs, opening new directions and ideas for drug screening and cardiotoxicity research.

Verification and Validation of Lower Body Negative Pressure as a Non-Invasive Bioengineering Tool for Testing Technologies for Monitoring Human Hemorrhage.

Since hemorrhage is a leading cause of preventable death in both civilian and military settings, the development of advanced decision support monitoring capabilities is necessary to promote improved clinical outcomes. The emergence of lower body negative pressure (LBNP) has provided a bioengineering technology for inducing progressive reductions in central blood volume shown to be accurate as a model for the study of the early compensatory stages of hemorrhage. In this context, the specific aim of this study was to provide for the first time a systematic technical evaluation to meet a commonly accepted engineering standard based on the FDA-recognized Standard for Assessing Credibility of Modeling through Verification and Validation (V&V) for Medical Devices (ASME standard V&V 40) specifically highlighting LBNP as a valuable resource for the safe study of hemorrhage physiology in humans. As an experimental tool, evidence is presented that LBNP is credible, repeatable, and validated as an analog for the study of human hemorrhage physiology compared to actual blood loss. The LBNP tool can promote the testing and development of advanced monitoring algorithms and evaluating wearable sensors with the goal of improving clinical outcomes during use in emergency medical settings.

The Erlangen model of tinnitus development-New perspective and treatment strategy

About one sixth of the population of western industrialized nations suffers from chronic, subjective tinnitus, causing socioeconomic treatment and follow-up costs of almost 22billion euros per year in Germany alone. According to the prevailing view, tinnitus develops as aconsequence of amaladaptive neurophysiological process in the brain triggered by hearing loss. The Erlangen model of tinnitus development presented here is intended to propose acomprehensive neurophysiological explanation for the initial occurrence of the phantom sound after hearing loss. Based on the model, anew treatment strategy will be developed. The model summarized here is based on various animal and human physiological studies conducted in recent years. The Erlangen model considers subjective tinnitus as aside effect of aphysiological mechanism that permanently optimizes information transmission into the auditory system by means of stochastic resonance (SR) even in the healthy auditory system. In fact, hearing-impaired patients with tinnitus hear better on average than those without tinnitus. This unfamiliar perspective on the phantom percept may already help affected patients to cope better with their suffering. In addition, based on the model, low intensity noise tinnitus suppression (LINTS) has been developed as anew, individually adapted treatment strategy for tonal tinnitus and has already been successfully tested in patients. Apossible limiting factor for the model and treatment strategy is the pitch of the tinnitus percept, which may require adjustments to the treatment strategy for frequencies above about 5 kHz.

Massively parallel base editing to map variant effects in human hematopoiesis

Systematic evaluation of the impact of genetic variants is critical for the study and treatment of human physiology and disease. While specific mutations can be introduced by genome engineering, we still lack scalable approaches that are applicable to the important setting of primary cells, such as blood and immune cells. Here, we describe the development of massively parallel base-editing screens in human hematopoietic stem and progenitor cells. Such approaches enable functional screens for variant effects across any hematopoietic differentiation state. Moreover, they allow for rich phenotyping through single-cell RNA sequencing readouts and separately for characterization of editing outcomes through pooled single-cell genotyping. We efficiently design improved leukemia immunotherapy approaches, comprehensively identify non-coding variants modulating fetal hemoglobin expression, define mechanisms regulating hematopoietic differentiation, and probe the pathogenicity of uncharacterized disease-associated variants. These strategies will advance effective and high-throughput variant-to-function mapping in human hematopoiesis to identify the causes of diverse diseases.

Estimates of persistent inward currents in lower limb motoneurons are larger in females than in males.

Noninvasive recordings of motor unit (MU) spike trains help us understand how the nervous system controls movement and how it adapts to various physiological conditions. The majority of participants in human and nonhuman animal physiology studies are male, and it is assumed that mechanisms uncovered in these studies are shared between males and females. However, sex differences in neurological impairment and physical performance warrant the study of sex as a biological variable in human physiology and performance. To begin addressing this gap in the study of biophysical properties of human motoneurons, we quantified MU discharge rates and estimates of persistent inward current (PIC) magnitude in both sexes. We decomposed MU spike trains from the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (SOL) using high-density surface electromyography and blind source separation algorithms. Ten participants of each sex performed slow triangular (10 s up and down) isometric contractions to a peak of 30% of their maximum voluntary contraction. We then used linear mixed-effects models to determine if peak discharge rate and estimates of PICs were predicted by the fixed effects of sex, muscle, and their interaction. Despite a lack of sex-differences in peak discharge rates across all muscles, estimates of PICs were larger [χ2(1) = 6.26, P = 0.012] in females [4.73 ± 0.242 pulses per second (pps)] than in males (3.81 ± 0.240 pps). These findings suggest that neuromodulatory drive, inhibitory input, and/or biophysical properties of motoneurons differ between the sexes and may contribute to differences in MU discharge patterns.NEW & NOTEWORTHY Sex-related differences in motoneuron analyses have emerged with greater inclusion of female participants, however, mechanisms for these differences remain unclear. Estimates of persistent inward currents (i.e., ΔF) in motoneurons of the lower limb muscles were larger in females than in males. This suggests neuromodulatory drive, monoaminergic signaling, intrinsic motoneuron properties, and/or descending motor commands may differ between the sexes, which provides a potential mechanism underlying previously reported sex-related differences in motoneuron discharge patterns.

USE OF EDUCATIONAL TRAINING IN THE PROFESSIONAL TRAINING OF FUTURE BIOLOGY TEACHERS IN THE STUDY OF HUMAN AND ANIMAL ANATOMY AND PHYSIOLOGY

The prospective and effective area of training is the use of training in the educational process for students of natural sciences. The relevance of introducing training technologies into the educational process for the study of human and animal anatomy and physiology by future biology teachers is emphasized. The latest domestic and foreign scientific researches and publications on the use of training technologies in the educational process are analyzed. The features of the use of training in the training of future specialists in a higher educational institution are characterized. It is shown that the training technology is a collective form of organization of training, helps to carry out training taking into account individual characteristics, promotes self-realization of the future professional and the choice of an adequate pace of learning. The expediency of using the training in the training of biology teachers in the development of certain aspects of the disciplines of human anatomy and physiology, namely: reproductive health, methods of contraception, rational nutrition, hardening, healthy lifestyle, is determined. A plan of training sessions for the cycle is proposed: «Show Awareness and Prudence», their topics and goals are defined. The questionnaire survey of higher education students of the Faculty of Natural Sciences and Geography of Pavlo Tychyna Uman State Pedagogical University, which took place before and after the training sessions on the following topics, was analyzed: «Human Immunodeficiency Virus (HIV) and Acquired Immunodeficiency Syndrome (AIDS)», «Sexually Transmitted Infections (STIs)». The effectiveness of the training in the educational activities of future biology teachers was evaluated. We consider it promising to introduce training technologies for the training of a biologist during higher education at the university.

Peptidomic Characterization and Amino Acid Availability after Intake of Casein vs. a Casein Hydrolysate in a Pig Model.

It is known that casein hydrolysis accelerates gastrointestinal transit in comparison to intact casein, although the effect of the protein hydrolysis on the composition of the digests is not fully understood. The aim of this work is to characterize, at the peptidome level, duodenal digests from pigs, as a model of human digestion, fed with micellar casein and a previously described casein hydrolysate. In addition, in parallel experiments, plasma amino acid levels were quantified. A slower transit of nitrogen to the duodenum was found when the animals received micellar casein. Duodenal digests from casein contained a wider range of peptide sizes and a higher number of peptides above five amino acids long in comparison with the digests from the hydrolysate. The peptide profile was markedly different, and although β-casomorphin-7 precursors were also found in hydrolysate samples, other opioid sequences were more abundant in the casein digests. Within the same substrate, the evolution of the peptide pattern at different time points showed minimal changes, suggesting that the protein degradation rate relies more on the gastrointestinal location than on digestion time. Higher plasma concentrations of methionine, valine, lysine and amino acid metabolites were found in animals fed with the hydrolysate at short times (<200 min). The duodenal peptide profiles were evaluated with discriminant analysis tools specific for peptidomics to identify sequence differences between both substrates that can be used for future human physiological and metabolic studies.

Basal tone: When is posterior myofascial tissue considered “basal”? An experimental study of human physiology

Basal tone: When is posterior myofascial tissue considered “basal”? An experimental study of human physiology

Recent Advances in Human Physiology

The study of human physiology provides important insight into the complex nature of the human body, increasing our understanding of the various systems and processes that occur to keep us alive. Developments in this field provide the basis for the development of novel treatments and therapies that are crucial for the advancement of medicine and improving the health and well-being of people around the world. Recent research into the pathogenesis of SAR-CoV-2 and the discovery of novel treatments for its symptoms have bought this field of science to the forefront. Yet there have also been several other recent advances that have increased our understanding of the human body and provided opportunities for the development of new medicines and therapies. Here we discuss the latest advances in this field, highlighting recent progression in our understanding of cancer metastasis, the development of the brain, and the use of organoids in the study of the human body. Finally, we examine the work of two physiologists that received the Nobel Prize in 2021 for their work in understanding the mechanism behind how humans feel the heat, cold, and mechanical force.

Of mice and men: Considerations on adipose tissue physiology in animal models of obesity and human studies

The ever-increasing burden of obesity demands a better pathophysiological understanding, especially regarding adipose tissue pathophysiology. Animal models of obesity are of great importance in investigating potential mechanisms and implications of obesity. Many issues should be considered while interpreting the preclinical results as anatomical and pathophysiological differences exist among species. Importantly, the natural history of obesity development differs considerably. An important example of conflicting results among preclinical models and human physiological studies is that of adipose tissue oxygenation, where rodent models almost unanimously have shown the presence of hypoxia in the adipose tissue of obese animals while human studies have yielded conflicting results to date. Other issues which require further clarification before generalizing preclinical data in humans include adipose tissue browning, endocrine function and fibrosis. The aim of this mini-review is to synopsize similarities and differences between rodent models and humans, which should be taken into consideration in obesity studies.

Human Hyoid Descent is Adaptive rather than Maladaptive for Swallowing Performance: An Experimental and Computational Argument

PurposeDistinctive features of the human supralaryngeal vocal tract have been argued to be adaptive for speech and maladaptive for swallowing. However, this maladaptive hypothesis has not been thoroughly tested and in fact is refuted by studies of normal human feeding physiology. This study validates a computational model of hyolingual biomechanics derived from an previously validated experimental nonhuman primate model system and examines how changes in hyolingual and craniofacial morphology over the course of human evolution and ontogeny affect hyolingual biomechanics during swallowing. In particular, we examine superoanterior excursion of the hyoid, which is often reduced in human patients with dysphagia and therefore used here as a surrogate for swallowing performance.MethodsA computational model of hyolingual range of motion during swallowing in Macaca mulatta was developed in R using in vivo data. Model outputs were compared against in vivo macaque data to determine the model's goodness of fit using a receiver operating characteristics (ROC) curve; model predictive power was estimated using the area under the ROC curve. Several aspects of macaque craniofacial and hyoid morphology were then altered towards the human condition with consequent adjustments made to muscle lengths, and the model calculated hyoid displacement during swallowing for each condition. A model was considered viable if its range of motion included a target of hyoid excursion derived from in vivo macaque and human data.ResultsThe macaque model had a sensitivity of 0.79, specificity of 0.82, and accuracy of 88% compared to in vivo data. A humanoid model predicts hyoid excursion within normal limits of healthy humans. Altering macaque morphology to approach the human condition by lowering the hyoid and making the symphysis vertical had little effect on model performance. Isolated shortening of the mandible significantly decreased hyoid excursion unless the symphysis was also made more vertical. Moreover, as the model was made more humanoid, both hyoid descent and physiologically maximal muscle shortening were necessary to approximate target hyoid excursion.ConclusionsHyoid descent—which is commonly assumed to be detrimental to swallowing—may actually maintain swallowing performance as the human face has shortened in recent evolutionary history. Moreover, because mandibular morphology affects suprahyoid muscle length, selective pressure to maintain swallowing performance may have resulted in the emergence of humanity’s characteristic vertical mandibular symphysis. Geometric constraints require human suprahyoid muscles to perform at their physiological limits when the hyoid is descended. Such extreme performance requirements may leave humans vulnerable to dysphagia not because of airway morphology alone but because of physiologic constraints.

Challenges in the Management of Atrial Fibrillation With Subclinical Hyperthyroidism.

Subclinical thyroid disorders have a high prevalence among older individuals and women. Subclinical hypothyroidism is diagnosed by elevated serum levels of thyroid-stimulating hormone (TSH) with thyroxine levels within the reference range, and subclinical hyperthyroidism is diagnosed by low TSH in conjunction with thyroxine and triiodothyronine levels within reference ranges. Atrial fibrillation is the most commonly diagnosed cardiac arrhythmia and has been associated with an increased risk of mortality, heart failure, stroke, and depression. Mechanistic data from animal and human physiology studies as well as observational data in humans support an association of subclinical hyperthyroidism with atrial fibrillation. Guidelines recommend the measurement of TSH in the evaluation of new-onset atrial fibrillation. All patients with overt hyperthyroidism should be treated, and treatment of subclinical hyperthyroidism should be considered in patients older than 65 years with TSH < 0.4 mlU/L, or in younger patients with TSH < 0.1 mlU/L. Guidelines also recommend screening for AF in patients with known hyperthyroidism. Wearable devices that measure the heart electrical activity continuously may be a novel strategy to detect atrial fibrillation in patients at risk. In this review, we explore the interplay between thyroid hormones and atrial fibrillation, management controversies in subclinical hyperthyroidism, and potential strategies to improve the management of atrial fibrillation in patients with subclinical hyperthyroidism.

Electrical Properties of Adult Mammalian Motoneurons.

Motoneurons are the 'final common path' between the central nervous system (that intends, selects, commands, and organises movement) and muscles (that produce the behaviour). Motoneurons are not passive relays, but rather integrate synaptic activity to appropriately tune output (spike trains) and therefore the production of muscle force. In this chapter, we focus on studies of mammalian motoneurons, describing their heterogeneity whilst providing a brief historical account of motoneuron recording techniques. Next, we describe adult motoneurons in terms of their passive, transition, and active (repetitive firing) properties. We then discuss modulation of these properties by somatic (C-boutons) and dendritic (persistent inward currents) mechanisms. Finally, we briefly describe select studies of human motor unit physiology and relate them to findings from animal preparations discussed earlier in the chapter. This interphyletic approach to the study of motoneuron physiology is crucial to progress understanding of how these diverse neurons translate intention into behaviour.