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Mechanics to pre-process information for the fine tuning of mechanoreceptors. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2019; 205:661-686. [PMID: 31270587 PMCID: PMC6726712 DOI: 10.1007/s00359-019-01355-z] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/27/2019] [Revised: 06/18/2019] [Accepted: 06/20/2019] [Indexed: 11/17/2022]
Abstract
Non-nervous auxiliary structures play a significant role in sensory biology. They filter the stimulus and transform it in a way that fits the animal’s needs, thereby contributing to the avoidance of the central nervous system’s overload with meaningless stimuli and a corresponding processing task. The present review deals with mechanoreceptors mainly of invertebrates and some remarkable recent findings stressing the role of mechanics as an important source of sensor adaptedness, outstanding performance, and diversity. Instead of organizing the review along the types of stimulus energy (force) taken up by the sensors, processes associated with a few basic and seemingly simple mechanical principles like lever systems, viscoelasticity, resonance, traveling waves, and impedance matching are taken as the guideline. As will be seen, nature makes surprisingly competent use of such “simple mechanics”.
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Feng J, Hu H. A novel player in the field: Merkel disc in touch, itch and pain. Exp Dermatol 2019; 28:1412-1415. [PMID: 31001848 DOI: 10.1111/exd.13945] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2018] [Revised: 04/03/2019] [Accepted: 04/12/2019] [Indexed: 12/30/2022]
Abstract
The mechanosensitive Merkel cell-neurite complex comprising two distinct cell types in both hairy and glabrous skin has been widely recognized as touch receptor for more than 100 years. In 2014, three elegant studies further demonstrated that the Merkel cell-neurite complex mediates touch transduction via the mechanosensitive Piezo2 channel. However, whether it is involved in genesis of itch and pain sensations, has been unclear. Recently, we reported that Merkel cells modulate the development of mechanical itch under the conditions of dry skin and aging, whereas two other studies demonstrated that Piezo2 channel mediates mechanical pain. In this assay, we summarized the current knowledge of Merkel disk under both normal and pathological conditions, with a focus on its role in touch, itch, and pain.
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Affiliation(s)
- Jing Feng
- Department of Anesthesiology, The Center for the Study of Itch, Washington University School of Medicine, St. Louis, Missouri
| | - Hongzhen Hu
- Department of Anesthesiology, The Center for the Study of Itch, Washington University School of Medicine, St. Louis, Missouri
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Severson KS, Xu D, Van de Loo M, Bai L, Ginty DD, O'Connor DH. Active Touch and Self-Motion Encoding by Merkel Cell-Associated Afferents. Neuron 2017; 94:666-676.e9. [PMID: 28434802 DOI: 10.1016/j.neuron.2017.03.045] [Citation(s) in RCA: 66] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2016] [Revised: 02/15/2017] [Accepted: 03/29/2017] [Indexed: 01/12/2023]
Abstract
Touch perception depends on integrating signals from multiple types of peripheral mechanoreceptors. Merkel-cell associated afferents are thought to play a major role in form perception by encoding surface features of touched objects. However, activity of Merkel afferents during active touch has not been directly measured. Here, we show that Merkel and unidentified slowly adapting afferents in the whisker system of behaving mice respond to both self-motion and active touch. Touch responses were dominated by sensitivity to bending moment (torque) at the base of the whisker and its rate of change and largely explained by a simple mechanical model. Self-motion responses encoded whisker position within a whisk cycle (phase), not absolute whisker angle, and arose from stresses reflecting whisker inertia and activity of specific muscles. Thus, Merkel afferents send to the brain multiplexed information about whisker position and surface features, suggesting that proprioception and touch converge at the earliest neural level.
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Affiliation(s)
- Kyle S Severson
- Kavli Neuroscience Discovery Institute, Brain Science Institute, The Solomon H. Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA; Neuroscience Training Program, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
| | - Duo Xu
- Kavli Neuroscience Discovery Institute, Brain Science Institute, The Solomon H. Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA; Neuroscience Training Program, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
| | - Margaret Van de Loo
- Kavli Neuroscience Discovery Institute, Brain Science Institute, The Solomon H. Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
| | - Ling Bai
- Department of Neurobiology, Howard Hughes Medical Institute, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115, USA; Neuroscience Training Program, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
| | - David D Ginty
- Department of Neurobiology, Howard Hughes Medical Institute, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115, USA
| | - Daniel H O'Connor
- Kavli Neuroscience Discovery Institute, Brain Science Institute, The Solomon H. Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
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Wang Y, Marshall KL, Baba Y, Lumpkin EA, Gerling GJ. Compressive viscoelasticity of freshly excised mouse skin is dependent on specimen thickness, strain level and rate. PLoS One 2015; 10:e0120897. [PMID: 25803703 PMCID: PMC4372409 DOI: 10.1371/journal.pone.0120897] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2014] [Accepted: 01/27/2015] [Indexed: 11/18/2022] Open
Abstract
Although the skin’s mechanical properties are well characterized in tension, little work has been done in compression. Here, the viscoelastic properties of a population of mouse skin specimens (139 samples from 36 mice, aged 5 to 34 weeks) were characterized upon varying specimen thickness, as well as strain level and rate. Over the population, we observed the skin’s viscoelasticity to be quite variable, yet found systematic correlation of residual stress ratio with skin thickness and strain, and of relaxation time constants with strain rates. In particular, as specimen thickness ranged from 211 to 671 μm, we observed significant variation in both quasi-linear viscoelasticity (QLV) parameters, the relaxation time constant (τ1 = 0.19 ± 0.10 s) and steady-state residual stress ratio (G∞ = 0.28 ± 0.13). Moreover, when τ1 was decoupled and fixed, we observed that G∞ positively correlated with skin thickness. Second, as steady-state stretch was increased (λ∞ from 0.22 to 0.81), we observed significant variation in both QLV parameters (τ1 = 0.26 ± 0.14 s, G∞ = 0.47 ± 0.17), and when τ1 was fixed, G∞ positively correlated with stretch level. Third, as strain rate was increased from 0.06 to 22.88 s−1, the median time constant τ1 varied from 1.90 to 0.31 s, and thereby negatively correlated with strain rate. These findings indicate that the natural range of specimen thickness, as well as experimental controls of compression level and rate, significantly influence measurements of skin viscoelasticity.
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Affiliation(s)
- Yuxiang Wang
- Department of Systems and Information Engineering, University of Virginia, 151 Engineers Way, Charlottesville, Virginia, 22903, United States of America
- Department of Mechanical and Aerospace Engineering, University of Virginia, 122 Engineers Way, Charlottesville, Virginia, 22903, United States of America
| | - Kara L. Marshall
- Department of Dermatology, Columbia University College of Physicians & Surgeons, 1150 St. Nicholas Ave., New York, New York, 10032, United States of America
| | - Yoshichika Baba
- Department of Dermatology, Columbia University College of Physicians & Surgeons, 1150 St. Nicholas Ave., New York, New York, 10032, United States of America
| | - Ellen A. Lumpkin
- Department of Dermatology, Columbia University College of Physicians & Surgeons, 1150 St. Nicholas Ave., New York, New York, 10032, United States of America
- Department of Physiology & Cellular Biophysics, Columbia University College of Physicians & Surgeons, 1150 St. Nicholas Ave., New York, New York, 10032, United States of America
| | - Gregory J. Gerling
- Department of Systems and Information Engineering, University of Virginia, 151 Engineers Way, Charlottesville, Virginia, 22903, United States of America
- Department of Biomedical Engineering, University of Virginia, 415 Lane Road, Charlottesville, Virginia, 22908, United States of America
- * E-mail:
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Abstract
The somatosensory system decodes a wide range of tactile stimuli and thus endows us with a remarkable capacity for object recognition, texture discrimination, sensory-motor feedback and social exchange. The first step leading to perception of innocuous touch is activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs). Here, we review the properties and functions of LTMRs, emphasizing the unique tuning properties of LTMR subtypes and the organizational logic of their peripheral and central axonal projections. We discuss the spinal cord neurophysiological representation of complex mechanical forces acting upon the skin and current views of how tactile information is processed and conveyed from the spinal cord to the brain. An integrative model in which ensembles of impulses arising from physiologically distinct LTMRs are integrated and processed in somatotopically aligned mechanosensory columns of the spinal cord dorsal horn underlies the nervous system's enormous capacity for perceiving the richness of the tactile world.
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Affiliation(s)
- Victoria E Abraira
- The Solomon H. Snyder Department of Neuroscience, Howard Hughes Medical Institute, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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Kim EK, Wellnitz SA, Bourdon SM, Lumpkin EA, Gerling GJ. Force sensor in simulated skin and neural model mimic tactile SAI afferent spiking response to ramp and hold stimuli. J Neuroeng Rehabil 2012; 9:45. [PMID: 22824523 PMCID: PMC3506479 DOI: 10.1186/1743-0003-9-45] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2011] [Accepted: 07/05/2012] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND The next generation of prosthetic limbs will restore sensory feedback to the nervous system by mimicking how skin mechanoreceptors, innervated by afferents, produce trains of action potentials in response to compressive stimuli. Prior work has addressed building sensors within skin substitutes for robotics, modeling skin mechanics and neural dynamics of mechanotransduction, and predicting response timing of action potentials for vibration. The effort here is unique because it accounts for skin elasticity by measuring force within simulated skin, utilizes few free model parameters for parsimony, and separates parameter fitting and model validation. Additionally, the ramp-and-hold, sustained stimuli used in this work capture the essential features of the everyday task of contacting and holding an object. METHODS This systems integration effort computationally replicates the neural firing behavior for a slowly adapting type I (SAI) afferent in its temporally varying response to both intensity and rate of indentation force by combining a physical force sensor, housed in a skin-like substrate, with a mathematical model of neuronal spiking, the leaky integrate-and-fire. Comparison experiments were then conducted using ramp-and-hold stimuli on both the spiking-sensor model and mouse SAI afferents. The model parameters were iteratively fit against recorded SAI interspike intervals (ISI) before validating the model to assess its performance. RESULTS Model-predicted spike firing compares favorably with that observed for single SAI afferents. As indentation magnitude increases (1.2, 1.3, to 1.4 mm), mean ISI decreases from 98.81 ± 24.73, 54.52 ± 6.94, to 41.11 ± 6.11 ms. Moreover, as rate of ramp-up increases, ISI during ramp-up decreases from 21.85 ± 5.33, 19.98 ± 3.10, to 15.42 ± 2.41 ms. Considering first spikes, the predicted latencies exhibited a decreasing trend as stimulus rate increased, as is observed in afferent recordings. Finally, the SAI afferent's characteristic response of producing irregular ISIs is shown to be controllable via manipulating the output filtering from the sensor or adding stochastic noise. CONCLUSIONS This integrated engineering approach extends prior works focused upon neural dynamics and vibration. Future efforts will perfect measures of performance, such as first spike latency and irregular ISIs, and link the generation of characteristic features within trains of action potentials with current pulse waveforms that stimulate single action potentials at the peripheral afferent.
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Affiliation(s)
- Elmer K Kim
- Department of Systems and Information Engineering, University of Virginia, Charlottesville, VA 22904, USA
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