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Deardorff AS, Romer SH, Sonner PM, Fyffe REW. Swimming against the tide: investigations of the C-bouton synapse. Front Neural Circuits 2014; 8:106. [PMID: 25278842 PMCID: PMC4167003 DOI: 10.3389/fncir.2014.00106] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2014] [Accepted: 08/17/2014] [Indexed: 11/19/2022] Open
Abstract
C-boutons are important cholinergic modulatory loci for state-dependent alterations in motoneuron firing rate. m2 receptors are concentrated postsynaptic to C-boutons, and m2 receptor activation increases motoneuron excitability by reducing the action potential afterhyperpolarization. Here, using an intensive review of the current literature as well as data from our laboratory, we illustrate that C-bouton postsynaptic sites comprise a unique structural/functional domain containing appropriate cellular machinery (a “signaling ensemble”) for cholinergic regulation of outward K+ currents. Moreover, synaptic reorganization at these critical sites has been observed in a variety of pathologic states. Yet despite recent advances, there are still great challenges for understanding the role of C-bouton regulation and dysregulation in human health and disease. The development of new therapeutic interventions for devastating neurological conditions will rely on a complete understanding of the molecular mechanisms that underlie these complex synapses. Therefore, to close this review, we propose a comprehensive hypothetical mechanism for the cholinergic modification of α-MN excitability at C-bouton synapses, based on findings in several well-characterized neuronal systems.
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Affiliation(s)
- Adam S Deardorff
- Boonshoft School of Medicine, Department of Neuroscience, Cell Biology and Physiology, Wright State University Dayton, OH, USA
| | - Shannon H Romer
- Boonshoft School of Medicine, Department of Neuroscience, Cell Biology and Physiology, Wright State University Dayton, OH, USA
| | - Patrick M Sonner
- Boonshoft School of Medicine, Department of Neuroscience, Cell Biology and Physiology, Wright State University Dayton, OH, USA
| | - Robert E W Fyffe
- Boonshoft School of Medicine, Department of Neuroscience, Cell Biology and Physiology, Wright State University Dayton, OH, USA
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Barry BK, Pascoe MA, Riek S, Carson RG, Enoka RM. Common input to different regions of biceps brachii long head. Exp Brain Res 2008; 193:351-9. [DOI: 10.1007/s00221-008-1631-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2008] [Accepted: 10/20/2008] [Indexed: 10/21/2022]
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Langenbach GEJ, van Wessel T, Brugman P, Korfage JAM, van Eijden TMGJ. Is fiber-type composition related to daily jaw muscle activity during postnatal development? Cells Tissues Organs 2007; 187:307-15. [PMID: 18089936 DOI: 10.1159/000112791] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/09/2007] [Indexed: 11/19/2022] Open
Abstract
AIM Muscles containing large numbers of slow-contracting fibers are generally more active than muscles largely composed of fast fibers. This relationship between muscle activity and phenotype suggests that (1) changes in fiber-type composition during postnatal development are accompanied by changes in daily activity and (2) individual variations in fiber-type composition are related to similar variations in daily muscle activity. METHODS The masseter and digastric muscles of 23 New Zealand White rabbits (young, juvenile and adult) were examined for their phenotype (myosin heavy chain content) and their daily activity (total daily number of activity bursts). RESULTS During development, the masseter showed a strong increase in the number of fast-type fibers compared to the number of slow-type fibers. During development, also the number of powerful bursts in the masseter increased. The digastric showed no significant changes in fiber types or burst numbers. Within each muscle, across individual animals, no significant correlations (R < 0.70) were found between any of the fiber types and daily burst numbers in any of the age groups. CONCLUSIONS The results suggest that activity-related influences are of relatively minor importance during development and that other factors are dominant in determining fiber-type composition.
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Affiliation(s)
- G E J Langenbach
- Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, The Netherlands.
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Cai L, Courtine G, Fong A, Burdick J, Roy R, Edgerton V. Plasticity of functional connectivity in the adult spinal cord. Philos Trans R Soc Lond B Biol Sci 2007; 361:1635-46. [PMID: 16939979 PMCID: PMC1664672 DOI: 10.1098/rstb.2006.1884] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
This paper emphasizes several characteristics of the neural control of locomotion that provide opportunities for developing strategies to maximize the recovery of postural and locomotor functions after a spinal cord injury (SCI). The major points of this paper are: (i) the circuitry that controls standing and stepping is extremely malleable and reflects a continuously varying combination of neurons that are activated when executing stereotypical movements; (ii) the connectivity between neurons is more accurately perceived as a functional rather than as an anatomical phenomenon; (iii) the functional connectivity that controls standing and stepping reflects the physiological state of a given assembly of synapses, where the probability of these synaptic events is not deterministic; (iv) rather, this probability can be modulated by other factors such as pharmacological agents, epidural stimulation and/or motor training; (v) the variability observed in the kinematics of consecutive steps reflects a fundamental feature of the neural control system and (vi) machine-learning theories elucidate the need to accommodate variability in developing strategies designed to enhance motor performance by motor training using robotic devices after an SCI.
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Affiliation(s)
- L.L Cai
- Bioengineering Option, California Institute of TechnologyPasadena, CA 91125-4100, USA
| | - G Courtine
- Department of Physiological Science, University of California1804 Life Sciences, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1527, USA
| | - A.J Fong
- Bioengineering Option, California Institute of TechnologyPasadena, CA 91125-4100, USA
| | - J.W Burdick
- Bioengineering Option, California Institute of TechnologyPasadena, CA 91125-4100, USA
- Mechanical Engineering Option, California Institute of TechnologyPasadena, CA 91125-4100, USA
| | - R.R Roy
- Brain Research Institute, University of CaliforniaLos Angeles, CA 90095, USA
| | - V.R Edgerton
- Brain Research Institute, University of CaliforniaLos Angeles, CA 90095, USA
- Department of Physiological Science, University of California1804 Life Sciences, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1527, USA
- Department of Neurobiology, University of CaliforniaLos Angeles, CA 90095-1763, USA
- Author for correspondence ()
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van Wessel T, Langenbach GEJ, Korfage JAM, Brugman P, Kawai N, Tanaka E, van Eijden TMGJ. Fibre-type composition of rabbit jaw muscles is related to their daily activity. Eur J Neurosci 2005; 22:2783-91. [PMID: 16324112 DOI: 10.1111/j.1460-9568.2005.04466.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Skeletal muscles contain a mixture of fibres with different contractile properties, such as maximum force, contraction velocity and fatigability. Muscles adapt to altered functional demands, for example, by changing their fibre-type composition. This fibre-type composition can be changed by the frequency, duration and presumably the intensity of activation. The aim of this study was to analyse the relationship between the spontaneous daily muscle activation and fibre-type composition in rabbit jaw muscles. Using radio-telemetry combined with electromyography, the daily activity of five jaw muscles was characterized in terms of the total duration of muscle activity (duty time) and the number of activity bursts. Fibre-type composition of the muscles was classified by analysing the myosin heavy chain content of the fibres. The amount of slow-type fibres was positively correlated to the duty time and the number of bursts only for activations exceeding 20-30% of the maximum activity per day. Furthermore, cross-sectional areas of the slow-type fibres were positively correlated to the duty time for activations exceeding 30% of the maximum activity. The present data indicate that the amount of activation above a threshold (> 30% peak activity) is important for determining the fibre-type composition and cross-sectional area of slow-type fibres of a muscle. Activation above this threshold occurred only around 2% of the time in the jaw muscles, suggesting that contractile properties of muscle fibres are maintained by a relatively small number of powerful contractions per day.
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Affiliation(s)
- T van Wessel
- Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam, Universiteit van Amsterdam and Vrije Universiteit, Meibergdreef 15, 1105 AZ, Amsterdam, Netherlands.
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Li AH, Zhang JM, Xie YK. Human acupuncture points mapped in rats are associated with excitable muscle/skin-nerve complexes with enriched nerve endings. Brain Res 2004; 1012:154-9. [PMID: 15158172 DOI: 10.1016/j.brainres.2004.04.009] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/01/2004] [Indexed: 10/26/2022]
Abstract
As part of our ongoing investigation into the neurological mechanisms of acupuncture, we have tried to correlate the distribution of afferent nerve endings with acupuncture points (AP) in the rat hind limbs. In vivo extracellular microfilament recordings of Aalpha/Abeta/Adelta fibers were taken from peripheral nerves to search for units with nerve endings or receptive fields (RF) in the skin or the muscles. The location of the RFs for each identified unit was marked on scaled diagrams of the hind limb. Noxious antidromic stimulation-induced Evans blue extravasation was used to map the RFs of C-fibers in the skin or muscles. Results indicate that, for both A- and C-fibers, the distribution of RFs was closely associated with the APs. In the skin, the RFs concentrate either at the sites of APs or along the orbit of meridian channels. Similarly, the majority of sarcous sensory receptors are located at the APs in the muscle. Results from our studies strongly suggest that APs in humans may be excitable muscle/skin-nerve complexes with high density of nerve endings.
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Affiliation(s)
- Ai-Hui Li
- Department of Physiology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences and Peking Union Medical College, #5 Dong Dan San Tiao, Beijing 100005, PR China
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Laouris Y, Bevan L, Reinking RM, Stuart DG. Associations between force and fatigue in fast-twitch motor units of a cat hindlimb muscle. Can J Physiol Pharmacol 2004; 82:577-88. [PMID: 15523515 DOI: 10.1139/y04-062] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Associations were quantified between the control force and fatigue-induced force decline in 22 single fast-twitch-fatigable motor units of 5 deeply anesthetized adult cats. The units were subjected to intermittent stimulation at 1 train/s for 360 s. Two stimulation patterns were delivered in a pseudo-random manner. The first was a 500-ms train with constant interpulse intervals. The second pattern had the same number of stimuli, mean stimulus rate, and stimulus duration, but the stimulus pulses were rearranged to increase the force produced by the units in the control (prefatigue) state. The associations among the control peak tetanic force of these units, 3 indices of fatigue, and total cumulative force during fatiguing contractions were dependent, in part, on the stimulation pattern used to produce fatigue. The associations were also dependent, albeit to a lesser extent, on the force measure (peak vs. integrated) and the fatigue index used to quantify fatigue. It is proposed that during high-force fatiguing contractions, neural mechanisms are potentially available to delay and reduce the fatigue of fast-twitch-fatigable units for brief, but functionally relevant, periods. In contrast, the fatigue of slow-twitch fatigue-resistant units seems more likely to be controlled largely, if not exclusively, by metabolic processes within their muscle cells.Key words: cat, catch-like property, fatigue, force, motor units, size principle.
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Affiliation(s)
- Y Laouris
- Department of Physiology, University of Arizona College of Medicine, Tucson, AZ 85724, USA
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Burke RE. Some unresolved issues in motor unit research. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2003; 508:171-8. [PMID: 12171107 DOI: 10.1007/978-1-4615-0713-0_20] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Abstract
The intrinsic properties of motoneurones, muscle units, and synaptic inputs exhibit correlated variations that subserve a wide range of functional demands. In large limb muscles, these correlations suggest distinct "types" of motor units, while in smaller, distal muscles their distributions are more continuous. The CNS mechanisms that control recruitment patterns are still unclear, particularly the organization of spinal interneurone circuits. We need new approaches to identify segmental interneurones by their inputs and output targets. However, functional circuitry is changeable, depending on the "state" of the system. Shifting alliances of interneurone groups can in principle produce virtually unlimited permutations of motor unit coactivation and suppression. Although such state-dependence plasticity is a challenge, it can also be a useful tool in unraveling interneurone organization.
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Affiliation(s)
- Robert E Burke
- Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20092-4455, USA.
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Abstract
Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.
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Affiliation(s)
- S C Gandevia
- Prince of Wales Medical Research Institute, Prince of Wales Hospital and University of New South Wales, Randwick, Sydney, Australia.
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Abstract
Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.
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Affiliation(s)
- R K Powers
- Department of Physiology & Biophysics, University of Washington School of Medicine, Box 357290, Seattle, Washington 98195-7290, USA
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Haftel VK, Prather JF, Heckman CJ, Cope TC. Recruitment of cat motoneurons in the absence of homonymous afferent feedback. J Neurophysiol 2001; 86:616-28. [PMID: 11495937 DOI: 10.1152/jn.2001.86.2.616] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
This study provides the first test in vivo of the hypothesis that group Ia muscle-stretch afferents aid in preventing reversals in the orderly recruitment of motoneurons. This hypothesis was tested by studying recruitment of motoneurons deprived of homonymous afferent input. Recruitment order was measured in decerebrate, paralyzed cats from dual intra-axonal records obtained simultaneously from pairs of medial gastrocnemius (MG) motoneurons. Pairs of MG motor axons were recruited in eight separate trials of the reflex discharge evoked by stimulation of the caudal cutaneous sural (CCS) nerve. Some reports suggest that reflex recruitment by this cutaneous input should bias recruitment against order by the size principle in which the axon with the slower conduction velocity (CV) in a pair is recruited to fire before the faster CV axon. Recruitment was studied in three groups of cats: ones with the MG nerve intact and untreated (UNTREATED); ones with the MG nerve cut (CUT); and ones with the MG nerve cut and bathed at its proximal end in lidocaine solution (CUT+). The failure of electrical stimulation to initiate a dorsal root volley and the absence of action potentials in MG afferents demonstrated the effective elimination of afferent feedback in the CUT+ group. Recruitment order by the size principle predominated and was not statistically distinguishable among the three groups. The percentage of pairs recruited in reverse order of the size principle was actually smaller in the CUT+ group (6%) than in CUT (15%) or UNTREATED (19%) groups. Thus homonymous afferent feedback is not necessary to prevent recruitment reversal. However, removing homonymous afferent input did result in the expression of inconsistency in order, i.e., switches in recruitment sequence from one trial to the next, for more axon pairs in the CUT+ group (33%) than for the other groups combined (13%). Increased inconsistency in the absence of increased reversal of recruitment order was approximated in computer simulations by increasing time-varying fluctuations in synaptic drive to motoneurons and could not be reproduced simply by deleting synaptic current from group Ia homonymous afferents, regardless of how that current was distributed to the motoneurons. These findings reject the hypothesis that synaptic input from homonymous group Ia afferents is necessary to prevent recruitment reversals, and they are consistent with the assertion that recruitment order is established predominantly by properties intrinsic to motoneurons.
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Affiliation(s)
- V K Haftel
- Department of Physiology, Medical School, Emory University, Atlanta, Georgia 30322, USA.
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