Properties of motor units in the first deep lumbrical muscle of the cat's foot

doi:10.1016/0006-8993(75)90508-9

Brain Research

Volume 98, Issue 1, 7 November 1975, Pages 37-55

https://doi.org/10.1016/0006-8993(75)90508-9 Get rights and content

Abstract

Isometric contractions of single motor units were studied in the first deep lumbrical muscle of the cat's hind-foot. Motor units with short twitch contraction times (15–20 msec) generally differed from those with longer ones (23–50 msec; contraction time measured in unpotentiated twitches) in showing (1) a greater maximum tetanic tension, (2) a smaller resistance to fatigue, (3) more post-tetanic potentiation of twitch tension, and (4) no post-tetanic occurrence of repetitive activity in response to single nerve stimuli (such ‘post-tetanic repetitive activity’ was seen in several of the slower units). The ratio between unpotentiated twitch tension and maximum tetanic tension was similar for units with brief and long contraction times. The peak-to-peak amplitude of a single motor unit spike, recorded with gross electrodes, tended to be directly proportional to the maximum tetanic tension of the same motor unit.

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The mathematical muscle models should include several aspects of muscle structure and physiology. First, muscle force is the sum of forces of multiple motor units (MUs), which have different contractile properties and play different roles in generating muscle force. Second, whole muscle activity is an effect of net excitatory inputs to a pool of motoneurons innervating the muscle, which have different excitability, influencing MU recruitment. In this review, we compare various methods for modeling MU twitch and tetanic forces and then discuss muscle models composed of different MU types and number. We first present four different analytical functions used for twitch modeling and show limitations related to the number of twitch describing parameters. We also show that a nonlinear summation of twitches should be considered in modeling tetanic contractions. We then compare different muscle models, most of which are variations of Fuglevand’s model, adopting a common drive hypothesis and the size principle. We pay attention to integrating previously developed models into a consensus model based on physiological data from in vivo experiments on the rat medial gastrocnemius muscle and its respective motoneurons. Finally, we discuss the shortcomings of existing models and potential applications for studying MU synchronization, potentiation, and fatigue.
The contractile properties of motor units in the rat flexor digitorum brevis muscle have continuous distribution
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Citation Excerpt :
However, small distal muscles seem to be an exception to this general rule. Kernell et al. (1975) analyzed the occurrence of sag in motor units in the first deep lumbrical muscle of the cat and found that this phenomenon occurs for both fast and slow motor units and depends on the frequency stimulation. Human thenar muscle motor units also show a lack of sag (Thomas et al. 1991).
The majority of motor unit studies were performed predominantly on calf muscles, where three types of units: S, FR and FF were found. These muscles are involved in postural activity, walking, running and jumping. The properties of foot muscles that perform other functions, e.g. scratching (in animals), and are purely co-active with calf muscles, are poorly known. The aim of the present study was to investigate the contractile properties of motor units in the flexor digitorum brevis. Fifty-six motor units were studied in male Wistar rats. Several methods of fast/slow motor unit categorization, presence of sag, contraction time values, and 20 Hz index, did not allow the separation of the studied motor units into discrete clusters. Therefore, motor units were divided into two groups: fatigable and resistant to fatigue, based on the fatigue index with the border value of 0.5 (although the distribution of the index was not bimodal). The fatigable motor units were stronger and faster compared to the resistant ones. In conclusion, the distribution of motor unit contractile properties in the studied foot muscle was continuous and indicated a lack of three separate physiological types of motor units that usually occurs for the majority of hindlimb muscles. This discrepancy appears to be associated with differences in the typical forms of motor unit activity in distinct muscles.
Motor Units and Muscle Receptors
2018, Muscle and Exercise Physiology
The motor unit is composed of the motoneuron, its axon branching within a muscle, and the innervated muscle fibers. There are three types of motor units: slow (S), fast resistant to fatigue (FR), and fast fatigable (FF). Differences between them concern morphological and electrophysiological properties of motoneurons, a number, morphology, biochemical characteristics, and metabolism of muscle fibers forming each unit, contractile times, forces and fatigue resistance. Recruitment of motor units into a contraction depends on the excitability of motoneurons and correlates with the properties of muscle fibers. The number of active units, their position and mutual relations within a muscle as well as frequencies and patterns of discharges generated by motoneurons determine the muscle force during subsequent phases of movement. The properties of motoneurons and motor units are subject to plastic modifications during development and aging, as a result of various forms of motor activity or disease. The motor unit activity is closely connected to the muscle receptors, primarily muscle spindles, and tendon organs which send information about the spatial position of the body and the limbs to the central nervous system, thus contributing to the control of precise movements.
Summation of motor unit forces in rat medial gastrocnemius muscle
2010, Journal of Electromyography and Kinesiology
Citation Excerpt :
All investigated MUs were classified as fast or slow on the basis of sag appearance, which is absent in slow MUs (Burke et al., 1973; Grottel and Celichowski, 1990). Fast MUs, with a sag, were divided into FF and FR types on the basis of the fatigue index, which was <0.5 for the FF and >0.5 for the FR (Kernell et al., 1975; Grottel and Celichowski, 1990). For each averaged MU twitch, a number of parameters were analyzed: the twitch force (TwF, measured from the baseline to the peak), the contraction time (CT, from the onset of the force increase to the peak) and the half-relaxation time (HRT, measured from the peak force to half of its maximum).
The summation of contractile forces of motor units (MUs) was analyzed by comparing the recorded force during parallel stimulation of two and four individual MUs or four groups of MUs to the algebraic sum of their individual forces. Contractions of functionally-isolated single MUs of the medial gastrocnemius muscle were evoked by electrical stimulation of thin filaments of the split L5 or L4 ventral roots of spinal nerves. Additionally, contractions of large groups of MUs were evoked by stimuli delivered to four parts of the divided L5 ventral root. Single twitches, 40 Hz unfused tetani, and 150 Hz fused maximum tetani were recorded. In these experimental situations the summation was more effective for unfused tetani than for twitches or maximum tetani. The results obtained for pairs of MUs were highly variable (more- or less-than-linear summation), but coactivation of more units led to progressively weaker effects of summation, which were usually less-than-linear in comparison to the algebraic sums of the individual forces. The variability of the results highlights the importance of the structure of the muscle and the architecture of its MUs. Moreover, the simultaneous activity of fast and slow MUs was considerably more effective than that of two fast units.
Time-related changes of motor unit properties in the rat medial gastrocnemius muscle after spinal cord injury. I. Effects of total spinal cord transection
2010, Journal of Electromyography and Kinesiology
The contractile properties of motor units (MUs) were electrophysiologically investigated in the medial gastrocnemius (MG) muscle in 17 Wistar three-month-old female rats: 14, 30, 90 and 180 days after the total transection of the thoracic spinal cord and compared to those in intact (control) rats. A sag phenomenon, regularly observed in unfused tetani of fast units in intact animals at 40 Hz stimulation, almost completely disappeared in spinal rats. Therefore, the MUs of intact and spinal rats were classified as fast or slow types basing on 20 Hz tetanus index, the value of which was lower or equal 2.0 for fast and higher than 2.0 for slow MUs. The MUs composition of MG muscle changed with time after the spinal cord transection: an increasing proportion of fast fatigable (FF) units starting one month after injury and a disappearance of slow (S) units within the three months were observed. In all MUs investigated the twitch contraction and half-relaxation time were significantly prolonged after injury (p < 0.01, Mann–Whitney U-test). Moreover, a decrease of the fatigue index for fast resistant (FR) and slow MUs was observed in subsequent groups of spinal rats. No significant changes were found between twitch forces in all MU types of spinal animals (p > 0.05). However, due to a decrease of the maximal tetanic force, a significant rise of the twitch-to-tetanus ratio of all MUs in spinal rats was detected (p < 0.01). The considerable reduction of ability to potentiate the force was noticed for fast, especially FF type MUs. In conclusion, the spinal cord transection leads to changes in the proportion of the three MU types in rat MG muscle. The majority of changes in MUs’ contractile properties were developed progressively with time after the spinal cord injury. However, the most intensive alterations of twitch-time parameters were observed in rats one month after the transection.
The image of motor units architecture in the mechanomyographic signal during the single motor unit contraction: in vivo and simulation study
2009, Journal of Electromyography and Kinesiology
The mechanomyographic (MMG) signal analysis has been performed during single motor unit (MU) contractions of the rat medial gastrocnemius muscle. The MMG has been recorded as a muscle surface displacement by using a laser distance sensor. The profiles of the MMG signal let to categorize these signals for particular MUs into three classes. Class MMG-P (positive) comprises MUs with the MMG signal similar to the force signal profile, where the distance between the muscle surface and the laser sensor increases with the force increase. The class MMG-N (negative) has also the MMG profile similar to the force profile, however the MMG is inverted in comparison to the force signal and the distance measured by using laser sensor decreases with the force increase. The third class MMG-M (mixed) characterize the MMG which initially increases with the force increases and when the force exceeds some level it starts to decrease towards the negative values. The semi-pennate muscle model has been proposed, enabling estimation of the MMG generated by a single MU depending on its localization. The analysis have shown that in the semi-pennate muscle the localization of the MU and the relative position of the laser distance sensor determine the MMG profile and amplitude. Thus, proposed classification of the MMG recordings is not related to the physiological types of MUs, but only to the MU localization and mentioned sensor position. When the distance sensor is located over the middle of the muscle belly, a part of the muscle fibers have endings near the location of the sensor beam. For the MU MMG of class MMG-N the deflection of the muscle surface proximal to the sensor mainly influences the MMG recording, whereas for the MU MMG class MMG-P, it is mainly the distal muscle surface deformation. For the MU MMG of MMG-M type the effects of deformation within the proximal and distal muscle surfaces overlap. The model has been verified with experimental recordings, and its responses are consistent and adequate in comparison to the experimental data.

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^*: Present address: Department of Neurophysiology, University of Amsterdam, Eerste Constantijn Huygensstraat 20, Amsterdam, The Netherlands.

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Properties of motor units in the first deep lumbrical muscle of the cat's foot

Abstract

Brain Research

Brain Research

Intramuscular branching of fusimotor fibres

J. Physiol. (Lond.)

The mechanical properties and innervation of fast and slow motor units in the intercostal muscles of the cat

J. Physiol. (Lond.)

Properties of fast and slow motor units in hind limb and tail muscles of the rat

Quart. J. exp. Physiol.

Motor units of the first superficial lumbrical muscle of the cat

J. Neurophysiol.

Isometric contractions of motor units in a fast twitch muscle of the cat

J. Physiol. (Lond.)

Post-tetanic effects in motor units of fast and slow twitch muscle of the cat

J. Physiol. (Lond.)

Relation entre la vitesse de conduction des fibres nerveuses motrices et le temps de contraction de leurs unite´s motrices

C.R. Acad. Sci. (Paris)

Motor fibres innervating extrafusal and intrafusal muscle fibres in the cat

J. Physiol. (Lond.)

A comparison between the effects of a tetanus and the effects of sympathomimetic amines on fast- and slow-contracting mammalian muscles

Brit. J. Pharmacol.

Fatigue and neuromuscular block in mammalian skeletal muscle

Factors affecting the differentiation of mammalian fast and slow muscle fibres

Motor units in cat soleus muscle: physiological, histochemical and morphological characteristics

J. Physiol. (Lond.)

Physiological types and histochemical profiles in motor units of the cat gastrocnemius

J. Physiol. (Lond.)

Mammalian motor units: Physiological-histochemical correlation in three types in cat gastrocnemius

Science

Anatomy and innervation ratios in motor units of cat gastrocnemius

J. Physiol. (Lond.)