SINGLE MOTOR UNITS OF MAMMALIAN MUSCLE

Force variability is mostly not motor noise: Theoretical implications for motor control

Variability in muscle force is a hallmark of healthy and pathological human behavior. Predominant theories of sensorimotor control assume 'motor noise' leads to force variability and its 'signal dependence' (variability in muscle force whose amplitude increases with intensity of neural drive). Here, we demonstrate that the two proposed mechanisms for motor noise (i.e. the stochastic nature of motor unit discharge and unfused tetanic contraction) cannot account for the majority of force variability nor for its signal dependence. We do so by considering three previously underappreciated but physiologically important features of a population of motor units: 1) fusion of motor unit twitches, 2) coupling among motoneuron discharge rate, cross-bridge dynamics, and muscle mechanics, and 3) a series-elastic element to account for the aponeurosis and tendon. These results argue strongly against the idea that force variability and the resulting kinematic variability are generated primarily by 'motor noise.' Rather, they underscore the importance of variability arising from properties of control strategies embodied through distributed sensorimotor systems. As such, our study provides a critical path toward developing theories and models of sensorimotor control that provide a physiologically valid and clinically useful understanding of healthy and pathologic force variability.

The rise and fall of fasciculations in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease with a median survival of 3 years from symptom onset. Accessible and reliable biomarkers of motor neuron decline are urgently needed to quicken the pace of drug discovery. Fasciculations represent an early pathophysiological hallmark of amyotrophic lateral sclerosis and can be reliably detected by high-density surface electromyography. We set out to quantify fasciculation potentials prospectively over 14 months, seeking comparisons with established markers of disease progression. Twenty patients with amyotrophic lateral sclerosis and five patients with benign fasciculation syndrome underwent up to seven assessments each. At each assessment, we performed the amyotrophic lateral sclerosis-functional rating scale, sum power score, slow vital capacity, 30-min high-density surface electromyography recordings from biceps and gastrocnemius and the motor unit number index. We employed the Surface Potential Quantification Engine, which is an automated analytical tool to detect and characterize fasciculations. Linear mixed-effect models were employed to account for the pseudoreplication of serial measurements. The amyotrophic lateral sclerosis-functional rating scale declined by 0.65 points per month (P < 0.0001), 35% slower than average. A total of 526 recordings were analysed. Compared with benign fasciculation syndrome, biceps fasciculation frequency in amyotrophic lateral sclerosis was 10 times greater in strong muscles and 40 times greater in weak muscles. This was coupled with a decline in fasciculation frequency among weak muscles of -7.6/min per month (P = 0.003), demonstrating the rise and fall of fasciculation frequency in biceps muscles. Gastrocnemius behaved differently, whereby strong muscles in amyotrophic lateral sclerosis had fasciculation frequencies five times greater than patients with benign fasciculation syndrome while weak muscles were increased by only 1.5 times. Gastrocnemius demonstrated a significant decline in fasciculation frequency in strong muscles (2.4/min per month, P < 0.0001), which levelled off in weak muscles. Fasciculation amplitude, an easily quantifiable surrogate of the reinnervation process, was highest in the biceps muscles that transitioned from strong to weak during the study. Pooled analysis of >900 000 fasciculations revealed inter-fasciculation intervals <100 ms in the biceps of patients with amyotrophic lateral sclerosis, particularly in strong muscles, consistent with the occurrence of doublets. We hereby present the most comprehensive longitudinal quantification of fasciculation parameters in amyotrophic lateral sclerosis, proposing a unifying model of the interactions between motor unit loss, muscle power and fasciculation frequency. The latter showed promise as a disease biomarker with linear rates of decline in strong gastrocnemius and weak biceps muscles, reflecting the motor unit loss that drives clinical progression.

Motor unit types of cat triceps surae muscle

1. Motor units, defined as including a motoneurone (cell body, dendrites and axon) plus the muscle unit innervated, have been examined in the triceps surae motor pool of pentobarbital anaesthetized cats.2. The technique of intracellular stimulation and recording which was used permitted measurement of the axonal conduction velocity, post-spike hyperpolarization duration and input resistance of individual motoneurones, and the correlation of these properties with the characteristics of the twitch and tetanus responses of the muscle unit innervated by the cell elicited by direct intracellular stimulation.3. On the basis of muscle unit speed of contraction, motor units were divided into two groups: (a) fast twitch, or F, type with twitch time to peak (TwTp) less than or equal to 30 msec, and (b) slow twitch, or S, type with TwTp of 40 msec or greater. The twitch tensions (TwTen) produced by type F units were significantly larger (median value = 18 g) than the tensions generated by type S units (TwTen median value = 1.6 g). Type F muscle units had much higher tetanus fusion frequencies (median = 85 pulses/sec) than the S type (median 25 pulses/sec), and tended to have smaller tetanus to twitch tension ratios (Tet/Tw) (median = 2.6) than type S units (median = 5.4).4. The gastrocnemius heads contained a mixture of F and S types of muscle units, the proportions found being about 3 to 1 respectively. Units encountered in the soleus muscle were uniformly of type S. The characteristics of gastrocnemius and soleus type S motor units were not identical but appeared to represent quantitative differences in units of the same qualitative type.5. Motoneurones innervating type F muscle units had faster axonal conduction velocities, shorter post-spike hyperpolarizations and lower input resistances than those supplying type S units. However, no combination of motoneurone properties alone was sufficient to separate unambiguously types F and S motor units.

Motor innervation, motor unit organization and afferent innervation of m. extensor digitorum communis of the baboon's forearm

1. One hundred and fifty efferent axons innervating m. extensor digitorum communis (EDC) were isolated in filaments of C7 and C8 ventral roots of baboons. Conduction velocities were measured antidromically by stimulating the muscle nerve and recording from the filaments, and fell into two groups: a fast (49-84 m/sec) and a slow (22-41m/sec), presumably fusimotor group. The threshold for these latter axons exceeded the strength needed to elicit the maximal motor twitch.2. Stimulation of ventral root filaments containing slow axons produced no contractile tension in EDC.3. Stimulation of ventral root filaments containing fast-group axons elicited all-or-nothing twitches of motor units of EDC. The twitch tensions of 66.3% of the units were < 2.0 g wt.; only 8.7% were > 5.0 g wt. Tetanus-twitch ratios were 1.4-4.7 in a sample of 14 units. Contraction times were between 15 and 35 msec in 97% of the units. There was no correlation between contractile properties and axonal conduction velocity.4. Afferent volleys from the stimulated EDC nerve were recorded from C6 or C7 dorsal roots. The threshold was below the threshold for a just-detectable motor twitch in ten out of eleven baboons. Conduction velocity of the earliest component of the muscle afferent volley was 67-83 m/sec.5. The conduction velocities of twenty-eight spindle afferents, identified by their responses to linear stretches of EDC and by their unloading by maximal twitches, were all < 70 m/sec. Higher dynamic sensitivity tended to be associated with higher conduction velocity.

Influence of acute lung volume change on contractile properties of human diaphragm

The effect of stimulus frequency on the in vivo pressure generating capacity of the human diaphragm is unknown at lung volumes other than functional residual capacity. The transdiaphragmatic pressure (Pdi) produced by a pair of phrenic nerve stimuli may be viewed as the sum of the Pdi elicited by the first (T1 Pdi) and second (T2 Pdi) stimuli. We used bilateral anterior supramaximal magnetic phrenic nerve stimulation and a digital subtraction technique to obtain the T2 Pdi at interstimulus intervals of 999, 100, 50, 33, and 10 ms in eight normal subjects at lung volumes between residual volume and total lung capacity. The reduction in T2 Pdi that we observed as lung volume increased was greatest at long interstimulus intervals, whereas the T2 Pdi obtained with short interstimulus intervals remained relatively stable over the 50% of vital capacity around functional residual capacity. For all interstimulus intervals, the total pressure produced by the pair decreased as a function of increasing lung volume. These data demonstrate that, in the human diaphragm, hyperinflation has a disproportionately severe effect on the summation of pressure responses elicited by low-frequency stimulations; this effect is distinct from and additional to the known length-tension relationship.

Individual excitatory post-synaptic potentials due to muscle spindle Ia afferents in cat triceps surae motoneurones

1. The monosynaptic projection of single Ia muscle spindle afferents to motoneurones of identified type has been examined by spike triggered averaging in barbiturate anaesthetized cats. 2. The amplitude of averaged individual monosynaptic e.p.s.p.s in medical gastrocnemius motoneurones from homonymous Ia afferents was clearly related to the motor unit mechanical properties. It was largest in type S units, intermediate in type FR and smallest in type FF. 3. The projection frequency of homonymous Ia afferents to medial gastrocnemius motoneurones followed the same order. 4. Similar trends were evident in both respects for the heteronymous projections of triceps surae. 5. The mechanical property of units which related most directly to e.p.s.p. size was muscle unit contraction strength, a single continuous relation existing throughout all three unit types. 6. On the basis of motor axon conduction velocity measurements, it appeared that motoneurone size could provide a simple explanation for e.p.s.p. size in type S and type FR units, but not in type FF. The latter had unduly small e.p.s.p.s relative to their axon conduction velocity, indicating a qualitatively different presynaptic organization for FF motoneurones from the others. 7. The data make it possible to predict that Ia afferent excitation alone would cause recruitment of units in order of increasing contraction strength and that this order would also obtain if a substantial part of the excitation came from other sources with uniform effect on all unit types. 8. It is shown that a consequence of the observed organization is that the gain of the Ia mediated stretch reflex would be approximately proportional to the developed force.

A commentary on muscle unit properties in cat hindlimb muscles

A broad survey of muscle unit properties in 14 muscles of the cat hind limb is presented which emphasizes some general features of unit properties in mammalian muscles. A more detailed analysis of muscle unit properties in three muscles of the posterior compartment of the lower leg is then presented using Burke's tetrapartite (FF, FI or F (Int.), FR, and S) unit classification scheme. Our data on the properties of motor units in cat tibialis posterior (TP) have been compared to those generated by Burke and colleagues on units in flexor digitorum longus (FDL) and medial gastrocnemius (MG). In all three muscles, twitch contraction time was distinctly slower for type S units and specific tension outputs were substantially greater for type FF units than for type S units. The innervation ratios of type FR units were slightly lower than for type S units but the specific tension of the FR units was closer to FF units than to type S units. The FF units controlled 70-74% of the cumulative force output of each muscles, indicating a substantial capacity for powerful rapid contractions of all three of these muscles despite their differences in "size," action, and force generation. Distinctive features of the three muscles included differences in the unit types' force producing capabilities and in the relative representation of "nonfatigable" type FR and S units in each muscle. In particular, TP is endowed with some unusually powerful type FF units and a high percentage (42%) of type S units. In contrast, FDL has units that develop relatively little force and an unusually high representation (56%) of type FR units. The possible relationships between these muscle features and their presumed role in posture and locomotion is discussed.

Disparity of motoneurone and muscle differentiation following spinal transection in the kitten

1. The spinal cord of kittens, 3--5 days of age, was transected at the lower thoracic level. Isometric contractions of the medial gastrocnemius and soleus muscles as well as intracellular potentials of their motoneurones were recorded after varying post-operative periods of up to 110 days. Similar observations were made 52--59 days after cord transection in adult cats. 2. In cord-transected kittens, contraction time of the gastrocnemius muscle showed normal development, whereas the soleus muscle failed to maintain slow contraction. In adult cats, cord transection increased the speed of contraction in the soleus muscle without significant changes in contraction times of the gastrocnemius muscle. 3. Soleus motoneurones showed a normal post-natal increase in the duration of afterhyperpolarization (a.h.p.) up to a certain stage (61--71 days in age) following cord transection. However, the subsequent increase in the duration of a.h.p. of soleus motoneurones observed in normal kittens was lacking in cord-transected kittens. It is suggested that soleus motoneurones show two stages of differentiation in terms of the duration of a.h.p. 4. In adult cats, cord transection caused a decrease in the duration of a.h.p. of soleus motoneurones approximately to the value observed at the end of the first stage of differentiation in kittens. 5. The duration of a.h.p. of gastrocnemius motoneurones remained virtually unchanged follwoing cord transection in both kittens and adult cats. 6. The positive correlation between the duration of a.h.p. of soleus motoneurones and contraction time of the innervated muscle fibres normally observed in kittens and adult cats was absent following cord transection. 7. It was assumed that alteration s in contraction time of the muscle following cord transection are due to virtual elimination of motoneurone discharge and that the duration of a.h.p. reflects the discharge pattern of motoneurones under normal conditions. Based on these assumptions, a possible process for normal post-natal differentiation of motoneurone and muscle is proposed.

Reaction of intact spinal motoneurones to partial denervation of the muscle

1. The properties of soleus motoneurones of the cat were examined with intracellular electrodes about 3 weeks after partial denervation of the soleus muscle. Soleus motoneurones whose axons had been left intact were distinguished from those whose axons had been sectioned by the presence or absence of muscle contraction in response to intracellular stimulation of each motoneurone. 2. The average twitch tension of motor units evoked by intracellular stimulation of intact soleus motoneurones after partial denervation of the muscle was not significantly different from that observed in control, unoperated cats. Therefore, it was assumed that the majority of intact motoneurones had not been subject to injuries in their axons upon partial denervation. 3. Soleus motoneurones whose axons had been sectioned showed a significant increase in overshoot of action potentials and a significant decrease in resting membrane potential, in axonal conduction velocity and in the duration of after-hyperpolarization. 4. Soleus motoneurones whose axons had been left intact also showed a significantly shorter after-hyperpolarization than that seen in control, unoperated cats. Other electrophysiological properties of the intact soleus motoneurones were indistinguishable from those observed in unoperated animals. 5. The decrease of the duration of after-hyperpolarization in intact soleus motoneurones was greater in highly denervated preparations than in moderately denervated preparations. 6. The decrease of the duration of after-hyperpolarization in intact soleus motoneurones was associated with a decrease in contraction times of the innervated muscle fibres, the former preceding the latter by one to two weeks. 7. It is concluded that motoneurone properties can be modified without injury to their axons and that alterations in the properties of intact motoneurones depend upon the degree of partial denervation of the muscle. The possible signal for alterations of motoneurone properties is discussed. 8. It is also concluded that the contractile properties of muscle fibres can be modified without cross-union of the nerves. It is suggested that the contractile properties of muscle fibres may be linked to the duration of after-hyperpolarization or to some mechanism related to this factor in the innervating motoneurones.

Differentiation of motoneurones and skeletal muscles in kittens

1. Isometric contractions of the medial gastrocnemius (fast switch) and soleus (slow twitch) muscles were recorded in kittens ranging in age from 3 to 112 days, as well as in adult cats. 2. It was confirmed that the speed of contraction of the gastrocnemius muscle becomes progressively faster during the first few weeks after birth, whereas contraction times of the soleus muscle show little changes or a slight prolongation during the period of post-natal development. 3. The properties of gastrocnemius (fast alpha) and soleus (slow alpha) motoneurons were examined with intracellular electrodes in kittens at three different stages; 16-20, 61-71 and 100-112 days in age. 4. The axonal conduction velocities of both gastrocnemius and soleus motoneurones increased monotonically throughout the period of development and showed no correlation with post-natal changes in contraction times of the innervated muscles. 5. The duration of after-hyperpolarization in soleus motoneurones became progressively longer with age, while that in gastrocnemius motoneurones remained virtually unchanged during development. 6. The relation between the duration of after-hyperpolarization and the axonal conduction velocity in kitten motoneurones was similar to that observed in axotomized motoneurones of adult cats. 7. It is suggested that fast and slow alpha motoneurones show postnatal differentiation in terms of the duration of after-hyperpolarization and that axotomy leads to 'dedifferentiation' of the motoneurone properties. 8. Post-natal changes in the contractile properties of skeletal muscles were independent of the changes in the duration of after-hyperpolarization of the innervating motoneurones. However, it remains uncertain whether muscle differentiation is independent of the discharge pattern of the innervating motoneurones.

Studies of some twitch and fatigue properties of different motor unit types in the ankle muscles of the adult cat

Contractile responses of motor units in the gastrocnemius, soleus and pretibial flexor muscles of adult cats were elicited by intracellular stimulation of motoneurones. The motor units were classified into types FF, FR and S (Burke et al. 1971) and their responses to the same stimulation patterns as those used in a previous investigation of whole muscles (Hammarberg and Kellerth 1975 a) were studied. The duration of motoneurone afterhyperpolarization was short in both the fast twitch FF and FR units; it was longer in the soleus S units than in the S units of the pale muscles. Twitch time-to-peak was less than 30 ms in the FF and FR units, but exceeded 40 ms in the S units. Soleus S units were slower than S units of the pale muscles. Potentiation was observed in the gastrocnemius units, but not in the soleus S units. A short rest allowed fatigued extensor units of the FF and FR types to regain some contractile strength. This was less evident in the S units which, on the other hand, were extremely resistant to fatigue. Differences in response patterns between corresponding motor unit types of the flexor and extensor muscles were observed. A few fast twitch units were identified in the slow soleus muscle.

The effect of peripheral nerve cross-union on connections of single Ia fibers to motoneurons

The distribution of Ia terminals to alpha-motoneurons has been investigated under conditions where a) the Ia fiber innervates a foreign muscle and b) the motor axon innervates a foreign muscle. This distribution has been assessed by examining which motoneurons develop an EPSP in response to stimulation of a single Ia afferent fiber. Cross-union of the medial gastrocnemius and lateral gastrocnemius-soleus nerves has been performed in 5--8 day old kittens and the effect of this procedure on Ia connections to alpha-motoneurons has been investigated in the adult about one year later. No rearrangement of synaptic connections has been observed under these conditions. The connections from self-unioned Ia afferents and to self-unioned motoneurons are also normal. The individual EPSP's in these motoneurons with regenerated axons have normal rise times. The findings of normal Ia connections as well as normal EPSP rise times in alpha-motoneurons with regenerated motor axons indicates reversibility of the changes seen in axotomized motoneurons.

Properties of fast and slow alpha motoneurones following motor reinnervation

1. The properties of medial gastrocnemius (fast alpha) and soleus (slow alpha) motoneurones of the cat were examined with intracellular electrodes 33-154 days after self- or cross-union of the muscle nerves.2. The original properties of fast and slow alpha motoneurones measured in terms of the axonal conduction velocity, the duration of after-hyperpolarization and the overshoot amplitude were restored at least in part after self-union of the muscle nerves.3. Slow alpha motoneurones recovered their original properties whether the muscle nerve was united to the original, slow red muscle or to the fast pale muscle. In two animals, some of the fast muscle fibres seemed to be doubly innervated by both fast and slow alpha motoneurones.4. The degree of restoration of the original motoneurone properties was closely related to the degree of motor reinnervation of the muscle.5. At a given stage of motor reinnervation, the degree of recovery of the motoneurone properties was approximately the same regardless of the presence or absence of functional motor connexions of the motoneurone under study.6. When the dorsal root ganglia were removed immediately before self-union of the muscle nerves, recovery of the conduction velocity of motoneurones was impeded without significant effect upon the process of motor reinnervation or upon recovery of the overshoot amplitude and the duration of after-hyperpolarization.7. It is concluded that the ;dedifferentiated' properties of fast and slow alpha motoneurones after nerve section may be ;redifferentiated' following motor reinnervation to any muscle. The possible signal for restoration of the motoneurone properties is discussed.

The Ferrier lecture, 1968. Motor apparatus of the baboon's hand

It is exactly 40 years since Ferrier died, and 39 years since Sherrington gave the first Ferrier Lecture in honour of his work. Sherrington (1906) had dedicated The integrative action of the nervous system to Ferrier ‘in token of recognition of his many services to the experimental physiology of the central nervous system’. At Aberdeen Ferrier had been taught by Bain, the Professor of Logic, who kept a model of the human brain upon his lecture table. Ferrier moved to London in 1870, and was excited by Hughlings Jackson’s revolutionary discoveries and ideas about the nature and localization of the sensory and motor functions of the human brain. Early in 1873 he discussed Fritsch & Hitzig’s (1870) pioneer galvanic stimulations of the dog’s cortex with his friend James Crichton-Browne. In the spring and summer of that year he performed his pioneer faradic stimulations of the brains of monkeys in the laboratory of the West Riding Lunatic Asylum, of which Crichton-Browne was Medical Director (Sherrington 1928). He sought to ‘put to experimental proof the views entertained by Dr Hughlings Jackson on the pathology of Epilepsy, Chorea and Hemiplegia, by imitating artificially the "destroying" and “discharging lesions” of disease which his writings have defined and differentiated’ (Ferrier 1873). 'The phenomena of localized and unilateral convulsive movements, depending, as Hughlings Jackson shows, on vital irritation of certain regions of the cortex, are essentially of the same nature as those caused by electrisation of the same regions’ (1876, p. 133). But Ferrier believed that many of the movements he mapped by electrical stimulation had evidently a purposive or volitional character’ (1876, p. 163). In 1876 he transferred his monkey map to the brain of man, and described of the relation between brain fissures and bony landmarks; and in 1883 he suggested that the time had come when localized cerebral lesions should be excised by the surgeon. For this, as he later recalled with amusement, he was criticized in an editorial in the Lancet (Ferrier, 1888), but in 1884 Godlee removed a localized tumour ‘of the size of a walnut’ (Sherrington 1928) from the Rolandic cortex through an opening exactly overlying it and but little larger than itself. Ferrier and Hughlings Jackson both witnessed this memorable operation (Trotter 1934). As time went on, experimentalists interested themselves more in defining details of localization than in developing concepts of function (Phillips 1966). ‘More penetrative modes and aims of analysis came to be little pursued,’ wrote Sherrington (1928). ‘A localization vogue reigned for nearly a quarter of a century, and became in due course tedious and relatively infertile.’

Firing patterns of gastrocnemius motor units in the decerebrate cat

1. The patterns of medial gastrocnemius (MG) motor unit firing in response to MG muscle stretch have been studied in decerebrate cats, using intracellular recording techniques. In most of the units, the motoneurone axonal conduction velocity and cell input resistance were measured, and the maximum amplitude and wave form of the group Ia composite EPSP evoked by MG nerve stimulation were determined. The mechanical properties of the muscle unit portion of each motor unit were also studied.2. Motor unit firing patterns were classified into two groups, ;tonic' and ;phasic'. With few exceptions, tonic motor units showed sustained firing throughout MG stretches of varying duration, while most phasic units did not fire at all to the same stimulus. Tetanization of the afferents did not convert any of the previously phasic units to tonic firing.3. MG motor units in this study were divided into three groups on the basis of the muscle unit twitch properties. Units with short twitch time to peak values (< 35 msec) were subdivided into two groups according to twitch tension output: (a) type F, with twitch tension > 1.5 g, and (b) type F(*), with twitch tension < 1.5 g. Units with slow twitch time to peak (> 35 msec) were classified as type S.4. The presence or absence of tonic firing during sustained MG stretch was found to be significantly related to the following factors: (a) the motor unit twitch type, in that tonic firing was observed in 100% of type S, 70% of type F(*), and only 10% of type F units; (b) the apparent motoneurone size, in that tonic units tended to have higher cell input resistance values and slower axonal conduction velocities than phasic units; and (c) the density and spatial organization of the group Ia synaptic input, in that the MG monosynaptic EPSPs found in tonic motor units tended to be both larger in amplitude and longer in duration than those found in phasic units.5. The intrinsic properties of MG motor units, the characteristics of the group Ia synaptic input to the motoneurones and the unit firing patterns elicited by MG muscle stretch appear to be mutually interrelated, forming a pattern of motor units within the MG pool which is analogous to the pattern thought to characterize the motor units belonging to ;slow' and ;fast' muscles. In most respects this pattern of MG motor units appears to be a continuum without clearly separable subgroups.