Motor unit types of cat triceps surae muscle

Fast and slow twitch units in a human muscle

A study has been made of the isometric twitches of single motor units in the extensor hallucis brevis (EHB) muscle in man. The twitch contraction times ranged from 35 to 98 msec and the pooled results indicated the presence of at least two types of unit. The recorded twitch tensions also varied considerably (from 2 to 14 g) but were not related to the twitch speeds. The twitch tension developed by the whole EHB muscle depended on the initial length of the muscle and on the age of the subject. The EHB muscle was estimated to contain approximately 56 motor units.

Electrophysiological estimation of the number of motor units within a human muscle

An electrophysiological method is described for estimating the numbers of motor units in the extensor digitorum brevis muscle in man. The results obtained are compared with counts of axons in the nerve to the muscle. The significance of the sizes of the evoked motor unit potentials is discussed.

Histochemical and functional correlations in anterior horn neurons of the cat spinal cord

The histochemical reaction for phosphorylase is completely lost from anterior horn neurons rich in phosphorylase within 72 hours after proximal or distal axonal section. Using this new type of axonal reaction as a marking technique in the anterior horn of the seventh lumbar spinal cord segment of the cat, we demonstrated that (i) alpha motor neurons of slow twitch motor units, like those of fast twitch motor units, are rich in phosphorylase and poor in succinate dehydrogenase, and (ii) interneurons and Renshaw neurons are rich in succinate dehydrogenase and poor in phosphorylase. Gamma motor neurons, because of their small size, are considered to be rich in succinate dehydrogenase and poor in phosphorylase. Thus, anterior horn neurons capable of higher firing frequencies (Renshaw neurons, interneurons, and gamma motor neurons) are richer in mitochondrial oxidative enzyme activity as marked by succinate dehydrogenase. Those firing at lower frequencies (both types of alpha motor neurons) are richer in phosphorylase activity and glycogen content and, thus, apparently better equipped for anaerobic glycolysis.

Electrotonic characteristics of alpha motoneurones of varying size

1. The neuronal membrane responses to long constant current pulses (essentially current steps) have been studied in cat triceps surae motoneurones identified as to the type of muscle fibres, fast twitch (type F) or slow twitch (type S), innervated by the cell being studied. For each motoneurone the membrane time constant, tau(M), and input resistance, R(N), were determined from the response to a current step. In addition, shorter time constants (;equalizing time constants') resulting from current spread into the dendrites were estimated by graphical analysis.2. The electrotonic length of the combined motoneurone soma and dendritic tree was estimated from the current step data using the neuronal equivalent cylinder model formulated by Rall (Rall, 1969). The mean electrotonic length of the motoneurone equivalent cylinder was approximately 1.5 in both type F and type S motoneurones. The mean membrane time constant of type F cells was 5.6 msec and that of type S motoneurones was 6.7 msec. This difference in mean tau(M) values was of border line statistical significance.3. The results indicate that the electrotonic length of the combined dendritic trees of both large type F and small type S motoneurones is essentially the same. The implication of this conclusion for interpretation of previous analyses of the monosynaptic EPSP is discussed.

The ultrastructure of the neuromuscular junctions of mammalian red, white, and intermediate skeletal muscle fibers

Distinct ultrastructural differences exist at the neuromuscular junctions of red, white, and intermediate fibers of a mammalian twitch skeletal muscle (albino rat diaphragm). The primary criteria for recognizing the three fiber types are differences in fiber diameter, mitochondrial content, and width of the Z line. In the red fiber the neuromuscular relationship presents the least sarcoplasmic and axoplasmic surface at each contact. Points of contact are relatively discrete and separate, and axonal terminals are small and elliptical. The junctional folds are relatively shallow, sparse, and irregular in arrangement. Axoplasmic vesicles are moderate in number, and sarcoplasmic vesicles are sparse. In the white fiber long, flat axonal terminals present considerable axoplasmic surface. Vast sarcoplasmic surface area is created by long, branching, closely spaced junctional folds that may merge with folds at adjacent contacts to occupy a more continuous and widespread area. Axoplasmic and sarcoplasmic vesicles are numerous. Both axoplasmic and sarcoplasmic mitochondria of the white fiber usually contain intramitochondrial granules. The intermediate fiber has large axonal terminals that are associated with the most widely spaced and deepest junctional folds. In all three fiber types, the junctional sarcoplasm is rich in free ribosomes, cisternae of granular endoplasmic reticulum, and randomly distributed microtubules.

A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae

1. Post-synaptic potentials (PSPs) evoked by electrical stimulation of a variety of input systems have been compared in triceps surae motoneurones innervating slow and fast muscle units, the speed of contraction of which was also determined.2. Stimulation of high threshold afferents in both flexor and extensor muscle nerves, and of joint afferents, evoked polysynaptic PSPs which were predominantly hyperpolarizing in both fast and slow twitch motor units.3. Volleys in cutaneous afferents in the sural and saphenous nerves evoked polysynaptic PSPs composed of mixtures of inhibitory and excitatory components. The inhibitory components were predominant in slow twitch motor units, while in fast twitch units there was a trend towards excitatory predominance.4. Repetitive stimulation of the red nucleus caused predominantly inhibitory PSPs in slow twitch units and mixed or predominantly excitatory PSPs in fast twitch units. There was a correlation in the excitatory/inhibitory balance between PSPs of cutaneous and rubrospinal origin in those motoneurones in which both types of PSPs were studied.5. The amplitudes of group Ia disynaptic inhibitory PSPs were found to be correlated with motor unit twitch type: IPSPs in slow twitch units were larger than those in fast twitch units. Rubrospinal conditioning volleys were found to facilitate group Ia IPSPs in both fast and slow twitch motor units.6. The results suggest that there may be several basic patterns of synaptic input organization to motoneurones within a given motor unit pool. In addition to quantitative variation in synaptic distribution, there is evidence that qualitative differences in excitatory to inhibitory balance also exist in the pathways conveying input from cutaneous afferents and rubrospinal systems to triceps surae motoneurones. These qualitative differences are correlated with the motor unit twitch type.

Catch property in single mammalian motor units

The tension output of single motor units in the cat triceps surae muscle was studied during patterned stimulation of the innervating motoneurons. With repetitive stimulation within a rather narrow frequency range, the tension output of slow-twitch (type S) motor units may be quite sensitive to the pattern of stimulus intervals in the train. The presence of only one stimulus interval that is much shorter than the others in the train can cause marked, long-lasting tension enhancement, provided that the stimulus repetition rate in the basic train is within certain limits.

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.’

Functional consequences of tenotomy in hind limb muscles of the cat

1. The chronic EMG activity and the contractile properties of normal, denervated or tenotomized cat triceps surae muscles were measured.2. Following Achilles tenotomy no measurable change in EMG activity occurred in either the soleus or medial gastrocnemius muscles.3. No change in the twitch time or tetanus-twitch ratio was seen following tenotomy in these muscles despite marked atrophy of the muscle. Measurements were made 3-10 weeks following the tenotomy procedures.4. Denervation results in some prolongation of the gastrocnemius twitch.5. The relationship between the contractile properties of a given muscle and the aggregate amount of activity imposed on that muscle by its innervation is discussed.

Analysis of synaptic efficacy in spinal motoneurones from 'quantum' aspects

1. Synaptic responses of triceps surae motoneurones of the cat following stimulation of single afferent fibres were examined by intracellular recording techniques.2. The mean quantum content (m) of monosynaptic excitatory postsynaptic potentials (EPSPs) was independent of the type of motoneurone recorded and of the afferent fibre stimulated. There was no significant difference in m value between homonymous and heteronymous synapses.3. A positive correlation was found between the amplitude of unit EPSPs and the input resistance of motoneurones. The difference in the amplitude of unit EPSPs appears to be responsible for the higher synaptic efficacy in slow-conducting motoneurones than in fast-conducting motoneurones.4. There was no significant difference in the time course of monosynaptic EPSPs evoked by impulses from homonymous and heteronymous afferent fibres.5. The ratio of monosynaptic connexions from a given afferent fibre was significantly greater on to homonymous than to heteronymous motoneurones. It is concluded that the difference in efficacy between homonymous and heteronymous synaptic transmission is due to the difference in the number of afferent fibres converging upon these motoneurones.

Differential histochemical effects of muscle contractions on phosphorylase and glycogen in various types of fibres: relation to fatigue

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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.