Compound maximal motor unit response is modulated by contraction intensity, but not contraction type in tibialis anterior

In 11 tibialis anterior muscles of the cat, a single motor unit was characterized physiologically and subsequently depleted of its glycogen through repetitive stimulation of an isolated ventral root filament. Muscle cross sections were stained for glycogen using a periodic acid-Schiff reaction, and single-fiber optical densities were determined to identify those fibers belonging to the stimulated motor unit. Innervation ratios were determined by counting the total number of muscle fibers in a motor unit in sections taken through several levels of the muscle. The average innervation ratios for the fast, fatigueable (FF) and fast, fatigue-resistant (FR) units were similar. However, the slow units (S) contained 61% fewer fibers than the fast units (FF and FR). Muscle fibers belonging to S and FR units were similar in cross-sectional area, whereas fibers belonging to FF units were significantly larger than fibers belonging to either S or FR units. Additionally, muscle fibers innervated by a single motoneuron varied by two- to eightfold in cross-sectional area. Specific tensions, based on total cross-sectional area determined by summing the areas of all muscle fibers of each unit, showed a modest difference between fast and slow units, the means being 23.5 and 17.2 N X cm-2, respectively. Variations in maximum tension among units could be explained principally by innervation ratio, although fiber cross-sectional area and specific tension did contribute to differences between unit types.

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Spatial distribution of motor unit fibers in the cat soleus and tibialis anterior muscles: local interactions

Journal of Neuroscience ◽

10.1523/jneurosci.08-06-02142.1988 ◽

1988 ◽

Vol 8 (6) ◽

pp. 2142-2152 ◽

Cited By ~ 129

Author(s):

SC Bodine ◽

A Garfinkel ◽

RR Roy ◽

VR Edgerton

Keyword(s):

Spatial Distribution ◽

Motor Unit ◽

Tibialis Anterior ◽

Local Interactions

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Further evidence of incomplete neural control of muscle properties in cat tibialis anterior motor units

AJP Cell Physiology ◽

10.1152/ajpcell.1995.268.2.c527 ◽

1995 ◽

Vol 268 (2) ◽

pp. C527-C534 ◽

Cited By ~ 13

Author(s):

G. A. Unguez ◽

R. R. Roy ◽

D. J. Pierotti ◽

S. Bodine-Fowler ◽

V. R. Edgerton

Keyword(s):

Motor Unit ◽

Tibialis Anterior ◽

Neural Control ◽

Periodic Acid ◽

Muscle Fibers ◽

Motor Units ◽

Fiber Types ◽

Muscle Properties ◽

Periodic Acid Schiff ◽

The Mean

To examine the influence of a motoneuron in maintaining the phenotype of the muscle fibers it innervates, myosin heavy chain (MHC) expression, succinate dehydrogenase (SDH) activity, and cross-sectional area (CSA) of a sample of fibers belonging to a motor unit were studied in the cat tibialis anterior 6 mo after the nerve branches innervating the anterior compartment were cut and sutured near the point of entry into the muscle. The mean, range, and coefficient of variation for the SDH activity and the CSA for both motor unit and non-motor unit fibers for each MHC profile and from each control and each self-reinnervated muscle studied was obtained. Eight motor units were isolated from self-reinnervated muscles using standard ventral root filament testing techniques, tested physiologically, and compared with four motor units from control muscles. Motor units from self-reinnervated muscles could be classified into the same physiological types as those found in control tibialis anterior muscles. The muscle fibers belonging to a unit were depleted of glycogen via repetitive stimulation and identified in periodic acid-Schiff-stained frozen sections. Whereas muscle fibers in control units expressed similar MHCs, each motor unit from self-reinnervated muscles contained a mixture of fiber types. In each motor unit, however, there was a predominance of fibers with the same MHC profile. The relative differences in the mean SDH activities found among fibers of different MHC profiles within a unit after self-reinnervation and those found among fibers in control muscles were similar, i.e., fast-2 < fast-1 < or = slow MHC fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

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Inter-rater reliability of motor unit number estimates and quantitative motor unit analysis in the tibialis anterior muscle

Clinical Neurophysiology ◽

10.1016/j.clinph.2009.02.168 ◽

2009 ◽

Vol 120 (5) ◽

pp. 947-952 ◽

Cited By ~ 13

Author(s):

S.G. Boe ◽

B.H. Dalton ◽

B. Harwood ◽

T.J. Doherty ◽

C.L. Rice

Keyword(s):

Motor Unit ◽

Tibialis Anterior ◽

Tibialis Anterior Muscle ◽

Rater Reliability ◽

Unit Analysis

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Synaptic organization of defined motor-unit types in cat tibialis anterior

Journal of Neurophysiology ◽

10.1152/jn.1980.43.6.1631 ◽

1980 ◽

Vol 43 (6) ◽

pp. 1631-1644 ◽

Cited By ~ 29

Author(s):

R. P. Dum ◽

T. T. Kennedy

Keyword(s):

Motor Unit ◽

Tibialis Anterior ◽

Muscle Afferents ◽

Medial Gastrocnemius ◽

Synaptic Organization ◽

Ia Afferents ◽

Group I ◽

Ia Epsp ◽

Group I Afferents ◽

Stimulation Of

1. Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF), and three segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR, and S (6, 16). 2. The sum of the monosynaptic EPSP amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. Type-FF, -FR, and -S motoneurons showed significant differences in the mean amplitude of their summed group Ia EPSPs. 3. The amplitudes of disynaptic IPSPs resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The observed relationships between motoneuron size and the monosynaptic group Ia EPSP amplitude or the disynaptic group I IPSP amplitude are compatible with the “size principle” of motor-unit recruitment (26). 4. The amplitudes of the monosynaptic EPSPs evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. A slight tendency of larger monosynaptic EPSPs to occur in motoneurons with larger tetanic tensions was observed. 5. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. Clearly, the polysynaptic cutaneous and muscle inputs and the monosynaptic MLF input onto TA motoneurons show a different pattern of synaptic organization than the group I inputs. 6. In general, the synaptic organization of the TA motor nucleus is similar to that of its extensor antagonist, medial gastrocnemius (MG) (2--5, 7, 8), when analogous neural circuits are compared. This parallel organization suggests a commonality of motor-control systems for both flexor and extensor muscles.

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