Task-dependent inhomogeneous muscle activities within the bi-articular human rectus femoris muscle

The motor nerve of the bi-articular rectus femoris muscle is generally split from the femoral nerve trunk into two sub-branches just before it reaches the distal and proximal regions of the muscle. In this study, we examined whether the regional difference in muscle activities exists within the human rectus femoris muscle during maximal voluntary isometric contractions of knee extension and hip flexion. Surface electromyographic signals were recorded from the distal, middle, and proximal regions. In addition, twitch responses were evoked by stimulating the femoral nerve with supramaximal intensity. The root mean square value of electromyographic amplitude during each voluntary task was normalized to the maximal compound muscle action potential amplitude (M-wave) for each region. The electromyographic amplitudes were significantly smaller during hip flexion than during knee extension task for all regions. There was no significant difference in the normalized electromyographic amplitude during knee extension among regions within the rectus femoris muscle, whereas those were significantly smaller in the distal than in the middle and proximal regions during hip flexion task. These results indicate that the bi-articular rectus femoris muscle is differentially controlled along the longitudinal direction and that in particular the distal region of the muscle cannot be fully activated during hip flexion.     

Ultrastructure of muscle fibers of an extraocular muscle of the pigeon

An electron microscopic analysis of the internal rectus muscle of the eye of the pigeon permitted identification of three types of muscle fibers: the first type shows the features previously described in vertebrate twitch fibers. The second type has very scarce sarcoplasmic reticulum at the A-band, their myofibrils fuse together at this level; the Z-line is large and the M-line is not present; the thick filaments are more abundant per unit area than in the first type of fibers, their hexagonal array is slightly disrupted and the fibers appear more opaque than the other two fiber types. The third type of fibers has bundles of myofibrils incompletely surrounded by sarcoplasmic reticulum at the A-band; the Z-line is large; the M-line is present and the hexagonal array of the thick filaments is maintained.     

Insect visceral muscle. Fine structure of the proctodeal muscle fibres

The fine structure of the longitudinal muscle fibres of the cockroach proctodeum was investigated by electron microscopy. The fibre is separated incompletely into fibrils, the resting sarcomere length is variable: about 5·8 to 7·3 μm, and the A- and I-bandings are not always clear in longitudinal sections. The ratio of thin and thick filaments at the overlapped region is about 4·1:1 when relaxed, and about 9·8:1 when fully contracted. The myofilament array is not well organized.The previously observed prolonged time course of muscle contraction seems to correlate with the present observations on the poorly developed sarcoplasmic reticulum and irregular distribution of transverse tubules. The Z-bands are irregularly aligned and discontinuous in longitudinal sections. The Z-band structure was studied in relation to the supercontractility. It was found that at maximal isotonic contraction (about 25 per cent rest length) the myofilaments pass through the expanded Z-regions.     

Chordate muscle actins differ distinctly from invertebrate muscle actins: The evolution of the different vertebrate muscle actins

A total of 30 actins from various chordate and invertebrate muscle sources were either characterized by full amino acid sequence data or typed by those partial sequences in the NH₂-terminal tryptic peptide which are known to be specific markers for different actin isoforms. The results show that most, if not all, invertebrate muscle actins are homologous to each other and to the isoforms recognized as vertebrate cytoplasmic actins. In contrast the actin forms typically found in muscle cells of warm-blooded vertebrates are noticeably different from invertebrate muscle actins and seem to have appeared in evolution already with the origin of chordates. During subsequent vertebrate evolution there has been a high degree of sequence conservation similar or stronger than that seen in histone H4. Urochordates, Cephalochordates and probably also Agnathes express only one type of muscle actin. Two types, a striated muscle-specific form and a smooth muscle form, are already observed in Chondrichthyes and Osteichthyes. Later in evolution, with the origin of reptiles, both muscle actins seem to have duplicated again; the striated muscle type branched into a skeletal- and cardiac-specific form, while the smooth muscle form duplicated into a vascular- and stomach-specific type. These findings support the hypothesis that each of the four muscle actins of warm-blooded vertebrates are coded for by a small number and possibly only one functional gene.     

Influence of muscle shortening on the geometry of gastrocnemius medialis muscle of the rat

For static and dynamic conditions muscle geometry of the musculus gastrocnemius medialis of the rat was compared at different muscle lengths. The dynamic conditions differed with respect to isokinetic shortening velocity (25, 50 and 75 mm/s) of the muscle-tendon complex and in constancy of force (isotonic) and velocity (isokinetic) during shortening. Muscle geometry was characterized by fibre length and angle as well as aponeurosis length and angle. At high isokinetic shortening velocities (50 and 75 mm/s) small differences in geometry were found with respect to isometric conditions: aponeurosis lengths differed maximally by -2%, fibre length only showed a significant increase (+3.2%) at the highest shortening velocity. The isotonic condition only yielded significant differences of fibre angle (-4.5%) in comparison with isometric conditions. No significant differences of muscle geometry were found when comparing isotonic with isokinetic conditions of similar shortening velocity. The small differences of geometry between isometric and dynamic conditions are presumably due to the lower muscle force in the dynamic condition and the elastic behaviour of the aponeurosis. It is concluded that, unless very high velocities of shortening are used, the relationship between muscle geometry and muscle length in the isometric condition may be used to describe muscle geometry in the dynamic condition.     

A not so plain muscle: the maturing of smooth muscle research

Today, smooth muscle research is a flourishing sub-discipline. This essay charts the trials and tribulations of early workers as they overcame technical limitations to establish this field by the end of the 1960s.