Development of Postural Muscles and Their Innervation

Control of posture is a prerequisite for efficient motor performance. Posture depends on muscles capable of enduring contractions, whereas movements often require quick, forceful muscle actions. To serve these different goals, muscles contain fibers that meet these different tasks. Muscles with strong postural functions mainly consist of slow muscle fibers with a great resistance against fatigue. Flexor muscles in the leg and arm muscles are mainly composed of fast muscle fibers producing relatively large forces that are rapidly fatigable. Development of the neuromuscular system continues after birth. We discuss in the human baby and in animal experiments changes in muscle fiber properties, regression from polyneural into mononeural innervation, and developmental changes in the motoneurons of postural muscles during that period. The regression of poly-neural innervation in postural muscles and the development of dendrite bundles of their motoneurons seem to be linked to the transition from the immature into the adult-like patterns of moving and postural control.

Vestibular Deprivation and the Development of Dendrite Bundles in the Rat

Motoneuronal pools of muscles that subserve postural tasks contain dendrite bundles. We investigated in the rat the development of these bundles in the pools of the long back muscles and related this to postural development. Motoneurons and their dendrites were retrogradely labeled by injecting unconjugated cholera toxin subunit B (CTB) into the muscles of 54 normal rats from birth until adulthood and into 18 rats that were vestibularly deprived from the 5th postnatal day (P5). Dendrite bundles coursing in a transverse direction already occurred at P1. From P4, the first longitudinal bundles could be observed, but the major spurt in development occurred between P6 and P9, when conspicuous bundles developed coursing in rostro-caudal and tranverse directions. This is the age when rats become able to stand freely and walk a few steps. Around P20, the dendrite bundles attained their adult characteristics. Vestibular deprivation by plugging both semicircular horizontal canals did not lead to a retarded development of dendrite bundles nor to a changed morphology. This finding is remarkable, as behavioral analysis showed a delay in postural development by about 3 days. We hypothesize that dendrite bundles in the pools of the long back muscles function to synchronize the motoneurons in different spinal cord segments.

Ultrastructure of the larval antenna of Tenebrio molitor L. (Coleoptera: Tenebrionidae): structure of the trichoid and uniporous peg sensilla

The antenna of the final instar larva of Tenebrio molitor has three segments. The reduced third (terminal) segment bears a large trichoid sensillum, four uniporous peg sensilla, one blunt tipped peg sensillum, and one papillate sensillum. The second segment bears a very large multiporous placoid sensillum, three uniporous peg sensilla, one blunt-tipped peg sensillum, and one papillate sensillum. The numbers and arrangement of these sensilla are usually stereotyped, but variations occur.The trichoid sensillum is a long, thin, unsocketed, aporous hair. It is innervated by two (sometimes one) bipolar neurones and has five sheath cells. The three sheath cells which distally delimit the large sensillar sinus have extremely elaborate microvillate inner borders. The uniporous peg sensillum is a short, stout, socketed peg with a single terminal pore. It is innervated by two to six (usually six) bipolar neurones. The dendrite from one of these always ends as a tubular body in the base, while the dendrites from the others extend to the tip of the peg. This sensillum has a small sensillar sinus and only four sheath cells. The inner sheath cell of both types of sensilla forms a cylindrical, nonlapped sleeve around the dendrite bundles.