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

Corticotropin-releasing factor receptor types 1 and 2 are differentially expressed in pre- and post-synaptic elements in the post-natal developing rat cerebellum

Corticotropin-releasing factor (CRF)-like proteins act via two G-protein-coupled receptors (CRF-R1 and CRF-R2) playing important neuromodulatory roles in stress responses and synaptic plasticity. The cerebellar expression of corticotropin-releasing factor-like ligands has been well documented, but their receptor localization has not. This is the first combination of a light microscopic and ultrastructural study to localize corticotropin-releasing factor receptors immunohistologically in the developing rat cerebellum. Both CRF-R1 and CRF-R2 were expressed in climbing fibres from early stages (post-natal day 3) to the adult, but CRF-R2 immmunoreactivity was only prominent throughout the molecular layer in the posterior cerebellar lobules. CRF-R1 immunoreactivity was concentrated in apical regions of Purkinje cell somata and later in primary dendrites exhibiting a diffuse cytoplasmic appearance. In Purkinje cells, CRF-R1 immunoreactivity was never membrane bound post-synaptically in dendritic spines while CRF-R2 immunoreactivity was found on plasmic membranes of Purkinje cells from post-natal day 15 onwards. We conclude that the localization of these receptors in cerebellar afferents implies their pre-synaptic control of the release of corticotropin-releasing factor-like ligands, impacting on the sensory information being transmitted from afferents. Furthermore, the fact that CRF-R2 is membrane bound at synapses, while CRF-R1 is not, suggests that ligands couple to CRF-R2 via synaptic transmission and to CRF-R1 via volume transmission. Finally, the distinct expression profiles of receptors along structural domains of Purkinje cells suggest that the role for these receptors is to modulate afferent inputs

Polyneural innervation in the psoas muscle of the developing rat

Polyneural innervation was studied in the psoas muscle in developing rats from P4 till P25 and at adult age, with the combined silver-acetylcholinesterase technique. Nerve endings were counted, and endplates were measured. These data were compared with such data in the human. The end of polyneural innervation in the rat (around P20) and in the human (around 12 weeks postterm age) in both cases coincides with a transformation in motor behavior and postural control. The rat's psoas muscle at early stages is less heavily innervated than this muscle in the human. Up to three axons per motor end-plate were counted at P4, but in the human up to five axons at 25 weeks of post menstrual age. This difference might be related to the lower percentage of type I muscle fibers in the rat. (C) 1998 John Wiley & Sons, Inc

Increased cell number in remaining cerebellar nuclei after cerebellar hemispherectomy in neonatal rats

After cerebellar hemispherectomy before the 10th day of life aberrant cerebello-rubral projections develop from the nuclei in the remaining hemisphere, which terminate on the ipsilateral side. These aberrant fibres stem from separate parent cells in the lateral and interposed nuclei. The problem addressed by the present study was whether the number of neurones in these nuclei is increased after early cerebellar hemispherectomy. Results demonstrate that the number of neurones was increased by 50-60%. The volumes of the nuclei were also increased (14-31%) but much less so, which indicates a relative decrease in neuropil per neuron

Polyneural innervation in the psoas muscle of the developing rat

Polyneural innervation was studied in the psoas muscle in developing rats from P4 till P25 and at adult age, with the combined silver-acetylcholinesterase technique. Nerve endings were counted, and endplates were measured. These data were compared with such data in the human. The end of polyneural innervation in the rat (around P20) and in the human (around 12 weeks postterm age) in both cases coincides with a transformation in motor behavior and postural control. The rat's psoas muscle at early stages is less heavily innervated than this muscle in the human. Up to three axons per motor end-plate were counted at P4, but in the human up to five axons at 25 weeks of post menstrual age. This difference might be related to the lower percentage of type I muscle fibers in the rat. (C) 1998 John Wiley & Sons, Inc

Reinnervation of muscles after transection of the sciatic nerve in adult rats

Functional recovery after transection of the sciatic nerve in adult rats is poor, probably because of abnormalities in reinnervation. Denervation and reinnervation patterns were studied morphologically in the lateral gastrocnemius (LGC), tibialis anterior (TA), and soleus (SOL) muscles for 21 weeks after nerve transection (motor endplates by acetylcholinesterase staining; nerves by silver impregnation). Motor endplates in the TA showed improving morphology with age, and, at 21 weeks, three-quarters of these were normal. Poorest recovery was observed in the SOL, as, at 21 weeks, only one-third of the motor endplates had a normal morphology. Polyneuronal innervation initially was more pronounced in the SOL, but, at 21 weeks, 10% of the motor endplates in all three muscles were still polyneuronally innervated. Our results indicate important differences in the reinnervation of these three hindleg muscles, and, even at 5 months, abnormalities were still present. These factors may in part explain the abnormal locomotion in rats as well as the limited recovery of function observed clinically in humans after nerve transection. (C) 2002 Wiley Periodicals, Inc