The number and size of motoneurons in the soleus motor nucleus of the normal and dystrophic (C57BL/6J dy2j/dy2j) mouse

Musculotopic organization of the motor neurons supplying forelimb and shoulder girdle muscles in the mouse

We identified the motor neurons (MNs) supplying the shoulder girdle and forelimb muscles in the C57BL/6J mouse spinal cord using Fluoro-Gold retrograde tracer injections. In spinal cord transverse sections from C2 to T2, we observed two MN columns (medial and lateral) both with ventral and dorsal subdivisions. The dorsolateral column consisted of the biceps brachii, forearm extensors, forearm flexors, and hand MNs, and the ventrolateral column consisted of the latissimus dorsi, trapezius, teres major, deltoid, and triceps MNs. The supraspinatus muscle MNs were located in the dorsomedial column, and pectoralis major and serratus anterior MNs were located in the ventromedial columns. MNs of the dorsolateral column innervated the biceps brachii in mid-C4 to mid-C7, forearm extensors in caudal C4 to mid-T1, forearm flexors in rostral C5 to mid-T1, and hand muscles in mid-C8 to mid-T2 segments. The MNs innervating the trapezius were located in mid-C2 to mid-C4, triceps brachii in mid-C6 to rostral T1, deltoid in rostral C4 to mid-C6, teres major in rostral C5 to mid-C8, and latissimus dorsi in mid-C5 to caudal C8. In addition, MNs innervating the supraspinatus were located from rostral C4 to caudal C8, pectoralis major in mid-C6 to mid-T2, and serratus anterior in rostral C5 to caudal C7/rostral C8 segments. While the musculotopic pattern of MN groups was very similar to that documented for other species, we found differences in the position and cranio-caudal extent of some MN pools compared with previous reports. The identification of mouse forelimb MNs can serve as an anatomical reference for studying degenerative MN diseases, spinal cord injury, and developmental gene expression.

Developmental changes in spinal motoneuron dendrites in neonatal mice

We examined the age-dependent morphological changes of lumbar spinal motoneurons (MNs) in neonatal Swiss-Webster mice during the first 2 weeks of postnatal life. Neurons labeled by intracellular injection of biocytin in hemisected lumbosacral spinal cords in vitro were reconstructed from serial sections. Digitized data were compared for young (P3; postnatal days 2-4; n = 9) and older animals (P11; postnatal days 10-13; n = 8). As expected, measures of dendritic size (e.g., stem branch diameter, total surface area, maximum distance to tips, and lateral tree spread) were all significantly greater for P11 than for P3 mice. In contrast, the number of dendrites per MN and parameters related to tree topology (e.g., terminations per tree and maximum branch order), although slightly greater for P11 animals, were not significantly different between the two ages. Dendrite growth appeared to be proportional throughout the tree because the ratios between average terminal and internal branch lengths were similar for the two groups. Furthermore, this elongation was proportional to enlargement of overall spinal cord dimensions. A variety of other morphometric measures showed no significant difference between age groups. The relative constancy of MN dendritic topology up to P13 was surprising, given the striking maturation in motor function during this time period.

Dependence of developing group Ia afferents on neurotrophin-3

At birth, group Ia proprioceptive afferents and muscle spindles, whose formation is Ia afferent-dependent, are absent in mice carrying a deletion in the gene for neurotrophin-3 (NT-3-/-). Whether Ia afferents contact myotubes, resulting in the formation of spindles which subsequently degenerate, or whether Ia afferents and spindles never form was examined in NT-3-/- mice at embryonic days (E) 10.5-18.5 by light and electron microscopy. Three sets of data indicate that Ia neurons do not develop and spindles do not form in NT-3-deficient mice. First, peripheral projections of Ia afferents did not innervate hindlimbs of NT-3-/- mice, as reflected by a deficiency of nerve fibers in limb peripheral nerves and an absence of afferent nerve-muscle contacts and spindles in the soleus muscle at E13.5-E18.5. Second, central projections of Ia afferents did not innervate the spinal cord in the absence of NT-3, as shown by an atrophy of the dorsal spinal roots and absence of afferent projections from limb musculature to spinal motor neurons at E13.5 or E15.5. Lastly, the lumbar dorsal root ganglia (DRGs) at E10.5-E14.5, the stages of development that precede or coincide with the innervation of the spinal cord and hindlimbs by Ia afferents, were 20-64% smaller in mutant than in wild-type mice, presumably because the cell bodies of Ia neurons were absent in embryos lacking NT-3. The failure of Ia neurons to differentiate and/or survive and Ia afferent projections to form in early fetal mice lacking NT-3 suggests that NT-3 may regulate neuronal numbers by mechanisms operating prior to neurite outgrowth to target innervation fields. Thus, developing Ia neurons may be dependent on NT-3 intrinsic to the DRGs before they reach a stage of potential dependence on NT-3 retrogradely derived from skeletal muscles or spinal motor neurons.

Relative efficacy of slow and fast alpha-motoneurons to reinnervate mouse soleus muscle

Contractile and histochemical properties of reinnervated motor units in soleus muscles of C57BL/6J mice were examined 1 mo after sectioning the soleus nerve. Fifty-one motor units were isolated by the technique of ventral root splitting. Their sizes ranged from 0.4 to 13.6% of whole muscle tetanic tension (Po) with a mean size of 5.3% Po corresponding to 19 motor units. In control unoperated mice, the range was 2.2-8.6% Po, with a mean size of 4.8% Po corresponding to 22 motor units. Although no clear relationship between unit time to peak tension and size was seen in control units, it appeared that in the reinnervated muscle the large units were also slow contracting, whereas the smaller units were predominantly fast contracting. Adenosinetriphosphatase (ATPase) staining revealed an increase in the proportion of muscle area occupied by type I fibers in reinnervated soleus compared with control soleus. Immunohistochemical staining of reinnervated soleus with monoclonal antibodies against type I and IIa myosin showed the presence of hybrid fibers containing both myosins. It is concluded that during reinnervation most motoneurons reinnervate the soleus muscle of the mouse. The hypothesis that slow motoneurons are more adept at expanding their innervating field than fast motoneurons is also supported by the data.

Morphometric analysis of neuronal soma size within the motor nucleus of a transplanted murine skeletal muscle

Since the number of motor neurons supplying a muscle graft is reduced, the peripheral field for the surviving motor neurons would be enlarged. This possible change in motor unit size may result in morphologic changes in the size of motor neurons within the motor neuron pool of the graft. The extensor digitorum longus (EDL) muscle from 19 transplanted and 17 age-matched normal 129 ReJ female mice were injected with horseradish peroxidase in order to examine the cell size distributions of the motor neuron pools supplying these muscles. Computer-assisted morphometric analysis of cell sizes within the motor neuron pool to the transplants indicated a significant shift in cell size, with the largest areas ranging between 630 and 1250 micron2. A number of the alpha neurons supplying the grafts were twice the average cell size for the control population (means = 592 micron2). The increase in the number of large motor neurons indicates an hypertrophy of neurons reinnervating the grafts. The long-term graft is reinnervated by a decreased population of motor neurons (means = 7), which vary in size from small to very large, reflecting both changes in the possible source of the nerve reinnervating the graft as well as alteration in the size of the motor unit, respectively.

Number and size of motoneurons in a forelimb motor nucleus of normal and dystrophic (C57BL/6J dy2j/dy2j) mice

The method of retrograde axonal transport of horseradish peroxidase (HRP) was used to identify the motoneurons that innervate the distal forelimb muscles via the ulnar nerve in normal and dystrophic (C56BL/6J dy2j/dy2j) mice. In both normal and dystrophic mice this motor nucleus was located in spinal segments C6 through T1. No clear division, on the basis of size, into alpha and gamma motoneuron populations was apparent. The motoneurons of dystrophic mice were fewer in number (26.5 vs. 35) but larger in cross-sectional area (780 vs. 674 microns2) than those of age-matched control mice. These results are quantitatively similar to those reported for the hind limb soleus muscle of dystrophic mice and suggest that the motoneuronal changes are a reflection of the dystrophic process rather than the associated spontaneous action potential generation seen in the dystrophic hind limb muscles.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Motor units in a fast-twitch muscle of normal and dystrophic mice

Isometric contractions of motor units, isolated functionally by ventral root splitting were recorded from extensor digitorum longus muscles of normal and dystrophic mice of the strain C57BL/6J dy2J/dy2J. Motor unit tetanic tension was significantly lower and both time to peak tension and to half-relaxation of the twitch were significantly prolonged in dystrophic mice relative to age-matched controls. In control mice, motor unit tetanic tension averaged 4.98% of whole muscle tension, corresponding to twenty motor units. Two out of fifteen dystrophic mice exhibited an apparent decrease in the number of motor units, but the data from the remaining thirteen mice indicated no change in relative motor unit size, and hence in the number of motor units. The two mice in which changes were seen were the most severely affected and it is suggested that the apparent reduction in the number of units might be due to some units becoming so small as to be unmeasurable. No evidence was obtained for a population of units with normal characteristics within the dystrophic muscles. There was no clear relationship between tetanic tension and the time to peak tension or to half-relaxation in units from control mice. In dystrophic mice, however, a significant correlation was seen. This possibly reflects two simultaneous effects of the dystrophic process, a loss of tension accompanied by slowing of the twitch.

Axonal transport of nerve growth factor and serum albumin in the dystrophic mouse

Specific (nerve growth factor, NGF) and non-specific (serum albumin, BSA) uptake followed by retrograde axonal transport was studied in the sciatic nerves of normal (C57BL/6J) and dystrophic (dy2J/dy2J) mice. In the dystrophic mouse there was a significant decrease in the amount of NGF transported to dorsal root ganglia or BSA accumulating at a ligature placed on the sciatic nerve.