An ultrastructural study of cat lumbosacral gamma-motoneurons after retrograde labelling with horseradish peroxidase

Fast Blue and Cholera Toxin-B Survival Guide for Alpha-Motoneurons Labeling: Less Is Better in Young B6SJL Mice, but More Is Better in Aged C57Bl/J Mice

Fast Blue (FB) and Cholera Toxin-B (CTB) are two retrograde tracers extensively used to label alpha-motoneurons (α-MNs). The overall goals of the present study were to (1) assess the effectiveness of different FB and CTB protocols in labeling α-MNs, (2) compare the labeling quality of these tracers at standard concentrations reported in the literature (FB 2% and CTB 0.1%) versus lower concentrations to overcome tracer leakage, and (3) determine an optimal protocol for labeling α-MNs in young B6SJL and aged C57Bl/J mice (when axonal transport is disrupted by aging). Hindlimb muscles of young B6SJL and aged C57Bl/J mice were intramuscularly injected with different FB or CTB concentrations and then euthanized at either 3 or 5 days after injection. Measurements were performed to assess labeling quality via seven different parameters. Our results show that tracer protocols of lower concentration and shorter labeling durations were generally better in labeling young α-MNs, whereas tracer protocols of higher tracer concentration and longer labeling durations were generally better in labeling aged α-MNs. A 0.2%, 3-day FB protocol provided optimal labeling of young α-MNs without tracer leakage, whereas a 2%, 5-day FB protocol or 0.1% CTB protocol provided optimal labeling of aged α-MNs. These results inform future studies on the selection of optimal FB and CTB protocols for α-MNs labeling in normal, aging, and neurodegenerative disease conditions.

Intramuscular Botulinum toxin A injections induce central changes to axon initial segments and cholinergic boutons on spinal motoneurones in rats

Intramuscular injections of botulinum toxin block pre-synaptic cholinergic release at neuromuscular junctions producing a temporary paralysis of affected motor units. There is increasing evidence, however, that the effects are not restricted to the periphery and can alter the central excitability of the motoneurones at the spinal level. This includes increases in input resistance, decreases in rheobase currents for action potentials and prolongations of the post-spike after-hyperpolarization. The aim of our experiments was to investigate possible anatomical explanations for these changes. Unilateral injections of Botulinum toxin A mixed with a tracer were made into the gastrocnemius muscle of adult rats and contralateral tracer only injections provided controls. Immunohistochemistry for Ankyrin G and the vesicular acetylcholine transporter labelled axon initial segments and cholinergic C-boutons on traced motoneurones at 2 weeks post-injection. Soma size was not affected by the toxin; however, axon initial segments were 5.1% longer and 13.6% further from the soma which could explain reductions in rheobase. Finally, there was a reduction in surface area (18.6%) and volume (12.8%) but not frequency of C-boutons on treated motoneurones potentially explaining prolongations of the after-hyperpolarization. Botulinum Toxin A therefore affects central anatomical structures controlling or modulating motoneurone excitability explaining previously observed excitability changes.

Monkey extensor digitorum communis motoneuron pool: Proximal dendritic trees and small motoneurons

Transverse sections of the monkey cervical spinal cord from a previous study (Jenny and Inukai, 1983) were reanalyzed using Neurolucida to create a three-dimensional display of extensor digitorum communis (EDC) motoneurons and proximal dendrites that had been labeled with horse radish peroxidase (HRP). The EDC motoneuron pool was located primarily in the C8 and T1 segments of the spinal cord. Small motoneurons (cell body areas less than 500 μm² and presumed to be gamma motoneurons) comprised about ten percent of the motoneurons and were located throughout the length of the motoneuron pool. Most small motoneurons were oblong in shape and had one or two major dendrites originating from the cell body in the transverse plane of section. The majority of the HRP labeled dendritic trees were directed either superiorly, dorsal-medially to the mid zone area between the base of the dorsal horn and the upper portion of the ventral horn, or medially to the ventromedial gray matter. The longer HRP labeled dendrites usually continued in the same radial direction as when originating from the cell body. As such we considered the radial direction of the longer proximal HRP labeled dendrites to be a reasonable estimate of the radial direction of the more distal dendritic tree. Our data suggest that the motoneuron dendritic tree as seen in transverse section has direction-oriented dendrites that extend toward functional terminal regions.

Swimming against the tide: investigations of the C-bouton synapse

C-boutons are important cholinergic modulatory loci for state-dependent alterations in motoneuron firing rate. m2 receptors are concentrated postsynaptic to C-boutons, and m2 receptor activation increases motoneuron excitability by reducing the action potential afterhyperpolarization. Here, using an intensive review of the current literature as well as data from our laboratory, we illustrate that C-bouton postsynaptic sites comprise a unique structural/functional domain containing appropriate cellular machinery (a “signaling ensemble”) for cholinergic regulation of outward K⁺ currents. Moreover, synaptic reorganization at these critical sites has been observed in a variety of pathologic states. Yet despite recent advances, there are still great challenges for understanding the role of C-bouton regulation and dysregulation in human health and disease. The development of new therapeutic interventions for devastating neurological conditions will rely on a complete understanding of the molecular mechanisms that underlie these complex synapses. Therefore, to close this review, we propose a comprehensive hypothetical mechanism for the cholinergic modification of α-MN excitability at C-bouton synapses, based on findings in several well-characterized neuronal systems.

Expression of postsynaptic Ca2+-activated K+ (SK) channels at C-bouton synapses in mammalian lumbar -motoneurons

Small-conductance calcium-activated potassium (SK) channels mediate medium after-hyperpolarization (AHP) conductances in neurons throughout the central nervous system. However, the expression profile and subcellular localization of different SK channel isoforms in lumbar spinal α-motoneurons (α-MNs) is unknown. Using immunohistochemical labelling of rat, mouse and cat spinal cord, we reveal a differential and overlapping expression of SK2 and SK3 isoforms across specific types of α-MNs. In rodents, SK2 is expressed in all α-MNs, whereas SK3 is expressed preferentially in small-diameter α-MNs; in cats, SK3 is expressed in all α-MNs. Function-specific expression of SK3 was explored using post hoc immunostaining of electrophysiologically characterized rat α-MNs in vivo. These studies revealed strong relationships between SK3 expression and medium AHP properties. Motoneurons with SK3-immunoreactivity exhibit significantly longer AHP half-decay times (24.67 vs. 11.02 ms) and greater AHP amplitudes (3.27 vs. 1.56 mV) than MNs lacking SK3-immunoreactivity. We conclude that the differential expression of SK isoforms in rat and mouse spinal cord may contribute to the range of medium AHP durations across specific MN functional types and may be a molecular factor distinguishing between slow- and fast-type α-MNs in rodents. Furthermore, our results show that SK2- and SK3-immunoreactivity is enriched in distinct postsynaptic domains that contain Kv2.1 channel clusters associated with cholinergic C-boutons on the soma and proximal dendrites of α-MNs. We suggest that this remarkably specific subcellular membrane localization of SK channels is likely to represent the basis for a cholinergic mechanism for effective regulation of channel function and cell excitability.

Sensorimotor function is modulated by the serotonin receptor 1d, a novel marker for gamma motor neurons

Gamma motor neurons (MNs), the efferent component of the fusimotor system, regulate muscle spindle sensitivity. Muscle spindle sensory feedback is required for proprioception that includes sensing the relative position of neighboring body parts and appropriately adjust the employed strength in a movement. The lack of a single and specific genetic marker has long hampered functional and developmental studies of gamma MNs. Here we show that the serotonin receptor 1d (5-ht1d) is specifically expressed by gamma MNs and proprioceptive sensory neurons. Using mice expressing GFP driven by the 5-ht1d promotor, we performed whole-cell patch-clamp recordings of 5-ht1d::GFP⁺ and 5-ht1d::GFP⁻ motor neurons from young mice. Hierarchal clustering analysis revealed that gamma MNs have distinct electrophysiological properties intermediate to fast-like and slow-like alpha MNs. Moreover, mice lacking 5-ht1d displayed lower monosynaptic reflex amplitudes suggesting a reduced response to sensory stimulation in motor neurons. Interestingly, adult 5-ht1d knockout mice also displayed improved coordination skills on a beam-walking task, implying that reduced activation of MNs by Ia afferents during provoked movement tasks could reduce undesired exaggerated muscle output. In summary, we show that 5-ht1d is a novel marker for gamma MNs and that the 5-ht1d receptor is important for the ability of proprioceptive circuits to receive and relay accurate sensory information in developing and mature spinal cord motor circuits.

Gamma motor neurons express distinct genetic markers at birth and require muscle spindle-derived GDNF for postnatal survival

BACKGROUND

Gamma motor neurons (gamma-MNs) selectively innervate muscle spindle intrafusal fibers and regulate their sensitivity to stretch. They constitute a distinct subpopulation that differs in morphology, physiology and connectivity from alpha-MNs, which innervate extrafusal muscle fibers and exert force. The mechanisms that control the differentiation of functionally distinct fusimotor neurons are unknown. Progress on this question has been limited by the absence of molecular markers to specifically distinguish and manipulate gamma-MNs. Recently, it was reported that early embryonic gamma-MN precursors are dependent on GDNF. Using this knowledge we characterized genetic strategies to label developing gamma-MNs based on GDNF receptor expression, showed their strict dependence for survival on muscle spindle-derived GDNF and generated an animal model in which gamma-MNs are selectively lost.

RESULTS

In mice heterozygous for both the Hb9::GFP transgene and a tau-lacZ-labeled (TLZ) allele of the GDNF receptor Gfralpha1, we demonstrated that small motor neurons with high Gfralpha1-TLZ expression and lacking Hb9::GFP display structural and synaptic features of gamma-MNs and are selectively lost in mutants lacking target muscle spindles. Loss of muscle spindles also results in the downregulation of Gfralpha1 expression in some large diameter MNs, suggesting that spindle-derived factors may also influence populations of alpha-MNs with beta-skeletofusimotor collaterals. These molecular markers can be used to identify gamma-MNs from birth to the adult and to distinguish gamma- from beta-motor axons in the periphery. We also found that postnatal gamma-MNs are also distinguished by low expression of the neuronal nuclear protein (NeuN). With these markers of gamma-MN identity, we show after conditional elimination of GDNF from muscle spindles that the survival of gamma-MNs is selectively dependent on spindle-derived GDNF during the first 2 weeks of postnatal development.

CONCLUSION

Neonatal gamma-MNs display a unique molecular profile characterized by the differential expression of a series of markers - Gfralpha1, Hb9::GFP and NeuN - and the selective dependence on muscle spindle-derived GDNF. Deletion of GDNF expression from muscle spindles results in the selective elimination of gamma-MNs with preservation of the spindle and its sensory innervation. This provides a mouse model with which to explore the specific role of gamma-fusimotor activity in motor behaviors.

Ultrastructural synaptic features differ between alpha- and gamma-motoneurons innervating the tibialis anterior muscle in the rat

We investigated the synaptology of retrogradely labeled spinal motoneurons after injection of horseradish peroxidase into the tibialis anterior (TA) muscle of adult rat. In total, 32 TA motoneurons were investigated in the electron microscope and demonstrated a bimodal size distribution with cell diameter peaks at 40 microm and 20 microm, likely representing alpha- and gamma-motoneurons, respectively. Both alpha- and gamma-motoneurons were apposed by S- and F-type synaptic boutons, whereas only alpha-motoneurons demonstrated inputs by the large M- and C-type boutons. The proportion of cell body membrane covered by synaptic inputs was surprisingly indistinguishable between alpha-motoneurons (72.2%) and gamma-motoneurons (63.5%). The ratio between the number of F- and S-type boutons in apposition with the motoneuron cell body (F/S ratio) and the ratio between the soma membrane coverage provided by F- and S-type boutons were both significantly higher in alpha- than in gamma-motoneurons. When comparing our data with previous findings in other species, we conclude that rat TA alpha-motoneurons are similar to cat and primate alpha-motoneurons with regard to synaptic terminal morphology, frequency, and distribution. However, rat gamma-motoneurons show a markedly higher total synaptic coverage and frequency than cat gamma-motoneurons, although both species exhibit appositions made by the same synaptic types and similar ratios between inhibitory and excitatory inputs.

Quantitative ultrastructural analysis of glycine- and gamma-aminobutyric acid-immunoreactive terminals on trigeminal alpha- and gamma-motoneuron somata in the rat

Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.

Exogenous brain-derived neurotrophic factor regulates the synaptic composition of axonally lesioned and normal adult rat motoneurons

Brain-derived neurotrophic factor has previously been shown to promote survival and axonal regeneration in injured spinal motoneurons and, also, to modulate synaptic transmission and regulate the density of synaptic innervation in a variety of neurons. The present light and electron microscopic study demonstrates synaptotrophic effects of exogenously applied brain-derived neurotrophic factor on the synaptic composition of both normal and axonally lesioned adult rat spinal motoneurons. After L5-L6 ventral root avulsion, a massive loss of all types of boutons occurred on the somata of the lesioned motoneurons which persisted for at least 12 weeks postoperatively. We found that (i) intrathecal infusion of brain-derived neurotrophic factor during the first postoperative week did not prevent the synaptic detachment and activation of glial cells; (ii) prolonged treatment for four weeks restored synaptic covering and significantly reduced microglial reaction; (iii) the synaptotrophic effect remained significant for at least eight weeks after cessation of the treatment; (iv) brain-derived neurotrophic factor mainly supported F-type boutons with presumably inhibitory function, while it had little effect on S-type boutons associated with excitatory action; and (v) in normal unlesioned motoneurons, four weeks of treatment with brain-derived neurotrophic factor induced sprouting of F-type boutons, a loss of S-type boutons and motoneuron atrophy. The present data show that exogenous neurotrophins not only help to restore synaptic circuitry in axonally injured motoneurons, but also strongly influence the synaptic composition in normal motoneurons.

Differences in the synaptic complement of thoracic motoneurons of adult and ageing cats after permanent or reversible axotomy

We have compared the effects of intercostal nerve crush (reversible axotomy) or nerve transection with proximal ligation (permanent axotomy) on the somatic synaptic terminals of thoracic motoneurons of adult (1-2 years) and ageing (10-15 years) cats. Retrograde axonal transport of horseradish peroxidase (HRP) was used to identify axotomised motoneurons; control motoneurons were labelled by the intramuscular injection of HRP. Synaptic frequency and cover of control motoneurons in adult and ageing cats was similar. In adults, 8-16 days following both types of axotomy, synaptic cover was halved without any significant change in synaptic frequency. By 32-64 days following reversible axotomy, synaptic frequency and cover were not significantly different from controls. By contrast, 32-64 days following permanent axotomy synaptic frequency and cover were reduced to 30-50% of adult control values. In ageing cats 2 months following reversible axotomy, synaptic frequency and cover were reduced to 40% and 33% of ageing control values, respectively, while no significant change from controls was found 2 months following permanent axotomy. The long-term synaptic response of axotomised motoneurons in ageing cats is therefore opposite to that of adults.

Morphological and morphometric characterisation of Onuf's nucleus in the spinal cord in man

In the absence of a systematic morphometric study of Onuf's nucleus in man, this investigation defines the limits of variation of segmental position and the range of length and volume of Onuf's nucleus in 6 normal humans displaying no neurological disease (2 males, 4 females). Serial section reconstruction methods in conjunction with the disector method provided information on the numbers, sizes and shapes of the constituent motor neurons of Onuf's nucleus. In contrast to previous descriptions, the cranial origin of Onuf's nucleus occurred in rostral S1 in 50% of subjects, and midcaudal S1 in the remaining subjects. Onuf's nucleus varied in length between 4 and 7 mm, and was 0.2-0.37 mm3 in volume. Differences in length or volume between males or females, or between the left and right side of the cord were not statistically significant. Neurons in Onuf's nucleus varied in diameter between 10 microns and 60 microns (mean 26 microns) and their mean number was 625 +/- 137. A higher density of neurons occurred at the cranial and caudal ends of the nucleus relative to the middle. While 37% of neurons were approximately spherical (shape index approximately 1), 44% were ellipsoid and 19% fusiform (shape indices varying between 0.26 and 0.8). These findings are compared with previous studies of Onuf's nucleus in man and animals. The results form a basis for further studies on Onuf's nucleus in normality and neurodegenerative diseases.

Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat

The aim of the present study was to provide a quantitative analysis of the synapses made onto the axon hillock and initial segment of rat abducens motoneurons retrogradely or intracellularly stained with HRP. GABA-immunoreactive terminals contacting these axons were visualized using a postembedding procedure. The presynaptic terminals contained either spherical or pleomorphic vesicles. gamma-Aminobutyric acid (GABA)-immunoreactive axon terminals, which belonged to this last category, were distributed both onto axon hillocks and the proximal part of initial segments. The percentage of axonal membrane covered by synapses ranged from 44.1 to 68.2%. A quantitative analysis performed on a series of ultrathin sectioned terminals contacting the axon of an intracellularly labeled motoneuron revealed a significant correlation between the length of membrane apposition of the terminals and their perimeter or surface area, and also between the area of membrane apposition and terminal volume. GABA-immunoreactive terminals had a mean perimeter and volume that were larger than those of unlabeled axon terminals. The number of active zones was correlated with the area of apposition. Some hypotheses concerning the functional role of the GABAergic innervation of this particular part of the neuron are discussed.

Ultrastructure and synaptology of the nucleus ambiguus in the rat: the semicompact and loose formations

The fine structure of the pharyngomotor semicompact and laryngomotor loose formations of the rat nucleus ambiguus was studied in single and serial sections by means of light and electron microscopy. Motoneurons and their dendrites were identified after retrograde labelling by injections of neuroanatomical tracers into pharyngeal and laryngeal muscles or nerves. Pharyngeal motoneurons measured 39 x 29 microns and had 2-25 axosomatic synapses per somatic profile, representing an estimated average of 182 synapses per soma. Laryngeal motoneurons measured 42 x 30 microns with 6-33 synapses per profile, or an average of 339 synapses per soma. In both subdivisions, axon terminals that contained round vesicles and formed symmetric junctions and terminals that contained pleomorphic vesicles and formed symmetric junctions were distributed in approximately equal proportions on somata and dendrites, forming over 90% of the synapse population. A small percentage (2-8%) of synapses had a subsurface cistern situated below the axon terminal (C type). Small, atypical motoneurons measuring 15 x 5 microns with an invaginated nucleus were also present in both subdivisions. The ultrastructure and synaptology of pharyngeal and laryngeal motoneurons are characterized by similarities to those of spinal motoneurons and by their relatively large numbers of axosomatic synapses in comparison to esophageal motoneurons of the compact formation of the nucleus ambiguus.

Postnatal development of alpha- and gamma-peroneal motoneurons in kittens: an ultrastructural study

Motoneurons innervating the peroneus brevis muscle of 1 week- and 3 week-old kittens were retrogradely labelled by HRP and examined by electron microscopy. At 1 week the distribution of mean cell body diameters was unimodal. Consequently alpha- and gamma-motoneurons could not be identified by their size. The aim of this study was to see whether the alpha- and gamma-motoneurons of kittens could be identified using the combination of ultrastructural criteria previously defined in the adult cat. Using these three criteria it was not possible to distinguish all the motoneurons as either alpha- or gamma in the kitten and a fourth criterion (frequency of F bouton profiles) was added to aid identification. However, with these four criteria, at 1 week six of 21 motoneurons and at 3 weeks two of 18 could still not be clearly identified as alpha or gamma (four were tentatively considered to be gamma, and four could not be identified). The maturation of alpha-motoneurons between 1 week and the adult was accompanied by an increase in somatic membrane area and a significant decrease in the somatic packing density of F boutons. On gamma-motoneurons there was a decrease in the somatic packing density of F boutons between 1 and 3 weeks. However, the numbers of F and S boutons remained stable for both motoneuron types. Age-related changes in apposition and active zone lengths of F and S boutons characterize the synaptic rearrangements which are occurring during the postnatal development of motoneurons.

Gaba-like immunoreactive terminals on lumbar motoneurons of the adult cat. A quantitative ultrastructural study

The aim of this ultrastructural study was to analyse quantitatively the distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in axon terminals apposed to somatic and proximal dendritic membranes of cat motoneurons in lumbar column 2. Preembedding immunocytochemistry was used to count the GABAergic terminals contacting profiles of eighteen alpha-and six gamma-motoneurons. Of the 1293 terminals counted on the somatic and proximal dendritic compartments of alpha-motoneurons, 197 were GABAergic. In contrast, a total number of only 62 terminals were counted on gamma-motoneurons, of which 8 were GABAergic. These populations of GABAergic terminals were less numerous than the population of glycinergic terminals observed in a previous study. The morphometric characteristics of GABAergic synapses were analyzed using postembedding immunocytochemistry. Most of the GABAergic terminals contained pleomorphic vesicles (F-type boutons, flattened or pleomorphic vesicles). All terminals presynaptic (P boutons) to large terminals containing sphericle vesicles (M-type boutons, characteristic of alpha-motoneurons), were GABA-immunopositive. These results suggest that there are different distributions of the GABAergic control of excitability on gamma- and alpha-motoneurons. GABA appears to be strongly involved in post-synaptic inhibition of alpha-motoneurons, whereas gamma-motoneurons receive very few GABAergic inhibitory inputs. Morphological correlates of GABAergic presynaptic inhibition were seen on alpha- but not on gamma-motoneurons.

Synaptic terminal coverage of primate triceps surae motoneurons

This study examined the synaptic terminal coverage of primate triceps surae (TS) motoneurons at the electron microscopic level. In three male pigtail macaques, motoneurons were labeled by retrograde transport of cholera toxin-horseradish peroxidase that was injected into TS muscles bilaterally and visualized with tetramethylbenzidine stabilized with diaminobenzidine. Somatic, proximal dendritic, and distal dendritic synaptic terminals were classified by standard criteria and measured. Overall and type-specific synaptic terminal coverages and frequencies were determined. Labeled cells were located in caudal L5 to rostral S1 ventral horn and ranged from 40 to 74 microns in diameter (average, 54 microns). The range and unimodal distribution of diameters, the label used, and the presence of C terminals on almost all cells indicated that the 15 cell bodies and associated proximal dendrites analyzed here probably belonged to alpha-motoneurons. Synaptic terminals covered 39% of the cell body membrane, 60% of the proximal dendritic membrane, and 40% of the distal dendritic membrane. At each of these three sites, F terminals (flattened or pleomorphic vesicles, usually symmetric active zones, average contact length 1.6 microns) were most common, averaging 52%, 56%, and 58% of total coverage and 56%, 57%, and 58% of total number of cell bodies, proximal dendrites, and distal dendrites respectively. S terminals (round vesicles, usually asymmetric active zones, average contact length 1.3 microns) averaged 24%, 29%, and 33% of coverage and 33%, 35%, and 36% of number at these three sites, respectively. Thus, S terminals were slightly more prominent relative to F terminals on distal dendrites than on cell bodies. C terminals (spherical vesicles, subsynaptic cisterns associated with rough endoplasmic reticulum, average contact length 3.5 microns) constituted 24% and 11% of total terminal coverage on cell bodies and proximal dendrites, respectively, and averaged 11% and 6% of terminal number at these two locations. M terminals (spherical vesicles, postsynaptic Taxi bodies, some with presynaptic terminals, average contact length 2.7 microns) were absent on cell bodies and averaged 3% and 7% of total coverage and 2% and 5% of terminals on proximal and distal dendrites, respectively. Except for M terminals, which tended to be smaller distally, terminal contact length was not correlated with location. Total and type-specific coverages and frequencies were not correlated with cell body diameter. Primate TS motoneurons are similar to cat TS motoneurons in synaptic terminal morphology, frequency, and distribution. However, primate terminals appear to be smaller, so that the fraction of membrane covered by them is lower.

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Quantitative synaptology of functionally different types of cat medial gastrocnemius alpha-motoneurons

The aim of this ultrastructural investigation was to study quantitatively the synaptology of the cell bodies and dendrites of cat medial gastrocnemius (MG) alpha-motoneurons of functionally different types. In electrophysiologically classified and intracellularly HRP-labelled MG alpha-motoneurons of the FF (fast twitch, fatigable), FR (fast twitch, fatigue resistant) and S (slow twitch, very fatigue resistant) types, the synaptic covering of the soma as well as that of dendritic segments located within 100 microns and at 300, 700, and 1,000 microns distance, respectively from the soma, was analyzed. The synaptic boutons were classified into the L-(apposition length > 4 microns) and S-types (< 4 microns) with spherical synaptic vesicles, and the F-type with flat or pleomorphic synaptic vesicles. The length of apposition towards the motoneuron membrane was measured for each bouton profile. Approximately 1,000 boutons contacted the soma and a similar number of boutons contacted the proximal dendrites within 50 microns from the soma. The number of dendritic boutons was larger at the 300 microns distance than at the 100 and 700 microns distances. The three types of motoneurons showed similar values for percentage synaptic covering and synaptic packing density in the proximal dendrites, while in the most distal dendritic regions the S motoneurons had more than 50% higher values for percentage covering, packing density and total number of boutons. The S motoneurons also exhibited a larger preponderance of F-type boutons on the soma. The ratio between the F- and S-types of boutons decreased somatofugally along the dendrites in the type FF and FR motoneurons, while in the S motoneurons it remained fairly constant.

Selective vulnerability of alpha motor neurons in ALS: relation to autoantibodies toward acetylcholinesterase (AChE) in ALS patients

The degenerative process in amyotrophic lateral sclerosis (ALS) concerns primarily alpha motor neurons in the spinal cord and brain stem, and neurons forming descending pathways to the cord, especially in the pyramidal tract. Some degeneration of large peripheral sensory nerve fibers often occurs too, but preganglionic autonomic neurons and gamma motor neurons are most often spared in the disease. The vulnerability of alpha motor neurons compared to other types of neurons in ALS is discussed in relation to retrograde axoplasmic transport from peripheral blood of foreign noxious macromolecules, interneuronal transport of such molecules, and neuronal surface structure properties relevant to uptake for retrograde axoplasmic transport. Certain differences in these aspects between alpha motor neurons and other neuronal types exist. Some differences concern the neuronal turnover of acetylcholinesterase (AChE), which could be of special interest in view of the recent demonstration of regular occurrence of autoantibodies towards this enzyme in ALS patients.

Alpha and gamma motoneurons in the peroneal nuclei of the cat spinal cord: an ultrastructural study

The aim of the present study was to investigate whether ultrastructural features can be used as a guide to identify alpha- and gamma-motoneurons among the intermediate-size neurons of the peroneal motor nuclei. The peroneus brevis and peroneus tertius muscles of adult cats were injected with horseradish peroxidase, and motoneurons labeled by retrograde axonal transport were examined by electron microscopy. In both nuclei, the distributions of cell-body diameters, measured in the light microscope, were bimodal covering the range of 28-84 microns, with a trough around 50 microns. The sample of 25 motoneurons selected for the ultrastructural study included not only large (presumed alpha) and small (presumed gamma) neurons but also intermediate-size cell bodies with diameters in the 40-60 microns range. For each motoneuron, 2-5 profiles were reconstructed from ultrathin sections taken at 6-8 microns intervals. Synaptic boutons were counted and their lengths of apposition were measured. On the basis of three criteria, namely: (1) bouton types present on the membrane, (2) percentage of membrane length covered by synapses, and (3) the aspect of the nucleolus, all the examined motoneurons, including those with intermediate sizes, fell into one of two categories. Fourteen motoneurons, with cell-body diameters in a range of 55-84 microns, were contacted by all types of boutons (mainly S-type with spherical vesicles, F-type with flattened vesicles, and C-type with subsynaptic cistern); the synaptic covering of the somatic membrane was over 40% and the nucleus contained a vacuolated nucleolus. These were considered alpha-motoneurons. Eleven motoneurons, with only S and F boutons, a synaptic covering under 30%, a compact nucleolus and a cell-body diameter ranging between 28 and 50 microns, were considered gamma-motoneurons. No other combination of the three criteria was observed. These results show that unequivocal distinction of alpha- and gamma-motoneurons is possible in the peroneal nuclei, on the basis of morphological differences independent of cell-body size.

Synapses on motoneuron dendrites in the brachial section of the frog spinal cord: a computer-aided electron microscopic study of cobalt-filled cells

Cobalt-labelled motoneuron dendrites of the frog spinal cord at the level of the second spinal nerve were photographed in the electron microscope from long series of ultrathin sections. Three-dimensional computer reconstructions of 120 dendrite segments were analysed. The samples were taken from two locations: proximal to cell body and distal, as defined in a transverse plane of the spinal cord. The dendrites showed highly irregular outlines with many 1-2 microns-long 'thorns' (on average 8.5 thorns per 100 microns 2 of dendritic area). Taken together, the reconstructed dendrite segments from the proximal sites had a total length of about 250 microns; those from the distal locations, 180 microns. On all segments together there were 699 synapses. Nine percent of the synapses were on thorns, and many more close to their base on the dendritic shaft. The synapses were classified in four groups. One third of the synapses were asymmetric with spherical vesicles; one half were symmetric with spherical vesicles; and one tenth were symmetric with flattened vesicles. A fourth, small class of asymmetric synapses had dense-core vesicles. The area of the active zones was large for the asymmetric synapses (median value 0.20 microns 2), and small for the symmetric ones (median value 0.10 microns 2), and the difference was significant. On average, the areas of the active zones of the synapses on thin dendrites were larger than those of synapses on large calibre dendrites. About every 4 microns 2 of dendritic area received one contact. There was a significant difference between the areas of the active zones of the synapses at the two locations. Moreover, the number per unit dendritic length was correlated with dendrite calibre. On average, the active zones covered more than 4% of the dendritic area; this value for thin dendrites was about twice as large as that of large calibre dendrites. We suggest that the larger active zones and the larger synaptic coverage of the thin dendrites compensate for the longer electrotonic distance of these synapses from the soma.

Morphology and frequency of axon terminals on the somata, proximal dendrites, and distal dendrites of dorsal neck motoneurons in the cat

The purpose of the present study was to compare the frequency of different classes of axon terminals on selected regions of the somatodendritic surface of dorsal neck motoneurons. Single motoneurons supplying neck extensor muscles were antidromically identified and intracellularly stained with horseradish peroxidase. By using light microscopic reconstructions as a guide, axon terminals on the somata, proximal dendrites (within 250 microns of the soma), and distal dendrites (more than 540 microns from the soma) were examined at the electron microscopic level. Axon terminals were divided into several classes based on the shape, density, and distribution of their synaptic vesicles. The proportion of axon terminals belonging to each axon terminal class was similar on the somata and proximal dendrites. However, there were major shifts in the relative frequency of most classes of axon terminals on the distal dendrites. The most common classes of axon terminals on the somata and proximal dendrites contained clumps of either spherical or pleomorphic vesicles. These types of axon terminals accounted for more than 60% of the axon terminals on these regions. In contrast, only 11% of the axon terminals found on distal dendrites belonged to these types of axon terminals. The most commonly encountered axon terminal on distal dendrites contained a dense collection of uniformly distributed spherical vesicles. These types of axon terminals accounted for 40% of all terminals on the distal dendrites, but only 5-7% of the axon terminals on the somata and proximal dendrites. Total synaptic density on each of the three regions examined was similar. However, the percentage of membrane in contract with axon terminals was approximately four times smaller on distal dendrites than somata or proximal dendrites. Axon terminals (regardless of type) were usually larger on somata and proximal dendrites than distal dendrites. These results indicate that there are major differences in the types and arrangement of axon terminals on the proximal and distal regions of dorsal neck motoneurons and suggest that afferents from different sources may preferentially contact proximal or distal regions of the dendritic trees of these cells.

Sensorimotor connections in the lumbar spinal cord of the young rat: a morphological study

A morphological investigation of sensorimotor connections was performed on the isolated lumbar spinal cord of 8-15-day-old rats using horseradish peroxidase labelling techniques. Horseradish peroxidase was applied to the filaments of dorsal and ventral roots and injected intracellularly into motoneurons. The labelled afferent fibres and their contacts on motoneurons were examined under a light microscope. Numerous afferent collaterals entered the lateral motor nuclei. In the medial motor nuclei a few afferent collaterals were found. Some fibres were visible passing through the ventral commissure. The number of boutons per afferent collateral in the motor nuclei was 40-60. A single terminal branch contained one to five boutons (average 1.5). Predominating axodendritic and apparent axosomatic contacts were found between afferent fibres and motoneurons belonging to the lateral motor nuclei. The contacting boutons were both terminaux and en passant. As a rule, the sensorimotor connection involved dorsally and rostrocaudally directed dendrites of the first to sixth orders.

Postnatal development of peroneal motoneurons in the kitten

In 1- to 72-day-old kittens, motoneurons of the 3 peroneal muscle nuclei were labeled by retrograde axonal transport of horseradish peroxidase from individual muscles. At birth, the locations of peroneal nuclei were similar to those of the adult cat. Counts of motoneurons at different ages indicated that postnatal cell death does not occur in peroneal motor nuclei. Primary dendrites were as numerous in motoneurons of newborn kittens as in adult motoneurons but they were thinner, shorter and poorly ramified. The number of recurrent axon collaterals was higher in the first postnatal week than at later stages. The growth of motoneurons followed similar rates in the 3 peroneal nuclei. Distributions of cell body diameters and volumes were unimodal at birth and became bimodal between 15 and 20 days postnatal. The separation of peroneal motoneurons in two size subgroups, presumably corresponding to alpha and gamma populations, was followed by an increase in growth rate which became faster for alpha than for gamma motoneurons.

Morphology of developing motoneurons innervating the medial gastrocnemius of the cat

The morphology of medial gastrocnemius (MG) motoneurons labeled by retrograde transport of horseradish peroxidase was quantified in 5 postnatal ages (3 to 79-86 days) and in adults. A bimodal distribution of somal volumes was evident at birth which permitted separating the motoneurons into alpha and gamma subpopulations for analysis. There was a significant increase in the axial dimensions, surface area and volume calculated for both alpha and gamma cell bodies between each of the age-groups studied. A greater relative growth of the major over minor axis for the gammas produced a significant decrease in the form factor (i.e. greater eccentricity) between the youngest and oldest age-groups. The number of primary dendrites observed remained constant throughout postnatal development. The surface area of alpha somata more than tripled while that of the gammas doubled from 3 days to the adult. The mean somal volume of an alpha motoneuron at birth was only 17% of its adult value while the gamma cell bodies were 33% of their adult volume. A positive correlation was found for both alpha and gamma motoneurons when their somal surface area was plotted against postnatal age and weight. The rate of growth of the MG somal surface area is compared to the changes found in axonal conduction velocity and axonal diameter for MG in the literature.

Ultrastructure of axotomized alpha and gamma motoneurons in the cat thoracic spinal cord

Using horseradish peroxidase as a retrograde marker, the ultrastructural response of alpha and gamma motoneuronal cell bodies in the cat thoracic spinal cord has been compared 1-8 days following intercostal nerve transection and ligation. By light microscopy, reduction of Nissl body size, together with nuclear and nucleolar alterations were seen in alpha motoneurons 4-8 days following axotomy, but not at any stage in axotomized gamma motoneurons. In the electron microscope, disorganization of Nissl body ultrastructure was seen in both alpha and gamma motoneurons 2 days following axotomy. Only in alpha motoneurons, however, did these disorganized Nissl bodies subsequently fragment into smaller pieces. Both alpha and gamma motoneurons lost synapses following axotomy, but the proportional loss from gamma motoneurons was two-fold greater than that from alpha motoneurons. Loss of synaptic terminals with flattened synaptic vesicles was two-fold higher than that of synaptic terminals with round synaptic vesicles from axotomized gamma motoneurons, whereas axotomized alpha motoneurons lost both types of synaptic terminal equally.

Ultrastructural study of large efferent neurons in the superior colliculus of the cat after retrograde labeling with horseradish peroxidase

The ultrastructure of large neurons in the stratum griseum intermedium of the cat superior colliculus was examined following injections of horseradish peroxidase (HRP) into the dorsal tegmental decussation. Four HRP-labeled cells were selected, and the synaptology of their cell bodies and selected regions of proximal and distal dendrites was examined. The four neurons represent four morphologically distinct cell types: multipolar radiating, tufted, large vertical, and medium-sized trapezoid radiating. These four neurons correspond with cell types X1, X2, X3, and T1 respectively, according to the recent classification of neurons in the superior colliculus of the cat by Moschovakis and Karabelas (J. Comp Neurol. 239:276-308, '85). The three X type neurons are similar in having 83% of their somata and over 74% of their proximal dendrites contacted by synaptic profiles. Distal dendrites of the X type neurons, however, receive fewer synaptic contacts. In contrast, in the T1 cell, only 69% of the soma membrane is contacted by synaptic profiles, and the synaptic coverage on proximal and distal dendrites does not vary much from this. Of the eight types of synaptic terminals described in the stratum griseum intermedium of the cat superior colliculus by Norita (J. Comp. Neurol. 190:29-48, '80), only five are found in contact with the X and T type efferent neurons described here. There are some regional differences in terminal distribution, although each terminal is represented on each cell. Type III terminals (small, contain mostly pleomorphic vesicles, and make symmetrical contacts) are the most abundant on cell bodies and dendrites of all four cell types. Terminal types II (medium-sized, containing round and flattened vesicles, and making asymmetrical contacts), and IV (medium to large in size, containing flattened vesicles, and making symmetrical contacts) are well represented. In general, terminal types I (small, containing densely packed round vesicles, and making asymmetrical contacts) and VI (small and irregular in shape, containing flattened vesicles and making symmetrical contacts) are found infrequently. The identity of different types of synaptic terminal is discussed.

Quantitative synaptology of feline motoneurones to external anal sphincter muscle

Motoneurones innervating the cat external anal sphincter muscle were labelled retrogradely following intramuscular injections with horseradish peroxidase (HRP). Labelled motoneurones were examined by correlative light and electron microscopy (LM and EM) with special regard to a qualitative and morphometric analysis of the axon terminals resident on the neuronal membrane. By LM, labelled motoneurones were (1) ipsilateral to the injections; (2) all in S1-S2; (3) found only in the superior dorsomedial region of Onuf's nucleus; and (4) exhibited a broad spectrum of diameters (25-72 micron, mean 47.4 +/- 11.3 micron). By EM, axon terminals on the neuronal membrane when classified according to size, vesicle shape, and synaptic complex ultrastructure conformed to the S-, F-, T-, M-, and C-type terminals previously described for cat lumbosacral motoneurones. C-terminals confirmed these sphincteric motoneurones to be skeletomotor. Pooled data from midnuclear sections through 15 random labelled motoneurones (20-64-micron diameter) revealed that S- and F-type terminals predominated, with numerically few M and C types. Notwithstanding their low frequency (0.3/100 micron membrane) C-terminals contributed 1% of the mean areal coverage by terminals, which implies a potentially larger synaptic influence relative to other terminal types. Linear relationships occurred between terminal frequency (or cover) and motoneurone diameter. While motoneurones greater than 40 micron in diameter exhibited all five terminal types, labelled motoneurones less than or equal to 30 micron generally possessed only S-, F-, and occasional T-type terminals, and in this respect resembled gamma motoneurones.

An ultrastructural study of the synaptology of gamma-motoneurones during the postnatal development in the cat

The postnatal development of cat triceps surae gamma-motoneurones, retrogradely labelled with horseradish peroxidase (HRP), was studied light and electron microscopically. The mean diameter of the cell bodies of the gamma-motoneurones increased by about 25% from birth to the adult stage, which was much less than the increase in cell body diameter of alpha-motoneurones (about 45%). Throughout development the only bouton types apposing the gamma-motoneurones were the F- and S-types, with flattened and spherical synaptic vesicles, respectively. Thus, the C-, M- and T-types of boutons seen on a alpha-motoneurones. The number of boutons on the gamma-motoneurone cell bodies seemed to decrease postnatally. This decrease was only moderate for S-type boutons but substantial for F-type boutons. In contrast, the number of boutons on the proximal dendrites appeared to increase and this was most evident for S-type boutons. The mentioned postnatal changes in synaptology were more differentiated with regard to bouton type and part of the neurones under study than what could be inferred from earlier studies on the postnatal development of alpha-motoneurones. These changes also occurred later than in alpha-motoneurones. The relative dominance of F-type boutons with probable inhibitory actions on the immature gamma-motoneurone may explain the previously demonstrated poor encoding of muscle length by muscle spindles during the first postnatal weeks in the kitten.

The dendrites of single brain-stem motoneurons intracellularly labelled with horseradish peroxidase in the cat. An ultrastructural analysis of the synaptic covering and the microenvironment

Two laryngeal motoneurons intracellularly stained with horseradish peroxidase were studied ultrastructurally. The precise position of the ultrastructural observations made along the dendrites was obtained from the computer-reconstruction of the motoneurons in three dimensions. The shape and the size of the synaptic boutons, the percentage of membrane covered by bouton appositions and active zones, the number of boutons per 100 microns2 (packing density) were analysed on the soma and on the labelled dendrites at different distances from the soma up to 1000 microns. The results revealed no important regional differences in the mean length of synaptic apposition. The packing density was in the range of 9.3-14.9 boutons per 100 microns2 and was not correlated with the distance from the soma. The percentage apposition covering was higher on the soma and the proximal part of the dendrites than on the remaining part of the dendritic arborization. Close appositions between labelled dendrite and unlabelled somata and/or dendrites together with dendro-dendritic synapses suggested the possibility that the dendrites may be involved in local cell-to-cell communication. Microdendrites emerging from the soma or the proximal dendrites were contacted by synaptic boutons which may be more efficient as revealed by computation.

Electron microscopic observations on the synaptology of cat sciatic gamma-motoneurons after intracellular staining with horseradish peroxidase

Four cat sciatic motoneurons with axon conduction velocities below 30 m/s, and thus considered to be of the gamma-type, were intracellularly labelled with horseradish peroxidase (HRP) and subsequently studied in the electron microscope. The labelled neurons were apposed by synaptic terminals with spherical (S-type) and flattened vesicles (F-type) but not by large terminals with spherical vesicles (M- and C-types) seen on alpha-motoneurons. Quantitative analysis of a complete series of ultrathin sections through one of the neurons showed that the synaptic covering on the cell body (24.2%) was considerably larger than what has been reported for triceps surae gamma-motoneurons, but within the range of values for gamma-motoneurons in the thoracic region of the spinal cord.