Quantitative analysis of the dendrites of cat phrenic motoneurons stained intracellularly with horseradish peroxidase

All the dendrites (N = 37) generated by four phrenic motoneurons were analyzed following intracellular injection of horseradish peroxidase. The dendritic arbors produced from each of these stem dendrites were studied in detail. The mean number of stem dendrites produced by a phrenic motoneuron was 9.7, their mean diameter was 6.0 micron, and their mean combined diameter was 58.3 micron. The length at which a phrenic motoneuronal dendrite terminated was 1,236 micron, with several end terminals extending more than 2 mm from the cell body. The mean value for the combined lengths of all segments originating from a single stem dendrite was 5.3 mm. A full spectrum of dendritic branching patterns was observed from simple (five unbranched) to complex, the latter producing up to ninth-order branches. Most terminal and nonterminal dendritic segments tapered, producing a mean diameter reduction of 34%, or approximately 9% per 100-micron length. All phrenic motoneurons exhibited a steady decrease in the combined dendritic parameter (sigma d3/2) with distance from the soma as a result of tapering and end-branch termination. The mean surface area and volume of a phrenic motoneuronal dendrite were 35.3 X 10(3) micron 2 and 25.9 X 10(3) micron 3, respectively. The dendrites constituted greater than 97% of the total phrenic motoneuronal surface area, with 75% of this area lying outside of a 300-micron radius from the cell body. The diameter of a stem dendrite was positively correlated with its combined dendritic length, number of terminal branches, dendritic surface area, and volume. Despite this strong correlation, the value of total dendritic surface area calculated using the power equation derived from the dendritic surface area versus stem dendritic diameter plot was not a consistent estimator of the total dendritic surface area directly measured for these four phrenic motoneurons. It is suggested that this inconsistency may be the result of a heterogeneity in the phrenic motoneuronal population and/or in the dendrites projecting to the different terminal fields.     

Morphometric analysis of phrenic motoneurons in the cat during postnatal development.

The dendritic geometry of 20 phrenic motoneurons from four postnatal ages (2 weeks, 1 and 2 months, and adult) was examined by using intracellular injection of horseradish peroxidase. The number of primary dendrites (approximately 11-12) remained constant throughout postnatal development. In general, postnatal growth of the dendrites resulted from an increase in the branching and in the length and diameter of segments at all orders of the dendritic tree. There was one exception. Between 2 weeks and 1 month, the maximum extent of the dendrites increased in parallel with the growth of the spinal cord; however, there was no increase in either combined dendritic length or total membrane surface area. In addition, there was a significant decrease in the number of dendritic terminals per cell (59.8 +/- 9.3 vs. 46.4 +/- 7.4 for 2 weeks and 1 month, respectively). The distance from the soma, where the peak number of dendritic terminals per cell occurred, ranged from 700-900 microns at 2 weeks and 2 months to 1,300-1,700 microns in the adult. The diameter of dendrites as a function of distance from the soma along the dendritic path increased with age. The process of maturation tended to increase the distance from the soma over which the surface area and dendritic trunk parameter (sigma d1.5/D1.5) remained constant. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes or hexants. This analysis revealed that the postnatal growth in surface area in the rostral and caudal hexants was proportionately larger than that in either the medial, lateral, dorsal, or ventral hexants. Strong linear correlations were found between the diameter of the primary dendrite and the combined length, surface area, volume, and number of terminals of the dendrite at all ages studied.     

The determination of dendrite morphology on lateral rectus motoneurons in cat

Previously developed morphometric analysis of motoneurons (Ulfhake and Kellerth, '81, J. Comp. Neurol. 202: 571-583) was applied to lateral rectus motoneurons (LRMs). Total dendrite size was approximated from a single stem dendrite measurement. Fifteen dendrites from nine LRMs of the principal abducens nucleus intracellularly stained with HRP were morphometrically analyzed. The diameters and lengths along the extent of the dendrite were measured to calculate the surface area, volume, and combined length of the process. Linear correlation of stem dendrite diameter to these size parameters produced r values of .80, .84, and .61, respectively. Although the regression lines could be used to estimate dendrite size from the stem dendrite diameter, two morphologically distinct types were found among the 83 dendrites of the nine cells. Six dendrites differed from the other 77. Therefore, these six and a representative sample of the more common dendrite (nine) were included in the measurements. The rare dendrites consistently branched at about 40 micron from the soma into a rostrally and a caudally directed secondary dendrite. The secondary dendrites branched less and reduced more in diameter by tapering. Also, these dendrites exhibited a higher than expected total dendrite size to stem diameter ratio compared to "regular" dendrites. Statistical correlations of the stem diameter to surface area or volume within each dendrite type showed clear increases in r values from those of all 15. Significant differences were found between the size parameters of the two types. These qualitative and quantitative differences should be considered in accurate motoneuron size determinations in the abducens nucleus.     

Morphology of lumbar motoneurons innervating hindlimb muscles in the turtle Pseudemys scripta elegans: an intracellular horseradish peroxidase study

Motoneurons in the turtle lumbar spinal cord, electrophysiologically identified as innervating a muscle belonging to a functional group, were injected with horseradish peroxidase by electrophoresis. A total of 45 motoneurons were reconstructed from transverse sections. Eleven motoneurons were identified as innervating knee extensor muscles, eight as innervating hip retractor and knee flexor muscles, 14 as supplying ankle and/or toe extensors, and 12 as innervating ankle and/or toe flexor muscles. The cell bodies were elongated and spindle-shaped in the transverse plane. The mean equivalent soma diameter was calculated to be 33.4 micrometers. The mean axon conduction velocity was 15.7 m/second. Significant, though rather weak, positive correlations were found between soma diameter, axon diameter, and axon conduction velocity. The axons of the reconstructed motoneurons did not reveal a recurrent axon collateral. However, a few unidentified motoneurons did possess such collaterals. The dendritic trees were restricted to the ipsilateral side of the cord, but reached out in lateral, ventral, and ventromedial directions to the subpial surface. Easily recognizable and characteristic dendrites were found both in the dorsal dendritic tree and in the dorsomedial dendritic tree. Correlations were calculated between the soma diameter and (1) the number of first-order dendrites, (2) the mean diameter of the first-order dendrites, and (3) the combined diameter of the first-order dendrites. In each case no correlations or only weak correlations were found. Fair correlations were observed between the diameter of a first-order dendrite and the number of terminal dendritic branches (r = .61) and the combined dendritic length (r = .78). However, correlations between the combined diameter of all first-order dendrites per neuron and the total number of terminal dendritic branches and the total combined dendritic length of a neuron were extremely weak. The overall appearance of turtle spinal motoneurons is comparable to that observed in other "lower" vertebrates such as frog and lizard. However, similarities are also observed between certain morphometric parameters in turtle and cat lumbar motoneurons.     

Membrane area and dendritic structure in type-identified triceps surae alpha motoneurons

The size and branching structure of the dendritic tree were studied in nine type-identified triceps surae alpha-motoneurons that were labeled intracellularly with horseradish peroxidase and reconstructed from serial sections in the light microscope. The average total membrane area (AN) for motoneurons of type S (slow-twitch) motor units was about 22% smaller than AN for cells of type F units (including both FF and FR motor unit types in this category) (480.1 X 10(3) microns 2 vs. 617.7 X 10(3) microns 2, respectively). Systematic correlations were found between stem dendrite diameter and three measures of dendritic size: dendrite membrane area, combined dendritic length, and number of terminations. All of these correlations were significantly different for the dendrites of F and S motoneurons. Power-function relations between stem diameter and dendritic membrane area were used to estimate AN for a sample of 79 type-identified motoneurons. Mean estimated AN values were significantly different for the F and S motoneuron groups, despite a large overlap in AN values between these groups. The branching structure of dendrites of F and S motoneurons also showed clear differences. Type S motoneuron dendrites showed less-profuse branching and a more-even radial distribution of branch points than found in type F cells. Examination of two forms of the "3/2 power rule" for the relation between the diameters of parent and daughter dendritic branches at branch points showed that the dendrites of type S motoneurons conform less well with the anatomical constraints necessary to represent binary branching trees as equivalent cylinders than do dendrites of type F cells. There was no systematic difference between F and S motoneuron dendrites in the degree of asymmetry of first-order daughter trees. The results overall indicate that the dendrites of F and S motoneuron groups are structurally different, giving rise to a systematic difference in AN between these groups. Such structural differences suggest that the F and S groups of alpha-motoneurons can be viewed as intrinsically distinct cell types and not just large vs. small variants of the same cell species.     

Triceps surae motoneuron morphology in the rat: A quantitative light microscopic study

The rat is now the model of choice for many studies of motor function. However, little quantitative information on the structure of rat motoneurons is available. In conjunction with efforts to define the physiologic and anatomic substates of operantly conditioned plasticity in the spinal cord, 13 physiologically identified triceps surae motoneurons in the rat lumbar spinal cord were labeled intracellularly with horseradish peroxidase and completely reconstructed and measured with a computer-based neuron-tracing system. Somata were all located in the ventral horn of lumbar segments 4–5, had and average diameter of 35 μm, and had 6–12 dendrites. Dendrites remified throughout the ventral horn and also penetrated the white matter. Their spread was greater in the rostocaudal and dorsoventral directions (1.53 ± 0.24 mm and 1.35 ±0.23 mm, respectively) than in the mediolateral direction (0.85 ± 0.14 mm). Regardless of soma location, dendritic fields usually extended throughout the ipsilateral coronal cross-section of the ventral horn. As a result, the ventral or lateral extent of the field was correlated strongly with the soma's distance from the ventral or lateral border, respectively, of the ventral horn. Furthermore, although soma locations in the coronal plane varied widely, the centers of the dendritic fields tended to cluster near the center of the ventral horn. Dendrites constituted 96.2–98.4% (mean ± SD = 97.3 ± 0.7%) of the total neuronal surface area. Each of the 104 dendrites studies had an average of 13 branch points and 27 segments. First-order segment diameters ranged from 1.4 to 11.7 μm (mean ±SD = 5.3 ± 2.1 μm). Total dendritic length, surface area, volume, number of dendritic segments, and maximum segment order were correlated strongly with diameter of the first-order segment. Proceeding distally between branch points, the mean decrease in dendritic diameter (i. e., tapering) ± the standard deviation was 22 ± 8% of the proximal diameter. The average ratio ± the standard deviation of the sum of the average diameters of each daughter segment raised to the 1.5 power to the average diameter of the parent segment raised to the 1.5 power (i. e., Rall's ratio; Rall, 1959) was 0.87 ± 0.08. In comparison with cat α-motoneurons, rat motoneurons had smaller soma diameters, fewer dendrites, smaller total surface areas, and shorter total dendritic lengths. However, the number of terminations per dendrite was similar in the two species, so that rat motoneurons had more terminations per unit dendritic length. This greater branching density indicates that the dendritic fields of rat motoneurons are more compact and their branching structures are more densely packed that those of cat motoneurons. © 1994 Wiley-Liss, Inc.     

Morphology of motoneurons in different subdivisions of the rat facial nucleus stained intracellularly with horseradish peroxidase

Horseradish peroxidase was injected into single facial motoneurons of the rat. Neurons were identified by antidromic stimulation of either the buccal or the marginal mandibular or the posterior auricular nerve branches. Motoneuronal cell bodies supplying the buccal branch were located in the lateral subdivision of the facial nucleus, those supplying the marginal mandibular branch were in the intermediate subdivision, and those supplying the posterior auricular branch were in the medial subdivision. Eleven motoneurons were reconstructed with a computer-assisted technique. Their soma diameters averaged 20 microns; the average number of primary dendrites was 7.9 and the combined lengths of the dendritic trees averaged 17,650 microns. There was no distinction between the three motoneuron groups in terms of these and other quantitative data. However, on the basis of reconstructed dendritic tree orientation (i.e., dendritic distribution), major differences were observed between motoneurons of the three groups. Dendrites from all groups extended beyond the boundaries of the facial nucleus into the reticular formation. The border between the intermediate and the lateral subdivision was crossed by some dendrites but the overlap was small. In contrast, no dendrite of a motoneuron in the medial subdivision entered the intermediate subdivision and vice versa. The dendritic extent was totally restricted by the borders between these two subdivisions. Outside the Nissl-defined nuclear border, however, dendrites from cells in adjacent subdivisions overlapped. It is concluded that the medial subdivision of the facial nucleus can be distinguished from the intermediate and lateral subdivisions not only by its sharp Nissl-defined border but also by the discrete organization of its dendritic field.     

The Extent of the Dendritic Tree and the Number of Synapses in the Frog Motoneuron

Frog motoneurons were intracellularly labelled with cobaltic lysine in the brachial and the lumbar segments of the spinal cord, and the material was processed for light microscopy in serial sections. With the aid of the neuron reconstruction system NEUTRACE, the dendritic tree of neurons was reconstructed and the length and surface area of dendrites measured. The surface of somata was determined with the prolate - oblate average ellipsoid calculation. Corrections were made for shrinkage and for optical distortion. The mean surface area of somata was 6710 microm2; lumbar motoneurons were slightly larger than brachial motoneurons. The mean length of the combined dendritic tree of brachial neurons was 29 408 microm and that of lumbar neurons 46 806 microm. The mean surface area was 127 335 microm2 in brachial neurons, and 168 063 microm2 in lumbar neurons. The soma - dendrite surface area ratio was 3 - 5% in most cases. Dendrites with a diameter of 600 microm from the soma. This suggests that about two-thirds of the synapses impinged upon distant dendrites >600 microm from the soma. The efficacy of synapses at these large distances is investigated on model neurons in the accompanying paper (Wolf et al., Eur. J. Neurosci., 4 1013 - 1021, 1992).     

A quantitative morphological study of HRP-labelled cat alpha-motoneurones supplying different hindlimb muscles

Cat alpha-motoneurones supplying the quadriceps (Q), posterior biceps (PB), gastrocnemius (G), soleus (SOL) and short intrinsic plantar foot (SP) muscles were studied after retrograde or intracellular labelling with HRP. The average soma sizes were rather similar for the different pools, the SOL cells being the smallest. The median number of first-order dendrites ranged from 10 (PB) to 12 (SOL). The median diameters of the first-order dendrites ranged from 6 (SOL) to 8.5 (PB, G) micrometer. The dendritic projection patterns were rather similar for the different motoneurone groups, except for a prominent dorsomedial projection of SP dendrites. A considerable fraction of the dendrites extended into the white matter. The diameter of the first-order dendrite correlated positively to the number of end branches as well as to the combined length, surface area and volume of the whole dendrite. These relations appeared to be independent of motoneurone group and dendritic orientation. The combined diameter of the first-order dendrites, which reflects the total dendritic size of a motoneurone, exhibited median values between 82 micrometers (SOL) and 112 micrometers (Q). With respect to the relative scaling of soma and dendrites, motoneurones with large somas tended to have proportionally larger dendritic trees. The distribution of dendritic diameters, number of branches, dendritic surface area and volume, and the combined dendritic parameter (epsilon d3/2) at various distances from the soma were quite similar for the different motoneurone groups.     

Dendrite distribution of identified motoneurons in the lumbar spinal cord of the turtle Pseudemys scripta elegans

Motoneurons in the turtle lumbar spinal cord were injected with HRP by electrophoresis after being electrophysiologically identified as innervating a muscle belonging to a functional group. The distribution of dendrites was studied in transverse reconstructions of 45 motoneurons, including 11 motoneurons identified as innervating knee extensor muscles, eight motoneurons innervating hip retractor and knee flexor muscles, 14 motoneurons innervating ankle and/or toe extensors and 12 motoneurons innervating ankle and/or toe flexor muscles. The dorsal dendritic tree of motoneurons innervating distally positioned musculature (ankle and/or toe extensors and flexors) was observed to contain significantly less terminal dendritic branches compared to the dorsal dendritic trees of motoneurons innervating proximally situated (hip and knee) muscles. The distribution of dendrites within the white matter was studied by measuring the total projected length of the dendritic branches within empirically defined sectors in the transverse plane. This kind of analysis also revealed differences between the dorsal dendrites of motoneurons innervating distally and proximally positioned muscles conforming to the counts of terminal dendritic branches. It is suggested that these apparent differences in the size of the dorsal dendrite may be related to the number of synapses made by primary afferents. In the white matter, the highest dendritic density for all four groups of mononeurons was found within the central part of the lateral funiculus. However, only in the ventral funiculus could slight indications be found that the dendritic density of functionally different motoneuron groups may bear some relation to the locations of the terminations of the descending pathways known to establish monosynaptic contacts with lumbar mononeurons.     

Morphological differences between wild-type and transgenic superoxide dismutase 1 lumbar motoneurons in postnatal mice

Quantitative analysis of the dendritic arborizations of wild-type (WT) and superoxide dismutase 1 (SOD1) postnatal mouse motoneurons was performed following intracellular staining and 3D reconstructions with Neurolucida system. The population of lumbar motoneurons was targeted in the caudal part of the L5 segment, and all labeled motoneurons were located within the same ventrolateral pool. Despite the similar size of the soma and the mean diameter of primary dendrites, the dendritic arborizations of the WT and SOD1 motoneurons showed significant differences in terms of their morphometric parameters. The metric and topological parameters of dendrites show that the total dendritic length and surface area and total number of segments, branching nodes, and tips per motoneuron were significantly higher in SOD1 motoneurons. Our main finding concerns a proliferation of dendritic branches starting at about 100 microm from the soma in the SOD1 motoneurons. However, the longest and mean dendritic paths from soma to terminations were similar, giving a comparable envelope of the dendritic fields. Indeed, the SOD1 motoneurons were larger as a result of abnormal branching. The results suggest that a defect in pruning mechanisms occurs during this developmental period. The abnormal growth of the dendritic arborizations and the reduced excitability of postnatal SOD1 motoneurons could be a neuroprotective response and would represent an early compensatory mechanism against the activity-induced toxicity.     

Reduced branching and length of dendrites detected in cervical spinal cord motoneurons of Wobbler mouse, a model for inherited motoneuron disease.

The Wobbler mouse (wr) has been proposed as a model for human inherited motoneuron disease (infantile spinal muscular atrophy). The primary defect is thought to be in the motoneurons. Therefore we undertook a survey of the qualitative and quantitative changes occurring in the cervical spinal motoneurons of Wobbler mice during a late stage of the motoneuron disease compared with age- and sex-matched normal phenotype (NFR/wr) littermates. The Rapid Golgi Method was applied. In control and Wobbler mice, four types of neurons were identified according to their dendritic patterns: multipolar, tripolar, bipolar, and unipolar cells. Unipolar cells were observed more often in the Wobbler specimens than the controls and may represent a final stage in the degeneration of other cell types with greater numbers of primary dendrites. Medium (300-999 microns 2) and large (greater than 1,000 microns 2) impregnated neurons (presumably alpha-motoneurons) showed strong indications of cell degeneration, including statistically significant reductions in the measurements for dendritic length, distribution, and branching, as well as the number of spines. In contrast, the small (less than 300 microns 2) neurons showed only mild signs of degeneration, including slight reductions in dendritic length, but no significant differences appeared in the distribution and branching of dendrites, or in the number of spines. Instead, a small increase could be detected in the number of primary and secondary dendritic branches emanating from the small neurons, as well as in the number of dendritic spines. These findings suggest that sprouting may occur to a slight extent. Although previous studies document that swelling with subsequent vacuolation of motoneurons is the predominant feature characterizing the Wobbler disease, the mean soma area (microns 2) calculated for the impregnated neurons of the Wobbler specimens showed no significant difference from the controls. It is hypothesized that the advanced signs of the Wobbler motoneuron disease are primarily reflected in the degeneration of the dendrites and spines on the medium and large alpha-motoneurons. The small neurons (presumably a mixed population of gamma-motoneurons, interneurons, and Renshaw cells) possess dendrites and spines that seem to be less affected, and instead show signs of sprouting.     

Morphology of HRP-injected spinocervical tract neurons: effect of dorsal rhizotomy

Twenty-five physiologically identified spinocervical tract (SCT) neurons in the sixth lumbar segment of the cat were filled with HRP by intracellular injection. All were reconstructed from sagittal sections using the camera lucida, and a subset (n = 18) was also reconstructed using a computer reconstruction system. Thirteen cells were in intact preparations, nine were in spared root preparations (L5, L6, S1, S2 cut; L7 spared), and three were in preparations with L5 through S2 cut. Analysis of the dendritic tree of these neurons revealed little change in gross morphology after partial deafferentation despite increased proportions sensitive to nociceptive input (Sedivec et al., 1983). The dendrites still largely respected the lamina II-III border, and relatively few dendrites were directed ventrally from the cell body, although the ratio of ventral to dorsal dendrites was greater than normal. The major change was an increase in surface area and volume caused by changes in diameter (but not length) of the dendrites. Larger-than-usual maximum branch order of individual dendritic trees of some cells was also observed after chronic deafferentation. Thus, SCT cells in deafferented segments do not undergo atrophy, but show, rather, limited signs of growth and the possibility of dendritic reorganization. We have also computed correlations between different parameters of these cells (cell body size, number and size of primary dendrites, total area and length of individual dendrites) and have found that, as in motoneurons, diameter of the primary dendrite measured 30 micron from the soma is significantly correlated with total dendritic surface area and length. SCT neurons tend to have more dendrites than spinal alpha-motoneurons, but total surface area is smaller for a given diameter of a proximal dendrite.     

Androgenic regulation of dendritic trees of motoneurons in the spinal nucleus of the bulbocavernosus: reconstruction after intracellular iontophoresis of horseradish peroxidase

Androgen-sensitive motoneurons in the spinal nucleus of the bulbocavernosus (SNB) in adult male rats were labeled after intracellular iontophoresis of horseradish peroxidase, after which they were fully reconstructed in three dimensions in order to measure their dendritic trees. Three groups of rats were compared: intact adult male rats and male rats castrated as adults and given Silastic tube implants containing either testosterone or nothing. In the high-androgen groups (intact males and testosterone-treated castrates), soma size and the diameter of the first-order dendrites were larger than in blank-treated castrates. Moreover, the terminal dendrites in all groups possessed growth cones, implying that the dendrites of these motoneurons are capable of growth in adulthood. However, there were no statistically significant group differences in the length, membrane surface area, or volume of the dendritic trees, or in the orientation or branching symmetry of dendrites. In general, there were positive correlations between the size of the motoneuronal soma and various measures of the size of the dendritic tree and between the diameter of individual stem (first-order) dendritic branches and the size of remainder of that dendrite. These data suggest that there may be a modest effect of androgen on the size of the dendritic trees of SNB motoneurons in adulthood, although the effect is much smaller than has previously been reported.     

Postnatal development of cat hind limb motoneurons. III: Changes in size of motoneurons supplying the triceps surae muscle

The postnatal changes of neuronal dimensions were studied in cat triceps surae motoneurons intracellularly labeled with horseradish peroxidase. Systematic correlations were observed in the analysis of single dendrites at each studied stage, from birth to 44-46 days post natum (d.p.n.) age, between size parameters intrinsic to the dendrites as the diameter of a 1st-order dendrite, the combined dendritic length, the dendritic membrane area, and the degree of branching. Some variability among samples was evident in each studied age group. The correlations were, however, sufficiently close to permit indirect estimations of both combined dendritic length and dendritic membrane area for larger samples of neurons from data on dendritic stem caliber. The total postnatal increase in dendritic membrane area was, on the average, 400%, i.e., from close to 100 X 10(3) microns2 to about 500 X 10(3) microns2. The corresponding increase in soma area amounted to 100%. Analysis revealed that there was a time lag between the increase in somatic and dendritic size. Thus, adult somatic dimensions were attained at age 44-46 d.p.n.; however, at this stage, the mean total dendritic membrane area was only about half of the adult value. The postnatal increase in size appeared to vary among neurons, yielding a wider neuronal size spectrum in the adult cat than that observed in kittens. The measured increase in size corresponded to a calculated average addition of dendritic membrane area of 3700 microns2/day from birth to 22-24 d.p.n. and from that stage to 44-46 d.p.n. of 2700 microns2 per day. Likewise, the increase in combined dendritic length could initially be as large as 1 mm/day down to 0.4 mm/day between 22-24 and 44-46 d.p.n., with a mean growth during the first 44-46 d.p.n. of 0.5 to 0.6 mm/day. The ratios of daughters to parent branch diameters (sigmadd1.5: dp1.5) and the dendritic trunk parameter (sigma d1.5) recorded along the proximodistal dendritic path distance revealed transient changes that might impact on the electrotonic properties of the dendrites during postnatal development. Computations from the measured changes in dendritic branch lengths and calibers indicated that if membrane and internal resistivity remain unaltered during postnatal development, the dendritic domain is electrotonically more compact in the newborn kitten than in the adult cat.(ABSTRACT TRUNCATED AT 400 WORDS)     

Early Postnatal Changes in the Somatodendritic Morphology of Ankle Flexor Motoneurons in the Rat

The development of locomotor function in the rat spans the first 3 postnatal weeks. We have studied morphological features of the soma and dendrites of motoneurons innervating the physiological flexor muscles of the ankle, tibialis anterior and extensor digitorum longus, by intracellular injection in vitro between the first and ninth postnatal days. We obtained serial optical sections of 96 adequately filled motoneurons in whole-mounted hemisected spinal cords by confocal microscopy, projected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such motoneurons were covered in growth-associated spiny, thorny or hair-like appendages. These had disappeared from the soma by the fourth postnatal day and from most proximal dendrites by day 7, but were still common distally on day 9. During this period there was little or no net growth of either the soma (which was still much smaller than in the adult) or the dendritic tree. A dorsal dendritic bias was present and 'sprays' of long, loosely bundled dorsal dendrites were often seen. The mean number of primary dendrites remained constant at about eight, and their combined diameter was already significantly correlated with mean soma diameter, as in the adult cat. Thus, the critical neonatal period during which these ankle flexor motoneurons are known to change their electrophysiological properties and to be particularly sensitive to interference with neuromuscular interaction is characterized by major changes in the neuronal surface, presumably linked to synaptogenesis.     

Dendritic distribution of motoneurons innervating the three heads of the trapezius muscle in the cat

The organization of the nuclei and dendritic architecture of motoneurons innervating the three heads of the trapezius muscle, clavotrapezius (CT), acromiotrapezius (AT), and spinotrapezius (ST), have been examined by using intracellular staining techniques. CT, AT, and ST motoneurons were found in the spinal accessory nucleus and were arranged in three overlapping subnuclei. CT motoneurons were primarily found in C2 and C3. In contrast, most AT motoneurons were found in C3, C4, and C5 and ST motoneurons were found in C4, C5, and the rostral parts of C6. Most dendrites of CT motoneurons, located in rostral C2, extended dorsally and many of these dendrites spread medially and laterally to encompass all of lamina VIII and the dorsolateral part of lamina VII. When viewed in the horizontal plane these motoneurons had a stellate appearance. The dendritic tree structure of CT motoneurons changed abruptly between rostral C2 and mid-C2. The majority of dendrites of CT motoneurons located in the central and caudal parts of C2 projected rostrally and caudally to form a complex bundle of dendrites in the motoneuron nucleus. Small numbers of dendrites were also found ventromedial and dorsal to the soma. The dendritic trees of CT motoneurons in C3 and C4 and AT and ST motoneurons located in C4 and the rostral parts of C5 also followed this fusiform distribution pattern. The dendritic trees of AT and ST motoneurons in caudal C5 were not fusiform but instead had a complex distribution pattern which consisted of dendrites projecting in several directions. Many dendrites projected rostrally and caudally, and in addition, there were major dendritic projections ventrolateral and dorsolateral to the soma. These results indicate that each head of the trapezius muscle is innervated by two structurally dissimilar groups of motoneurons which occupy different spinal segments. Trapezius motoneurons at the same segmental level, regardless of which head of the trapezius muscle they innervated, have similar dendritic trees whose structure differs from those of neighbouring dorsal neck muscle motoneurons in C2, C3, and C4. Thus, the organization of motoneuron dendritic trees appears to be governed by several factors including the muscle innervated by the motoneuron and the transverse and segmental position of the motoneuron's soma.     

Light microscopic observations on cat Renshaw cells after intracellular staining with horseradish peroxidase. II. The cell bodies and dendrites

The cell bodies and dendritic trees of five lumbosacral Renshaw cells of adult cats were studied in the light microscope (LM) after intracellular injection with horseradish peroxidase (HRP). The cell bodies were all located in the ventral part of lamina VII. The dendrites extended up to 0.7 mm from the cell body into the neighbouring parts of laminae VIII and IX as well as into more dorsal parts of lamina VII. The dendritic branching was sparse and about half the dendrites were unbranched. The mean diameter of the cell body was positively correlated to both the combined and mean diameters of the first-order dendrites. Between four and eight dendrites originated from the cell bodies. The number of dendritic end-branches, the combined dendritic length, the mean dendritic length from the cell body to the termination of the end branches, the distance from the cell body to the termination of the most remote end-branch, the dendritic surface area, and the dendritic volume all correlated positively with the diameter of the parent first-order dendrite. The dendritic tapering was somewhat more pronounced in the Renshaw cells than previously observed in alpha- and gamma-motoneurons. The present data are discussed in relation to previous morphological observations on Renshaw cells and alpha- and gamma-motoneurons.     

Quantitative morphological analysis of the motoneurons innervating muscles involved in tongue movements of the frog Rana esculenta

We give an account of an effort to make quantitative morphological distinctions between motoneurons of the frog innervating functionally different groups of muscles involved in the movements of the tongue. The protractor, retractor, and inner muscles of the tongue were considered on the basis of their major action during the prey-catching behavior of the frog. Motoneurons were selectively labeled with cobalt lysin through the nerves of the individual muscles, and dendritic trees of successfully labeled neurons were reconstructed. Each motoneuron was characterized by 15 quantitative morphological parameters describing the size of the soma and dendritic tree and 12 orientation variables related to the shape and orientation of the dendritic field. The variables were subjected to multivariate discriminant analysis to find correlations between form and function of these motoneurons. According to the morphological parameters, the motoneurons were classified into three functionally different groups weighted by the shape of the perikaryon, mean diameter of stem dendrites, and mean length of dendritic segments. The most important orientation variables in the separation of three groups were the ellipses describing the shape of dendritic arborization in the horizontal, frontal, and sagittal planes of the brainstem. These findings indicate that characteristic geometry of the dendritic tree may have a preference for one array of fibers over another.     

Normal dendritic morphology of frog spinal motoneurons: A Golgi study

Normal dendritic morphology of frog (Rana pipiens) lumbar motoneurons was studied using Golgi silver impregnation. Branching characteristics and quantitative measurements of dendrites were obtained using computeraided serial reconstruction of a typical lumbar motoneuron over seven adjacent 80-μm transverse sections. Dendrites were classified based upon site of dendrite origin from the soma and distribution of the dendritic array within the spinal cord. Eight possible sites of dendritic origin from the soma were identified. Two dendrites, D1 and D2, are planar dendrites which arise from the dorsal aspect of the soma. They are moderately complex, reaching branch order 5–6, and are oriented predominantly in the transverse plane. Input to these dendrites is primarily segmental via dorsal root projections. Three dendrites, D3, D4, and D5, arise laterally from the soma and extend through the lateral funiculus toward the subpial region. Two dendrites, D6 and D7, arise ventrally. D6 extends ventrolaterally and is a simple dendrite reaching branch order 3–4. D7 aborizes extensively in the ventral funiculus and in the central gray, reaching a branch order of 8–9. This dendrite extends rostrally and caudally over a distance of at least 560 μm. Another dendrite (D8) arises from the medial aspect of the soma and projects toward the central canal. Four sites (D1, D2, D6, and D7) almost invariably give rise to dendrites. Dendrites arise at D4 in 66% of the cells examined. Dendrites are found at D3, D5, and D8 much less frequently (6–21%). Total dendritic length (12,043 μm) and lengths of the individual dendrites, branch length versus branch order, and number of branches at increasing radii were examined, and Sholl analysis was performed.