Relations between Cell Body Size, Axon Diameter and Axon Conduction Velocity of Triceps Surae Alpha Motoneurons during the Postnatal Development in the Cat S. CULLHEIM AND B. ULFHAKE Department o f d n a t o m y , Karolinska Znstitutet, 104 01 Stockholm 60, Sweden

ABSTRACT Triceps surae a-motoneurons in cats of different postnatal ages were stained intracellularly with horseradish peroxidase (HRP) and studied light microscopically. In individual neurons, the mean diameter of the cell body and the intramedullary axon diameter were measured and related to the axon conduction velocity. The mean diameter of the cell body grew from 39.6 pm a t birth to 57.6 p m in the adult cat, while the corresponding figures for the intramedullary axon diameters were 2.4 pm and 6.7 pm. During the same period of time, the axon conduction velocity increased from 11.3 m/s to 93.5 m/s, and the ratio between the conduction velocity and the intramedullary diameter of the axon (CV/d ratio) increased from 4.6 to 14.1. The results indicate that the growth of the cell body is smaller and completed earlier than the growth in diameter of the intramedullary and, in particular, the peripheral parts of the axon. The considerable change of the CVld ratio during the postnatal development may be explained by previously described immature morphological properties of the axons in very young kittens, and by a changing relation between the dimensions of the intramedullary and peripheral parts of the axon. A number of morphological studies has been devoted to the postnatal development of mammalian alpha motoneurons. The results from studies on the postnatal growth of the cell body (Donaldson and Nagasaka, '18; Ngowyang, '30; Ford and Cohan, '68; Mellstrom and Skoglund, '69; Sat0 et al., '77) and the ventral root fibres (Dunn, '12; Donaldson and Nagasaka, '18; Rexed, '44; Skoglund and Romero, '65) seem to indicate that the postnatal increase in cell body diameter is smaller than the increase in axon diameter. In contrast, Martinez and Friede ('70) reported that the postnatal growth of axoplasmic and cell body volumes of alpha motoneurons in the rat seemed to occur in parallel. With the introduction of horseradish peroxidase (HRP) as an intraneuronal marker substance (Muller and McMahan, '75; Purves, '75; Snow et al., '76; Kitai et al., '76; Jankowska et al., '76; Cullheim and Kellerth, '76) it has become possible to relate directly single cell physiology with detailed morphological studies of the cell body, dendrites and J. COMP. NEUR. (1979)188: 679-686.

axons of the very same neuron. With the aid of this technique, the present study was undertaken to investigate the postnatal development of triceps surae alpha motoneurons in the cat and to see whether there is a differential growth of the cell body and the axon of these neurons. In each motoneuron three parameters were studied, namely the cell body diameter, the intramedullary axon diameter and the axon conduction velocity, which should indirectly reflect the dimension of the long, peripheral part of the axon (e.g., Hursh, '39; Boyd, '64). The motoneurons were collected from kittens up to six t o seven weeks of age. This age was chosen, since it has been held to constitute a transition point between immature and adult properties in the motor system of the cat (e.g., Hammarberg and Kellerth, '75). MATERIALS AND METHODS

Eleven kittens and 9 young adult cats (7-12 months of age) were used in the present study. The kittens were collected in 5 age groups (0-

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Only neurons which were intact as judged 1,7-8, 14-15,22-24and 44-46 days of postnatal age) and were anaesthetized with ketamine from light microscopic examination were conchloride and xylazine chloride. The initial sidered in the present investigation. The mean doses, given intramuscularly were 20 mg/kg of diameter of the cell bodies was determined as ketamine chloride and 2.0 mg/kg of xylazine the mean of the major and minor diameter of chloride. The anaesthesia was maintained the largest possible oval which could be placed with subsequent doses of ketamine chloride within the margins of the cell in a focal plane (2.0-10.0m g k g ) and xylazine chloride (0.2-2.0 where the cell body showed its largest outline mg/kg), given intravenously with intervals (Cullheim, '78). The diameters of the stained ranging between 20 and 60 minutes in dif- motor axons in the white matter were meaferent animals, the doses and intervals being sured on light micrographs taken with an oil kept throughout the experiment. The adult immersion objective. The final magnification cats were anaesthetized with Nembutal (40 of the micrographs was 3,000 x . The diameter of the axons was obtained as the mean of 8 m g k g intraperitoneally). After preparation of the medial gastrocne- measurements along each axon a t intervals of mius and the lateral gastrocnemius-soleus 10 p m (kittens 0-8 days of age), 20 p m (kitnerves in the left popliteal fossa, a lum- tens 14-24 days of age), 30 p m (kittens 44-46 bosacral laminectomy was performed and the days of age) or 10 measurements a t intervals dorsal roots L,-S, on the left side were cut. The of 50 p m (adults). The conduction velocity of the motoneurons animals were paralyzed with tubocurarine chloride and respired artificially. The expired was determined by measuring the axonal conCO, was monitored with a Beckman Gas Ana- duction time and the nerve length between lyzer and kept within normal limits. The tem- the stimulation site of the peripheral nerve perature of the body and paraffin oil pools was and the ventral root exit from the spinal cord kept within 36-38°C by use of infrared light. a t the rostro-caudal level of the position of the Triceps surae motoneurons were identified neurons. All injected motoneurons were considered by antidromic invasion after stimulation of t h e peripheral nerves and impaled with to be of the alpha type, since their axons conbevelled glass microelectrodes (tip diameters ducted faster than what has been reported for 1.5-2.5 pm) containing 25% HRP (type VI; gamma motor axons both in kittens (SkogSigma) dissolved in 0.1 M NaOH. The electrode lund, '60; Westerman et al., '73) and in adult resistances ranged between 60 and 100 M R . cats (cf. Leksell, '45; Kuffler et al., '51). The injections of the motoneurons were made RESULTS by passing a constant positive current Table 1 presents some morphological data through the microelectrodes. In kittens, 10-15 nA was passed during 5-10 minutes while 20- and the axon conduction velocity of HRP30 nA was maintained for 10-20 minutes in stained triceps surae alpha motoneurons a t the adult cats. After 1-5 hours the animals different postnatal ages. At birth, the mean were perfused with buffered 5% glutaralde- diameter of the motoneuronal cell bodies was hyde through the descending aorta according about 40 p m and the intramedullary axon dito Berthold ('68) and Conradi ('69).The spinal ameter was 2.4 pm. The mean conduction cords were cut in transverse sections with a velocity of the axons at this age was 11.3 m/s. thickness of 30 p m (adult cats and kittens 2 All values, in particular t h e diameter and the 14 days of age) or 50 p m (kittens < 8 days of conduction velocity of the axons, showed a age) by use of a Vibratome (Oxford Lab., substantial range already in the newborn kitUSA). The sections were processed for HRP ten. After a very small increase of all paramewith diaminobenzidine according to Graham ters during the first postnatal week, the mean and Karnovsky ('66) or with a mixture of p- diameter of the cell body gradually increased phenylenediamine (PPD) and pyrocatechol to reach the adult value (about 58 p m ) a t 44(PC) according to Hanker et al. ('77). After 46 days of postnatal age. The rapid phase of osmication and dehydration in acetone, the growth was found during the second and third sections were embedded between plastic foils weeks of life when about 60%of the total post(Hollander, '70) in Vestopal. Detailed descrip- natal increase in diameter occurred. At 44-46 tions of the technical procedures have been days of age, the intramedullary axon diameter published earlier (Cullheim and Kellerth, '76, had reached a n average of 5.7 p m which was '78). about 80%of the adult value (6.7 wm). In this

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POSTNATAL DEVELOPMENT OF CAT ALPHA MOTOR AXONS TABLE 1

Cell body diameter, intramedullary axon diameter, axon conduction velocity and axon conduction velocity/ diameter ratio of cat triceps surae u-motoneurons at different postnatal ages. For each parameter the mean ualue has been indicated with the total range of ualaes within parenthesis No. of animala

No. of cells

0-1 days

2

10

7-8 days

3

11

14-15days

2

14

22-24days

2

7

44-46days

2

15

Adult

9

33

Age

Mean diameter of cell body (pm)

Mean diameter of axon (d) (crm)

39.6 (35.0-43.5) 40.4 (36.0-46.0) 47.8 (44.5-51.5) 51.6 (46.0-57.5) 58.2 (47.0-65.5) 57.6 (47.0-65.5)

I/ 1

2.4 (2.0-3.1) 2.7 (2.1-3.0) 3.4 (2.9-4.1) 4.2 (3.9-4.7) 5.7 (3.8-7.1) 6.7 (4.4-8.4)

Axon conduction velocity (CV) (rnla)

11.3 (6.8-15.0) 12.3 (9.0-16.6) 25.6 (21.0-31.2) 31.7 (26.0-36.3) 41.0 (29.5-54.0) 93.5 (57-113)

CVid ratio

4.6 (3.4-6.0) 4.6 (3.4-5.5) 7.6 (5.9-10.1) 7.4 (6.6-8.3) 7.3 (6.0-9.5) 14.1 (11.3-17.3)

80

70

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50

40

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10

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and the diameter of the axon in the spinal white matter ( 0 ) Fig. 1 The mean diameter of the cell body (0) of tricepa surae alpha motoneurona in the cat plotted against postnatal age.Each bar illustrates the range of observed values, while the superimposed symbols indicate the mean values. The number of observations in each age group is given in table 1.

case, the rapid phase of growth seemed to include not only the second and third postnatal weeks but also the subsequent period up to 4446 days of age. The total percental increase in the mean diameter of the cell body and the intramedullary diameter of the axon was 45% and 180%, respectively. The relationship be-

tween the postnatal changes of these two parameters is shown graphically in figure 1. Regarding the axon conduction velocity during the postnatal period, it showed a very large increase during the second week of life and then a comparatively moderate increase until 44-46 days of age, when it had attained

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Fig. 2 The axon conduction velocity ( x ) and the diameter of the axon in the spinal white matter ( 0 )of triceps surae alpha motoneurons in the cat plotted against postnatal age. Each bar illustrates the range of observed values, while the superimposed symbols indicate the mean values. The number of observations in each age group is given in table 1.

on an average 41.0 m/s which was about 45% of the adult value (93.5 m/s). Thus, a very substantial part of the postnatal change in conduction velocity seemed to occur after 6-7 weeks of age. The total percental increase in conduction velocity postnatally amounted t o 730%,which was much more than what was found when studying the postnatal changes of the intramedullary axon diameter and, in particular, the cell body diameter. The relationship between the conduction velocity and the intramedullary diameter of the axon a t different postnatal stages is shown graphically in figure 2. The ratio between the axon conduction velocity and the diameter of the intramedullary part of the axon (CVld ratio) was 4.6 both a t birth and after one week of life, but then raised to 7.6 a t 14-15 days of age. After this

age no change seemed to occur until after the kittens were 44-46 days old. Thereafter, there was almost a doubling of the CVld value which reached on an average 14.1 in the adult cats. Figure 3 shows the mentioned changes graphically. DISCUSSION

Since this paper presents a number of data on dimensions of HRP-stained neurons it is adequate to ask whether the used technique may produce artefacts which have had influence on the results. With regard to adult material, this has been extensively discussed in a previous paper (Cullheim and Kellerth, '78) where it was suggested that no substantial errors due to shrinkage or swelling of the neurons had been introduced by the staining technique. The present material on kitten moto-

POSTNATAL DEVELOPMENT OF CAT ALPHA MOTOR AXONS

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neurons has not given any indications that immature neurons would be more susceptible to the staining procedure than adult neurons. Even if there are differences between the results in the present study and those obtained in some earlier investigations, they might well be explained by differing sampling of the neurons or by the various fixation techniques (see below) instead of being interpreted as signs of artefactual volume changes induced by the staining of the neurons. The postnatal changes in cell body diameter in the present study indicate that the cell body volume of the triceps surae alpha motoneurons of newborn kittens should be approximately one third of the volume in adult cats. In comparison, Sat0 et al. ('77) found the cell bodies of adult cat soleus and medial gastrocnemius motoneurons to have about four times the volume of motoneuronal cell bodies in the newborn kitten. Mellstrom and Skoglund ('69) found the mean cell body volume of one hundred neurons in lamina IX to be 4.4 times bigger in the adult cat than a t the new-

born stage. As to the absolute size of the cell bodies, Conradi and Skoglund ('69) reported somewhat smaller values of presumed alpha motoneurons in the kitten than were found in our study, while Conradi and Ronnevi ('77) found values in good correspondence with the present results. In contrast to all previous studies, only neurons which were positively identified as alpha motoneurons have been considered in the present study. Thus, the differences between our results and some of the earlier findings might be explained by different sampling of the neurons. Also, the use of different fixation techniques with varying degree of tissue shrinkage might have had influence on both the absolute values and the relative postnatal increase in size of the motoneurons. The demonstrated postnatal changes in axon conduction velocity are in accordance with earlier investigations (Ridge, '67; Nystrom, '68; Bagust et al., '74; Huizar et al., '75). In agreement with Ridge ('67), the present results also indicate that only a very small

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increase in the conduction velocity occurs during the first postnatal week. During the postnatal period, the percental increase in axon conduction velocity was about four times larger than the growth of the intramedullary axon diameter. The ratio between the conduction velocity and the diameter of large adult medullated fibres has been proposed to be around 6 (Hursh, '39; Boyd, '641, while the ratio of small-diameter fibres (c:a 2 to 6 pm) has been reported t o be only slightly smaller (4.5-4.7) (Boyd, '64; Ekholm, '67). This small difference cannot explain for the large discrepancy between the increase in axon conduction velocity and the growth of the intramedullary axon diameter. However, when comparing our values on the intramedullary axon diameter with previous results obtained from fibres in the ventral root (Eccles and Sherrington, '30; Rexed, '44; Skoglund and Romero, '65), a clear difference is indicated between kittens and adult cats. While the intramedullary axon diameter seems to represent the dimension of the peripheral part of the axon fairly well in kittens up to 6-7 weeks of age, the intramedullary part appears to be considerably thinner than the ventral root axons in the adult cat (see also Cullheim and Kellerth, '78). Moreover, in adult mammals, there is physiological evidence for an increase in conduction velocity as one proceeds distally along a peripheral nerve (Clough et al., '68; Mendell et al., '761, which may indicate an irregularity of the axon diameter also within the periphery, the distal part of the peripheral axon being thicker than the proximal part. If these fluctuations in conduction velocity are the result of developmental processes, they should certainly change the relation between axon conduction velocity and intramedullary axon diameter even more than predicted from the growth of the ventral root axons. In any way, the present findings suggest a larger postnatal increase in the axon diameter peripherally than intramedullarly, resulting in a thicker peripheral part of the axon in the adult. This should explain the surprisingly high CVld ratio of the adult alpha motor axons in the present study. Such high CVld ratios have been found also in thin and slowly conducting gamma motor axons in adult cats (Cullheim and Ulfhake, '79) which indicates that the changing relation between the intramedullary and peripheral parts of the alpha motor axons during the development is

not determined simply by the different dimensions of the axons a t various ages. In our material the postnatal growth of the diameters of the alpha motor axons by far exceeded the growth of the diameter of the cell bodies. This is in accordance with the findings by Donaldson and Nagasaka ('18) who reported the diameters of ventral root fibres in the rat to increase much more than the large, ventrolaterally situated spinal cord cell bodies during the later phases of growth (from 17 days of postnatal age). If also considering the increase in axonal length postnatally (in the present material about 300%),it is obvious that the difference in growth of volume between the axoplasm and the cell body is even larger. This is in con.trast to the findings by Martinez and Friede ('70) who reported that the postnatal growth of axoplasmic and cell body volumes in the rat seemed to occur in parallel. The present results suggest that the growth of various parts of the alpha motoneurons differs not only in magnitude but also in time course. Thus, the size of the cell bodies of the motoneurons had attained adult values at 6-7 weeks of age, while earlier studies on the cat have indicated a minor increase in cell body volume from 45-50days of postnatal age t o the adult stage (Mellstrom and Skoglund, '69; Sat0 et al., '77). In comparison, after a roughly parallel growth of the intramedullary and peripheral parts of the axons up to 44-46 days of age, the further growth of the intramedullary axon diameter seemed to be about 25%, while previous investigations suggest the growth of the diameter of the largest fibres in the ventral root and in the medial gastrocnemius nerve to be 60-100%during the same period of time (Skoglund and Romero, '65; Nystrom, '68). These findings may indicate that the proximal parts of the alpha motoneurons a t tain maturity earlier than the distal parts. In spite of the suggested parallel growth of the intramedullar and peripheral parts of the alpha motor axons from birth to 6-7 weeks of age, the CVld ratio of the youngest kittens appeared to be smaller than in the older kittens. This may be explained by the existence of very short (10-50pm) internodes and axonal segments completely devoid of myelin in the kitten ventral root during the first postnatal weeks (Berthold and Skoglund, '68a,b; Berthold, '73; Fraher, '78) which might lead to a conduction velocity of these axons which is

POSTNATAL DEVELOPMENT OF CAT ALPHA MOTOR AXONS

lower than predicted from their diameter (McDonald and Sears, ’70). ACKNOWLEDGMENTS

This study was supported by grants from the Swedish Medical Research Council (project 02886) and the Karolinska Institutet. We wish to thank Ms. Gunilla Linder and Ms. Lillebil Stuart for their skillful technical assistance. LITERATURE CITED Bagust, J., D. M. Lewis and R. A. Westerman 1974 The properties of motor units in a fast and a slow twitch muscle during postnatal development in the kitten. J. Physiol., 237: 75-90. Berthold, C.-H. 1968 A study on the fixation of large mature feline myelinated ventral lumbar spinal-root fibres. Acta SOC.Med. Upsal., 73: 1-36. 1973 Local “demyelination” in developing feline nerve fibres. Neurobiology, 3: 339-352. Berthold, C.-H., and S. Skoglund 1968a Postnatal development of feline paranodal myelin-sheath segments. I. Light microscopy. Acta Soc. Med. Upsal., 73: 113-126. 1968b Postnatal development of feline paranodal myelin-sheath segments. 11. Electron microscopy. Acta SOC.Med. Upsal., 73: 127-144. Boyd, I. A. 1964 The relation between conduction velocity and diameter for the three groups of efferent fibres in nerves to mammalian skeletal muscle. J. Physiol., 175: 33-35P. Clough, J. F. M., D. Kernel1 and C. G. Phillips 1968 Conduction velocity in proximal and distal portions of forelimb axons in the baboon. J. Physiol., 198: 167-178. Conradi, S. 1969 Ultrastructure and distribution of neuronal and glial elements on the motoneuron surface in the lumbosacral spinal cord of the adult cat. Acta Physiol. Scand., Suppl. 332: 5-48. Conradi, S . , and L.-0. Ronnevi 1977 Ultrastructure and synaptology of the initial axon segment of cat spinal motoneurons during early postnatal development. J. Neurocytol., 6: 195-210. Conradi, S.,and S. Skoglund 1969 Observations on the ultrastructure and distribution of neuronal and glial elements on the motoneuron surface in the lumbosacral spinal cord of the cat during postnatal development. Acta Physiol. Scand., Suppl. 333: 5-52. Cullheim, S. 1978 Relations between cell body size, axon diameter and axon conduction velocity of cat sciatic a motoneurons stained with horseradish peroxidase. Neurosci. Lett., 8: 17-20. Cullheim, S., and J.-0. Kellerth 1976 Combined light and electron microscopic tracing of neurons, including axons and synaptic terminals, after intracellular injection of horseradish peroxidase. Neurosci. Lett., 2: 307-313. 1978 A morphological study of the axons and recurrent axon collaterals of cat sciatic a-motoneurons after intracellular staining with horseradish peroxidase. J. Comp. Neur., 178: 537-558. Cullheim, S., and B. Ulfhake 1979 Observations on the morphology of intracellularly stained gamma motoneuTons in relation to their axon conduction velocity. Neurosci. Lett., 13: 47-50. Donaldson, H. H., and G. Nagasaka 1918 On the increase in the diameters of nerve-cell bodies and of the fibers arising

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Relations between cell body size, axon diameter and axon conduction velocity of triceps surae alpha montoneurons during the postnatal development in the cat.

Relations between Cell Body Size, Axon Diameter and Axon Conduction Velocity of Triceps Surae Alpha Motoneurons during the Postnatal Development in th...
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