The combined use of immunohistochemistry and intracellular staining with horseradish peroxidase for light and electron microscopic studies of transmitter-identified inputs to functionally characterized neurons

Physiologically identified triceps surae alpha motoneurons in the cat were stained intracellularly with horseradish peroxidase (HRP). After fixation with 2% glutaraldehyde and treatment with sodium borohydride, spinal cord sections were incubated with rabbit antiserum against thyrotropin-releasing hormone (TRH) and rabbit peroxidase-antiperoxidase complex. Light microscopically detected close contacts between immunoreactive nerve terminals and intracellularly HRP-stained profiles were studied under the electron microscope. In this way, synaptic contacts between TRH-immunoreactive boutons and functionally characterized alpha motoneurons could be demonstrated.

Thyrotropin-releasing hormone (TRH)-immunoreactive boutons and nerve cell bodies in the dorsal horn of the cat L7 spinal cord

With the use of the peroxidase-anti-peroxidase (PAP) technique, thyrotropin-releasing hormone (TRH)-like immunoreactivity was found in axon terminals and cell bodies in the dorsal horn of the cat spinal cord L7 segment. In particular, a conspicuous band-shaped region of axon terminals was observed in laminae II and III, while the cell bodies were most frequent in lamina III. Electron microscopic analysis showed that the TRH-immunoreactive terminals made synaptic contacts with cell bodies and dendrites of various sizes.

Ultrastructural observations on beaded alpha-motoneuron dendrites

Beaded dendrites of alpha-motoneurons intracellularly labelled with horseradish peroxidase (HRP) were studied ultrastructurally in eight adult cats. For comparison, adjacent unlabelled beaded dendrites of unknown origin were also included in the study. Electron microscopy revealed no signs of degeneration or poor fixation according to common criteria. With the exception of the HRP-reaction product no difference in structure was observed between labelled and unlabelled beaded dendrites. Both the beads and their interconnecting segments were postsynaptic to boutons of normal appearance containing spherical (S-type boutons) or flattened vesicles (F-type boutons). The values for synaptic covering and synaptic packing density of the beaded dendritic regions, which usually were located in the periphery of the dendritic trees, were clearly lower than values obtained previously for cell bodies and proximal dendrites of alpha-motoneurons.

The effects of tenotomy and overload on the postnatal development of medial gastrocnemius motor units in the cat

Five-to 7-day-old kittens were subjected to tenotomy of either the medial gastrocnemius muscle (MG) or its synergists within the Achilles tendon. The effects of these operations on the postnatal differentiation of MG motor units were investigated when the cats had reached the adult stage. The MG tenotomy produced a substantial weight loss, while tenotomy of synergists induced only a minor weight gain of the MG muscle. Tenotomy of the MG synergists induced a marked prolongation of motoneuronal AHP durations in the overloaded MG. This prolongation affected equally motoneurones of the S and F types. The twitches of the tenotomized motor unit group showed a relatively slower relaxation than those of the overloaded group. The muscle unit properties of the tenotomized MG muscles showed a less distinct differentiation than those of the overloaded muscles. The basic features of the various motor unit types were, however, normal in both groups, and there was no evidence of a major shift in the proportions of different motor unit types. It is concluded that the postnatal differentiation of all types of MG motor units is largely unaffected by the abnormal situations introduced in the present study.

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.

The effects of tenotomy and compensatory hypertrophy on the postnatal development of soleus motor units in the cat

Kittens, 5-7 days old, were subjected to tenotomy of either the soleus muscle or all its Achilles tendon synergists. When the cats had reached the adult stage, the physiological properties of the soleus motor units were investigated in both the atrophic and hypertrophic situations. Tenotomy resulted in a marked muscle weight loss and overload due to tenotomy of synergists in a marked gain in muscle weight compared to the contralateral side. The motor units of the tenotomized soleus muscles exhibited a moderate shortening of the twitch contraction time, and also a change in twitch shape, related to the degree of atrophy. In the motor units of the hypertrophic soleus muscles, an increased fatiguability could be demonstrated. Further, in this group, there was a tendency towards differentiation of the normally uniform motor units into two groups with different features. The functional implications of this process are discussed. The development of motor units in both the tenotomized and the hypertrophic muscles were only marginally influenced by the operations, and it is concluded that the basic features of their development are largely unaffected by the functional manipulations imposed in the present study.

Electron microscopic observations on recurrent axon collateral boutons of a triceps surae gamma-motoneuron in the cat

Boutons from the recurrent axon collaterals of an adult cat gastrocnemius gamma-motoneuron were studied after intracellular labelling with horseradish peroxidase (HRP). Light and electron microscopic observations revealed that the studied gamma-motoneuron possessed 6 dendrites with 39 dendritic end branches totally and that its axon gave off two axon collaterals with together 8 synaptic boutons of either en passant or terminal type. Both collateral trees were confined to the ventral part of lamina VII. Four of the synaptic boutons were studied electron microscopically. They were all found to be S-type boutons containing spherical vesicles. In two cases the bouton could be further subclassified as a type T bouton. All of the studied boutons made synaptic contact with thin dendrites of unknown origin.

Dimensions and branching patterns of triceps surae alpha-motor axons and their recurrent axon collaterals in the spinal cord during the postnatal development of the cat

Triceps surae alpha motoneurons in the cat were stained intracellularly with horseradish peroxidase (HRP) at different postnatal ages from birth to the adult stage. The motor axons and axon collaterals were studied with regard to length, diameter and branching pattern. The postnatal increase of internodal length, measured as the distance between two subsequent axon collateral origins, was about 100% which paralleled the total length increase of the main axon in the grey matter. The axon collaterals were unmyelinated at birth and branched exclusively dichotomously until after 3 weeks of age when a substantial fraction of the branching points gave off 3-5 daughter branches. This was interpreted as signs of a fusion between neighboring branching points during the period of myelination of the axon collaterals. The length analysis of the collaterals indicated that the postnatal elimination of collateral branches described previously is preferentially located in the distal parts of the collateral tree.

A morphometric study of the soma, first-order dendrites and proximal axon of cat lumbar alpha-motoneurones intracellularly labelled with HRP

Intracellularly HRP-labelled cat hindlimb alpha-motoneurones were reconstructed light microscopically from a series of 1 micron or 2 micron thick consecutive sections. The volume and surface area of the soma as well as the size of the very proximal part of the dendritic and axonal processes were estimated morphometrically. Similar measurements were also made on adjacent unlabelled neurons in the same series of sections. A close relation was found between the soma volume and surface area on one hand the combined cross-sectional area of the proximal dendrites and axon on the other. The combined axonal and dendritic bases occupied on the average 16% of the soma surface. The accuracy in using the diameters and cross-sectional area of the cell body as indirect estimates of soma volume and surface area was analyzed. Combined measurements in both the transversal and sagittal planes were then found to yield more satisfactory estimates then when the measurements were confined only to the transversal plane. Several different formulas using the soma axes for indirect calculations of the soma volume and surface area were compared with respect to the accuracy of the results.

Electrophysiological and morphological measurements in cat gastrocnemius and soleus alpha-motoneurones

Intracellular recording and staining with HRP was used to study the electrical properties and anatomical size of medial gastrocnemius (MG) and soleus (SOL) alpha-motoneurones in curarized cats. The MG motoneurones were divided into two groups on the basis of their input resistance (RN), namely low-resistance MG-LR cells (RN less than 1.0 M omega) and high-resistance MG-HR cells (RN greater than 1.0 M omega). Analysis of the voltage transients following applied current pulses indicated that the SOL neurones had longer membrane time constants (tau o) than the MG-LR cells, while the MG-HR group exhibited intermediate values. Using Rall's equivalent cylinder model, a difference in specific membrane resistivity (Rm) between the MG-LR (low Rm) and SOL (high Rm) cells was obtained. This difference was observed also in neurones of similar anatomical size, and was consistent with the observed difference in tau o. In two neurones Rm was in addition calculated directly from anatomy and input resistance according to the general solution for a continuous neurone model with arbitrary geometry given by Rall. The latter method was found to yield significantly lower values for Rm, although the observed difference between the neurone types remained similar. Also the values for electrotonic length (L) were found to differ considerably between the calculations based on voltage transient analysis and those obtained from combined physiological and anatomical measurements. The observed variations in results are discussed in relation to possible sources of error in the experimental techniques and/or in the theoretical assumptions, particularly that of Rm being uniform over the entire soma-dendritic membrane. It is suggested that Rm might be larger in the dendritic regions than in the soma. A crude approximation of the dendrite to soma conductance ratio (Q) indicated that most cells (80%) had Q greater than 5.

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.

Does alpha-motoneurone size correlate with motor unit type in cat triceps surae?

The cell bodies and first-order dendrites of alpha-motoneurones supplying different functional types of muscle units in the cat gastrocnemius (type FF, FR and S units) and soleus (type SOL-S units) muscles, were studied after intracellular injection of horseradish peroxidase. The SOL-S neurones had smaller values for cell body diameter in comparison with both the FF and FR neurones. The SOL-S neurones also had significantly thinner first-order dendrites than the FF, FR and S neurones. In the gastrocnemius pool the S neurones had smaller values for dendritic diameters than the FF and FR cells. The values for combined diameter of the first-order dendrites indicated that the dendritic trees of the FF and FR neurones are, on the average, larger than those of the S and SOL-S neurones. Furthermore, the relationship between the combined dendritic diameter and the mean soma diameter, indicated that a difference in relative scaling of soma and dendrites exists between the FF and FR neurones on the one hand and the S and SOL-S neurones on the other. Similar results were obtained also when relating the combined dendritic parameter sigma d3/2 to the soma surface area. Although a certain statistical relation seems to exist between motoneurone size and motoneurone type, it should be emphasized, however, that the range of values for each parameter studied overlapped considerably between the different types of motoneurones.

Evidence for a postnatal elimination of terminal arborizations and synaptic boutons of recurrent motor axon collaterals in the cat

Triceps surae alpha motoneurons in cats of different postnatal ages were stained intracellularly with horseradish peroxidase (HRP). The recurrent axon collateral trees of the neurons were studied light microscopically. A large reduction of the number of axon collateral end branches and swellings, interpreted as synaptic boutons, was found to occur during the first two weeks of postnatal life.

A quantitative light microscopic study of the dendrites of cat spinal alpha-motoneurons after intracellular staining with horseradish peroxidase

The cell bodies and dendrites of cat spinal alpha-motoneurons were studied after intracellular staining with horseradish peroxidase. The mean diameter of the soma was positively correlated to both the mean diameter and the combined diameter of the first-order dendrites, but not to the number of first-order dendrites. On the average, 11.2 dendrites originated from the soma. The dendritic trees were more extensive than has been described previously. The mean value for the combined length of a whole dendrite was 4.7 mm, while the mean values for the total surface area and volume of a dendrite were 33.0 x 10(3) micron (2) and 27.2 x 10(3) micron (3), respectively. The diameter of the first-order dendrite was positively correlated to the combined length of the entire dendrite, the number of dendritic branching points, and the number of dendritic end branches. The diameter of the first-order dendrite was also directly proportional to the volume and the surface area of the entire dendrite. About 75% of the dendritic surface area and 55% of the dendritic volume was located more than 300 micron away from the soma. The dendrites constituted about 97% of the surface area and about 75% of the volume of the entire motoneuron (excluding the axon). The dendritic tapering was moderate. On the average, the distal decrease in dendritic diameters caused a reduction in the combined dendritic parameter (sigma d 3/2) by 1.5% and 15% at 500 micron and 800 micron distance, respectively, from the soma.

A quantitative light microscopic study of the dendrites of cat spinal gamma -motoneurons after intracellular staining with horseradish peroxidase

By use of intracellular staining with horseradish peroxidase (HRP), the dendritic systems of spinal gamma-motoneurons of the adult cat were studied with a light microscope. The dendrites extended in various directions up to 1.5 mm from the cell body. The dendritic branching was sparse and even unbranched dendrites were occasionally seen. The number and combined diameter of the first-order dendrites increased in parallel with the mean cell body diameter. The number of dendritic end branches, the combined dendritic length, the membrane surface area, and the volume of the entire dendrite correlated positively with the diameter of the parent first-order dendrite. In comparison with the alpha -motoneurons (Ulfhake and Kellerth, '81) the gamma -motoneurons had smaller values for mean cell body diameter and mean diameter of the first-order dendrites and they also had a smaller number of first-order dendrites. The dendrites of the gamma-motoneurons were also found to have fewer branching points and larger values for combined dendritic length. The relation between the diameter of the first-order dendrite and the surface area of the entire dendrite was almost identical for the two types of motoneurons.

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

Triceps surae alpha-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 micrometer at birth to 57.6 micrometer in the adult cat, while the corresponding figures for the intramedullary axon diameters were 2.4 micrometer and 6.7 micrometer. 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 CV/d ratio during the postnatal development may be explained by previously described immature morphological properties of the axons in very kittens, and by a changing relation between the dimensions of the intramedullary and peripheral parts of the axon.

Observations on the morphology of intracellularly stained gamma-motoneurons in relation to their axon conduction velocity

Hindlimb gamma-motoneurons of adult cats were stained intracellularly with horseradish peroxidase. The gamma-motor-axons had intramedullary diameters between 2.0 micron and 2.4 micron and lacked recurrent collaterals. The conduction velocity of the gamma-motor-axons (20-29 m/sec) was close to what could be predicted from the relationship between conduction velocity and intramedullary diameter of much thicker adult alpha-motor-axons. However, the gamma-motor-axons were conducting much faster than alpha-motor-axons of 1-week-old kittens in spite of the fact that these two types of axons had about the same dimensions intramedullarly.