Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. I. Effects of microelectrode impalement and intracellular staining with the fluorescent dye "Procion Yellow"

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

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.

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.

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

Twelve retrogradely horseradish peroxidase (HRP)-labelled triceps surae motoneurons of gamma size (mean cell body diameter less than 38 micron) were studied ultrastructurally. The contours of the cell bodies, as observed in the transverse midnucleolus plane, were elongated to rounded. The axons identified all originated from the cell body. The mean diameter of the stem dendrites was 4.5 micron. A substantial part of the cell membrane was covered by glial extensions. The boutons and synaptic contacts apposing the gamma-motoneurons could be classified into two categories on the basis of the type of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles. In each neuron, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S+F-type boutons were estimated on the cell body and in two dendritic compartments. In comparison with alpha-motoneurons and Renshaw cells, the cell bodies of the gamma-motoneurons were covered by smaller and strikingly fewer boutons of both the S- and F-types. The values for percentage synaptic covering and packing density of boutons on the proximal dendrites were also lower for gamma-motoneurons than for both alpha-motoneurons and Renshaw cells, although the differences were less pronounced than on the cell body. No boutons of the C-, M-, and T-types described for alpha-motoneurons were found on the gamma-motoneurons.

The ultrastructure of group Ia afferent fiber synapses in the lumbosacral spinal cord of the cat

Ia synapses in laminae VI and IX of the cat's spinal cord were examined in the electron microscope following iontophoretic injection of horseradish peroxidase (HRP) into single, identified, Ia afferent fibers from gastrocnemius muscles. Ia boutons contacting motoneuron dendrites in lamina IX contained spherical synaptic vesicles and generally contacted only one postsynaptic profile. The Ia boutons were often postsynaptic to smaller P-type axonal terminals. Consequently Ia boutons may be classified as S-boutons with axo-axonic contacts.

Electron microscopic observations on the synaptic contacts of group Ia muscle spindle afferents in the cat lumbosacral spinal cord

After intra-axonal injection of horseradish peroxidase (HRP) into afferent fibers originating from muscle spindle primary endings of the cat gastrocnemius, group Ia boutons located in the ventral horn of the spinal cord were identified and studied electron microscopically. The Ia boutons were invariably found to contain spherical synaptic vesicles (S-type boutons), and a number of them were also postsynaptic to smaller P-type boutons (large S-type boutons with axo-axonic contacts). None of the present Ia-boutons belonged to the previously described M-type. The vast majority of the studied boutons were considered to be located at less than 500 microns distance from the alpha-motoneuron soma. The results are discussed in relation to previous light and electron microscopic data.

An ultrastructural study of serially sectioned Renshaw cells. III. Quantitative distribution of synaptic boutons

The quantitative distribution of synaptic boutons on 17 presumed Renshaw cells has been studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons and were located in lamina VII, ventromedially to the main motor nuclei. In each of the presumed Renshaw cells, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S + F-type boutons were estimated on the cell body and in two dendritic compartments. The F/S percentage synaptic covering ratio was also calculated. The previously demonstrated differences within the present group of neurons, with respect to the site of axonal origin, were not accompanied by any corresponding differences in the quantitative distribution of synaptic boutons. However, it is suggested that the presumed Renshaw cells may possibly fall into two categories with respect to the F/S percentage synaptic covering ratio. The results are discussed in relation to previous studies on the neuronal architecture and synaptic types on the same presumed Renshaw cells, as well as in relation to earlier observations on the quantitative distribution of boutons on central neurons, particularly spinal alpha-motoneurons.

A horseradish peroxidase labeling technique for correlation of motoneuron metabolic activity with muscle fiber types

Modifications in the tetramethylbenzidine technique by Mesulam et al. (1980) for demonstration of retrogradely transported horseradish peroxidase (HRP) in alpha-motoneurons have been made. The methodology was designed to permit use of quantitative enzyme histochemical techniques on sections serial to those used for demonstration of HRP. Injections of HRP into different muscles on contralateral limbs, each of which are composed almost exclusively of one muscle fiber type, have allowed us to determine relative enzyme activities in the motoneurons innervating specific muscle fiber types. The modifications should be useful to others using HRP as a peripheral or central nervous system tracer since significant time and chemical costs are saved, the slide-mounted sections are easier to handle, and the sensitivity is superior to currently available methods.

Perisomatic changes in the maturing hypoglossal nucleus after axon injury

The perisomatic retrograde reactions to peripheral nerve injury during postnatal maturation were investigated by quantitative light and electron microscopy. The hypoglossal nerve was crushed, ligated or transected in 10 and 21 day postnatal (dpn) rats. Only crush injury was made in 7 dpn rats. Survival periods ranged from 3 to 40 days postoperative (dpo). Normal and sham operated animals of corresponding ages served as controls. A remarkable, transient response of neuronal somata in the young (7-10 dpn) rats, following all three types of axon injury, was the formation of excrescences which engulfed neuronal processes: boutons, dendrites, small neurites and a few myelinated axons in adjacent neuropil. The somal engulfment was rarely evident after nerve injury to 21 dpn rats. Bouton displacement from the somal surface, accompanied by glial incursion, followed each type of nerve injury but was less extensive and occurred later in the young rats. There seemed to be no association in the amount of boutons displaced from neuronal somata with the type of nerve injury for any of the three experimental age groups. However, the rate and intensity of the perineuronal glial reaction were related to the severity of the nerve injury in the older (21 dpn) but not in the younger (7-10 dpn) rats. Substantial loss of neurons occurred in the affected nucleus after each type of trauma to the young neurons. The degree of neuronal loss was related to the age and the volume of axoplasm disrupted. Only nerve transection in 21 dpn rats resulted in appreciable neuronal loss. The responses of axotomized neurons, their afferent axon terminals and the surrounding glia are quantitatively and qualitatively different in the hypoglossal nucleus of rats of increasing postnatal ages.

A technique combining intracellular dye-marking, immunocytochemical identification and ultrastructural analysis of physiologically identified single neurons

A new method which produces an insoluble osmophilic polymer within Lucifer Yellow-injected neurons has allowed us to develop a technique for the ultrastructural examination of electrophysiologically characterized, immunocytochemically identified single neurons. In this initial report, we examine the light- and electron-microscopic features of neurophysin-containing, pituitary-projecting neurons in the goldfish nucleus.

An ultrastructural study of serially sectioned Renshaw cells. I. Architecture of the cell body, axon hillock, initial axon segment and proximal dendrites

Seventeen neurons which were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons were studied ultrastructurally. All 17 neurons were situated in lamina VII, ventro-medially to the main motor nuclei. This and other facts support the assumption that the observed neurons are morphological correlates to the physiologically defined Renshaw cells. The contours of the cell bodies, as observed in the midnucleolus plane, were elongated. The axons originated either from the cell bodies or from dendrites. The number of dendrites of each neuron varied between 3 and 7. The appearance of the presumed Renshaw cells was also compared with that of a larger sample of neurons from the ventral part of lamina VII which was studied light microscopically in semithin sections. It was suggested that the Renshaw cells belong to the larger and more elongated neurons in the area.

Intracellularly injected cobaltous ions accumulate at synaptic densities

Physiologically identified neurons in the locust were iontophoretically injected with a mixture of cobaltous and potassium ions. After being fixed for electron microscopy, 2.5-micrometer sections of the epoxy-embedded ganglia were intensified with silver. The intensified material was resectioned and examined in the electron microscope. The cobalt-silver precipitate appeared as discrete densities. Localized accumulations of the precipitate were seen within the injected cell along the neuronal membranes and especially at synapses. Location and recognition of the stained neuron in the electron microscope was facilitated by the tendency of the cobaltous ions to aggregate at the synaptic sites.

Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. II. A method for the description of architecture and synaptology of the cell body and proximal dendritic segments

The paper presents a method for ultrastructural analysis and description of neuronal architecture and synaptology of cat spinal alpha-motoneurons from complete series of consecutive ultrathin sections through the cell body and proximal parts of the dendrites. The method implies that sections are selected for analysis only at certain constant intervals in the series. The occurrence of boutons of different morphological types on the neuronal surface was expressed by their percentage covering of the neuronal membrane. The neuronal surface was divided into a number of compartments and the synaptic covering was calculated separately for each compartment. An interval of 6 micrometer between the sections was used for these calculations, and the obtained values for synaptic covering were found not to differ significantly from those obtained in controls at 3 micrometer intervals. The number and location of individual large boutons (C- and M-types) were studied at 3 micrometer section intervals, and the escape of boutons connected to this procedure was estimated from control observations at 1 micrometer intervals. It is concluded that detailed information on neuronal synaptology can be obtained with this method, which will be used in three subsequent studies on functionally identified and intracellularly stained cat alpha-motoneurons.

Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. IV. Motoneurons innervating slow-twitch (type S) units of the soleus muscle

Two intracellularly stained cat alpha-motoneurons of the soleus-S type (Burke et al., '74) were studied ultrastructurally. The architecture and synaptology of the cell body and proximal parts of the dendrites were analyzed from a long series of consecutive sections (Conradi et al., '79). Only few of the dendrites had a base diameter exceeding 10 micrometer. The proportion of the membrane surface of the cell body and dendrites covered by boutons was 40-45% and 50-80%, respectively. Out of this, 15-20% was constituted by S-boutons and 70-75% by F-boutons in both regions. In contrast to the situation in the gastrocnemius FR-neurons (Kellerth et al., '79) the large boutons of the C-type showed no clustering around the dendritic roots and were absent on the axon hillock. In addition, the M-boutons of dorsal root origin were more sparse on the proximal parts on the dendrites of the soleus-S neurons.

Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. III. Motoneurons innervating fast-twitch (type FR) units of the gastrocnemius muscle

Two intracellularly stained cat gastrocnemius alpha-motoneurons of the FR-type (Burke et al., '73) were studied ultrastructurally. The architecture and synaptology of the cell body and proximal parts of the dendrites were analyzed from a long series of consecutive sections, according to a method presented in a preceding paper (Conradi et al., '79a). Several of the dendrites had a base diameter exceeding 10 micrometer. The proportion of the surface covered by boutons was 40-50% for the cell body and about 80% for the proximal dendrites. In both regions, about 20% of the boutons were of the S-type and 70% of the F-type. The large C-boutons were clustered around the dendritic roots and were also present on the axon hillock. M-boutons of dorsal root origin were located on the proximal parts of the majority of the dendrites.