Number Of Synaptic Contacts Research Articles

Synaptic terminals from an identified motoneuron in locust muscle: Comparison between first instar larva and adult

Dimensions and innervation of muscle fibers in locust jumping muscle from 1st instar larva and adult are compared. Fibers innervated by only one motoneuron were investigated. During postembryonic development, the size, but not the number, of muscle fibers increases. The number of synaptic contacts per fiber increases by at least one order of magnitude while the number per μm 2 of surface membrane undergoes little change. Nerve terminals in the larva are on average shorter and separated by longer distances than in the adult. Synaptogenesis seems to be initiated by characteristic projections from the muscle attracting glial and axonal processes.

Regional differences in pigeon optic tract, chiasm, and retino-receptive layers of optic tectum.

Electron-microscopic examination of the pigeon optic chiasm, tract, stratum opticum, and retino-receptive layers of the optic tectum revealed regional differences at each level. Axonal size in the fiber pathways paralleled that previously reported for pigeon optic nerve, with mean diameter values of 0.96 micrometer for optic chiasm and 1.06 micrometer for optic tract. The dorsolateral aspects of these pathways contained a heterogeneous population of fibers (mean diameter congruent to 1.44 micrometer) similar to that found in the nasal portion of optic nerve, while the ventromedial regions were occupied by a more homogeneous population of smaller fibers (mean diameter congruent to 0.82 micrometer) resembling those observed in the temporal portion of the nerve. The retino-receptive layers of anteroventral optic tectum (avT) differed ultrastructurally from those of posterodorsal tectum (pdT) with respect to the thickness of horizontal dendrites in layer 2-3, the size of optic terminals in layers 2-7, and the number of synaptic contacts per terminal. These findings point towards a regional variation in the processing of visual information throughout the retino-tectal system and suggest that neurons in avT vs. pdT should show differences in the way they modify the neurophysiological characteristics of their respective optic inputs.

Quantal nature of neuromuscular transmission in crayfish phasic abdominal flexors

1. The electrophysiology and structure of the crayfish (Procambarus clarkii) deep abdominal flexor neuromuscular system were studied at several stages of development for the purpose of establishing and analyzing the quantal nature of synaptic transmission in these muscles. 2. In adult crayfish we were unable to obtain evidence for quantized neurotransmitter release in this system; spontaneous miniature synaptic potentials were never observed and evoked synaptic potential amplitude fluctuations suggested a mean quantal size of no more than a few microvolts. By recording from very young crayfish, so as to select fibers of higher input resistance, we were able to reveal the existence of a population of miniature potentials with a mean amplitude of less than 100 μV. Their amplitude distributions were always positively skewed, with the mode buried in the background noise. The time intervals between successive events were approximately exponentially distributed. 3. The extremely small quantal size was accounted for in terms of the input resistance (RN) of the muscle fibers. The exceptionally lowRN resulted from a low specific membrane resistivity combined with an unusual organization of the muscle fibers into a system of electrically coupled subunits. Thus the total membrane area of each fiber is disproportionately large for its diameter. The developmental origin and functional significance of this form of neuromuscular organization are discussed. 4. The mean quantal content per muscle fiber was estimated to be several thousand in adult animals, an order of magnitude higher than has been reported for other crayfish muscles. The large quantal content appears to result from a greater than usual total number of synaptic contacts. 5. Synapses formed by the motor giant (MoG) and non-giant (NG) motor neurons displayed the same use-dependent labilities in young crayfish as previously reported in adults. With repetitive stimulation at low to moderate rates the NG synapses facilitated mildly due to an increase in mean quantal content, while the MoG synapses depressed even at rates of once per minute or less.

The relationship between some measures of synaptic ultrastructure as a function of distance from the soma on lamprey reticulospinal neurons.

Synaptic junctions located on the dendrites of lamprey (Petromyzon marinus) reticulospinal neurons labelled with intracellularly-injected horseradish peroxidase were studied. The normal ultrastructure of the synaptic junctions was defined and several quantitative measures made from each junction in order to test the hypothesis that distally-located synapses are ultrastructurally different from those located at proximal dendritic sites. A total of 820 contacts from one neuron and 279 from a second neuron ranging from 20 to 340 microns from the soma were quantified. The vast majority of the presynaptic endings contained round, clear-cored vesicles and formed an asymmetrical membrane differentiation with the postsynaptic dendrite. A small fraction of the population contained flattened or pleomorphic vesicles and these synapses were equally distributed with respect to distance from the soma. Many of the terminals contained a few large dark- and clear-cored vesicles. Four quantitative measures of each synaptic contact were made. These included vesicle number, length of differentiated membrane, vesicle area and terminal area. Four ratios relating the different quantitative measures were also calculated. Each ratio or measurement from the synaptic junctions was plotted as a function of distance from the soma to determine if differences existed at any distance. It was found that synaptic junctions are uniformly similar and that distal junctions did not differ significantly (P greater than 0.05) from those at proximal dendritic sites. It is concluded that if distal synapses do compensate for their remote location they do this is some other way, possibly by increasing the number of synaptic contacts made by each presynaptic axon.

Postnatal development of the suprachiasmatic hypothalamic nucleus of the rat

Light and electron microscopy of newborn, four day, one, two, three and five week old rats revealed principally a progressive increase in the diversity and number of synaptic contacts in the suprachiasmatic nucleus (SCN). The major increase in synaptic diversity occurred between four days and one week of age. Correlation between this finding and the adult synaptic morphology of SCN (Gülder, 1976) on the one hand, and the ontogeny of circadian rhythms on the other were made. This suggested that the retinal afferents arriving on day four form asymmetrical contacts with dendrites. While increase in synaptic number was progressive, it was most marked between three and five weeks of age. By five weeks, most features of the adult SCN were present. No significant morphological effects were evident as a result of neonatal retinal lesions.

An electron microscopic study of the developing caudate nucleus in euthyroid and hypothyroid states.

Thyroid hormone exerts a powerful influence on CNS growth and maturation. Hypothyroidism early in life has long been known to cause disturbances in innate behavior, motor performance, severe and frequently irreversible mental retardation. In this deficiency, depressed caudate neurogenesis, cell migration and neuropil development during the rapid period of CNS growth may contribute to the clinical picture of perceptual handicaps often seen in cretins. Light microscopic and Golgi studies of the developing caudate nucleus in thyroid deficiency have been carried out to help attain insights into the mechanisms whereby the extrapyramidal system regulates motor function. The ultrastructural study of caudate nuclear cytogenesis and synaptogenesis in normal and hypothyroid states provides more detailed information for further analysis of the problem. Hypothyroidism was induced from birth by adding prophylthiouracil to the food and drinking water of lactating dams. Linear development of the caudate nucleus of both normal and hypothyroid rats at ages 8, 14, 20, 30 and 42 days was studied by electron microscopy. Thyroid glands were examined by light microscopy to assess the normal and deficient states. Immature cells, primitive processes and synapses were the characteristic features of the 8-day-old normal caudate nucleus. Distinctively wide cisternae of the rough endoplasmic reticulum, loosley packed Golgi apparatus and chromatin clumps throughout the nuclei of the neurons were significant early morphologic variations. The dramatic cytoarchitectural maturation in the 14- and 20-day normal caudate neuropil points to the rapidity of developmental rate. After the growth spurt of the first three weeks a maturational plateau occurs which is characterized by well-formed neuronal cytoplasmic organelles, myelinated and non-myelinated axons, axon terminals, dendrites and their spines, and synapses. Thyroid deficiency causes a marked maturational delay of approximately 7 days in caudate neuronal proliferation, the elaboration of neuronal networks and the attainment of mature synaptic contents and membranes. This delay is evidenced by comparison of the structural similarities between 8-day-old normal and 14-day-old deficient rats; and additional comparisons between the 14-day-old normal and 20-day-old hypothyroid rats. A rapid "catch up" process in fine structural morphogenesis takes place in the period between days 14 and 30 in the deficient animals. Repression of thyroid function does not entirely prevent development of the caudate nucleus but allows a fairly extensive, though critically incomplete degree of maturation. This imperfection is manifested by a decrease in the number of synaptic contacts that persists even after the rapid "catch up" phenomenon of caudate synaptogenesis.