Search

Brain development is determined by neuronal differentiation including changes of cell polarity and asymetric growth of neuronal processes. Although, there are many unkown factors contributing to changes of lenght of neuronal cones, mounting experimental and review papers focus on changes of growth conus and role of axonal transport. In particular, mechanisms of actin/microtubule polymerisation and depolymerisation are important. Role of intracellular calcium is also significant. Normal and properly timed changes of lenght of axons and dendrites are dependent on interaction of neurons and glia. Moreover, regeneration of injured axons is dependent on growth factors secreted from glial cells. The aim of the present study is characterisation of the most important mechanisms underlying changes of lenght of neurites.

The dendritic tree, this morphologically remarkably structured and typically diversified portion of the neuron reveals gradually its till recently little known complex functional possibilities. The size of the dendritic membrane participates in the latter similarly as their morphological pattern and their electronic structure which has a marked effect also due to its passive cable properties on the spread and interaction of potential changes on the membrane. Moreover the presence of a complex, differentiated mosaic of ion channels of different types provides dendrites with non-linearities with a wide scope of dynamic phenomena and active properties. The dendritic membrane can e.g. actively reinforce signals proceeding along the thin dendrite fibres and may make the development of action potentials or their retrograde movement from the membrane of the cell body possible. The possible operations above the spatial and temporal structures of signals equipped by differently located excitation and inhibition synapses are enriched by the action of ions entering via ion channels into the cell when the membrane potential changes. Manifestations of neuroplasticity lend these properties further temporal and functional dimensions. The neuron is no longer a one-way polarized and passive transmitter of synaptic input signals four output action potentials; it is a complicated non-linear hierarchic dynamic system with possible complex temporal and spatial analogue signal transformations in dendritic structures. New findings are the result of the cooperative use of empirical and theoretical procedures based on the morphophysiological methods with a high differentiating capacity and the methodology of studies of dynamic systems and computer simulation.

Pyramidal neurons of the CA3 hippocampal area were eliminated by means of the kainic acid. Subsequent transplantation of the embryonal hippocampal tissue suspension brought about an increase of the number of neurons. Groups of nerve cells were located at the tips of the implantation tracts. Neurons formed dendritic trees of various complexity. Most of them had an irregular dendritic branching pattern. Only those located within the pyramidal layer, had the typical arrangement of several basal and one apical dendrite.

The authors describe the development of dendrite structure of pyramidal cells of the CA 1 area of the hippocampus in the laboratory rat. The postnatal dendrite growth comprises their eventual branching and increasing number of dendritic segments. The final number of segments is reached first at the terminal branching of the apical dendrite and at the proximal lateral fibres (by the 10th day). In basal dendrites the branching is terminated on the 15th day of postnatal life. On the main stem and preterminal fibres of the apical dendrite the position comparable with the adult brain is reached at the age of 25 days. Repeated hypobaric hypoxia influenced the mentioned dendrite development in a quantitative way. In particular the number of dendritic branches and the density of dendritic spikes of pyramidal cells was lower. Hypoxia influenced in a similar way also interneurones of the hippocampus even when their number increased. The authors assume therefore that exposure to intermittent hypoxia during development leads to reduced input information. Conversely interneurones of the hippocampus are more resistant to oxygen deficiency, but the thus created disbalance obviously is the basis of the unbalanced character of stimulation and inhibition phenomena.