Long-lasting morphological changes in dendritic spines of dentate granular cells following stimulation of the entorhinal area

Stimulation of the perforant path induces a long-lasting increase in the area of dendritic spines, which are sites of termination of the stimulated pathway in the distal third of the dentate molecular layer. No enlarged spines were found in the proximal third of the dentate molecular layer, where the commissural afferents terminate. Following a single tetanic stimulus of 30 sec duration at 30/sec, spines became significantly larger by 15%, 38%, 35% and 23% within poststimulation intervals of 2-6 min, 10-60 min, 4-8 h, and 23 h, respectively. Axon terminals decreased their area by 15% within the 2-6 min interval and the vesicle density was decreased by 19% within the 10-60 min interval. Both changes were reversible and terminals resumed their prestimulation condition at longer intervals (greater than 4 h). The initial enlargement of spines was interpreted as being due to a glutamate-induced increase in the sodium permeability of the spine membrane, whereas for the long-lasting enlargement an increase in protein synthesis was postulated. The long-lasting enlargement of dendritic spines in the dentate molecular layer following a short train of stimuli delivered to the perforant path, supports the postulate which links such a change to the mechanism of long-lasting postactivation potentiation observed in this pathway.

The neuropil of the glomeruli of the olfactory bulb

The neuropil of the glomeruli of the rat olfactory bulb has been studied with the electron microscope with a view to elucidating the type of processes involved - dendrites, appendages and axons - their cellular identity, and the synaptic relationships they establish. The problems encountered in defining these are considered and criteria based on the previous study of neuron types and on examination of serial sections are put forward. The glomeruli are large structures containing many thousands of processes and are the sole site of termination of the olfactory receptor axons. The terminals of the latter are characteristically electron-dense, allowing identification in normal material; they run through the glomeruli making many synapses by means of spherical vesicles and asymmetrical thickenings on to all types of dendritic profile. The glomerular dendritic arborizations of mitral and tufted cells, which are indistinguishable from each other, start as large, fairly regular, pale profiles but become increasingly varicose as they branch and diminish in size. They regularly show groups of spherical vesicles, often in association with asymmetrical synaptic thickenings directed from the dendrite; these are typically associated with return, reciprocal, synapses of the symmetrical type from profiles containing large flattened vesicles. These latter profiles are those of the dendrites and gemmules of periglomerular cells; the dendrites are of irregular outline and give rise to many appendages, mostly gemmules making synaptic contact with mitral or tufted cell dendrites. A small number of pale axon terminals containing either small or large flattened vesicles, derived from short-axon and periglomerular cells respectively, synapse with symmetrical thickenings on to the periglomerular cell dendritic processes. Close associations of particular types of axo-dendritic and dendro-dendritic synapses on interconnecting processes, termed synaptic patterns, are described and their significance considered. The nature of the glomerular interactions is discussed and then placed in the context of other, smaller glomeruli in the central nervous system; certain common principles of glomeruli are suggested.

Transmitter release induced by injection of calcium ions into nerve terminals

Calcium ions injected into the presynaptic nerve terminal in the giant synapse of the squid, evoked transmitter release while similar doses of Mg and Mn were ineffective. The transmitter release induced by intracellular application was still observed when Ca was replaced in the external fluid by Mn, in spite of the fact that this abolished transmitter release in response to presynaptic depolarization.

The site of termination of afferent fibres in the caudate nucleus

An electron microscopic study has been made of the axon terminal degeneration in the caudate nucleus in the cat after lesions in either the cerebral cortex, the thalamus, the cerebral cortex and the thalamus, the midbrain or within the caudate nucleus. Degenerating axon terminals can be recognized after a survival period of 4 days as dark, shrunken profiles with indistinct vesicles. After shorter survival periods the degenerating terminals contain swollen vesicles and have pale cytoplasm. After lesions in all the above sites there is degeneration of fine myelinated and nonmyelinated fibres. The degenerating terminals of all the afferent fibres to the caudate nucleus have asymmetrical membrane thickenings and end mainly on dendritic spines with a small proportion in contact with peripheral dendrites; after damage of the cerebral cortex or thalamus a few of the degenerating terminals also end upon main stem dendrites and cell bodies. The projection from the ipsilateral cerebral cortex is greater than that from the thalamus, which in turn is heavier than that from the contralateral cortex or midbrain. After lesions within the caudate nucleus degenerating terminals with symmetrical membrane thickenings are found in a region extending approximately 450 pm from the damaged part of the nucleus. These terminals make contact with nerve cell somata, main stem and peripheral dendrites and the initial segments of axons. After such a lesion of the caudate nucleus degenerating axon terminals with symmetrical membrane thickenings are also seen in the globus pallidus and the substantia nigra.

The connexions of the striatum and globus pallidus: synthesis and speculation

Recent findings on the afferent and efferent connexions of the striatum (the caudate nucleus and putamen) and globus pallidus have been summarized in an orderly sequence. The striatum receives afferent fibres from three main sources, the cerebral cortex, the intralaminar nuclei of the thalamus and the midbrain; the major features of each of these pathways are outlined. The striatum sends efferent fibres to the globus pallidus and the substantia nigra, and the two segments of the globus pallidus in turn project upon the subthalamic nucleus and upon the thalamus and midbrain tegmentum. Through the thalamus the major influence of the striopallidum is upon the motor area of the cerebral cortex, and it is suggested that through the midbrain tegmentum there may also be a descending influence upon the spinal cord. These findings from light microscopical investigations are synthesized with observations made in electron microscopic studies of the striatum and globus pallidus. On the basis of present knowledge of their structure and connexions attention is drawn to several marked similarities between these parts of the basal ganglia and the cerebellum.

The termination of fibres from the cerebral cortex and thalamus upon dendritic spines in the caudate nucleus: a study with the Golgi method

The termination of fibres from the cerebral cortex and thalamus upon the dendritic spines of the medium spiny cell of the caudate nucleus has been studied with the Golgi method. Lesions were placed in the cerebral cortex, thalamus or cerebral cortex and thalamus of adult cats and kittens. After survival periods of between 6 and 52 weeks the animals were perfused with a mixture of formaldehyde and glutaraldehyde and the caudate nuclei impregnated by a Golgi technique. The distribution of spines along the dendrites of the medium spiny cell was determined in normal material by counting them over 20 /xm lengths of the dendrites, and was compared with their number and distribution after the various lesions. The density of spines on the dendrites varies with the distance from the cell body. The first 20 /xm length of dendrite is spine free, but thereafter the number increases to a peak between 60 and 80 /xm from the cell body after which the number per 20 /xm length decreases. The distribution pattern does not alter after any of the lesions, although the overall number of spines decreases. The decrease after lesions in the cerebral cortex or thalamus is the same, and after a combined lesion of thalamus and cortex is twice as great indicating that the fibres from both these regions end upon spines of the same cells. Statistical analysis shows that these results are significant.