Synapses on the Axon Hillocks and Initial Segments of Pyramidal Cell Axons in the Cerebral Cortex

1969 ◽  
Vol 5 (2) ◽  
pp. 495-507
Author(s):  
E. G. JONES ◽  
T. P. S. POWELL
Keyword(s):  
Pyramidal Cell ◽  
Synaptic Vesicles ◽  
Initial Segment ◽  
Sensory Cortex ◽  
Axon Terminals ◽  
Dense Bodies ◽  
Inner Surface ◽  
Spine Apparatus ◽  
Membrane Bound ◽  
Somatic Sensory Cortex

The axon hillocks and initial segments of pyramidal cell axons can be clearly recognized in electron micrographs of the somatic sensory cortex. The initial segment is characterized by three features: bundles of neurotubules linked together by electron-dense bands, a layer of dense material attached to the inner surface of the plasma membrane, and small membrane-bound dense bodies. All of these elements and the few ribosomes usually present disappear at the commencement of the myelin sheath. The initial segment of the axon often contains a cluster of cisternae similar to the spine apparatus, and this part of the axon sometimes gives off small branches. Axon terminals end on both the axon hillock and the initial segment, and there is an increase in number on the latter as the distance from the hillock increases. All of these terminals are relatively large, contain a high proportion of small flattened or pleomorphic synaptic vesicles and terminate in symmetrical synaptic contacts. These morphological features suggest that the synapses may be inhibitory in function.

An electron microscopic study of terminal degeneration in the neocortex of the cat

1970 ◽  
Vol 257 (812) ◽  
pp. 29-43 ◽  
Keyword(s):  
Synaptic Vesicles ◽  
Electron Microscopic ◽  
Axon Terminals ◽  
Dense Bodies ◽  
Sensory Area ◽  
Experimental Degeneration ◽  
Synaptic Region ◽  
Terminal Degeneration ◽  
Somatic Sensory Cortex ◽  
Degenerating Axons

The nature and immediate postoperative course of experimental degeneration of axon terminals have been studied in the somatic sensory cortex. The first somatic sensory area was examined at intervals of 2 to 6 days following lesions in the thalamus, opposite cortex or ipsilateral second somatic sensory area. There is a characteristic sequence of degenerative changes which affects the terminals of each of the afferent fibre systems studied. This commences as a simple, though marked, increase in electron density of the axoplasm with no loss of synaptic vesicles and little alteration in the size or shape of the terminal. Following this, there is a progressive loss of vesicles and disruption of the mitochondria with shrinkage of the terminal and its compression, invasion and fragmentation by astroglial processes. There is evidence that many fragments are phagocytosed by the invading astroglia but a thin sliver always remains attached at the synaptic contact zone. Within the range of survival periods used, no changes affect the synaptic region nor the postsynaptic profile and if the latter is a dendritic spine, it is not detached from the parent dendrite. Changes in degenerating axons are similar, except that the largest thalamo-cortical fibres show a stage of neurofilamentous hyperplasia. In the cortex at a distance from the lesion only smaller astrocytic processes are involved in breaking down the degenerating products; close to a lesion, however, all astrocytic processes and perikarya become involved and many atypical glial cells which are difficult to classify as astrocytes or oligodendrocytes become visible; the vascular pericytes also display large heterogeneous dense bodies and other inclusions.

scholarly journals THE SMALL PYRAMIDAL NEURON OF THE RAT CEREBRAL CORTEX

1968 ◽  
Vol 39 (3) ◽  
pp. 604-619 ◽  
Author(s):  
Alan Peters ◽  
Charmian C. Proskauer ◽  
Ita R. Kaiserman-Abramof
Keyword(s):  
Cerebral Cortex ◽  
Dendritic Spines ◽  
Pyramidal Neuron ◽  
Synaptic Vesicles ◽  
Initial Segment ◽  
Axon Terminals ◽  
Axon Hillock ◽  
Synaptic Junctions ◽  
Cytological Features ◽  
Synaptic Complexes

The axon of the pyramidal neuron in the cerebral cortex arises either directly from the perikaryon or as a branch from a basal dendrite. When it arises from the perikaryon, an axon hillock is present. The hillock is a region in which there is a transition between the cytological features of the perikaryon and those of the initial segment of the axon. Thus, in the hillock there is a diminution in the number of ribosomes and a beginning of the fasciculation of microtubules that characterize the initial segment. Not all of the microtubules entering the hillock from the perikaryon continue into the initial segment. Distally, the axon hillock ends where the dense undercoating of the plasma membrane of the initial segment commences. Dense material also appears in the extracellular space surrounding the initial segment. The initial segment of the pyramidal cell axon contains a cisternal organelle consisting of stacks of flattened cisternae alternating with plates of dense granular material. These cisternal organelles resemble the spine apparatuses that occur in the dendritic spines of this same neuron. Axo-axonal synapses are formed between the initial segment and surrounding axon terminals. The axon terminals contain clear synaptic vesicles and, at the synaptic junctions, both synaptic complexes and puncta adhaerentia are present.

Electron microscopy of the somatic sensory cortex of the cat III. The fine structure layers III to VI

1970 ◽  
Vol 257 (812) ◽  
pp. 23-28 ◽  
Keyword(s):  
Fine Structure ◽  
Dendritic Spines ◽  
Pyramidal Neurons ◽  
Pyramidal Cells ◽  
Progressive Increase ◽  
Sensory Cortex ◽  
Axon Terminals ◽  
Clear Cut ◽  
Unmyelinated Axons ◽  
Somatic Sensory Cortex

Variations in the fine structure of layers III to VI of the somatic sensory cortex have been described. Layers III and IV may be readily distinguished from one another and from layers V and VI, but within the latter two layers there is such a slow gradient of change that no clear-cut line of junction can be drawn between them. Layer III is characterized by the presence of many large apical dendrites ascending vertically through it from pyramidal cells in all layers to reach layer I. In parallel with these are many small unmyelinated axons which contain flattened synaptic vesicles and terminate on transversely orientated dendrites in symmetrical synaptic complexes. The remainder of the neuropil is filled by large numbers of dendritic spines receiving axon terminals which contain spherical vesicles and which terminate asymmetrically. In layer IV there is a marked increase in the number of small myelinated axons ascending from below and ramifying within it. Embedded in the neuropil among these are many small non-pyramidal neurons whose somata and small, irregular dendrites are covered in axon terminals. Also present, and particularly concentrated at the junction with layer III, is a meshwork of fine unmyelinated axons which contain flattened vesicles and terminate in an en passant manner as symmetrical type synapses. Most of these axons are orientated transversely. A larger axon terminal which ends in asymmetrical complexes on small dendritic shafts and spines and which may be the terminal of thalamo-cortical axons is only found in any quantity in this layer. On descending into layers V and VI there is a progressive increase in the number of large myelinated fibres and glial cells, and a progressive diminution of neuronal elements, particularly dendritic spines. Some large non-pyramidal cells resembling the smaller ones on layer IV are present in layer VI.

scholarly journals THREE-DIMENSIONAL ULTRASTRUCTURE OF THE CRAYFISH NEUROMUSCULAR APPARATUS

1974 ◽  
Vol 63 (2) ◽  
pp. 599-613 ◽  
Author(s):  
S. S. Jahromi ◽  
H. L. Atwood
Keyword(s):  
Surface Area ◽  
Synaptic Vesicles ◽  
Physiological Activity ◽  
Three Dimensional ◽  
Total Surface Area ◽  
Inhibitory Synapses ◽  
Axon Terminals ◽  
Dense Bodies ◽  
Branching Patterns ◽  
Neuromuscular Apparatus

The synapse-bearing nerve terminals of the opener muscle of the crayfish Procambarus were reconstructed using electron micrographs of regions which had been serially sectioned. The branching patterns of the terminals of excitatory and inhibitory axons and the locations and sizes of neuromuscular and axo-axonal synapses were studied. Excitatory and inhibitory synapses could be distinguished not only on the basis of differences in synaptic vesicles, but also by a difference in density of pre- and postsynaptic membranes. Synapses of both axons usually had one or more sharply localized presynaptic "dense bodies" around which synaptic vesicles appeared to cluster. Some synapses did not have the dense bodies. These structures may be involved in the physiological activity of the synapse. Excitatory axon terminals had more synapses, and a larger percentage of terminal surface area devoted to synaptic contacts, than inhibitory axon terminals. However, the largest synapses of the inhibitory axon exceeded in surface area those of the excitatory axon. Both axons had many side branches coming from the main terminal; often, the side branches were joined to the main terminal by narrow necks. A greater percentage of surface area was devoted to synapses in side branches than in the main terminal. Only a small fraction of total surface area was devoted to axo-axonal synapses, but these were often located at narrow necks or constrictions of the excitatory axon. This arrangement would result in effective blockage of spike invasion of regions of the terminal distal to the synapse, and would allow relatively few synapses to exert a powerful effect on transmitter release from the excitatory axon. A hypothesis to account for the development of the neuromuscular apparatus is presented, in which it is suggested that production of new synapses is more important than enlargement of old ones as a mechanism for allowing the axon to adjust transmitter output to the functional needs of the muscle.

Electron microscopic study of the somatic sensory cortex of the cat. II. The fine structure of layers I and II

1970 ◽  
Vol 257 (812) ◽  
pp. 13-21 ◽  
Keyword(s):  
Dendritic Spines ◽  
Pyramidal Neurons ◽  
Sensory Cortex ◽  
Electron Microscopic ◽  
Axon Terminals ◽  
Unmyelinated Axons ◽  
Asymmetrical Synapse ◽  
Layer I ◽  
Somatic Sensory Cortex ◽  
Apical Dendrites

Layers I and II of the somatic sensory cortex are clearly distinguishable with the electron microscope because of characteristic differences in the number, type and orientation of neurons and dendritic and axonal ramifications. Layer I may be subdivided into: (i) a subpial astrocytic layer immediately deep to the basement membrane of the cerebral surface; (ii) a superficial quarter consisting of bundles of small myelinated axons and large numbers of small axon terminals which contain spherical vesicles and end in asymmetrical synaptic complexes mainly on large dendritic spines. Most of these terminals are derived from a dense feltwork of fine unmyelinated axons which are especially concentrated at the junction of the superficial and deep parts of layer I; (iii) a deeper three quarters with similar features to the above but with the additional characteristic of many obliquely orientated large dendrites which are the diverging branches of apical dendrites ascending from deeper layers. Small pyramidal neurons dominate layer II, but among them are a small number of non-pyramidal neurons whose beaded dendrites are covered with axon terminals. Large apical dendrites traverse this layer, and in addition to the typical asymmetrical synapse on dendritic spines, a few symmetrical types appear. These are derived from thin unmyelinated axons orientated horizontally within the layer, and the terminals contain many small flattened or pleomorphic synaptic vesicles.

Electron microscopy of the somatic sensory cortex of the cat: I. Cell types and synaptic organization

1970 ◽  
Vol 257 (812) ◽  
pp. 1-11 ◽  
Keyword(s):  
Pyramidal Neurons ◽  
Pyramidal Cells ◽  
Cell Types ◽  
Sensory Cortex ◽  
Synaptic Organization ◽  
Axon Terminals ◽  
Membrane Contacts ◽  
Nonpyramidal Neurons ◽  
Somatic Sensory Cortex ◽  
Spherical Vesicles

Two main types of neuron may be distinguished electron microscopically in the somatic sensory cortex. Pyramidal neurons have a characteristically triangular perikaryon with a high content of ribonucleoprotein consisting mainly of free ribosomes; the nucleus usually shows a single small indentation. Nonpyramidal neurons, which may be large or small, have a higher concentration of all intracytoplasmic organelles and particularly of long cisternae of rough-surfaced endoplasmic reticulum forming Nissl bodies. The nucleus is often deeply indented and crenellated. The two cell types differ also in the nature of their dendritic ramifications and particularly in their synaptic relationships. The majority of axon terminals ending on pyramidal neurons contact dendritic spines and relatively few end on the shafts of dendrites or on the perikaryon. Synapses on spines are typically of the type in which the synaptic thickenings are asymmetrical and the synaptic vesicles spherical. Such synapses, even when they occur on the shafts of pyramidal cell dendrites, are usually associated with a ‘spine apparatus’. Most of the few synapses on the dendritic shafts and somata of pyramidal cells are associated with symmetrical membrane contacts and small, flattened or pleomorphic vesicles. Terminals of this type are commonly en passant endings of long, thin unmyelinated axons oriented vertically or transversely within the cortex. The somata and the usually irregular dendrites of non-pyramidal neurons are typically covered in axon terminals most of which contain flattened vesicles and end in symmetrical complexes, but a few may contain spherical vesicles and end asymmetrically. The axon hillocks and initial segments of both types of cell are postsynaptic to axon terminals containing small, flattened vesicles and ending symmetrically.

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