Neuron arborization

With very few exceptions, the neurons in the medial, lateral, and anterior cell groups of the AL fall into two main classes (19,65,67,72,73). Projection neurons (PNs or output neurons) have dendritic arborizations in the AL neuropil and axons that project out of the AL, and local... 

Axons of male-specific antennal ORCs specialized to detect components of the sex pheromone project exclusively to the MGC (64, 89), and all AL neurons that respond to antennal stimulation with sex pheromone components have arborizations in the MGC (65, 72, 73). The MGC in M. sexta has two major, easily distinguishable divisions a donut-shaped neuropil structure (the "toroid") and a globular structure (the "cumulus") adjacent to the toroid and closer to the entrance of the antennal nerve into the AL (74). AL PNs that respond to antennal stimulation with sex pheromone component A have arborizations in the toroid and PNs responsive to component B, in the cumulus (74). Thus first-order synaptic processing of sensory information about these key components of the sex pheromone apparently is confined to different, distinctive neuropil regions of the MGC. 

Many pheromone-responsive protocerebral neurons have arborizations in the lateral accessory lobes (LALs), which are situated lateral to the central body on each side of the protocerebrum and appear to be important for processing of olfactory information (86). Each LAL is linked, by neurons with arborizations in it, to the ipsilateral superior protocerebrum as well as the lateral protocerebrum, where axons of AL... 

PNs terminate (67, 72, 73). The LALs are also linked to each other by bilateral neurons with arborizations in each LAL. Neuropil adjacent to the LAL contains branches of many neurons that descend in the ventral nerve cord. Local neurons link the LAL to this adjacent neuropil. Some descending neurons also have arborizations in the LAL. Thus, the LAL is interposed in the pathway of olfactory information flow from the AL through the lateral protocerebrum to descending neurons. 

The excitatory transmitter released from these cells is, in most instances, glutamate. Local circuit neurons are typically smaller than projection neurons, and their axons arborize in the immediate vicinity of the cell body. Most of these neurons are inhibitory, and they release either GABA or glycine. They synapse primarily on the cell body of the projection neurons but can also synapse on the dendrites of projection neurons as well as with each other. Two common types of pathways for these neurons (Figure 21-6A) include recurrent feedback pathways and feed-forward pathways. A special class of local circuit neurons in the spinal cord forms axoaxonic synapses on the terminals of sensory axons (Figure 21-6B). In some sensory pathways such as the retina and olfactory bulb, local circuit neurons may actually lack an axon and release neurotransmitter from dendritic synapses in a graded fashion in the absence of action potentials. 

Wayman, G. A., Impey, S., Marks, D., Saneyoshi, T., Grant, W. F., Derkach, V. and Soderling, T. R., 2006, Activity-Dependent Dendritic Arborization Mediated by CaM-Kinase I Activation and Enhanced CREB-Dependent Transcription of Wnt-2, Neuron, 50, pp 897-909. 

Some 3500 dopaminergic neurons located in the zona compacta of the substantia nigra innervate the entire neostriatum 0>, 7). Within the neostriatum, the axons of these neurons form an extremely dense terminal arborization that can be visualized with fluorescence histochemistry and immunocytochemical techniques ( 8, 9). This terminal arborization contains approximately 1 billion dopaminergic varicosities and forms about 20% of all the varicosities (10) present in the neostriatum. 

The dendrites represent all the processes of the cell body except for the specialized axonal process (axon). They are usually numerous and serve to increase the surface area of the neuron available for receiving synaptic input. Neurons will have one or more main dendrites that successively branch and arborize to form many smaller processes. 

Neurons assume a vast array of forms in accordance with the functions they serve. In most neurons the cell body and dendrites are separated from the axonal terminal by a very long tube, the axon. This creates problems unique to nerve cells. In the motor neurons that innervate hands and feet, for example, more than 90 percent of the mass of the neuron is in the cell processes. An often given example of this relationship is that if the cell body of a motor neuron were enlarged to the size of a baseball, the corresponding axon would be about 1 mile long, and the dendrites and their branches would arborize throughout a large amphitheater. 

The most significant of the abnormalities observed in a hypothyroid brain is a hypoplastic neuropile, i.e., a marked reduction in the number of connections between neurons [102], This has been observed both in the cerebrum and the cerebellum. For instance a permanent and dramatic reduction in the arborization of the dendritic tree of the Purkinje cell is observed in the hypothyroid cerebellum [103]. The length of the primary dendritic trunk is increased and a deficit in the number, density and branching of the dendritic spines is noticed. In contrast neonatal hyperthyroidism accelerates development of spines. Similar findings have been reported for the cerebrum, i.e., reduction in length and branching of pyramidal neurons, of the density of axonal terminals and of the number of spines [102],... 

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