Axonal branches

I R LN Synopse formation Cell signalins Neuronal migration axonal branching... 

Another reason to consider a novel role for the cholinergic basal forebrain is that cholinergic axons exhibit exquisite structural plasticity, similar to that of monoaminergic axons, and in marked contrast to that of glutamatergic axons (Farris et ah, 1993 Woolf, 1996). Wholly new cholinergic axon branches bear-... 

Streppel, M. et al. (2002). Focal application of neutralizing antibodies to soluble neurotrophic factors reduces collateral axonal branching after peripheral nerve lesion. Eur. J. Neurosci. 15(8), 1327-1342. 

Microscopic studies of the structure of the terminal axons of the autonomic nerves have shown that the axons branch many times on entering the effector tissue,... 

As depicted in Fig. 4.1, neurons, despite a high region-dependent variabilit) in shape, have a common basic structure. Each neuron consists of the cell bod (soma, which contains the cell nucleus), several dendrites and an axon that spreads out distally into axon branches. 

The sensory fibers in the nonadrenergic, noncholinergic systems are probably better termed "sensory-efferent" or "sensory-local effector" fibers because, when activated by a sensory input, they are capable of releasing transmitter peptides from the sensory ending itself, from local axon branches, and from collaterals that terminate in the autonomic ganglia. These peptides are potent agonists in many autonomic effector tissues. 

Enlarged butlonllke structures at the ends of axon branches. 

Axon terminal (or terminal button) Enlarged buttonlike structures at the ends of axon branches (Chapter 3). 

Sawai, H., Clarke, D. B., Kittlerova, P., Bray, G. M., andAguayo, A. J., Brain-derived neurotrophic factor and neurotrophin-4/5 stimulate growth of axonal branches from regenerating retinal ganghon cells, J. Neurosci., 16, 3887, 1996. 

Hrycyshyn AW, Ghazi H, Flumerfelt BA (1989) Axonal branching of the olivocerebellar projection in the rat A double-labeling study, J. Comp. Neurol, 284, 48-59. 

Qvist H (1989b) Demonstration of axonal branching of fibres from certain pre-cerebellar nuclei to the cerebellar cortex and nuclei a retrograde fluorescent double-labelling study in the cat. Exp. Brain Res., 75, 15-27. 

Wharton SM, Payne JN (1985) Axonal branching in parasagittal zones of the rat olivocerebellar projection a retrograde fluorescent double-labeling study. Exp. Brain Res., 58, 183-189. 

All afferent inputs to the striatum that have been studied so far have formed axonal fields in which the individual axonal branches cross over the dendrites of individual spiny neurons, making synapses mostly en passant. This is the cruciform axodendritic pattern of innervation (Fox et al. 1971), which places each axon into position to contact the maximum number of neurons but minimizes the number of synapses possible with each postsynaptic cell. This is in contrast to the longitudinal axodendritic synaptic arrangement formed by striatopallidal fibers (Fox and Rafols 1976), in which individual axonal branches form multiple synaptic contacts on the dendrites of postsynaptic neurons. 

In the antennal lobe ORN axons terminate in structures known as glomeruli (Figure 4). The ORNs innervate the glomeruli - spheroids of tightly packed dendritic and axonal branches. These first-order synaptic neuropil are where ORNs, and the two major classes of AL neuron, principal neurons (PNs) and local intemeurons (LNs) interconnect. The moth M. sexta (male) has around 66 glomeruli, 360 LNs and 900 PNs [16]. There are many different receptor types, since most of the ORNs are tuned to respond to a variety of chemicals. 

While the navigation of axons in invertebrates is often accurate, it is not invariably error-free. In many situations, axon branches have been found on occasion to extend down pathways in which they are not found in the mature animal. Such er-... 

Amongst the insects, there appears to be a trend for more accurate initial axon navigation within the CNS inappropriate branching is more often seen in the periphery (Myers et al., 1990 Sink and Whitington, 1991a). However, this does not hold for the leech. Widespread inappropriate axon branching within the CNS is a feature of axon development for the RPE motoneurons (Baptista and Macagno, 1988). 

Whatever the reasons for the initial formation of inappropriate axonal branches, the neuron must have a mechanism for their removal. Whether the environmental cues responsible for this pruning process are the same as those that initially guide the growth cone in a particular direction remains to be determined, as does the operation of the sub-cellular machinery involved in the response of the neuron to those cues. This matter is discussed in more depth in Section 4.2. 

Fig. 5. The axon morphology of the motoneuron FETi in the grasshopper embryo following ablation of its target limb bud prior to axonogenesis. These two examples illustrate the variability observed in the pattern of axon branching from this motoneuron. Camera lucida drawings of intracellular LY fills. Scale bar = 50//m, 75% embryo (reproduced with permission from Whitington and Seifert, 1984).

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