Axons, concepts

Traditionally concepts of ion selective permeation of biological membranes have centered on differences in the effective radii of hydrated nuclei. An example of that perspective derives from consideration of the resting membrane potential, E, which in the squid axon is approximated by the Nernst equation... 

Anatomical evidence can also be presented to support the concept of presynaptic inhibition and examples of one axon terminal in contact with another are well documented. These do not show the characteristics of either type I or II synapses but... 

FIGURE 1 — 1. This is an artist s concept of how a neuron is organized in order to send synaptic information. It does this via a long axon, which sends its information into numerous branches called terminal axon fibers. Each of these axon terminals can potentially make presynaptic contacts with other neurons. Also shown is the cell body, which is the command center of the nerve, contains the nucleus of the cell, and processes both incoming and outgoing information. The dendrites are organized largely to capture information from other neurons (see also Fig. 1—2). 

FIGURE 1 —4. The anatomically addressed nervous system is the concept that the brain is a series of hard-wired connections between neurons, not unlike millions of telephone wires within thousands and thousands of cables. Shown in the figure is a cable of axons from many different neurons, all arriving to form synaptic connections with the dendritic tree of the postsynaptic neuron. 

Discussion of vulnerability of the axon and its terminal to toxicants must be based on an appreciation of specialized aspects of the metabolism of the neuron. This cell type has been discussed in the chapter dealing with the CNS. The concepts of chemically mediated neurotransmission, second messenger systems, specialized ion channels, and conduction of waves of depolarization with consequent high demand for energy to restore Na+ and K+ ion gradients are applicable to the PNS as well as the CNS. Of particular relevance is the fact that the neuromuscular junction is cholin-... 

Also important in understanding the function of the nervous system are the one-way flow of information across synapses and stimulatory versus inhibitory impulses Most synapses in the CNS will conduct an impulse (usually via a neurotransmitter) in one direction only that is, from the axon of the presynaptic neuron to the dendrite or cell body (soma) of the postsynaptic neuron (see Figures 1.2 and 1.3). This unidirectionality is critical in preserving the integrity of the information flow in the CNS. A particular postsynaptic neuron will have anywhere from 10,000 to 200,000 terminals, or receptor sites, that interact with presynaptic neurons. These terminals are activated by neurotransmitters. Activating some terminals will cause the postsynaptic neuron to fire an impulse to its neighbor neurons. When other terminals are activated, the same postsynaptic neuron will be especially quiet and will not fire off any impulses. This concept of stimulation versus inhibition and inhibitory impulses from one neuron to another is key in our discussion of mental illness and its treatment. 

With the use of proteins and peptides derived from ECM, artificial implants can already mimic the signals that regenerating axons encounter in the environment of the lesioned PNS. However, the concept of targeted use of factors that specifically... 

Bishop GA, Blake TL, O Donoghue DL (1987) The distribution pattern of Purkinje cell axon collaterals variations on a theme. In King JS (Ed.), New Concepts in Cerebellar Neurobiology. Alan R. Liss, New York, 29-56. 

Analysis of the functions of the olfactory bulb would be greatly simplified if the coding of information in primary olfactory axons were known. Unfortunately, widely differing concepts of olfactory coding can be supported by available information. 

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