Non-myelinated axon

Elliott P. Action of antiepileptic and anaesthetic drugs on Na- and Ca-spikes in mammalian non-myelinated axons. Eur J Pharmacol 1990 175 155-163. 

Lemkey-Johnston, N. and Dekirmenjian, H. (1970) The identification of fractions enriched in non-myelinated axons from rat whole brain. Exp. Brain Res., 11, 392-410. 

The characteristic diameter of an immature, nonmyelinated axon tip is of order 1 pm. " Other factors remaining constant, a decrease in pore size of a CG matrix is accompanied by an increase in specific surface of pore channels (cm /g). For example, we calculate that a decrease in pore size from 300 pm to 5 pm of the CG copolymers synthesized in our laboratory is accompanied by an approximately 50-fold increase in specific surface of the substrate. Therefore, we hypothesize that (a) the optimal pore diameter for nerve regeneration is of order 1 pm, (b) a pore diameter of about 1 pm is critical because it provides opportunity for maximum contact between the surface of the immature, non-myelinated axon and the inner surface (lumen) of the pore channel of the matrix and (c) when the pore diameter becomes much smaller than about 1 pm, axonal elongation is physically hindered. 

West, D.C. and Wolstencroft, J.H. Strength-duration characteristics of myelinated and non-myelinated bulbospinal axons in the cat spinal cord. /. Physiol. 1983 337 37 50. 

The central nerv ous system of insects consists essentially of a double nerve cord situated ventrally and punctuated by segmental ganglia from which the peripheral ner es arise. The axon of such a nerve measures up to 10 /xm in diameter and is enclosed in a thin non-myelinated lipoprotein sheath. These axons are bundled into nerves which are surrounded by dove-tailed layers of neuroglial cells, and the whole is enclosed by a protein lamella. The polarization of a resting nerve is very similar to that of vertebrate nerve (see Section 7.5.1). On electrical stimulation, successive spikes can be obtained at intervals of a millisecond but the action potentials are propagated only at about 2 m s , i.e. some 50 times slower than in the larger myelinated axons of vertebrates. 

In contractile experiments the time from the stimulation of the nerve to the CMAP recorded in muscle provides an estimate of NCV. The length of the nerve from the stimulating electrode to the muscle can simply be measured and divided by the time. However, the time recorded in this way includes the delay for synaptic transmission, which may be increased in models with synaptic defects. If this is a concern or if the only parameter desired is NCV, then the measurement can be obtained non-invasively with a relatively simple setup (e.g., (3)). Using the sciatic nerve, NCV can be calculated by measuring the latency of compound motor action potentials recorded in the muscle of a rear paw. Action potentials are produced by subcutaneous stimulation at two separate sites proximal stimulation at the sciatic notch and distally at the ankle. NCV is then calculated by using the two latencies and conduction distance. Decreases in nerve conduction velocity most often reflect defects in myelination, but may also be the result of changes in internodal distance, decreased axon diameters, or altered excitability. 

One example of Na channel plasticity in central pathways subsequent to a non-ion channel gene deletion is revealed by analysis of the mutant mouse shiverer, which bears a major disruption of the myelin basic protein gene, leading to absence of the glial-specific intracellular membrane protein myelin basic protein (MBP) (Roach et al 1985). Lack of MBP prevents developmental formation of the major dense line essential for the compaction of glial wrappings that form the mature myelin sheath. As a result, large calibre axons in the shiverer brain are... 

Layer III Smooth and neurogliaform, multipolar complex appendage and giant cells The most numerous non-pyramidal cell in layer III are neurons that have relatively few dendritic spines or knobs. These cells are distributed in the middle and deep parts of layer III. The somata and dendrites of this cell type vary considerably. The dendrites emerge from numerous sites on the soma and usually branch once or twice near the soma and then extend radially in all directions with few subsequent bifurcations. The dendritic tree typically respects the boundaries of layer III. The axons of layer III smooth cells are difficult to stain, in contrast to layer I smooth cells, possibly because of myelination. 

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