Unmyelinated axon

Figure 2.12 From voltage-clamp to current-clamp micro-electrode recordings of synaptic current (/, lower trace) and synaptic potential with superimposed action potential (V, upper trace) from a neuron in an isolated rat superior cervical sympathetic ganglion following a single stimulus (S) applied to the preganglionic nerve trunk. The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk. (SJ Marsh and DA Brown, unpublished)...

Saltatory conduction results in a significant increase in the velocity of conduction of the nerve impulse down the axon compared to that of local current flow in an unmyelinated axon (see Table 4.2). The speed of conduction is... 

Recent data also indicate that peripheral NMDA receptors are involved in inflammatory somatic and visceral pain (Teem et al. 2001). Peripheral glutamate receptors are associated with unmyelinated axons (Carlton et al. 1995), and the number of somatic sensory axons containing ionotropic glutamate receptors... 

Zhang, J.-M, Donnelly, D. F., Song, X.-J., LaMotte, R. H. Axotomy increases the excitability of dorsal root ganglion cells with unmyelinated axons, J. Neurophysiol. 1997, 78, 2790-2794. 

The axon is effectively insulated from the surrounding medium by the myelin sheets except for special regions, the nodes of Ranvier, which lie at 1-to 2-mm intervals along the nerve. The nerve impulse in effect jumps from one nerve to the next. This saltatory conduction occurs much more rapidly (up to 100 m / s) than conduction in unmyelinated axons. It depends upon Na+ and K+ channels that are concentrated in the nodes of Ranvier. 

Electron microscopic examination of the NTS demonstrated peroxidase immunoreactivity in small unmyelinated axons and axon terminals (159). A similar distribution of the 5-HT3 immunoreactivity to fine axons and axon terminals was subsequently documented in the frontal and entorhinal cortex, amygdala, hippocampus, spinal nucleus of the trigeminal nerve, and dorsal hom of spinal cord (183). Immunostaining within cell bodies, dendrites, and/or dendritic spines was then observed in all of these regions as well as in the NTS, but less frequently than the axonal labeling, except in the hippocampus. 

In the single electron microscopic immunocytochemical study thus far carried out with these antibodies (242), 5-HT7 receptors were described as located in neuronal somata and dendrites, fine unmyelinated axons, and axon terminals of the mouse suprachiasmatic nucleus (SCN). After double immunoperoxidase (5-HT7) and immunogold labeling, some of these SCN soma-dendrites and axon terminals could be shown to be GABA, vasoactive intestical polypeptide (VIP), or vasopressin (VP) immunoreactive. Astrocytes in the SCN, characterized by their numerous filaments, were also reported as immunopositive for 5-HT7 receptors. 

Figure 30.2. Peripheral nerve. This diagram shows a peripheral nerve in cross section. The nerve contains three bundles (fascicles), with each fascicle containing a mixture of myelinated and unmyelinated axons. (From H. H. Schaumburg et al. Disorders of Peripheral Nerves, F.A. Davis Co., Philadelphia, 1983.)...

Garlton, S. M., Hargett, G. L., and Coggeshall, R. E. (1995). Localization and activation of glutamate receptors in unmyelinated axons of rat glabrous skin. Neurosd. Lett. 197, 25-28. 

Unmyelinated axons generally have a smaller diameter than myelinated axons. 

The axon ari.ses from a thickened area of the cell body called the axon hillock. Its membrane is mainly composed of lipids and proteins and is known as the axitlentnttt. Many of the axons of the CNS and PNS ate partly covered from near the axon hillock to the. synaptic knob by a sheath of myelin (myelinated axons), but some axons do not have this type of covering (unmyelinated axons). The myelin sheath of PNS myelinated axons is not continuous but is broken at about I-mm intervals to expose the axulenima to the extracellular fluid. These exposed areas, which are about I /[Pg.679]

In the rat hippocampus, mGluR2/3 immunoreactivity was strong in terminal zones of the mossy fibers and perforant path, and was most dense in the lacunosum moleculare of the CAl area electron-microscopically it was found frequently in small unmyelinated axons, especially in preterminal portions of axons rather than in axon terminals (Yokoi et al., 1996 Shigemoto et al., 1997). 

Immunolabeling for GluRl, GluR5/6/7 and NRl is found in unmyelinated axons in glabrous skin of the rat hindpaw (Carlton et al., 1995). They are believed to act as autoreceptors for secreted glutamate that may regulate the response to pain. 

In addition to vertical bundles of myelinated axons, the cerebral cortex of monkeys (e.g., DeFelipe et al., 1990) and of humans (e.g., del Rio and DeFelipe, 1995) also contains vertically oriented bundles of unmyelinated axons that are referred to as horsetails. These horsetails are the axonal plexuses of the inhibitory double bouquet cells and can be demonstrated in monkey neocortex by immunolabeling with antibodies to calbindin and tachykinin. As shown by DeFelipe et al. (1990), in the monkey these axonal bundles are widespread and form a regular columnar system descending from layer 2 to layers 3-5. The bundles are most evident in tangential sections taken at the level of layer 3, where they can be seen to have a center-to-center spacing of 15-30 fim. In a later study of the calbindin labeled double bouquet cells in monkey striate cortex, Peters and Sethares (1997) showed that there is one double bouquet cell, and therefore one vertically oriented double bouquet cell axonal plexus, or horsetail, per pyramidal cell module (Fig. 7). Within layer 2/3 the double bouquet axons run alongside the apical dendritic clusters, while in layer 4C they are closely associated with the vertical myelinated axonal bundles. DeFelipe et al. (1989 1990) proposed that the axon terminals of the double bouquet cell synapse with the shafts and spines of basal dendrites and oblique shafts of apical dendrites of pyramidal cells, but the exact role of these vertical bundles of inhibitory axons is not known. It is likely that they constitute a vertical inhibitory system that acts upon pyramidal cells within the minicolumns. 

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