Block of Na current and intramembrane charge movment in myelinated nerve fibres poisoned with a vegetable toxin

In nerve membrane, the non-linear capacity current (displacement current) is assumed to reflect the movement of intrinsic membrane charges which control the opening of specific pathways for sodium ions (Na channels). However, various discrepancies have been reported between the effects of pharmacological agents on sodium and displacment currents (for a review see ref. 1). It is generally supposed that the opening and closing of Na channels constitutes one step of multi-step system in which each configuration change may or not give rise to a measurable charge movement. New drugs affecting either sodium or displacement currents may elucidate the relationship between charge movement and the control of sodium conductance. We therefore now report a comparison of the effects of a vegetable toxin (oenanthotoxin or OETX) on both sodium current (INa) and intra-membrane charge movement (Q) in Ranvier nodes. We show that OETX reversibly blocks both sodium and displacement current. Studies of INa and Q during partial supression by the toxin reveal differences in the effects of OETX on the remaining INa and Q. The findings are discussed in relation to recent models for the Na-channel gating process and for Na-channel block by local anaesthetics.     

Potassium channels in the nodal membrane of rat myelinated fibres

Following some preliminary reports, mammalian fibres from rabbit and rat have recently been successfully studied in detail by means of the voltage clamp. The early transient or sodium conductance system was found to be similar to that in frog and squid axons. However, the delayed conductance or potassium currents were found to be negligible. Only after chemical and osmotic manipulations, which were said to expose channels buried under the myelin, did Chiu and Ritchie find delayed currents in rabbit fibres. If confirmed, this would mean that the membrane conductance system of mammalian fibres is so different from that of invertebrate and amphibian axon models as to make the data base gathered from amphibian myelinated fibres (frog and toad) and invertebrate giant axons (squid and myxicola) irrelevant to human nd other mammalian fibres. However, we show here that it is possible to find in the normal nodal membrane of rat myelinated fibres potassium currents that flow through channels which are similar in many respects to those found in the frog node of squid axons.     

Potassium channels in nodal and internodal axonal membrane of mammalian myelinated fibres

Horakova et al. were the first to observe that the phase of late outward current carried by potassium ions in frog and squid nerve is virtually absent in voltage-clamped rat nodes of Ranvier. This observation has been recently confirmed by Chiu et al. in rabbit nodes of Ranvier, suggesting that the nodal membrane in the mammal generally has few if any potassium channels. The present voltage-clamp experiments show that large potassium currents can, however, be produced in normal rabbit nodes of Ranvier by acute treatment designed to loosen the myelin from the axonal membrane. From this we conclude that whereas potassium channels are absent in the mammalian nodal membrane, they are normally present in the internodal axonal membrane at least in the paranodal region.     

Peripheral nerve injury triggers central sprouting of myelinated afferents.

The central terminals of primary afferent neurons are topographically highly ordered in the spinal cord. Peripheral receptor sensitivity is reflected by dorsal horn laminar location: low-threshold mechanoreceptors terminate in laminae III and IV (refs 2, 3) and high-threshold nociceptors in laminae I, II and V (refs 4,5). Unmyelinated C fibres, most of which are nociceptors, terminate predominantly in lamina II (refs 5, 7). There is therefore an anatomical framework for the transfer of specific inputs to localized subsets of dorsal horn neurons. This specificity must contribute to the relationship between a low-intensity stimulus and an innocuous sensation and a noxious stimulus and pain. We now show that after peripheral nerve injury the central terminals of axotomized myelinated afferents, including the large A beta fibres, sprout into lamina II. This structural reorganization in the adult central nervous system may contribute to the development of the pain mediated by A-fibres that can follow nerve lesions in humans.     

Non-retinotopic arrangement of fibres in cat optic nerve

Fibres in the mammalian optic nerve are generally thought to be organised retinotopically. Recording electrophysiologically from the cat optic nerve, we found little evidence to support this notion, which led us to investigate the problem by anatomical methods. We made a localised injection of horseradish peroxidase into the lateral geniculate body of the cat, labelling a small clump of retinal ganglion cells and their axons in the optic nerve. These fibres, emanating from neighbouring cells in the retina, became widely scattered through the optic nerve, indicating that retinotopic order is essentially lacking.     

Requirement for nerve growth factor in the development of myelinated nociceptors in vivo

In adult animals, sensory neurons innervating the skin are phenotypically diverse. We have now investigated whether nerve growth factor (NGF) has a physiological role in the development of this diversity. We gave antisera against NGF to rats from postnatal day 1 (PND 1) to adulthood (5 weeks). We found a virtually complete depletion of high threshold mechanoreceptors conducting in the A delta range (2-13 ms-1) in the sural nerve. This afferent type, normally present in large numbers, appeared to have been replaced by D-hair afferents, sensitive mechanoreceptors which normally are relatively rare. NGF deprivation had this effect only in early postnatal life; treatment from postnatal day 14 to adulthood had no effect. We conclude that the presence of NGF postnatally in skin is necessary for the proper phenotypic development of A delta cutaneous nociceptors.     

Guidance of optic nerve fibres by N-cadherin adhesion molecules

The dendritic branches (neurites) of developing neurons migrate along specific pathways to reach their targets. It has been suggested that this migration is guided by factors present on the surface of other neurons or glial cells. The molecular nature of such factors, however, remains to be elucidated. N-cadherin is a cell-surface glycoprotein which belongs to the cadherin family of cell-cell adhesion molecules. This adhesion molecule is expressed in various neuronal cells as well as in glial cells of the central and peripheral nervous systems in vertebrate embryos and recent immunological studies suggested that N-cadherin may play a role in guiding the migration of neurites on myotubes or astrocytes. To further examine this possibility, we used a molecular-genetic approach; that is, we examined the outgrowth of chicken embryonic optic axons on monolayer cultures of Neuro 2a or L cells transfected with the complementary DNA encoding chicken N-cadherin. The data indicate that N-cadherin is used as a guide molecule for the migration of optic axons on cell surfaces.     

周氏新对虾有髓鞘神经纤维的超微结构

周氏新对虾(Metapenaeus joyneri)的有髓鞘神经纤维为椭圆型、圆型,椭圆型长轴直径40-65μm,短轴直径18-25μm．轴突直径6-10μm,位于髓鞘的一侧,轴突壁由微管组成．在轴突壁与膜层之间存在液体间隙．在液体间隙外的髓鞘厚约1μm,由3层膜层和2层微管层组成,在部分髓鞘的结构中,微管层增厚．在髓鞘结构中观察到高嗜锇性区域,由排列规则且明暗交替的膜层组成,     

Long-term regenerated nerve fibres retain sensitivity to potassium channel blocking agents

Mammalian myelinated peripheral nerve fibres display a remarkable degree of regeneration following a discrete nerve crush. Nerve crush disrupts the axon cylinder, but leaves the basement membrane of the Schwann cell intact. These intact endoneurial tubes provide pathways to guide the regenerating axon sprouts. After contact with the periphery is established, the regenerating fibres enlarge and myelinate. Conduction velocity recovers to nearly normal and functional recovery is, in many cases, nearly complete. A distinct feature of normal mature myelinated axons is the insensitivity of these fibres to potassium channel blocking agents. In contrast, immature myelinated axons are exquisitely sensitive to the K channel blocking agent 4-aminopyridine (4-AP). Application of 4-AP to immature myelinated fibres leads to a delayed membrane depolarization with action potential burst activity in response to a single stimulus. This sensitivity to 4-AP is attenuated as the fibres mature. Previous studies have demonstrated a sensitivity to 4-AP in regenerating nerve fibres; this sensitivity differentiates the regenerating axon segments from their normal parent axon segments. Such studies have not, however, examined the question of whether regenerated fibres, which have re-established peripheral connections and are functionally active, fully recapitulate the functional organization of normal mature myelinated fibres. We demonstrate here that while sensitivity to the potassium channel blocking agents 4-AP and 3, 4-diaminopyridine (3, 4-DAP) is lost in the normal course of myelinated axon maturation, this property is present in long-term regenerated axons. This suggests that long-term regenerated mammalian axons are characterized by a functional organization that bears a closer resemblance to that of immature myelinated fibres than to that of adult myelinated fibres.     

Intrinsic differences in the growth rate of early nerve fibres related to target distance

Target field innervation in the developing vertebrate nervous system coincides with the onset of important trophic interactions. Two factors that determine the timing of this event are the distance axons have to grow to reach their targets, which are known to vary, and the rate at which they grow. There have been few studies of axonal growth rate at this stage of development and no comparative study of the relationship between growth rate and target distance. Embryonic chick cranial sensory neurons are located in discrete ganglia and the distance axons have to grow to reach their targets is different for each ganglion, ranging from several hundred to several thousand microns. Here, I show that these neurons differ in their in vivo growth rates; neurons with more distant targets growing faster. In vitro, single isolated neurons from each of these populations grow at a similar rate to that observed in vivo, indicating that growth rate is an intrinsically determined property of neurons before they reach their targets.