Nonmyelinated nerve fibres

Vertebrates have two main types of nerve fibre, the larger fast-conducting axons, 1 to 25 fxm in diameter, being myelinated, and the small slowly conducting ones (under 1 fxm) being non-myelinated. Most of the fibres of the autonomic system are non-myelinated, as are peripheral sensory fibres subserving sensations like pain and temperature where a rapid response is not required. Almost all invertebrates are equipped exclusively with non-myelinated fibres, but where rapid conduction is called for, their diameter may be as much as 500 or even 1000 fxm. As will be seen in subsequent chapters, the giant axons of invertebrates have been extensively exploited in experiments on the mechanism of conduction of the nervous impulse. The major advances made in electrophysiology during the last fifty years have very often depended heavily on the technical possibilities opened up by the size of the squid giant axon.

All nerve fibres consist essentially of a long cylinder of cytoplasm, the axo-plasm, surrounded by an electrically excitable nerve membrane. Now the electrical resistance of the axoplasm is fairly low, by virtue of the K+ and other ions that are present in appreciable concentrations, while that of the membrane is relatively high; and the salt-containing body fluids outside the membrane are again good conductors of electricity. Nerve fibres therefore have a structure analogous to that of a shielded electric cable, with a central conducting core surrounded by insulation, outside which is another conducting layer. Many features of the behaviour of nerve fibres depend intimately on their cable structure.

The layer analogous with the insulation of the cable does not, however, consist solely of the high-resistance nerve membrane, owing to the presence of Schwann cells, which are wrapped around the axis cylinder in a manner which varies in the different types of nerve fibre. In the case of the olfactory nerve

Myelinated Nerve Fiber

Fig. 1.2. Electron micrograph of a section through the olfactory nerve of a pike, showing a bundle of non-myelinated nerve fibres partially separated from other bundles by the basement membrane B. The mean diameter of the fibres is 0.2 ^m, except where they are swollen by the presence of a mitochondrion (M). Reproduced by courtesy of Prof. E. Weibel. Magnification 54 800 x.

Fig. 1.2. Electron micrograph of a section through the olfactory nerve of a pike, showing a bundle of non-myelinated nerve fibres partially separated from other bundles by the basement membrane B. The mean diameter of the fibres is 0.2 ^m, except where they are swollen by the presence of a mitochondrion (M). Reproduced by courtesy of Prof. E. Weibel. Magnification 54 800 x.

(Fig. 1.2), a single Schwann cell serves as a multi-channel supporting structure enveloping a short stretch of thirty or more tiny axons. Elsewhere, each axon may be more or less closely associated with a Schwann cell of its own, some being deeply embedded within the Schwann cell, and others almost uncovered. In general, as in the example shown in Fig. 1.3, each Schwann cell

Fig. 1.3. Electron micrograph of a cross-section through a mammalian nerve showing non-myelinated fibres with their supporting Schwann cells and some small myelinated fibres. Reproduced by courtesy of Professor J. D. Robertson.

supports a small group of up to half a dozen axons. In the large invertebrate axons (Fig. 1.4) the ratio is reversed, the whole surface of the axon being covered with a mosaic of many Schwann cells interdigitated with one another to form a layer several cells thick. In all non-myelinated nerves, both large and small, the axon membrane is separated from the Schwann cell membrane by a space about 10 nm wide, sometimes referred to by anatomists as the mesaxon. This space is in free communication with the main extracellular space of the tissue, and provides a relatively uniform pathway for the electric currents which flow during the passage of an impulse. However, it is a pathway that can be quite tortuous, so that ions which move out through the axon membrane in the course of an impulse are prevented from mixing quickly with extracellular ions, and may temporarily pile up outside, thus contributing to the after-potential (see p. 84). Nevertheless, for the immediate purpose of describing the way in which nerve impulses are propagated, non-myelinated fibres may be regarded as having a uniformly low external electrical resistance between different points on the outside of the membrane.

Fig. 1.4. Electron micrograph of the surface of a squid giant axon, showing the axoplasm (A), Schwann cell layer (SC), and connective tissue sheath (CT). Ions crossing the excitable membrane (M, arrowheads) must diffuse laterally to the junction between neighbouring Schwann cells marked with an arrow, and thence along the gap between the cells into the external medium. Magnification 22 600 x. Reproduced by courtesy of Dr F. B. P. Wooding.

Fig. 1.4. Electron micrograph of the surface of a squid giant axon, showing the axoplasm (A), Schwann cell layer (SC), and connective tissue sheath (CT). Ions crossing the excitable membrane (M, arrowheads) must diffuse laterally to the junction between neighbouring Schwann cells marked with an arrow, and thence along the gap between the cells into the external medium. Magnification 22 600 x. Reproduced by courtesy of Dr F. B. P. Wooding.

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Responses

  • cory
    How is impulse conducted along a non myelinated nerve fibre?
    3 years ago
  • joseph
    How is nerve impulse condected along a non myelinated nerve fibre?
    3 years ago
  • felix
    How the nerve pulse is propagated along the nonmyelinated nerve fibre?
    2 years ago
  • Maurizio
    How nerve impulse is conducted along a non myelinated nerve fiber?
    2 years ago
  • MERCEDES
    Which of the following is nonmyelinated nerve fiber?
    1 year ago
  • abdullah
    What is the funtion of schwann cell in non myelinated nervd fibre?
    8 months ago

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