In vertebrates, only the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly-spaced intervals, enabling an especially rapid mode of electrical impulse propagation called saltation. The demyelination of axons is what causes the multitude of neurological symptoms found in the disease Multiple Sclerosis. The axons of some neurons branch to form axon collaterals, that can be divided into a number of smaller branches called telodendria. Along these the bifurcated impulse travels simultaneously to signal more than one other cell.
There are several types of sensory- as well as motorfibers. Other fibers not mentioned in table are e.g. fibers of the autonomic nervous system
|Type||Diameter||Conduction velocity||Associated muscle fibers|
|α||Extrafusal muscle fibers|
|γ||4-24 m/s||Intrafusal muscle fibers|
|Type||Diameter||Conduction velocity||Associated sensory receptors|
|Ia||Receptors of muscle spindle|
|Ib||Golgi tendon organ|
|Aβ(II)||6-12 µm diameter||33-75 m/s||All cutaneous mechanoreceptors|
|Aδ||1-5 µm||3-30 m/s|| Free nerve endings of touch and pressure |
Nociceptors of neospinothalamic tract
|C||0.2-1.5 µm||0.5-2.0 m/s|| Nociceptors of paleospinothalamic tract |
Growing axons move through their environment via the growth cone, which is at the tip of the axon. The growth cone has a broad sheet like extension called lamellipodia which contain protrusions called filopodia. The filopodia are the mechanism by which the entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Environments with high levels of cell adhesion molecules or CAM's create an ideal environment for axonal growth. This seems to provide a "sticky" surface for axons to grow along. Examples of CAM's specific to neural systems include N-CAM, neuroglial CAM or NgCAM, TAG-1, MAG, and DCC, all of which are part of the immunoglobulin superfamily. Another set of molecules called extracellular matrix adhesion molecules also provide a sticky substrate for axons to grow along. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Some of these are surface bound to cells and thus act as short range attractants or repellents. Others are difusible ligands and thus can have long range effects.
Cells called guidepost cells assist in the guidance of neuronal axon growth. These cells are typically other, sometimes immature, neurons.
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