Mechanosensitivity

Demyelinated axons may become not only spontaneously active but also mechanosensitive, so that they generate trains of impulses if they are deformed. In this respect, demyelinated axolemma acquires properties similar to those of mechanoreceptors, and the discharge patterns generated resemble the familiar phasic and/or tonic bursts of activity

Figure 10 Records obtained using a micropipette positioned in four different central demyelinated axons, showing ongoing activity obtained from sites either remote (left) or at (right) the location of a demyelinating lesion (indicated). Different axons show either a bursting discharge (upper records) or a more even discharge (lower records). The changes in membrane potential responsible for generating the impulses can be seen in the records obtained at the lesion. (Redrawn from Felts et al., 1995, and Kapoor et al., 1997.)

Figure 10 Records obtained using a micropipette positioned in four different central demyelinated axons, showing ongoing activity obtained from sites either remote (left) or at (right) the location of a demyelinating lesion (indicated). Different axons show either a bursting discharge (upper records) or a more even discharge (lower records). The changes in membrane potential responsible for generating the impulses can be seen in the records obtained at the lesion. (Redrawn from Felts et al., 1995, and Kapoor et al., 1997.)

recorded from such receptors coincident with the deformation (Smith and McDonald, 1980, 1982). In patients with demyelinating lesions in the optic nerves or the cervical posterior columns of the spinal cord, eye and neck movements, respectively, can deform the demyelinated axons, provoking bursts of activity. This activity is perceived as flashes of light (Davis et al., 1976) or an "electric shock" or tingling sensation that radiates down the limbs and body (i.e., Lhermitte's phenomenon) (Lhermitte et al., 1924; Kanchandani and Howe, 1982). Recordings from cutaneous nerve fascicles have established that the tingling sensations associated with

Lhermitte's sign occur simultaneously with a volley of impulses in sensory axons, presumably representing impulses conducted antidromically following their ectopic generation at a cervical lesion (Fig. 12) (Nordin et al., 1984).

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