When the inhibitory tone in gamma motor neurons provided by the inhibitory interneuron in the internuncial pool is removed, the spontaneous excitability of the lower motor neurons is unleashed. This phenomenon of unrestrained excitation of contraction of the skeletal muscles is known as tetanus. In the most common generalized form of tetanus, all skeletal muscle, both agonists and antagonists as well as diaphragm, paravertebral, and respiratory muscles, contract uncontrollably in paroxysms of painful muscle spasms (tetanic seizures) (see T.a.ble.J.S-S ). In the absence of any inhibitory check, any environmental stimuli increase the intensity of the paroxysms, preventing coordinated respiratory movements. Tetanus may remain localized to the area of the wound (local, or cephalic, tetanus if wound is of the face or head). Tetanus is produced by impairment of function of the neurotransmitters employed by the inhibitory interneurons (i.e., GABA and glycine). Tetanus toxin blocks glycine receptors. Antibodies against glutamic acid decarboxylase (GAD), the GABA-synthesizing enzyme, are produced by certain cancers and diabetes mellitus, resulting in the stiff-person syndromey (see Chapter.34 ).
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