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The pathophysiology of a spider bite is due to the effect of its venom. A spider envenomation occurs whenever a spider injects venom into the skin. Not all spider bites inject venom – a dry bite, and the amount of venom injected can vary based on the type of spider and the circumstances of the encounter. The mechanical injury from a spider bite is not a serious concern for humans. Some spider bites do leave a large enough wound that infection may be a concern. However, it is generally the toxicity of spider venom that poses the most risk to human beings; several spiders are known to have venom that can cause injury to humans in the amounts that a spider will typically inject when biting. Only a small percentage of species have bites that pose a danger to people. Many spiders do not have mouthparts capable of penetrating human skin. While venoms are by definition toxic substances, most spiders do not have venom that is toxic to humans (in the quantities delivered) to require medical attention. Of those that do, fatal outcomes are exceedingly rare. Spider venoms work on one of two fundamental principles; they are either neurotoxic (impairing the nervous system) or necrotic (dissolving tissues surrounding the bite). In some cases, the venom targets vital organs and systems. ==Neurotoxic venom== Spiders paralyze prey with neurotoxic venom of some sort. A few have a venom that cross reacts with mammalian nervous system, though the specific manner in which the nervous system is attacked varies from spider to spider. When motor neruons are excited the symptoms include muscle spasms, cramps, and twitching. When autonomic nerves are involved sweating, drooling, gooseflesh. In the extreme unstable blood pressure and heart rate can result. *Widow spider venom contains components known as latrotoxins, which cause the massive release of the neurotransmitters causing muscle contractions, sweating, and gooseflesh. This can affect the body in several ways, including causing painful abdominal cramps *The atracotoxins of Australian funnel-web spiders work by opening sodium channels, causing excessive neural activity including strange sensations (paresthesias), muscle contractions, unstable blood pressure (hypertension or hypotension). The venom may cause fluid to accumulate in the lungs (pulmonary edema) that can be fatal. *The venom of Brazilian wandering spiders is also a potent neurotoxin, which attacks multiple types of ion channels. Principally generating severe pain that travels up the limb automonic effects including painful erections occur with moderate envenomation. With severe envenomation heart and lung failure can result in death In addition, the venom contains high levels of serotonin, making an envenomation by this species particularly painful. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pathophysiology of spider bites」の詳細全文を読む スポンサード リンク
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