DrVirgo
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Tetany occurs during HYPOcalcemia... That means decreased Calcium extracellularly, and thus increased Calcium intracellularly.
so both Kaplan and Goljan are right. 
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| DrVirgo
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Also:
Calcium blocks Voltage gated Na Channels on the mucle cell at the NMJ.
In cases of Hypercalcemia - more Ca extracellularly, and block of voltage gated sodium channels, so decreases excitability.
In cases of Hypocalcemia - less calcium extracellularly, so less blockage of sodium channels, so sodium can enter the cell, so increased excitability and Tetany...
Edited by DrVirgo on 02/19/06 - 01:17 PM
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| mildus
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I agree with drVirgo. Just to add: when we say hypocalcemia we think of concentration of Ca in ECT (blood) not ICT
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| DrVirgo
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Right mildus... HYPO or HYPER-anything means the levels in the Extracellular Space.
Just a few more points about tetany while we are on the topic:
-Tetany can also be induced by multiple action potentials which will increase release of Ca from the SR thus causing more cycling of cross bridges.
-Complete Tetanus is obtained when enough Ca is avilable for continuous cycling of all available cross bridges.
-Can anyone explain the difference between complete and incomplete tetanus and how we incomplete tetanus?
Thanks.
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| star1
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Tetany with carpopedal spasm * Pathophysiology o Abnormal Ca++ control of Na+ channels o Axonal hyperexcitability: Due to + Membrane depolarization + Reduced action potential threshold o Spontaneous repetitive discharges + Frequency: 5 to 15 Hz most common + Source: Arise in peripheral axons * Causes o Ion changes + Hypocalcemia + Hypoparathyroidism + Hypomagnesemia o Alkalosis: Respiratory o Normocalcemic tetany: Rare + Acquired: Associated with neoplasm + Hereditary: Dominant with epilepsy * Exacerbated by: Hyperventilation; Ischemia * EMG o Spontaneous axonal discharges at high rates: Up to 300 Hz o Repetitive discharges: Doublets or Multiplets o Intense spasm: Maximal interference pattern o Normal doublets occur at < 10 Hz * Diagnosis: Ischemic forearm test * Treatment: Calcium; Magnesium; Phenytoin
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