Tetany (action potential summation)
Encyclopedia
Tetany is a case of involuntary muscle convulsion. It is a muscular physical state at which action potential
s from nerve
s arrive to the skeletal muscle
motor end plate rapidly enough in succession to cause a steady contraction. This differs from individual twitches in that the twitches are summed. A single twitch may elicit from one action potential at the motor end plate.
Tetany can be demonstrated by applying quick electrical pulses to the skin
close to where a nerve lies (such as by the elbow near the ulnar nerve
), or the sciatic nerve. If the frequency of charge is once per second, the hand muscles (which the ulnar nerve supplies) will flex once per second. If the frequency is increased, the muscle contractions will sum and appear as one smooth contraction. The hand will smoothly close.
Low serum levels of calcium can produce a medical sign called tetany
. In this case, the skeletal muscle fibers are persistently contracted because they are not persistently depolarized, and this is occurring because the serum calcium is very low - without replacement. If serum calcium was low with replacement with another divalent
cation, such as magnesium
, then there would not be persistent contraction because magnesium would competitively inhibit the voltage gated calcium channels, thereby inhibiting calcium's entry. However, in tetany, there are simply low calcium levels and the cell thinks that it is depolarized.
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s from nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s arrive to the skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
motor end plate rapidly enough in succession to cause a steady contraction. This differs from individual twitches in that the twitches are summed. A single twitch may elicit from one action potential at the motor end plate.
Tetany can be demonstrated by applying quick electrical pulses to the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
close to where a nerve lies (such as by the elbow near the ulnar nerve
Ulnar nerve
In human anatomy, the ulnar nerve is a nerve which runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest unprotected nerve in the human body , so injury is common...
), or the sciatic nerve. If the frequency of charge is once per second, the hand muscles (which the ulnar nerve supplies) will flex once per second. If the frequency is increased, the muscle contractions will sum and appear as one smooth contraction. The hand will smoothly close.
Low serum levels of calcium can produce a medical sign called tetany
Tetany (medical sign)
Tetany is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disease or other conditions that increase the action potential frequency...
. In this case, the skeletal muscle fibers are persistently contracted because they are not persistently depolarized, and this is occurring because the serum calcium is very low - without replacement. If serum calcium was low with replacement with another divalent
Divalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....
cation, such as magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, then there would not be persistent contraction because magnesium would competitively inhibit the voltage gated calcium channels, thereby inhibiting calcium's entry. However, in tetany, there are simply low calcium levels and the cell thinks that it is depolarized.
