Paroxysmal depolarizing shift
Encyclopedia
A paroxysmal depolarizing shift (PDS) is a cellular manifestation of epilepsy
. First, there is a Ca++ mediated depolarization, which causes voltage gated Na+ to open, resulting in action potential
s. This depolarization
is followed by a period of hyper-polarization
mediated by Ca++-dependent K+ channels or GABA-activated Cl- influx.
It is noteworthy that unmasking persistent sodium current in presence of Calcium channel blocker
has been well established. It is likely that calcium channel blocker will block voltage and ligand gated Calcium channel, thereby affecting calcium-activated potassium channel
in invertebrate
model system. The initiation of PDS without blocking any channel are much more prevalent in mammal
ian neuron
s, for example, thalamocoritical neuron, CA3 pyramidal neuron, and some hypothalamic neurons. The possibility of spontaneous bursting in these neurons are implicated in regulating hormonal secretion. The significance of PDS may increase the signal-to-noise ratio
, plays vital role in information processing
, synaptic plasticity
and among others. In contrast, the PDSs could be generated by electrical or chemical stimulation to single neurons.
Depending on influx of ion
s, PDS can be theoretically categorized into two type. Ca2+ dependent PDS requires the entry of Ca2+ while Na+ dependent PDS is presumed to be non-synaptic.
The PDS found in invertebrates like Helix, and higher vertebrate
s are assumed to predominantly generated by activation of AMPA receptor
subsequently leading to activation of NMDA receptor
. The evidence shows that there is probable increase in intracellular calcium ions, which sustained calcium-dependent PDS. As usual, this Ca-ions will activate calcium dependent potassium channel and PDS will terminate. This is the case of that provides clue for synaptic transmission.
Alternatively the PDS can still occur and less frequently studied in blocking calcium channel
with heavy metals such as Ni2+ . The further evidence for Na dependent PDS are highlighted in leech with the possibility to study PDS in detail . It is likely that such type of PDS sustained in absence of Calcium
, the case represents the non-synaptic nature of PDS. Finally, the Na/K pump and Calcium activated potassium channel might play a role in terminating PDS. Paradoxically, there might arise the argument whether intracellular calcium could able to repolarize the single neuron while blocking these calcium entry from extracellular milleu. However, the other opportunity such as Na+-Ca2+ exchange as well as small contribution from intracellular stores need to be explored.
If several million neurons discharge at once, it shows up on a scalp
EEG
as a focal interictal epileptiform spike. Paroxysmal depolarizing shifts can lead to an epileptic seizure if there is an underlying predisposition, and recording the spike can be an important aid in distinguishing seizure types.
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
. First, there is a Ca++ mediated depolarization, which causes voltage gated Na+ to open, resulting in action potential
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. This depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
is followed by a period of hyper-polarization
Hyperpolarization (biology)
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization.Hyperpolarization is often caused by efflux of K+ through K+ channels, or influx of Cl– through Cl– channels. On the other hand, influx of cations, e.g...
mediated by Ca++-dependent K+ channels or GABA-activated Cl- influx.
It is noteworthy that unmasking persistent sodium current in presence of Calcium channel blocker
Calcium channel blocker
A calcium channel blocker is a chemical that disrupts the movement of calcium through calcium channels.CCB drugs devised to target neurons are used as antiepileptics. However, the most widespread clinical usage of calcium channel blockers is to decrease blood pressure in patients with...
has been well established. It is likely that calcium channel blocker will block voltage and ligand gated Calcium channel, thereby affecting calcium-activated potassium channel
Calcium-activated potassium channel
Calcium-activated potassium channels are divided into BK channels, IK channels, and SK channels based on their conductance ....
in invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
model system. The initiation of PDS without blocking any channel are much more prevalent in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s, for example, thalamocoritical neuron, CA3 pyramidal neuron, and some hypothalamic neurons. The possibility of spontaneous bursting in these neurons are implicated in regulating hormonal secretion. The significance of PDS may increase the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
, plays vital role in information processing
Information processing
Information processing is the change of information in any manner detectable by an observer. As such, it is a process which describes everything which happens in the universe, from the falling of a rock to the printing of a text file from a digital computer system...
, synaptic plasticity
Synaptic plasticity
In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...
and among others. In contrast, the PDSs could be generated by electrical or chemical stimulation to single neurons.
Depending on influx of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, PDS can be theoretically categorized into two type. Ca2+ dependent PDS requires the entry of Ca2+ while Na+ dependent PDS is presumed to be non-synaptic.
The PDS found in invertebrates like Helix, and higher vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s are assumed to predominantly generated by activation of AMPA receptor
AMPA receptor
The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system . Its name is derived from its ability to be activated by the artificial glutamate analog AMPA...
subsequently leading to activation of NMDA receptor
NMDA receptor
The NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....
. The evidence shows that there is probable increase in intracellular calcium ions, which sustained calcium-dependent PDS. As usual, this Ca-ions will activate calcium dependent potassium channel and PDS will terminate. This is the case of that provides clue for synaptic transmission.
Alternatively the PDS can still occur and less frequently studied in blocking calcium channel
Calcium channel
A Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
with heavy metals such as Ni2+ . The further evidence for Na dependent PDS are highlighted in leech with the possibility to study PDS in detail . It is likely that such type of PDS sustained in absence of Calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, the case represents the non-synaptic nature of PDS. Finally, the Na/K pump and Calcium activated potassium channel might play a role in terminating PDS. Paradoxically, there might arise the argument whether intracellular calcium could able to repolarize the single neuron while blocking these calcium entry from extracellular milleu. However, the other opportunity such as Na+-Ca2+ exchange as well as small contribution from intracellular stores need to be explored.
If several million neurons discharge at once, it shows up on a scalp
Scalp
The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.-Layers:It is usually described as having five layers, which can conveniently be remembered as a mnemonic:...
EEG
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
as a focal interictal epileptiform spike. Paroxysmal depolarizing shifts can lead to an epileptic seizure if there is an underlying predisposition, and recording the spike can be an important aid in distinguishing seizure types.