Depolarisation potential

Polarised electrode. An electrode is polarised if its potential deviates from the reversible or equilibrium value. An electrode is said to be depolarised by a substance if that substance lowers the amount of polarisation. 

The nitrate ion is reduced to ammonium ion at a lower (i.e. less negative) cathode potential than that at which hydrogen ion is discharged, and, therefore, acts to decrease hydrogen evolution. The nitrate ion acts as a cathodic depolariser. 

Daniell cell 64 d.c. arc source 763, 771 Dead-stop end points 635 Decantation 119 Decomposition potential 504 Degreasing agent 80 Delves cup 788 Demasking agents 312, 334 Densitometers 231, 232 Depolariser anodic, 515 cathodic, 509... 

An effect of opening K+ channels is to hypetpolarise the primary sensory neurons. Similarly to local anaesthetics, this makes the cell less likely to produce an action potential because more depolarising stimuli are needed to overcome the block. NS 1619 is an example of this type of drug which has initially shown antitussive activity in a variety of experimental systems. 

Both disodium cromoglycate and nedocromil sodium have antitussive effects in humans. In this instance, their activity occurs by increasing the depolarisation of sensory nerves, which increases the threshold for an action potential and therefore inhibits the activity of these neurons. 

The LVA channels are expressed in a wide variety of tissues. In the cardiac sinus node and the thalamus, activation of LVA channels seems to be necessary to generate action potentials upon depolarising the membrane. 

Voltage-dependent inactivation is channel inactivation at depolarised membrane potentials. 

Such clear postsynaptic potentials can be recorded intracellularly with microelectrodes in large quiescent neurons after appropriate activation but may be somewhat artificial. In practice a neuron receives a large number of excitatory and inhibitory inputs and its bombardment by mixed inputs means that its potential is continuously changing and may only move towards the threshold for depolarisation if inhibition fails or is overcome by a sudden increase in excitatory input. 

Figure 1.4 Ionic basis for excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). Resting membrane potential ( — 70 mV) is maintained by Na+ influx and K+ efflux. Varying degrees of depolarisation, shown by different sized EPSPs (a and b), are caused by increasing influx of Na. When the membrane potential moves towards threshold potential (60-65 mV) an action potential is initiated (c). The IPSPs (a b ) are produced by an influx of Cl. Coincidence of an EPSP (b) and IPSP (a ) reduces the size of the EPSP (d)...

Depolarisation of (ii) from normal resting potential leads to normal (maximal) change in potential and release of NT... 

Partial depolarisation of (ii) by presynaptic inhibition reduces potential change induced by action potential and the release of NT... 

Figure 1.6 Presynaptic inhibition of the form seen in the dorsal horn of the spinal cord, (a) The axon terminal (i) of a local neuron is shown making an axo-axonal contact with a primary afferent excitatory input (ii). (b) A schematic enlargement of the synapse, (c) Depolarisation of the afferent terminal (ii) at its normal resting potential by an arriving action potential leads to the optimal release of neurotransmitter, (d) When the afferent terminal (ii) is already partially depolarised by the neurotransmitter released onto it by (i) the arriving acting potential releases less transmitter and so the input is less effective...

SKca and M channels are not the only K+ channels regulated by transmitters. As noted above, transmitters can also close, or open, other K+ channels that do not directly regulate excitability but instead determine the resting potential of the neuron, and hence depolarise or hyperpolarise the neuron. 

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