Membrane Potentials

Membrane potentials are described by various ionic concentration configurations outside and inside the membrane of a cell. These potentials are:


 * 1) Resting Membrane Potential: the membrane potential at rest, steady-state conditions.
 * 2) Action Potential: a non-graded potential, much like binary code (on/off).
 * 3) Post-Synaptic Potentials: graded potentials, that can be summated/subtracted by modulation from presynaptic neurons.

Ionic Basis of Postsynaptic Excitation
At an excitatory synapse, neurotransmitter opens channels in the postsynaptic membrane that are permeable to cations, principally Na+ and K+. The net ionic current (IEPSP) that gives rise to an EPSP is a sum of the individual currents carried by all the ions that permeate the channel.

EPSP reversal potentials. Reversal potential [Nernst potential] is the membrane potential Em at which the net ionic current through a channel is zero.

IEPSP = INa + IK

Erev = gNa ENa + gK EK / gNa + gK

gNa, gK - conductances proportional to the permeability of the channel to Na+ and K+

ENa, EK - the Nernst potentials for these ions

Rearranging:

Ionic Basis of Postsynaptic Inhibition
When the resting membrane potential is -65 mV, the direction of IEPSP is outward; an outward Cl- current represents an influx of the negatively charged Cl-, which will hyperpolarize the cell. The outward synaptic current will increase as the membrane potential is made less negative (depolarized) and decrease as the potential is made more negative (hyperpolarized).

Another possibility is that the inhibitory synapse channel is permeable to K+, with a Nernst potential of -90 mV. An increased permeability to K+ can bring the membrane potential close to the Nernst potential for K+ and thus hyperpolarize the cell.

General properties of Post-Synaptic Potentials

 * Postsynaptic potentials (like end-plate and receptor potentials) are a graded potential change. The amplitude of the postsynaptic potential increases with the amount of neurotransmitter released. In other words, it increases with the number of receptors bound by neurotransmitter, given that these receptors will lead to depolarization of the cell, leading to influx of Ca2+ which will eventually cause the exocytosis of the neurotransmitter in the following synaptic cleft.
 * Since post-synaptic potentials are graded, individual potential changes can summate. The summation of postsynaptic potentials can be either temporal or spatial:
 * 1) Temporal summation arises when the same input is stimulated repetitively, such that the second synaptic potential arrives before the postsynaptic cell has recovered from the first synaptic potential. There is a margin of 15ms since the onset of the action potential, during which the second post-synaptic potential can be superimposed and add on the first one before it fades out to -65mV (resting). This gives rise to a staircase increase in the postsynaptic potential change.
 * 2) During spatial summation the second synaptic potential is provided by a second input simultaneously, which makes synaptic contact in close proximity to the first input. If the two inputs have the same sign (both excitatory or both inhibitory) the postsynaptic response evoked by stimulating will be larger than if either input is stimulated alone.
 * Single excitatory PSPs are not greater than 0.5 - 1mV.
 * In both excitatory and inhibitory PSPs, the ion conductivity change (peak) lasts for 1-2ms, and then it takes up to 15ms for restoration of the resting potential, because this is the time it takes for the excess positive/negative charges to leak out/in from/to the cell.
 * The threshold for excitation averages around -45mV. To reach this threshold we need +20mV summated EPSP. Spatial and Temporal summation of single PSPs (40-80 EPSPs) from the dendrites and soma can yield such value.
 * However, even if this value (+20mV) is reached and the soma potential > -45mV (threshold), the action potential cannot be elicited. This threshold applies only for the axon hillock, where there is the maximal density of Na+-VGCs. For an action potential to be elicited from the soma, the summated EPSP must be between +30mV and 40mV instead.

The importance of summation is evident when the spinal motor-neuron model is considered: Each motoneuron carries about 20,000 individual synaptic boutons, which represent about 6000 separate inputs (each input forms more than one synapse). In the real situation, neurons are under a constant influence of both inhibitory and excitatory synaptic inputs. At any moment the membrane potential of the neurons reflects the sum of all these inputs. A decrease in the level of the tonic excitatory input will hyperpolarize the cell, as well as an increase in the inhibitory input. Similarly an increase in the tonic excitatory input will depolarize the cell, which may be achieved also by a decrease in the power of the tonic inhibitory input.

Facilitation of neurons
Facilitation of neurons means getting closer to the threshold. Often, the summated postsynaptic potential is excitatory but has not risen high enough to reach the threshold for firing by the postsynaptic neuron. When this happens, the neuron is said to be facilitated. That is, its membrane potential is nearer the threshold for firing than normal, but not yet at the firing level. Consequently, another excitatory signal entering the neuron from some other source can then excite the neuron very easily. Diffuse signals in the nervous system often do facilitate large groups of neurons so that they can respond quickly and easily to signals arriving from other sources.

"Excitatory State"
The "excitatory state" of a neuron is defined as the summated degree of excitatory drive to the neuron. If there is a higher degree of excitation than inhibition of the neuron at any given instant, then it is said that there is an excitatory state. Conversely, if there is more inhibition than excitation, then it is said that there is an inhibitory state. When the excitatory state of a neuron rises above the threshold for excitation, the neuron will fire repetitively as long as the excitatory state remains at that level.

Some neurons in the central nervous system fire continuously because even the normal excitatory state is above the threshold level. Their frequency of firing can usually be increased still more by further increasing their excitatory state. The frequency can be decreased, or firing can even be stopped, by superimposing an inhibitory state on the neuron. This is similar to carrier-wave type modulation on continuous firing rates.

Thus, different neurons respond differently, have different thresholds for excitation, and have widely differing maximum frequencies of discharge.

Presynaptic inhibition
Presynaptic inhibition is caused by release of an inhibitory substance onto the outsides of the presynaptic nerve fibrils before their own endings terminate on the postsynaptic neuron. In most instances, the inhibitory transmitter substance is GABA (γ-aminobutyric acid). This has a specific effect of opening anion channels, allowing large numbers of chloride ions to diffuse into the terminal fibril. The negative charges of these ions inhibit synaptic transmission because they cancel much of the excitatory effect of the positively charged sodium ions that also enter the terminal fibrils when an action potential arrives.Presynaptic inhibition occurs in many of the sensory pathways in the nervous system. In fact, adjacent sensory nerve fibers often mutually inhibit one another, which minimizes the sideway spread and mixing of signals in sensory tracts. It is the basis of Lateral Inhibition.