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2.5 Mode(s) of Communication among Neurons

As discussed earlier, a neuron is activated by the flow of chemicals across the synaptic junctions from the axons leading from other neurons. These electrical effects which reach a neuron may be excitatory (meaning they cause an increase in the soma potential of the receiving neuron) or inhibitory (meaning that they either lower the receiving neuron’s soma potential or prevent it from increasing) postsynaptic potentials. If the potential gathered from all the synaptic connections exceeds a threshold value in a short period of time called the period of latent summation, the neuron fires and an action potential propagates down its output axon which branches and communicates with other neurons in the network through synaptic connections. After a cell fires, it cannot fire again for a short period of several milliseconds, known as the refractory period.

Neural activation is a chain-like process. A neuron is activated by other activated neurons and, in turn, activates other neurons. An action potential for an activated neuron is usually a spiked signal where the frequency is proportional to the potential of the soma. The neuron fires when the neuron’s soma potential rises above some threshold value. An action potential may cause changes in the potential of interconnected neurons. The mean firing rate of the neuron is defined as the average frequency of the action potential. The mean soma potential with respect to the mean resting soma potential is known as the activation level of the neuron.


Figure 2.4  Equivalent circuit of a cell membrane

P: Polarization; DP: Depolarization
Vc: Intracellular potential with respect to cell exterior
VK: Nernst potential due to K ion differential concentration across the cell membrane
VNa: Nernst potential due to Na ion differential concentration across the cell membrane
RK: Relative membrane permeability to the flow of K ions
RNa: Relative membrane permeability to the flow of Na ions when the cell is polarized
R: Relative membrane permeability to the flow of Na ions when the cell is depolarizing
C: Capacitance of the cell

The dendrites have synaptic connections on them which receive signals from other neurons. From these synapses, the signals are then passed to the cell body where they are averaged over a short-time interval; and, if this average is sufficiently large, the cell “fires”, and a pulse travels down its axon passing on to succeeding cells. Thus, the neurons relay information along structured pathways, passing messages across synapses in the traditional viewpoint, as explained above.


Figure 2.5  Microelectrode recording of a typical action potential
P: Polarization regime; DP: Depolarization regime; R: Regenerative breakdown regime; W: Minimum width of amplitude invariant current stimulus required for action potential generation; VDP: Depolarized cell potential; VB: Baseline potential; VT: Threshold potential; VRP: Polarized cell resting potential; Duration of spike: About 1 ms; Decay time: Up to 100 ms

Besides this classical theory, Agnati et al. [29] have advocated volume transmission as another mode of neural communication across the cellular medium constituted by the fluid-filled space between the cells of the brain; and the chemical and electrical signals travel through this space carrying messages which can be detected by any cell with an appropriate receptor. The extracellular space, which provides the fluid bathing of the neurons, occupies about 20% of the brain’s volume. It is filled with ions, proteins, carbohydrates, and so on. In volume-transmission, these extracellular molecules are also regarded as participants in conveying signals. Accordingly, it has been suggested [29] that electrical currents or chemical signals may carry information via extracellular molecules also. Relevant electrical effects are conceived as the movement of ions (such as potassium, calcium and sodium) across the neuronal membrane. The chemical mode of volume transmission involves the release of a neuroactive substance from a neuron into the extracellular fluid where it can diffuse to other neurons. Thus, cells may communicate with each other (according to Agnati et al. [29]) without making intimate contact.


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