Back-propagating axon potentials

The primary role of the sodium channels is to generate action potentials in excitable cells. In case of neurons, the sodium channel density is high at axon hillocks or axon initial segment where action potentials start to propagate. The sodium channels are also present in dendrites. The sodium channels contribute to amplifying synaptic inputs (particularly those distally located) and are actively involved in back propagation of action potentials into dendrites. Subtle differences in properties of sodium channels influence the dendritic processes of synaptic integration in and complex ways. 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

When a stimulus depolarises the transmembrane potential in a spiking axon above the threshold level, an all-or-none action potential in a spiking axon is activated. The action potential propagates unattenuated to the nerve terminal where ion fluxes activate a mobilisation process leading to transmitter secretion.3 The neurotransmitter binds reversibly to receptor proteins embedded in the membrane of a neuron, which triggers a certain effect. There are two types of receptors known, presynaptic receptors or autoreceptors which are present on the neurotransmitter releasing neurons , and postsynaptic or heteroreceptors, which are present on the neurotransmitter receiving neuron . The former are supposed to perform a feed-back function, and slow down the release of neurotransmitter from these neurons when they are stimulated.4... 

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