Question

Synapses at distal apical dendrites of Layer 5 pyramidial neurons have a relevancy problem. Due to attenuation of excitatory postsynaptic potentials, excitatory signals generated at the most distal regions of the apical dendritescna be attenuated 100-fold by the time they reach the soma. Define and discuss the "passive" and "active" properties of dendrites that ultimately help to "democratize" proximal and distal dendrites

Answer 1

Dendrites are appendages that are designed to receive communications from other cells. They resemble a tree-like structure, forming projections that become stimulated by other neurons and conduct the electrochemical charge to the cell body.

Passive conduction is an important behaviour associated with the dendritic and terminal branches of the neuron. In the passive regime, the synaptic potential propagates with attenuation which prompted Hermann to ascribe a correlation with the theory which describes “lossy” transmission lines.

Passive properties of Dendrites

1. Concentration gradient
2. Specific membrane conductance
3. Membrane is selectively permeable to Sodium
4. Membrane as capacitors
5. Membrane as resistor

Passive Properties With the advent of the in vitro slice preparation and intracellular recordings, a number of studies explored the passive electrotonic properties of hippocampal neurons (CAI and CA3 pyramidal neurons and dentate granule cells). Although the neurons and dendrites are not expected to be entirely passive, knowledge about the electrotonic structure of a neuron is a necessary first step in understanding the spread of electrical signals. The overall conclusions from these studies were that hippocampal neurons have long membrane time constants (T,,,), large input resistances, and high specific resistivities, and the electrotonic length (L) of the dendrites is significantly less than one. Moreover, the “electrical compactness” of the neurons was introduced an electrotonically compact neuron is one in which the attenuation of DC or slowly changing potentials is minimal. The use of patch-clamp techniques and accurate morphometric reconstructions added refinements to these general overall conclusions. What is the significance for synaptic integration of short L and long rm? For example, will synaptic potentials generated in distal dendrites be attenuated at all upon reaching the soma? Spruston et al addressed this question and demonstrated that distal EPSPs will indeed diminish in amplitude by passive propagation in dendrites. Dendritic EPSPs may attenuate fivefold or more upon reaching the soma in pyramidal neurons. The explanation for this apparent paradox, that is, the fact that a neuron can be electrotonically compact and yet still produce large attenuations of potentials in its dendrites, is that a neuron may be compact for DC potentials but electrically “loose” for AC or rapidly changing signals such as those that occur during EPSPs. These findings are important not only for understanding passive spread of electrical signals in dendrites, but also suggest a possible role for voltage-gated channels in dendrites

Active Properties of Dendrites:

1. Action potentials in hippocampal dendrites
2. Dendrites express voltage-gated channels (Na+, Ca2+, K+)
3. - Supporting active AP backpropagation –
4. Supporting active boosting of distal EPSPs