iPsychology

Dendrite

There is wide variation in the number and organization of these cell processes in the central nervous system. The character of the dendritic tree reflects the afferent neuronal connections and consequently indicates the functional activity of the cell. The complex configuration of dendrites is best appreciated in Golgi preparations; these have allowed the classification of neurons into three groups based on their dendritic arborisations. Isodendritic neurons issue straight dendrites which run in all directions, whilst allodendritic cells are distinguished by shorter, branched den­drites which are restricted in their wavy course. Idiodendritic neurons have a unique dendritic tree characteristic determined by their location .A further, more detailed classification of neurons has been achieved by the dendritic branching pattern. Although clear differences exist, it may not be always easy to distinguish a dendrite from an axon. Dendrites have irregular contours, taper gradually, branch at relatively acute angles, are unmyelinated and contain Nissi substance. Axons, in contrast, display a smoother contour, have a relatively even diameter along their course, branch at obtuse or at right angles, can be myelinated or unmyclinated and lack Nissl substance. The smaller axons and dendrites become, the more difficult is their positive identification; the presence of clustered ribosomes, particularly in association with cisternac of rough-surfaced endoplasmic reticulum, remains the most reliable identifying feature of small dendritic processes. The dendritic tree represents the most sub­stantial receptive area of the nerve cells Synapses are formed either on the dendritic trunks them­selves or on the dendritic spines, the specialized structures projecting from them. The spines are composed of a stalk or neck which connects the dendrite with the ovoid bulb or head. There is a considerable variation in their configuration, but their overall length appears to be relatively con-stant at 2 um. Spines can also be present on the perikaryon and they are responsible for 43()() of the total surface area of the dendrites and the cell body: there are 4000 spines on a single pyramidal neuron. The function of the dendritic spines is still largely unknown. Although they synapse with axon terminals, their role is not restricted to increasing receptive surfaces; they may regulate the excitatory input to a neuron.

Dendrites are covered by minute dendritic spines--projections of cytoplasm-- which are postsynaptic receptors. These stain nicely in a Golgi* stain and can be counted to give an estimate of the synaptic convergence onto a neuron.