EXCITATORY AND INHIBITORY PROCESSES 287 



nucleus is not always the largest or most prominent part of the cell. As 

 shown by Florey and Florey (1955), some dendrites are the most conspicuous 

 part of the soma-dendrite complex. The axonal segment, formed by that 

 portion which connects the soma-dendrite complex with the ventral cord, is 

 less difficult to define. The axon hillock region is sometimes not distinct 

 (Fig. 1). From a functional point of view this classification may be quite 

 arbitrary. It has been suggested that the soma-dendrite complex differs from 

 neighboring axonal regions in that the soma membrane has a larger resistance 

 (cf. Edwards and Ottoson, 1958). Also, intracellular potentials (evoked by 

 antidromic stimulation) and recorded in the soma, have a different recovery 

 time course than simple axonal potentials evoked in a similar manner 

 (Eyzaguirre and Kuffler, 1955b). However, the situation becomes complicated 

 if one tries to separate functionally the soma and the dendrites. The larger 

 dendritic trunks behave functionally as the soma while the distal and very 

 fine dendritic portions have different properties with regard to excitation and 

 conduction properties. Indeed, dendritic terminals are depolarized by stretch 

 and they do not seem able to conduct propagated impulses (see later). The 

 length of the terminal dendritic membrane presenting such properties is not 

 known. This picture is comphcated even more since inhibitory synaptic 

 endings seem to occur over a fairly large dendritic area (Kuffler and Edwards, 

 1958). This innervated area includes dendritic portions which respond 

 actively to stretch excitation, but it may include also other parts of the 

 dendrites. 



The Dendritic Endings on the Receptor Muscle 



The manner by which the dendritic processes terminate on the receptor 

 muscles varies in different species and also depends on the type of cell that 

 is being studied. In addition, the muscle receptor organ of lobsters loses its 

 striation somewhere around its middle and the dendrites are embedded in this 

 apparently non-contractile tissue whereas in crayfish the striations continue 

 across the region where the dendrites are embedded. 



In the crayfish one type of neuron {RM 1) has a dendritic system which 

 consists mainly of three parts: (1) a long dendrite extending rostrally rather 

 parallel to the muscle receptor; (2) a stout or rather short dendrite whose 

 main direction is perpendicular to the extension of the muscle fibers; and 

 (3) a system of from one to four thin dendrites which leave the cell body at 

 the opposite side from the first long dendrite. Furthermore, silver staining has 

 revealed that these dendrites send their final endings to different muscle 

 fibers and that branches from one dendritic system run toward the main area 

 of branching of another dendritic system. When the dendritic branches reach 

 their designated muscle fiber they bifurcate in a characteristic T-shape, the 

 ends running in opposite directions along the muscle fiber. If further bifurca- 



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