EXCITATORY AND INHIBITORY PROCESSES 289 



the axon of this preparation. Furthermore, sustained stretch produced con- 

 tinuous firing of the RM 1 receptor while the same type of stimulus appUed 

 to the RM 2 receptor produced only a short burst of discharges. This shows 

 that RM 1 is a slowly adapting receptor while RM 2 is of a fast-adapting type. 

 These observations have been extended in a series of papers by Kufiier (1954), 

 Eyzaguirre and Kuffler (1955a, b), Kuffler and Eyzaguirre (1955). From these 

 studies several facts have emerged : (i) that stretch deformation is capable of 

 depolarizing that portion of the dendrites which is in close contact with the 

 muscle, and (ii) that a series of intermediate processes lead to the initiation 

 of conducted sensory impulses. 



Stretch Deformation of the Dendrites 



The dendrites are deeply embedded in the muscle mass, and stretch produces 

 deformation of the dendritic endings with consequent changes in sensory 

 frequency. The depolarization produced by deformation seems to be restricted 

 to the terminal dendritic filaments because bending of thick dendritic trunks, 

 the cell soma or the axon does not produce any changes in the frequency of 

 sensory impulses. 



Stretch-deformation of the dendrites can be accomplished also by stimula- 

 tion of motor nerve fibers which innervate the muscular portion of the 

 receptor. These fibers were described histologically by Alexandrowicz (1951, 

 1952), and by Florey and Florey (1955). Upon stimulation of the motor 

 axons the muscle contracts visibly and tension can be recorded by a sensitive 

 transducer. The muscle of RM 1 contracts in a fashion similar to "slow" 

 crustacean muscles; only junctional potentials, similar in appearance to end- 

 plate potentials of vertebrates, are recorded by means of intracellular micro- 

 electrodes. On the other hand, stimulation of the motor fibers innervating 

 RM 2 produces a fast, twitch-Hke contraction and intracellular recording 

 from the muscle fibers reveals propagated action potentials superimposed on 

 local junctional potentials. Contraction of the receptor muscle element 

 puUs on the dendrites and the ultimate effect is quahtatively similar to that of 

 externally apphed stretch (cf. Kuffler, 1954; Eyzaguirre and Kuffler, 1955a). 



Little is known about the basic processes underlying stretch-excitation in 

 dendrites since it is not known to what extent the dendrites are deformed by 

 stretch and what structural changes occur while the organ is being pulled. 

 Electron microscope studies of receptors fixed at different degrees of stretch 

 might yield some pertinent information concerning the relation between 

 stretch-deformation and the production of the generator potential (but, see 

 later). 



The dendrites of RM 1 are thin and long; such an arrangement might 

 permit effective and sustained deformation during stretch and might explain 

 the fact that this receptor is slowly adapting. On the other hand, RM 2 has 



