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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOC3Y 



FIG. i6. Lejl: Graphs of the impulse frequency in a single aflTercnt fiber innervating the capsule of 

 the knee joint of the cat, showing frequency of discharge against time during flexion at a rate of lo 

 degrees per sec. carried through three different angles: open triangles, lo degrees; open circles, 12 

 degrees; closed circles, 14 degrees. The upper curves show the frequencies of the impulses, the lower 

 ones the angular displacements from a position of 1 32 degrees of extension, where this receptor did 

 not discharge. Note that steady state frequency is higher for greater joint displacements. Right: 

 Similar graphs for the same afferent neuron during movements of the joint between the same posi- 

 tions at four different rates : closed triangles, 35 degrees per sec. ; closed circles, 1 7 degrees per sec. ; 

 open triangles, 10 degrees per sec; open circles, 6 degrees per sec. The displacements are indicated 

 by thin lines. Note that while onset transients differ, the steady impulse frequency in the final posi- 

 tion is the same in each case. [From Boyd & Roberts (29).] 



By far the most common are the ' spray-type' endings 

 which resemble those described in the skin by Ruffini. 

 They are located in the connective tissue capsule of 

 the joints but not in its synovial lining membrane and 

 are supplied by myelinated fibers ranging in diameter 

 from 7 to 10 /i (225)- They are well fitted by location 

 and response properties to signal the steady position 

 of the joint and the direction, rate and extent of joint 

 movement (29, 225). They respond at low threshold 

 with a rapid onset transient as the joint moves in a 

 direction which causes their excitation (fig. 15). The 

 rate of discharge during the movement is a function 

 of its speed and extent (fig. 16); the steady state of 

 discharge at a given excitatory displacement is inde- 

 pendent of the rate at which the initial displacement 

 occurred (fig. i 7). 



These .slowly adapting receptors .subserve angles of 

 aljout 15 degrees. For any given joint different mem- 

 bers of the population of receptors have their excita- 

 tory angles located at different positions along the 



range of joint movement. Some have excitatory angles 

 placed at one end of this range, responding at maximal 

 rate at either full flexion, or full extension (figs. 17, 

 18). At least that is true for the knee joint of the cat 

 which has been most intensively studied, but there is 

 no reason to believe that qualitatively different condi- 

 tions exist in other joints or other species, including 

 man, for the articular innervation has been found to 

 be remarkably uniform in all species studied. 



A second slowly adapting receptor resembling in 

 appearance the Golgi tendon organ has been found 

 associated with the ligaments of the joints, and has 

 been found to be innervated by fibers 7 to 10 fj in 

 diameter. This type is much less numerous than the 

 Ruffini type endings described above and possesses 

 similar discharge properties (16, 225). Very rarely, 

 first order afferents are observed which adapt very 

 quickly to joint movement which e.xcites them. Al- 

 though some disagreement exists (76) they are thought 

 to arise from modified \'ater-Pacinian corpuscles 



