i->6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



REPETITIVE RESPONSES AND TONIG RECEPTORS 



Stimulus-Frequency Relations 



There are many sensory units, the function of 

 which is to signal to the central nervous system the 

 properties of a steady state, e.g. temperature, con- 

 centration or intensity of illumination. At any time, 

 except shortly after an aljrupt change from one state 

 to another, the frequency of the impulse discharge 

 of the unit will depend on the value of the physical 

 or chemical function in question; and a particular 

 frequency will, in the working range of any one unit, 

 be consistently related to a particular value. One 

 example is seen in figure 2 where the frequency of 

 impulse discharge in five single fibers from pressure 

 receptors of the carotid sinus is plotted against pres- 

 sure in the sinus. Another example appears in figm-e 

 22 of Chapter XVIII on Thermal Sensations in this 

 volume (p. 452), in which the impulse frequencies in 

 two units from the cat's tongue responding to thermal 

 stimuli are plotted against temperature. The curves 

 in the two figures are clearly quite different; the 

 pressure units, while showing individual variations, 

 all start to fire at a certain pressure above which the 

 frequency of discharge increa.ses as the pressure in- 

 creases until an upper limit of frequency is reached 

 (12, 60). The temperature units on the other hand 

 both show maxima in their temperature-frequency 

 relationship, but these maxima occur at two widely 

 different temperatures. The two types of response 

 represent the activity of two distinct groups of units 

 found in cats (22, 46) and it is presumed that the 



FIG. 2. Responses of five single pressure sensitive units QA 

 to £) from the cat's carotid sinus. Abscissa: intrasinusal pres- 

 sure in mm Hg. Ordinate: impulse frequency per sec. [From 

 Landgren (60).] 



activity of these two types of unit bears a close causal 

 relationship with the subjective sensations of cold 

 and warmth. 



Looking at the two examples shown, it would seem 

 improbable that any relationship between 'stimulus' 

 and frequency having general relevance to sensory 

 units of all types could be found. This is strictly true, 

 but there is a relationship that has been found to de- 

 scribe reasonably well the response characteristics of 

 certain types of unit in their working range. This is 

 what is known as the Weber-Fechner law. This law 

 derives from an observation made by Weber that the 

 smallest difference in the weight of two objects bears 

 a constant relation to the weight of the objects. It is 

 usually given as AI/I = C, where / is intensity of 

 stimulus. A/ the smallest detectable difference in 

 intensity and C a constant. Fechner developed this 

 observation in a theoretical way by making the as- 

 sumption that each discriminable step of stimulus 

 intensity corresponds to a imit increase in sensation, 

 that is to say he stated that AI/I = kAS where AS 

 is the increase in sensation. From this it follows that 

 d5/d/ = I /k/ and S = a log / + A. This equation 

 was originally put forward in an attempt to quanti- 

 tate .sensation, a thing we are not concerned with 

 here; howe\er, we are concerned with its relevance 

 to 'stimulus' -frequency relations. The relation be- 

 tween the applied force and impulse frequency re- 

 corded from a frog's muscle spindle has been found 

 to be consistent with this relationship (95). The cor- 

 responding relationship between intensity of illumina- 

 tion and response from an ommatidium of the eye of 

 Limiilus is also consistent with the equation under 

 certain specific conditions (41). These findings have 

 inevitably raised the question of whether this rela- 

 tionship indicates anything about the mechanisms 

 invoked in the initiation of impulses or whether it 

 must be regarded simply as an empirical description 

 (31). The fit between equation and experiment is not 

 sufficiently good to suggest that the fundamental 

 processes depend on a simple logarithmic relation- 

 ship, but if, as seems possible, the.se processes are 

 related to ionic equilibria across cell membranes, a 

 logarithmic term might he expected to appear in the 

 relationship. 



Effect fij a Reduction oj Excitation 



It has already been pointed out that while a tonic 

 sensory unit will respond to a certain steady state 

 with a certain frequency, a sudden increase in, for 



