INITIATION OF IMPULSES AT RECEPTORS 



12- 



units that are described as tonic, the frequency of 

 the impulse discharge declines relatively slowly to a 

 steady value which is characteristic of the applied 

 force (fig. i); the frequency of the discharge from 

 other units, those called phasic, adapts more rapidly 

 and finally falls to zero (5). In an extreme case a 

 sensory unit may only discharge a single impulse 

 during the change in the applied forces and will 

 then remain silent until another change takes place. 

 Adaptation is also observed in sensory units specifi- 

 cally sensitive to forms of energy other than mechani- 

 cal. 



Adaptation is a word that describes the response of 

 a sensory unit to a particular function of the type of 

 energy concerned. When adaptation is rapid, it can 

 be said that the unit is not signalling a pressure, a 

 temperature or a concentration; but it does not tell 

 us what particular function in respect to time is 

 signalled. 



To say that a function is signalled means that a 

 constant value of the function gives rise to a constant 

 and repeatable frequency of impulse discharge in the 

 fiber of the sensory unit. In most situations it is very 

 difficult to maintain a constant velocity or accelera- 

 tion for a sufficiently long time to see whether or not 

 a constant frequency of discharge is in fact set up 

 [a notable exception has appeared in the experiments 

 on the semicircular canals using constant angular 

 velocities and accelerations (72)]. Even if such ex- 

 periments were performed it is by no means certain 

 that simple relations would be found. This is there- 



fore a situation in which it is necessary to continue to 

 use an empirical description. 



It should be noted that in the first sentence of this 

 section, reference is made to the tissue surrounding 

 the receptors. Even in the instances in which a recep- 

 tor has been isolated, e.g. the muscle spindle and 

 Pacinian corpuscle, there is far more supporting tis- 

 sue than active element. These supporting tissues 

 may be of fundamental importance in the adapta- 

 tion of 'simple' receptors in the same way as the 

 structures of the middle ear and cochlea cause 

 'adaptation' of the ear to steady pressures applied 

 to the tympanic membrane. This problem will be 

 considered at a later stage when all the relevant 

 evidence has been discussed. 



Receptive Fields 



A sensory unit has a particular situation and par- 

 ticular size of receptive field, i.e. the area from which 

 the single afferent fiber receives branches. The size 

 of these receptive fields can vary quite considerably, 

 for example up to 9 by 5 cm, not mm, in cat's skin 

 (73) and up to 100 sq. mm in frog's skin (3); while 

 other sensory units have receptive fields which com- 

 prise only a single end organ. Variation of size of re- 

 ceptive fields occurs with different types of unit in 

 skin and also in specialized organs such as the eye. 

 There is wide overlap of receptive fields and it is 

 clear that spatial discrimination must depend on the 

 coordination of information supplied through a con- 

 sideraljle number of primary channels. 



FIG. I. Response of cat muscle spindle to stretch. Abscissa: 

 time in sec. Ordinate: impulses frequency per sec. Each curve 

 for a diflferent force. [From Matthews (75).] 



Information 



Sensory units constitute independent channels 

 which signal to the central nervous system informa- 

 tion about the physical and chemical environment of 

 the organism. This information is conveyed by the 

 pattern of activity in any one unit and by the charac- 

 teristics and organization of each channel. These 

 factors can be classified as follows: 

 a) Factors related to time 



i) Interval between impulses 

 ii) Duration of activity 

 h) Factors related to the properties of units 



i) Characteristics of the 'normal' stimulus, 

 e.g. the nature of the energy and other 

 relevant factors 

 ii) Size and position of the receptive field 

 iii) SensitivitN' of the unit 



