INTRODUCTION 5 



(putting parts together into a whole) takes place in the receptor, so it is 

 probably desirable to anticipate a bit and to present a general picture 

 of events as they occur in a receptor cell. 



Energy impinging upon the cell initiates changes that ultimately 

 generate a propagated all-or-none nerve impulse. As Bullock (1958, 

 1959) has pointed out, 'The determination of firing is not merely the 

 build-up of an adequate stimulus or its transduced and amplified 

 resultant to a critical level, but rather it is a sequence of labile couplings 

 between graded events, each occurring in a limited fraction of the 

 neuron.* A series of separate steps with alternate pathways lead to 

 firing. Some are reflected in changes in the cell membrane potential 

 (Fig. 1). Others are excitability cycles and the presence or absence of 

 after-effects not reflected in the membrane potential. Between the 

 advent of the physical stimulus from outside the receptor, therefore, 

 and the final production of a nerve impulse, there is a long complicated 

 series of events. The importance of these is that the receptor, a highly 

 complex, physiologically non-uniform cell, not only detects energy 

 and transduces it but also carries out a considerable amount of 

 integration. 



When it comes to the transmission of information from the receptor, 

 one begins the long and difficult intellectual trek into the central 

 nervous system. This is beyond the scope of our endeavour here ; 

 however, it is worth pointing out that much additional integration is 

 possible long before the central nervous system is reached. Different 

 channels (nerves) from the receptor may have very different trans- 

 mitting characteristics - different gain, time constants, and maximum 

 output (cf. Bullock, 1953). 



Thus, the sensory physiologist is interested in the way in which 

 accessory structures filter or direct the energy going to a receptor, the 

 way in which the receptor transduces this energy into another form, 

 and how there is generated a propagated nerve impulse which is a 

 code relating to the central nervous system or to special effectors the 

 nature and parameters of a stimulus. And in seeking answers to these 

 questions, it is of no import to the sensory physiologist whether the 

 stimulus originates outside of the animal or from within or whether or 

 not it is ever perceived at a conscious level. 



