EXCITATION BY HYPERPOLARIZING POTENTIALS. 

 A GENERAL THEORY OF RECEPTOR ACTIVITIES* 



Harry Grundfest 



Department of Neurology, College of Physicians and Surgeons, 

 Columbia University, New York 



Studies on sense organs which have been classically designated as "primary 

 sense cells" (cf. Antrum, 1959) are exemplified by the analysis of the mechano- 

 receptors of muscle spindles (Katz, 1950; Paintal, 1959), the Pacinian cor- 

 puscles (Alvarez-Buylla and Ramirez de Arellano, 1953; Gray, 1959; Loewen- 

 stein, 1959), and the crayfish stretch receptor (Eyzaguirre and Kufiler, 1955). 

 The latter cell is the only primary receptor neuron thus far studied v/ith intra- 

 cellular recording (Fig. 1). It produces a graded depolarizing "generator 

 potential" (Bernhardt and Granit, 1946) which may be sustained as long as 

 the stimulus is appUed. Spikes are evoked in the axon (Edwards and Ottoson, 

 1958) and are conducted centripetally as a frequency-number coded message 

 of all-or-none pulses. 



The eccentric cell of Limulus eye, strictly speaking, should not be con- 

 sidered as a primary receptor neuron (Grundfest, 1961) since the photo- 

 sensory process probably takes place in the retinular cells or their rhabdomes 

 (cf. Fuortes, 1959). However, it, too, produces a depolarizing potential in its 

 dendrite which may be recorded in the soma (Hartline et al., 1952; Tomita, 

 1956; Fuortes, 1959). This potential is graded with the intensity of the light 

 and lasts as long as the stimulus does. 



The data obtained by studies with intracellular techniques confirm the 



deductions of earlier work (Adrian, 1932; Hartline, 1942; Bernhardt, 1942; 



Katz, 1950), on the linkage between the stimulus and message formation 



through the intermediate of the' generator potential. Conceptually, they raise 



two questions: (1) How is it that one and the same cell can produce two 



different kinds of potentials — one a graded sustained depolarization, the other 



a sequence of pulses ? (2) What happens in receptor systems of the secondary 



kind in which receptor cells without axons are involved and precede a neuron 



which may be regarded as the final common path for the centripetal sensory 



message ? 



* The researches at the author's laboratory were supported in part by funds from the 

 following sources: Muscular Dystrophy Associations of America, National Institutes of 

 Health, National Science Foundation and United Cerebral Palsy Research and Educational 

 Association. 



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