38o 



HANDBOOK OF PH'SSIOLGGY 



NEUROPHYSIOLOGY I 



in the impulse frequencies in the afferent nerves (27, 

 41, 43, 59). The Qio in the statocysts of the lobster is 

 4.5 for the nonadapting resting activity of the large 

 spikes, according to Cohen et al. (27). This tempera- 

 ture sensitivity is therefore lower than that of the 

 temperature receptors in the tongue of mammals, 

 studied by Hensel & Zotterman (55), and of the 

 ampullae of Lorenzi in rays, investigated by Hensel 

 (54). On the other hand Bullock found marked me- 

 chanical irritability of the thermoreceptors on the 

 head of the rattlesnake (21). The frequency of the 

 afferent signals from many receptors of cold blooded 

 animals depends not only on the adequate stimulus 

 but also on the temperature. This temperature sensi- 

 tivity of the sense organs of invertebrates raises a 

 physiological problem which has hardly been investi- 

 gated [cf. Bullock (20)]. 



A summary of the literature on the reactions of 

 lower animals toward temperature is given by von 

 Buddenbrock (130). 



MECHANORECEPTORS: TACTILE SENSE. The receptois for 

 this modality are well known only in a few cases. 

 Sense cells of soft skinned invertebrates which are lo- 

 cated in the epidermis and carry one or more long, 

 hair-like spines are termed hypothetical tactile re- 

 ceptors. However, no full proof has been given in any 



case [cf. MiilkT (87)]. Passano & Pantin (91) adopt 

 the view that a basal network of the sensory cells or 

 the nerve net or the circular and parietal muscle 

 sheets can be considered as receptors despite the fact 

 that many primary sensory neurons occur in the 

 ectoderm of the actinians. The tactile receptors of the 

 insects are definitely known to involve long, movable 

 hairs with joints in the chitinous skeleton at the base 

 of which one or many peripheral fibers of bipolar 

 sensory neurons end (fig. 13). They adapt rapidly if 

 their resting position is changed. The adaptation is 

 slow in certain spine-like hairs located on the legs of 

 insects. The initial frequency of the impulses in the 

 sensory a.xon depends on the velocity of displacement, 

 according to Pumphrey (loi). The transducer func- 

 tion of this sensory element was analyzed by Pringle 

 & Wilson (99). They were able to show that the maxi- 

 mum of response (recorded by the frequency of im- 

 pulses) precedes in phase the maximum tension of the 

 stimulus upon application of harmonic, sinusoidally 

 varying mechanical stimuli. This is a corollary of the 

 adaptation shown by the sensory response to a tran- 

 sient stimulus. 



MECH.JiNORECEPTORS: VIBRATION SENSE. Specific, highly 

 sensitive viisration receptors were found in the ex- 

 tremities of insects by Autrum (8, 1 1), by Autrum & 

 Schneider (15) and by Schneider (115). These are 

 groups of sensory cells which are spread in a sail-like 

 fashion in the Ijody fluid of the legs (fig. 16). They 

 are furnished with peculiarly differentiated bodies, 

 such as apical bodies, or scolopidia, as shown in fig. 6. 

 Adequate stimuli are provided by vibrations of the 

 ground. The subgenual organs respond preferentially 

 to vibrations between 200 and 6000 cycles per sec, 

 with maximum sensitivity between 1000 and 2000 

 cycles per sec. (fig. 1 7). The amplitude at threshold 



is about 4 X 



" cm at 1500 cycles per sec. for 



Fio. 16. Scheme of the subgenual organ in the leg of the 

 ant Formica, ac, accessory cells; ch, chitin cuticle; dc, cap cells; 

 ec, enveloping cell; hy, hypodermis; nc, nerve; nl, nucleus of a 

 neurilemma cell; sc, sense cells. [From Weber (137).] 



Periplaneta; consequently they are smaller than atoinic 

 dimensions.- The adequate physical stimulus is ac- 

 celeration. These organs cannot distinguish between 

 different frequencies. The high optimal frequency of 

 the vibration receptors of many insects given in figure 

 I 7 can be understood if their small size is considered. 

 Such high frequencies do not occur under natural 

 conditions. It is therefore not necessary to distinguish 

 the frequencies. Pulses and pulse-like vibrations of 

 the ground are important for reactions of insects in 

 the natural environment. These pulses possess high 

 frequency components and during the initial tran- 



- The amplitudes of movement of the human tympanum are 

 of the same order of magnitude at the threshold of hearing. 



