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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



the supersonic region, above 20,000 cycles per sec, as 

 shown by VVever & Bray (139), Pumphrey (102), 

 Autrum (8, 9, 10), and Schaller & Timm (i 1 1). The 

 upper limit of hearing in some species is at 1 75 

 kilocycles per sec. Electrophysiological analysis has 

 shown that the tympanal organs cannot distinguish 

 the frequency of the sound. They consequently are 

 not able to analyze sounds but react very sensitively 

 to modulations of amplitude (53, 105). In the natural 

 environment the stimulus is a sound of high frequency 

 which is amplitude-modulated. Such a sound is im- 

 portant for their behavior. For example, the songs of 

 crickets consist of sound with certain rhythms of 

 modulation. The impulses in the tympanal nerve do 

 not follow the frequency of the sound since these 

 frequencies are too high but follow the modulation 

 frequency to about 300 per sec, as shown in Tetti- 

 gonia by Autrum (unpublished observations). The 

 ultrasound therefore serves as a carrier frequency. 



Acoustically these tympanal organs are displace- 

 ment receptors as are the hair sensillae (8, 10, 102). 

 The sensitivity of all acoustic displacement receptors 

 depends on the direction from which the sound comes. 

 The tympanal organs have for this reason, therefore, 

 an 8-shaped polar diagram, which is important for 

 the localization of the source of the sound (10, 13). 

 In the same way sound reception by means of hairs is 

 dependent on the direction (125). 



ST.^TOCYSTS. The statocyst of the invertebrates consists 

 of a little ectodermal bag invaginated into the interior 

 of the body. It is filled with fluid, has hairs in certain 

 parts and contains one or more statoliths which are 

 little stones or concretions of greater specific gravity 

 than the fluid. According to the general scheme of 

 function the statocysts are analogous to the otolith- 

 containing organs of the vertebrates. Considerable 

 differences are found in the anatomical details. Thus, 

 the sensory neurons in the invertebrates are primary 

 sense cells with an afferent axon and there are addi- 

 tionally nonliving cuticular hairs but no equivalent of 

 the hair cells; in the vertebrates, by contrast, the hair 

 cells are secondary sense cells and lack an axon. 



The variety of the details is also very great in the 

 invertebrates [cf. Hanstrom (50) and Plate (92)]. 

 Statocysts are the first sense organs to appear in the 

 phylogeny of the animal kingdom, being present in 

 the coelenterates. 



The adequate stimuli are gravity (static stimuli), 

 acceleration (dynamic stimuli), or both. Compensa- 

 tory reactions of the whole body, tonic reactions to 

 certain muscles, or both, are directed from the stato- 



cysts. The statoliths are important for static reactions 



(73). 



The statocysts of crustaceans have been most care- 

 fully analyzed both by physiological behavior experi- 

 ments by Schone (117) and Dijkgraaf (34), and by 

 electrophysiological studies by Cohen et al. (27) and 

 Cohen (26). The adequate stimulus for the nerve end- 

 ing is a bending of the cuticular hair, not pressure or 

 pull of the statolith upon the hairs. Bending of the 

 hairs medially causes a refle.x rotational movement 

 about the long axis in the same direction; bending 

 towards the outside causes a rotation of the animal in 

 the opposite direction. The sensory epithelium gives 

 rise to a tonic impulse train which is independent 

 of stimulation of the statolith. The stimulus bending 

 the sense hairs produces either its own impulse or may 

 modify the tonic impulses. Between the intensity of 

 the stimulus (the bending) and the reaction (measured 

 in the tonic reactions of the eye stalk) there exists a 

 linear relationship. If the statoliths are removed on 

 one side, the zero position will move to this side since 

 the area of the statocyst which is adjacent to the sense 

 hairs is inclined outward and therefore the statoliths 

 in their normal position bend the hairs outward. 

 Compensatory processes counteract these changes of 

 the zero position. The position receptors are hook- 

 shaped hairs in Carcinus and Maja, while the receptors 

 for angular displacements are very thin thread-like 

 hairs, 300 y. long (34). The statocysts of Astacus do not 

 work antagonistically to each other, but in the same 

 lateral position each produces the same tendency 

 towards rotation. The impulses from both sides are 

 simply added up in the central nervous system. 



The results of Schone (i i 7) and Dijkgraaf (34) are 

 in accordance with the electrophysiological findings 

 of Cohen et al. Q2'f) and Cohen (26) in that the stato- 

 cysts react to rotatory acceleration around the axes 

 [cf. Dijkgraaf (34)], to linear acceleration and to 

 static position. A tonic discharge exists which remains 

 even after removal of the statoliths. There exist rapidly 

 adapting phasic elements and also tonic elements. 

 The excitation of the latter depends upon the position 

 and they adapt only slightly. Cohen found four types 

 of afferent fibers, each reacting differently. The type 

 I position receptor shows a nonadaptive impulse fre- 

 quency which depends (within a certain angle to the 

 normal position) upon the angle between the trans- 

 verse axis and the normal position. The position re- 

 ceptor of type II may well be not a single receptor 

 but may result from the coordination of several re- 

 ceptors. It a) maintains a characteristic nonadapting 

 impulse frequency for each constant deviation from 



