NONPHOTIC RECEPTORS I.\ LOWER FORMS 



379 



ing each bundle of muscles separately; b') a thick 

 accessory fiber which is ramified into two branches in 

 the vicinity of SN2 (fig. 15), one branch innervating 

 the area of the dendritic terminals of SNi, the other 

 branch innervating the dendritic terminals of SNj; 

 c) an additional thin accessory fiber innervating the 

 terminals of SNi and SN2 in Homarus (this is absent in 

 Astacus); and (f) a number of thinner fibers, the origin 

 and terminals of which are not known exactly but 

 mainly innervating RMj. 



The details may differ in the different species of 

 the crustaceans and may be found in the discussions 

 by Alexandrowicz (i, 2, 5) and Florey & Florey (42). 



The adequate stimulus for these receptors is stretch- 

 ing of the muscle bundles. One of the two receptor 

 cells of this organ (SNo, the slow cell) has tonic quali- 

 ties. The other one (SNj, the fast cell) has phasic 

 qualities. The same is valid for the corresponding 

 muscle bundles on efferent stimulation, RMi yielding 

 a fast twitch, RM2, a slow response (74). The tonic 

 receptor cell has a very low threshold to the adequate 

 stimulus (stretch) and yields uninterrupted discharges 

 with a low adaptation rate. The phasic cell has a high 

 threshold and adapts very rapidly and completely. 

 Excitation of the efferent nerve fibers leads to a con- 

 traction of the muscle bundles and in this way causes 

 afferent responses of the sensory neurons (74, 140). 

 The normal excitation originates in the dendrites. 

 These become depolarized by stretch deformation 

 producing a generator potential (37, 38). The gener- 

 ator potentials in the dendrites spread electrotonicalh' 

 and reduce the resting potential of the cell soma. Re- 

 duction of the resting potential (70 to 80 mv with 

 relaxed receptor) by 8 to 12 mv in the slow cell and 

 by 17 to 22 mv in the fast cell causes propagated 

 impulses. The neuron therefore works according to 

 the following scheme: stretch deformation of dendrite 

 terminals— ^generator potential^electronic spread to- 

 wards the cell .soma (prepotential) — >dendrite-soma 

 impulse— >axon impulse (37, 38). 



Excitation of the inhibitory fiber (fig. 15) acts upon 

 the generator mechanism in the dendrites and stops 

 the discharge of the sensory neuron within a few 

 milliseconds, even upon application of a strong stimu- 

 lus (large stretch). This effect is caused by the follow- 

 ing sequence of events. The impulses in the inhibitory 

 fiber cause a postsynaptic effect in the dendrites of the 

 sensory neuron. This drives the potential of the re- 

 ceptor cell towards an equilibrium level. The inhibi- 

 tory effect can therefore be a postsynaptic depolariza- 

 tion or a hyperpolarization depending upon the 

 existing state of the receptor cell. Through the stretch 



stimulus or its absence this may Ije pushed to either 

 side of the equilibrium potential (37, 38). 



The stretch receptors in the abdominal segments of 

 the crayfish are physiologically similar to the muscle 

 spindles of the vertebrates. Nevertheless all described 

 proprioceptors are considerably different from the 

 analogous organs of the vertebrates; they indicate not 

 the functional condition of a single muscle, but of a 

 whole body segment. These proprioceptors signalize 

 the relative position of the parts of an appendage, 

 e.g. the hair sensillae or the multipolar cells oi Limulus, 

 or they indicate the relative position of different seg- 

 ments of the body, e.g. the stretch receptors of the 

 crayfish or the muscle receptors of insects. The cam- 

 paniform and lyriform organs and the slit sense organs 

 are located in such a way that they can register the 

 forces which arise in the chitinous shell of the legs 

 upon contact of the extremity with the ground. These 

 organs therefore control the behavior and position of 

 the animals (78, 93, 94, 95, 98). 



Many of these organs are not designed to react to a 

 single specific mode of stimuli. The proprioceptors of 

 Carcinus respond both to the movement of the ex- 

 tremity and to vibration (22). The proprioceptors of 

 Dytiscus (which are not localized anatomically) yield 

 spikes during inspiration, expiration and to airborne 

 sounds of about 100 cycles per sec. (62). The stretch 

 receptors of Cambarus respond strongly to temperature 

 changes with a frequencv change of the discharges 

 (40-^ 



THERMORECEPTORS. Lower animals may lack tempera- 

 ture sensiti\'ity completely. The sea anemone CalUactis 

 is very sensitive to mechanical and chemical stimuli; 

 however, a glass tube which touches the body wall 

 can be heated so much that it causes burning of the 

 ectoderm without producing any reaction (91). On 

 the other hand most animals react to temperature. As 

 a rule the parasites of warm Islooded animals are 

 especially sensitive, beintr attracted by warm objects. 

 This has been shown for the leech and some insects, 

 e.g. Rhodnius by Wigglesworth & Gillet (142) and 

 Cimex by Sioli (119). 



The temperature receptors of invertebrates have 

 never been anatomically localized with precision. It 

 is assumed to be highly probable that the pointed 

 hairs on the antennae of insects are thermoreceptors. 

 This is the case in at least some species, including 

 Rhodnius (142) and Pyrrhocoris (46). At the base of 

 these hair sensillae lie six sensory neurons. 



The mechano- and chemoreceptors of the inverte- 

 brates very often respond to temperature by changes 



