CHAPTER 14 



Mechano' and Equilibrium-Reception 



T 



■ HE POSITION OR ATTITUDE assumcd by an animal in its environment 

 I is usually the result of a complex of reactions. Several kinds of 

 _ ■ stimuli— light, temperature, mechanical contact, gravity— may simul- 

 taneously affect different sense organs. One sense modality may at times be 

 dominant, or the sum total of sensory stimulation may determine the attitude 

 the animal assumes. The orienting effects of light, heat and cold, and of chem- 

 ical and auditory stimuli have been discussed in previous chapters. Animals 

 which live on a firm substratum are continually subjected to contact stimuli; 

 motile organs and appendages are regularly stretched or extended and re- 

 tracted, thus subjecting moving tissues to tension. Most animals orient with 

 respect to gravity, and manv have special equilibrium receptors. Thus me- 

 chanical stimuli are common, and all animals are sensitive in some way to 

 mechanical stimulation. Sound is high-frequency mechanical stimulation; 

 we are here concerned, however, with stimulation by low-frequency mechan- 

 ical stimuli and bv gravity or equilibrium alteration. 



MECHANORECEPTION 



The sensitivity of cells to mechanical deformation of their surface is a 

 more widespread phenomenon than is commonly supposed. Some cells, the 

 phonoreceptors, are specialized in sensitivity to low amplitude vibration; 

 others, tactile receptors, respond to the greater deformation of touch; still 

 others, tension receptors, are specifically sensitive to distortion by stretch. 

 Tactile and tension receptors are more widely distributed than anv other 

 type of sensory cells, especially in appendages; they are rarely restricted to 

 particular organs. Changes in equilibrium may be detected by special organs, 

 statocysts, but these are supplemented by scattered tension receptors. 



Nature of Mechanical Stimulation. The cellular phenomena associated 

 with mechanical stimulation are poorly known. Nerve axons have been 

 stimulated mechanically with an air jet, and the relation between pressure 

 and duration of stimulus needed to elicit a response has been shown to re- 

 semble a conventional strength-duration curve as obtained with electrical 

 stimulation.** Presumably the mechanical deformation causes local potential 

 changes which initiate impulses electrically. An amoeba stimulated by a 

 weight dropped on the coverglass stops locomotion with a latency which 

 varies inversely with strength of stimulus; after one response a recovery time 

 is required before the amoeba can respond again. '^^ It would be of interest 

 to examine quantitatively the responses of many types of cell to mechanical 

 stimulation of various kinds. 



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