MECHANORECEPTION 27 



orientation, postural relations, and touch are associated with compli- 

 cated and highly specific accessory structures whose function is to 

 transduce the energy of the stimulus into mechanical deformation of 

 the protoplasm of the neuron. Many cells are sensitive to mechanical 

 deformation, but the mechanoreceptors are highly specialized in this 

 respect. The different accessory structures and the different response 

 characteristics (e.g., rate of adaptation) of the receptor determine the 

 sensitivity of the sense organ and to which parameter of the stimulus 

 it will respond. 



THE TACTILE SENSE-SENSILLA TRICHODEA 



A non-living, tough, and most often rigid cuticle effectively insulates 

 tissues from all but the grossest mechanical disturbances of the 

 external environment. To provide the surface sensitivity denied by the 

 exocuticle there are numerous thin, hollow extensions, the sensilla 

 trichodea, on all surfaces of the body. They are most numerous on 

 those areas, such as legs, which come most frequently into contact 

 with the substrate, those, such as the antennae and mouth-parts, which 

 are employed in palpating or manipulating the environment, and on 

 such extended portions of the body as the antennae and cerci which 

 represent perimeter guards of the body. They also occur abundantly 

 on surfaces between joints, segments, and other appressed areas of the 

 body where their function is proprioceptive. Their shape, mechanical 

 properties, and the physiological properties of the neurons associated 

 with them vary considerably, depending upon the service they are 

 designed to perform. Those at the joints are short and slow adapting; 

 those on the cerci, extraordinarily long and delicate, and hence 

 sensitive to the most gossamer disturbance ; the thick tibial leg spines, 

 gross and slow adapting. 



The sensilla trichodea are set in membraneous sockets. The shaft is 

 so rigidly constructed that any force appUed to the structure is trans- 

 mitted to the socket. It is here that movement takes place and, because 

 of the leverage, is greatly ampHfied. The simplest sensilla trichodea are 

 those that are innervated by one neuron. These are simple mechano- 

 receptors. There are others, however, that are innervated by more 

 than one neuron, and these are compound sense organs. In the labellar 

 hairs of the blowfly, for example, one neuron is the mechanoreceptor 

 responding when the hair is moved in its socket ; the remaining neurons 

 are chemoreceptors responding when certain chemicals touch the tip 

 of the hair (cf. Chapter V). 



The exact manner in which the dendrite of the neuron is associated 



