Mechano- and Equilibrium-Reception 



513 



mm. Hg, when a few impulses appear, and at higher perfusion pressures the 

 impulse frequency in single fibers is directly proportional to the pressure 

 (Fig 183). There may be a slight fall in frequency after the initial dis- 

 charge on increasing pressure, but the equilibrium frequency is main- 

 tained more or less indefinitely.''' ^- 



In the preceding account mechanoreceptors are arranged in a series from 

 fast to slow adaptation. It is evidently useful in behavior that tactile mes- 

 sages be brief and tension reception persistent. There is no clear histological 

 correlation with speed of sensorv adaptation, and the cellular mechanism 

 underlving these differences in adaptation remains to be discovered. 



Pain. Tactile and proprioceptive impulses travel in relatively large 

 fibers (A fibers of the classification of Erlanger and Gasser, as given in 

 Chapter 23). Free nerve endings of the vertebrate skin are stimulated by 



Fig. 183. Steady discharge in single fiber of carotid sinus nerve at the following 

 perfusion pressures: A, 40 mm. Hg; B, 80 mm. Hg; C, 140 mm. Hg; D, 200 mm. Hg. 

 Time marked in 0.2 sec. From Bronk and Stella.'^ 



various noxious stimuli, heat, prick, heavy pressure, acid, etc., and give rise 

 to impulses in small fibers.' The pain impulses are of small amplitude and 

 low velocity. In the cutaneous nerves of the frog, for example, narcotics 

 abolish the slow impulses first, KCl eliminates the fast impulses first; the 

 slow are conducted at 1.5-4.5 M./sec, the fast at 10-12 M./sec.,^" and the 

 sensory areas in the skin for light touch and heavy pressure are different. 

 In the cat, impulses from sensory hairs travel at 30-60 M./sec, and pain im- 

 pulses are conducted at less than 5 M./sec.^*' The maximum frequency of 

 discharge in the small fibers is less than from the tactile receptors; the re- 

 sponse gradually builds up and adapts very little. 



Similarly, mechanical stimulation of the intestine of a cat elicits impulses 

 in medium-size fibers from the pacinian corpuscles and in small fibers prob- 

 ably from pain endings.^- There are also pain endings in the pulp of teeth 



