630 IRRITABILITY OF NERVES. 



In mixed nerves the reflex-conducting power is abolished earlier than the motor 

 power. 



Nerve-stretching is a mechanical procedure that has been employed for thera- 

 peutic purposes. If the exposed nerve is stretched, the tension acts as an irritant 

 when it reaches a certain degree. After slight stretching the reflex irritability is 

 at first increased ; stronger stretching causes for a time diminution of irritability, 

 as well as of reflex activity, and even temporary paralysis. The most extreme 

 degree of stretching finally gives rise to permanent paralysis. It appears that 

 the centripetal fibers (sciatic nerve) lose their conductivity earlier than the cen- 

 trifugal fibers. In the process of stretching mechanical changes are induced in 

 the nerve-tubes or in the end-organs that bring about alteration in irritability. 

 The effect of the stretching may be propagated also to the central nervous system. 

 Paralysis following forced stretching may undergo a marked degree of recovery. 

 If, therefore, a nerve is in a state of excessive irritability, for example in a case 

 of neuralgia, if this be due to inflammatory fixation or constriction of a nerve 

 in its course, nerve-stretching may be useful partly by diminishing the irritability 

 of the nerve, partly by breaking up the inflammatory adhesions. Nerve-stretching 

 may be useful also in cases in which irritation of a centripetal nerve gives rise 

 to reflex or epileptic convulsions by diminishing the peripheral irritability (in 

 addition to the action described). In the case also of diseases of the spinal cord 

 that have not yet advanced to a state of gross degeneration nerve-stretching is 

 not to be neglected as a therapeutic agent. 



For physiological purposes R. Heidenhain's tetanomotor is employed to 

 induce mechanical nerve-stimulation. This consists of a vibrating ivory hammer 

 attached to an extension of the Neef 's hammer of the induction-apparatus, which 

 by a rapid succession of blows upon the underlying nerve develops a condition 

 of tetanus lasting up to two minutes. 



Naturally, other mechanical stimuli of a similar nature will yield analogous 

 results, such as contact with a vibrating tuning-fork, or with a sounding string, 

 stroking with a bow-like apparatus, rhythmic stretching of the nerve (longitudinal 

 traction) . 



Thermal Stimuli. If a frog's nerve be heated to 45 C. its irritability at first 

 increases and then declines. The higher the temperature the greater is the 

 increase in irritability, but the shorter is its duration. Heated for a short time 

 to 50 C. the irritability and the conductivity of the nerve are abolished; but on 

 cooling, the frog's nerve is capable of recovering its irritability. Heat increased 

 above 65 C. destroys the irritability, without preceding contraction, with degenera- 

 tion of the myelin. A gradually frozen nerve retains its irritability when thawed. 

 The cooled nerve retains its irritability for a long time. In motor nerves the irri- 

 tability is increased, but the contractions are slighter and more prolonged and 

 the conduction in the nerve continues for a longer time. Sudden cooling of nerves 

 by a temperature of 5 C. and below excites contraction, as does also sudden 

 warming by a temperature of from 40 to 45 C. and above. At still higher tem- 

 peratures persistent tetanus occurs instead of the contraction. All such irritat- 

 ing variations in temperature if continued rapidly destroy the nerve, and probably 

 give rise to chemical and mechanical alterations in the nerve-substance. 



Of the nerves of mammals only the centripetal fibers and the dilators of 

 the blood-vessels exhibit the effects of irritation by temperatures between 45 

 and 50 C. The remainder exhibit merely a change in irritability. Cooling 

 to +5 C. diminishes the irritability of all of the nerve-fibers. Cooling of the 

 ulnar nerve by immersion of the elbow-ioint in cold water causes pain and con- 

 traction in the peripheral distribution of the nerve, such as is brought about by 

 prolonged pressure. Local cooling of nerves increases the irritability to the 

 constant current lasting for a considerable time (at least for ^o second) , and to 

 mechanical and some chemical stimuli. A marked lowering of the temperature 

 locally may abolish the conductivity of a nerve at the point of application. 

 Local heating of the nerve to 35 C. increases its irritability to the faradic cur- 

 rent, as well as to constant currents of shorter duration (of less than ^ second). 



According to Howell the extremes at which irritability is still present in 

 motor nerves are 4 (cat), in the inhibitory nerve of the heart below 15, in vaso- 

 motor nerves between 2 and 51, in the sweat-nerves between 3 and 45, in 

 the respiratory fibers of the vagus 7, and in the pressor fibers of the sciatic in 

 rabbits about 2 C. 



Chemical stimuli (chemical muscle-stimuli are discussed on p. 556) give rise 

 to irritation in nerves when they cause alteration in the constitution of the latter 



