NERVE RESPONSE— RROOKS 



261 



42 C 



38°C 



32°C 



Fig. 1. — The effect of cooling the spinal cord on reflex response: Incoming volley (top beam), 

 ventral root potentials (middle) and dorsal root reflex (lower). Time in 5 msec, intervals. 

 Amplification signs := 1 mv. 



thermia. A stimulus, such as a muscle stretch, which evokes a pure monosynaptic 

 response at normal temperature elicits a reflex with a polysnyaptic component if 

 the spinal cord or the whole animal is cooled. Incoming impulses create central 

 effects which "spill over" or involve other pathways not normally activated (tig. 2). 

 In this connection it should be pointed out that polysynaptic reflexes are more 

 greatly augmented by cooling than are the monosynaptic reflexes (Koizumi, Mal- 

 colm and Brooks, 1954). 



This interaction during cooling plus the slowed conduction and greater ampli- 

 tude of potentials in afferent terminals is probably responsible for the very great 

 increase in dorsal root potentials and the dorsal root reflex (Brooks and Koizumi, 

 1955). The antidromic firing in adjacent roots and fibers occasioned by incoming 

 volleys in the hypothermic animal is considerable and is capable of blocking by 

 collision incoming afferent discharges. This may be of physiological importance to 

 somatic response and might even contribute to phenomena such as ataxia and 

 shivering. 



As progressive cooling continues a stage is reached at which a single afferent 

 volley evokes multiple motoneuron discharges. This repetitive firing of the cooled 

 motor nerves is explainable on the basis of the prolonged and greater central action 

 of the single stimulus. This is another example of hyperresponsiveness which is 

 present though generally unseen because of the common use of anesthetics, the 

 cooling of effectors and their reduced ability to respond. 



Finally, it has been observed that cooling will produce a tetanus. The rhythm of 

 this tetanic activity is much slower than that of a strychnine or metrazol-induced 



