REFLEX SPASMS AND SUMMATION OF STIMULI. 



637 



[Fig. 458 shows the mechanism of simple and complex reflex movements. Suppose the skin 

 to be stimulated at P, an impulse is sent to A and from it to a muscle 1 on the same side, result- 

 ing in a unilateral simple reflex movement the resistance being less in this direction than in 

 the other channels. If the impulse be stronger, or the transverse resistance in the cord dimin- 

 ished, the impulse may pass to B, thence to 2, resulting in a symmetrical reflex movement on 

 both sides. But if a very strong impulse reach the cord, or if the excitability of the grey 

 matter be increased, e.g., by strychnin, the resistance to the diffusion of the impulse is dimin- 

 ished, and it passes upward to C and D, resulting in more complex movements thus there is 

 irradiation or it may even affect the centres in the medulla oblongata, E, giving rise to 

 general convulsive movements.] 



General spasms usually manifest themselves as "extensor tetanus," because the extensors 

 overcome the flexor muscles. Nerves which arise from the medulla oblongata may be excited 

 through the stimulation of distant afferent nerves, without general spasms being produced. 



Strychnin is the most powerful reflex-producing poison we possess, and it acts upon the grey 

 matter of the spinal cord. [An animal poisoned 

 with strychnin exhibits tetanic spasms on the 

 application of the slightest stimulus. All the 

 muscles become rigid, but the extensors overcome 

 the flexors.] If the heart of a frog be ligatured, 

 and the poison afterwards applied directly to the 

 spinal cord, reflex spasms are produced, proving 

 that strychnin acts upon the spinal cord. During 

 the spasm the heart is arrested in diastole, owing 

 to the stimulation of the vagus, while the arterial 

 blood-pressure is greatly increased, owing to stimu- 

 lation of the central vaso-motor centres of the me- 

 dulla oblongata and spinal cord. Mammals may 

 die from asphyxia during the attack ; and, after 

 large doses, death may occur, owing to paralysis 

 of the spinal cord, due to the frequently recurring 

 spasms. Fowls are unaffected by comparatively 

 large doses. [We can prove that strychnin does 

 not produce spasms by acting on the brain, 

 muscle, or nerve. Destroy the brain of a frog, 

 divide one sciatic nerve high up, and inject a small 

 dose of strychnin into the dorsal lymph-sac ; in 

 a few minutes all the muscles of the body, except 

 those supplied by the divided nerve, will be in 

 spasms, showing that, although the poisoned blood 

 has circulated in the nerves and muscles of the 

 leg, it does not act on them. Destroy the spinal 

 cord, and the spasms cease at once.] 



Summation of Stimuli. By this term 

 is meant, that a single weak stimulus, 

 which in itself is incapable of discharging 

 a reflex act, may, if repeated sufficiently 

 often, produce this act. The single im- 

 pulses are conducted to the spinal cord, in which the process of " summation" 

 takes place. According to J. Rosenthal, 3 feeble stimuli per second are capable 

 of producing this effect, although 16 stimuli per second are most effective. On 

 increasing the number of stimuli per second, no further increase of the reflex act is 

 possible. Other observers (Stirling, Ward) have found that stimuli, such as in- 

 duction shocks, are active within mueh wider limits, e.g., from 0-05 to 0*4 second 

 interval. W. Stirling has shown it to be extremely probable that all reflex acts 

 are due to the repetition of impulses in the nerve-centres. 



[Strychnin interferes with the summation of stimuli, but the reflex excitability is so greatly 

 exalted that a minimal stimulus is at the same time a maximal one.] .. 



Ffluger's Law of Reflex Actions.- (1) The reflex movement occurs on the same side on which 

 the sensory nerve is stimulated ; while only those muscles contract whose nerves arise from the 

 same segment of the spinal cord. (2) If the reflex occur on the other side, only the corre- 

 sponding muscles contract. (3) If the contractions be unequal upon the two sides, then the 

 most vigorous contractions always occur on the side which is stimulated. (4) If the reflex 

 excitement extend to other motor nerves, those nerves are always art'ected which lie in the 



Fig. 458. 

 Scheme of mode of propagation of reflex 

 . movements. P, skin ; A, B, C, D, motor 

 cells in spinal cord ; 1, 2, 3, 4, 5, muscles. 



