6o 



ESSENTIALS OF PHYSIOLOGY. 



L.3 



L.4. 



S.2 



the stimulus is increased, the muscular response may extend to the 

 opposite leg and to other parts of the body. This experiment shows 

 that an impulse reaching the spinal cord finds its way most easily 

 along a certain path, and that its spread up or down the cord is 

 normally prevented by the resistance offered by neighbouring synapses. 

 After the injection of strychnine this resistance disappears, and an 

 impulse reaching the cord at any level evokes generalised muscular 

 movements which are incoordinate. The normal limitation of the 



reflex response is thus 

 an important part of 

 the means by which the 

 object of the reflex is 

 obtained. 



Inhibition. - - Re- 

 flexes may be restrained 

 or completely inhibited 

 by impulses, voluntary 

 or involuntary, from the 

 higher nerve centres. If 

 a reflex is elicited in a 

 frog from which the 

 cerebral hemispheres 

 have been removed, and 

 then a crystal of com- 

 mon salt is applied to 

 the optic lobes and the 

 stimulus is repeated, the 

 reflex movement may 

 not occur, owing to in- 

 hibitory impulses result- 

 ing from the stimulation caused by the salt. A reflex act, which is 

 in progress, may also be checked or inhibited by the advent of another 

 stimulus to the central nervous system/ Further, inhibition forms a 

 constituent of many reflex actions, the movement of certain muscles 

 being accompanied by the simultaneous inhibition of others. 



Reciprocal Innervation. When firm pressure is applied to the 

 sole of the foot in a spinal dog, it responds by rapid extension of the 

 leg, due partly to contraction of the extensor muscles of the thigh. 

 It is evident that this extension is only possible if the flexor muscles 

 (hamstrings) are at the same time relaxed, and their relaxation is not 

 a passive but an active process and forms an essential part of the ex- 

 tensor reflex. This can be shown by separating the extensor- and flexor 



FIG. 13. Diagram to show muscles and nerves con- 

 cerned in Sherrington's experiment on the 

 reciprocal innervation of antagonistic muscles. 

 (Starling's Principles of Physiology. ) 



L.3, L.4, L.5, 3rd, 4th, and 5th lumbar nerve roots. S.I, S.2 : 

 1st and 2nd sacral roots. 



