636 



SPINAL REFLEX ACTIONS AND SPASMS. 



on being applied to an afferent nerve, sets up a state of excitement (nervous 

 impulse) in that nerve, which state of excitement is transmitted or conducted m a 

 central direction along the nerve to the centre (spinal cord in this case) ; where 

 the nerve-cells represent the nerve-centre in the cord, the impulse is transferred 

 to the motor efferent or centrifugal channel. Three factors, therefore, are essential 

 for a reflex motor act-a centripetal or afferent fibre, a transferring centre a centri- 

 fugal or efferent fibre ; these together constitute a reflex arc (fig. 456). In a purely 

 reflex act, all voluntary activity is excluded. 



Reflex movements may be divided into the three following groups : 

 I The simple or partial reflexes, which are characterised by the fact that 

 stimulation of a sensory area discharges movement in one muscle only, or at least 



Fig. 456. Fig. 457. 



Fig. 456. Scheme of a reflex arc. S, skin ; M, muscle; 1J, nerve-cell, with af, afferent, and 

 cf, efferent fibres. Fig. 457. Section of a spinal segment, showing a unilateral and crossed 

 reflex act. A, anterior, and P, posterior surface; M, muscle ; S, skin ; G, ganglion. 



in one limited group of muscles. Examples: A blow upon the knee causes a 

 contraction in the quadriceps extensor cruris ; contact with the conjunctiva causes 

 closure of the eyelids. In the former case, the afferent channels arise in the tendon 

 of the quadriceps, and the efferent channels lie in the nerve which supplies the 

 quadriceps ; in the latter case, the afferent nerve is the 5th and the efferent the 7th 

 cranial nerve. In the former case the centre is in the lumbar region of the cord ; 

 in the latter, in the grey matter of the medulla oblongata. 



II. The extensive inco-ordinate reflexes, or reflex spasms. These movements 

 occur in the form of clonic or tetanic contractions ; individual groups of muscles, 

 or all the muscles of the body may be implicated. Causes: A reflex spasm 

 depends upon a double cause (a) Either the grey matter or the spinal cord is in a 

 condition of exalted excitability, so that the nervous impulse, after having reached 

 the centre, is easily transferred to the neighbouring centres. This excessive 

 excitability is produced by certain poisons, more especially by strychnin, brucia, 

 caffein, atropin, nicotin, carbolic acid, &c. The slightest touch applied to an 

 animal poisoned with strychnin is sufficient to throw the animal at once into spasms. 

 Pathological conditions may cause a similar result ; thus, there is excessive 

 excitability in hydrophobia and tetanus. On the other hand, the central organ 

 may be in such a condition that extensive reflexes cannot take place ; thus, in the 

 condition of apncea, the spasms that occur in poisoning with strychnin do not take 

 place (J. Rosenthal and Leube), and the same result is brought about by passive 

 artificial respiratory movements ( 361, 3). The performance of other passive 

 periodic movements in various parts of the body also produces a similar condition 

 (Buchheim). If the spinal cord be cooled very considerably, reflex spasms may not 

 occur (Kunde). (b) Extensive reflex movements may also take place when the 

 discharging stimulus is very strong. Examples of this condition occur in man, 

 thus intense neuralgia may be accompanied by extensive spasmodic movements. 



