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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



of inleraction of spinal refiexes (87, 89, 92) and ulti- 

 mately to the nature of reflex transmission. It was 

 shown that for each type of reflex response a particu- 

 lar or 'adequate' type of stimulus was necessary, and 

 that in circumstances of concurrent multiple stimuli, 

 the nociceptive stimulus was 'prepotent/ Reflexes 

 exhibited the phenomena of facilitation and spatial 

 summation. At some point in the reflex network the 

 effects of different afferents converged on the same 

 motor neuron — the final common path. The discon- 

 tinuity of afferent, interrelated neurons and motor 

 cells demonstrated by Kolliker, Ramon y Cajal, 

 Golgi and van Gehuchten, with the postulate of 

 'dynamic polarization' of these histologists was in- 

 corporated into the 'synapse' for which Ramon y 

 Cajal demonstrated the 'end feet' on the surface of 

 the nerve cell and its dendrites. Sherrington also 

 proved that spinal inhibition is an active process and 

 demonstrated its reciprocal relation to excitation of 

 the motor neurons of antagonists. Like excitation, 

 inhibition is capable of temporal and spatial summa- 

 tion. Where neurons already discharging in response 

 to activation by one path were blocked from any 

 exciting effect by the prevdous activation of another 

 path converging on them, the effect was shown to be 

 passive (occlusion) and altogether different from in- 

 hibition (16). Of the neurons available to either of 

 such converging afferent pathways, few were excited 

 by weak stimulation of each path; yet the others were 

 still excited subliminally and could be discharged by 

 weak stimulation of both paths together. For each 

 reflex response there was therefore a wide 'subliminal 

 fringe' (29) of affected but unresponsive neurons. 



From such principles there emerged a concept of a 

 network of anatomically predetermined connections, 

 the effectiveness of each depending on the number of 

 end feet supplied to each pathway. The muscles, 

 being collections of motor units, are related to 

 columns of motor cells in the spinal gray matter, 

 their functional significance residing in the fi.xed 

 synaptic relationship supplied to the motor neurons 

 in the course of development. Earlier attempts to 

 prove a plasticity of central connection were dis- 

 proved by the experiments of Cunningham (18) and 

 others and lately of Sperry (94) who showed that 

 transplantation of nerves or muscles so as to perform 

 antagonistic function in the postnatal mammal could 

 not determine change of refle.x effect. 



Postural Reflexes 



Following the complete flaccidity of spinal shock 

 the posture of the limbs is at first an intermittent 



flexion. As the flexion reflex becomes more active 

 its after-discharge is at first more prolonged. A pluck 

 of a flexor tendon (e.g. that of the tibialis anterior) 

 may elicit a flexor jerk at this time. The emergence 

 of the reflexes of extension is associated with curtail- 

 ment of flexor after-discharge and intermittent ex- 

 tension of the limbs. For a time alternating stepping 

 then appears. In full reflex recovery the hind limbs 

 of the spinal dog are sufficiently extended to support 

 the hind quarters, although they have to be propped 

 passively in this position (39, 87). The posture is 

 liable to sudden lapses owing to intermittent brief 

 flexions or to the onset of stepping. Lateral stability 

 is impaired by overactive adduction. These spinal 

 antigravity postural responses are due to the presence 

 of the spinal stretch reflex (27) which can be sus- 

 tained for long periods and is associated with very 

 brisk tendon reflexes, ankle and patellar clonus. 

 Under certain circumstances the stretch reflex may 

 be demonstrated in fle.xor muscles, but this response 

 is very inactive in the spinal state. 



The stretch reflexes in both spinal and decerebrate 

 preparations are affected to some extent by cutaneous 

 stimulation, although they are in no way dependent 

 upon it. Thus if the animal is caused to sit up on the 

 rump, both lower limbs tend to extend stiffiy, more 

 so if the spine is tilted backwards. If the animal is 

 tilted forwards so as to flex the hip joints, the knee 

 and ankle tend to flex. There is little change if the 

 hind quarters are laid on one side, although the 

 lower limb may then tend to flex briefly. Pressure on 

 the pads of the feet increases the extensor resistance 

 and, if the limbs are previously flexed, to elicit an 

 extensor thrust. These effects from cutaneous pres- 

 sure are evidently spinal fragments of what in the 

 thalamic animal are identified as body-on-body 

 righting reflexes (68, 77) and contactual 'positive 

 supporting reaction' (69). They are particularly well 

 developed in the .skin of the feet and lateral thighs 

 as the reflex of 'ipsilateral extension' analyzed by 

 Hagbarth (45). A decapitated insect can right itself 

 by contact reactions (10) and the spinal segments of 

 an eel can right themselves; but the mammalian 

 spinal refle.xes preserve a more local character. The 

 relationship of the spinal extensor thrust to the 

 gallop mode of progression, and to the powerful 

 leap reflex of the thalamic animal (68), is the same as 

 that of the stretch reflex to spinal stepping and of 

 both to the more fully developed progression at 

 higher levels of transection of the neuraxis. The 

 spinal mechanism thus provides all the elements of 

 postural responses. It is their integration with the 

 whole of the organisin that is lacking. 



