SECTION III 



GENERAL CHARACTERISTICS OF REFLEX 

 ACTIONS 



THERE are certain features common to all reactions, carried out 

 through the intervention of the central nervous system, which must 

 be regarded as determined by the properties of the neurons, i.e. the 

 conducting links in the chain of excitatory tissues intervening 

 between the stimulated spot on the exterior and the reacting tissue, 

 muscle or gland. These characteristics may be roughly classified 

 as follows : 



(1) LOCALISATION. In a simple system of neurons a given 

 stimulus will nearly always produce the same reaction. In a frog 

 possessing only a spinal cord, the upper parts of the central nervous 

 system having been destroyed, any harmful stimulus applied to a toe 

 will cause a lifting of the leg. If the motor nerve to the gastroc- 

 nemius be excited, the whole muscle contracts. If one of the nerve- 

 roots entering into the formation of the sciatic nerve be excited, only 

 certain fibres of the gastrocnemius contract, the locality of the reacting 

 fibres being determined by their connection with the excited nerve 

 fibres. In the same way the contraction of certain muscles of the 

 leg, in response to a stimulus applied to the skin of the foot, is deter- 

 mined by the fact that the nerve fibres, which carry the impulses from 

 the toe into the spinal cord, divide there and make connections with 

 the motor neurons, whose axons are distributed to the several muscles 

 involved in the reaction. The connection of the sensory with the motor 

 neuron may be direct, but in most cases the impulse has to pass through 

 intermediate neurons before arriving at the motor neurons. The path 

 of the impulse, however, in spite of its enormous extension, is as definite 

 as is the path from an excited motor nerve-root to a muscle fibre. 



(2) DELAY. Instead of one nerve-ending intervening between 

 the stimulated nerve and the reacting tissue, there will be, in the 

 case of the reflex action, two, three, or more nerve-endings interpolated 

 in the path of the impulse. These nerve-endings are the fields of con- 

 junction, the synapses, between the axon of one neuron and the den- 

 drites and cell body of the neuron next in the chain. 



We have seen that there is a distinct difference between the latent 



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