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 charac- 

 teristics 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 gastrocnemius be excited, the whole muscle 

 con-tracts. 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 f 

 the impulse. These nerve-endings are the fields of conjunction, the synapses, 

 between the axon of one neuron and the dendrites and cell body of the neuron 

 next in the chain. 



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

 of a muscle excited through its nerve as compared with the latent period 

 when excited directly, and we ascribed this latent period to a delay in the 

 motor end-plate. We should expect therefore to find that the delay or 



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