Variations in Reflex Sensitivity

Now another class of factors which enter into the determination of a reflex are those which influence its sensitivity, or threshold. Perhaps the simplest of these is exemplified in the principle of summation of successive stimuli. A brief stimulus may be quite inadequate to arouse a reflex, but at the same time it increases the sensitiveness of the system to the action of a second brief stimulus, and this effect accumulates until the reaction appears. Thus, the history of stimulation during immediately preceding intervals may be a factor in determining the effectiveness of a stimulus. An influence which is the reverse of summation is found in phenomena of fatigue or refractory phase. Fatigue may occur in the receptors, due to the continued action of the stimulus, so that adequate afferent nerve currents are not set up or, on the other hand, it may be localized at the central, or synaptic point and interfere with the arousal of the motor sector of the reflex arc even when the afferent sector is operating normally. Refractory phase is a temporary condition, immediately following the discharge of the motor tendency, under which it is very difficult to rearouse the response. The degree of difficulty may vary from complete impossibility, during the period just subsequent to the discharge, to a relatively slight increase in difficulty later on when the normal condition has nearly been reëstablished.

Although the continued exercise of the nerve center causes fatigue, at a much later time an increase of sensitivity may be noted as a consequence of exercise. In addition to these influences, we must also consider the effect of changes in the chemical constitution of the lymph which surrounds the nerve cells. The presence of such drugs as alcohol, strychnine, or morphine in these fluids has a tremendous influence upon the central response to afferent nerve impulses; and variations in the oxygen and carbon dioxide content are also accompanied by important alterations of sensibility. Lack of oxygen tends to prolong the refractory and fatigue phases, while strychnine greatly increases the excitability. Different nerve centers or transfer points exhibit different degrees of sensitiveness to such chemical agencies, so that the chemical condition of the medium may determine the outcome of a conflict between two simultaneously aroused response tendencies.