CHAPTER XL 

 REFLEX ACTION 



Reflex action has already been referred to in Chap. XXX; it exists 

 in animals of the grade of flatworms and those higher in organization. 

 The structure in the earthworm is so easily correlated with the elements 

 in this type of action that it is usually studied in detail in this connection. 



280. Nervous Functions. — The functions of the nervous system are 

 the reception of stimuli, the conduction of nervous impulses, and the 

 stimulation of other cells of the body (Sec. 126). A nervous stimulus 

 is any outside influence exerted upon any part of a nerve cell and produc- 

 ing an effect. This effect transmitted through the cell or any of its 

 branches is termed a nervous impulse, and when this impulse is com- 

 municated as a stimulus to any other cell through an appropriate point 

 of contact, it starts an impulse in the other cell if it is a nerve cell or 

 causes it to act if it is a muscle or gland cell. 



The unit of structure in the nervous system is the neuron, which may 

 be defined as a nerve cell including all of its branches, or fibers. In 

 development the branches grow out from the cells to their ultimate point 

 of distribution. A neuron which receives a stimulus is known as a 

 receptor or sensory neuron, and one which conducts the impulse from the 

 receptor to the acting cell or cells is known as a motor neuron. The cells 

 which act and which are not neurons but muscle or gland cells are termed 

 effector cells, or simply effectors. Neurons generally show polarity. 

 which is the abihty to transmit impulses more readily in one direction 

 than in another. This is related to their position with reference to other 

 neurons, to sense cells, or to effectors. 



The impulse is passed from a fiber of one neuron to that of another 

 at one or more points where the fibers or their branches come in intimate 

 contact. It is believed that they do not become structurally continuous. 

 Such a contact area is known as a synapse. An impulse is communicated 

 to the effector cell usually through some form of nerve ending, such as 

 a motor end plate, in which the fiber comes in intimate contact with the 

 cell. 



All fibers which transmit impulses toward the cell body of the neuron 

 are termed dendrites, of which there may be several ; the one which trans- 

 mits an impulse in the opposite direction is termed the axon, or the axis 

 cylinder fiber. The path by which impulses pass from the periphery to 

 a nerve center is termed the afferent path, while that by which impulses 



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