NERVE IMPULSES AND REFLEX ACTION 



311 



THE NEURONS AS TRANSMITTERS OF IMPULSES 



Ingoing Paths. We have already learned that the pos- 

 terior roots of the spinal nerves carry messages inward and 

 that upon each posterior root 

 there is a swelling, the so-called 

 spinal ganglion; Fig. 158. 



Inside this spinal ganglion are 

 the cell bodies of a large 

 number of neurons. From each 

 ceil body extends a single pro- 

 jection or axis cylinder, which 

 soon divides, one branch passing 

 inward to the spinal cord, the 

 other outward in the nerve trunk 

 into the body, finally ending in 

 some sensitive part, e.g. the skin; 

 Fig. 163. If the skin is touched 

 in any way at that point, an 

 impulse will start and go rapidly 

 inward on the fibre, pass the 

 neuron cell body in the ganglion 

 and continue into the cord. In 

 the cord, as shown also in Figure 

 164, the fibre again divides, one 

 fibre going upward and the other FIG. 163. DIAGRAM ILLUSTRAT- 

 downward. The branch passing 

 up the cord soon divides into a 

 number of twigs, forming at c 

 a brush of fibrils called arboriza- 

 tions. Close to these arborizations 

 begin similar divisions of another 

 fibre whose cell body is higher up, possibly in the brain itself: 

 Fig. 164dL The impulse which enters the cord mav thus 



ING A SIMPLE REFLEX ACTION 

 The impulse from the sense organ 

 passes in over the nerve fibre, d, 

 and may pass up to the brain over 

 a-c. But a part may be switched 

 off through e down the fibre / to 

 the muscle, causing it to contract 

 without the aid of the brain. 



