76 The Conduction of Impulses by Nerves /4 : 3 



first cell body in ganglia at the dorsal roots of the spinal cord, just out- 

 side the spinal column. In these ganglia, synapses occur; the axons 

 entering the spinal cord are those of the second neuron. 



In efferent pathways leading to muscles, glands, and other target 

 organs, the axon conducts impulses from the nerve cell body. These 

 pathways may be conveniently divided into the voluntary motor path- 

 ways and the autonomic systems. The voluntary motor pathways have 

 all their cell bodies within the central nervous system. For example, the 

 cell bodies for the axons which control the toe muscles are within the 

 spinal cord. 



In contrast to the voluntary pathways, the autonomic pathways almost 

 always have a synapse and cell body outside the central nervous system. 

 The autonomic system is divided into two parts on a functional and 

 anatomical basis. Those pathways leaving the spinal cord in the nerves 

 between the thoracic and lumbar vertebrae comprise the thoracolumbar 

 division, or sympathetic system. The other part of the autonomic 

 system, with pathways which leave the central nervous system via the 

 sacral region of the spinal column or the cranium itself, is called the 

 craniosacral division, or parasympathetic system. Most organs are 

 supplied by the craniosacral division as well as the thoracolumbar 

 division. 



The thoracolumbar division, in most of its activities, prepares an 

 animal for "fight or flight." The effects of stimulation of the thoraco- 

 lumbar system include accelerating heart and respiratory rates, sup- 

 pressing digestion, increasing blood flow to striated muscles, increasing 

 blood pressure, and decreasing blood flow to the skin and smooth 

 muscle. In general, stimulation of the craniosacral system produces 

 effects which are opposite to those produced by the thoracolumbar 

 division. 



Although the neurons within the central nervous system look similar 

 to those without, they are different in many respects. If a nerve fiber 

 is injured within the central nervous system, the entire neuron degener- 

 ates. On the contrary, if a peripheral nerve is severed, the fibers will 

 regrow out of the old nerve trunk from the central ends. Another 

 difference is the sensitivity to oxygen. If a neuron within the central 

 nervous system (of an adult) is deprived of oxygen for a short period of 

 time, it will be irreversibly destroyed. In contrast, axons outside the 

 central nervous system will continue to conduct impulses for more than 

 an hour in the absence of oxygen. This is in part due to the difference 

 between the sensitivity of the nerve cell bodies and the axons. These 

 differences hold for the larger axons which are heavily myelinated 

 outside the central nervous system, as well as for the smaller, less myelin- 

 ated axons. 



