328 THE NERVOUS SYSTEM 



REFLEX ARCS OF THE SPINAL CORD 



Neuron I. Primary sensory neurons, with cell bodies in the spinal ganglia, 

 convey impulses from the sensory endings to the spinal cord, then along the 

 ascending and descending branches resulting from the bifurcation of the dorsal 

 root fibers within the cord, and along the collaterals of these branches to the 

 primary motor neurons, either directly or through an intercalated central unit 

 (Figs. 66-68). 



Neuron II. The central neurons have their cell bodies in the posterior gray 

 column and may belong to Golgi's Type II, having short axons restricted to the 

 gray matter; or their axons may be long, running through the fasciculi proprii 

 to the ventral horn cells at other levels of the cord. Some of these central axons 

 cross the median plane in the anterior commissure. 



Neuron III. Primary motor neurons, with cell bodies in the anterior gray 

 column, send their axons through the ventral roots and spinal nerves to the 

 skeletal musculature. Or in the case of visceral reflexes, the motor neuron has 

 its cell body located in the intermediolateral cell column, and its axon runs as a 

 preganglionic fiber to a sympathetic ganglion, whence the impulses are relayed 

 by a fourth or postganglionic neuron to involuntary muscle or glandular tissue. 



The reflex paths of the cranial nerves are similarly constituted, except that 

 rarely if ever do the sensory fibers form synapses directly with the motor cells. 

 The central neuron, which has its cell located in the sensory nucleus of a given 

 nerve, sends its axon through the reticular formation to the motor nucleus of 

 the same or of some other nerve (Figs. 92, 111). Two of the reflex circuits con- 

 nected with the vestibular nerve require special attention. 



VESTIBTJLAR REFLEX ARC THROUGH THE MEDIAL LONGITUDINAL BUNDLE 



Neuron I. The bipolar cells of the vestibular ganglion in the external audi- 

 tory meatus send peripheral processes to the cristae of the semicircular canals 

 and maculae of the saccule and utricle. Their central processes run through 

 the vestibular nerve to the vestibular nuclei (Figs. 135, 244). 



Neuron II. Cells in the lateral and superior vestibular nuclei send their axons 

 to the medial longitudinal fasciculus of the same or the opposite side, where they 

 divide into ascending and descending branches, which run in this bundle. From 

 these branches twigs are given off to the nuclei of the oculomotor, trochlear, and 

 abducens nerves and to the motor cells of the cervical portion of the spinal cord 

 (Fig. 244). 



Neuron III. Primary motor neurons of the oculomotor, trochlear, abducens, 



