372 



CHORDATE ANATOMY 



descending motor fibers. A fasciculus may contain both kinds. Fibers 

 may have their origin or termination at any level and relatively few 

 extend the entire length of the cord. (Fig. 329) 



Neuron Relations in the Cord. The spinal cord is both the center 

 for reflexes and the pathway for impulses towards and away from the 

 brain. 



In the simplest possible reflex action where only two neurons are 

 involved, the synaptic connexions between the two lie in the gray matter 

 of the cord. The cell body of the afferent neurone is in the sensory 

 gangHon of the afferent nerve. That of the effector neuron is in the 

 central column of the spinal cord, and its neurite extends by way of the 

 ventral root to a muscle fiber or a gland. 



Usually, however, more than two neurons are chained together in a 

 reflex act, for there may be one or more association neurons, located 

 in the gray matter of the cord, which carry the impulse from the receptor 

 neuron to the effector neuron. But it should not be understood that a 

 somatic motor nerve cell located in the ventral column has synaptic 

 connexions with the telodendria of only a single neurite. On the contrar}-, 

 many neurites may have synaptic relations with the dendrites of each 

 somatic motor neuron. The motor neuron is simply "the final common 

 path." 



Within the gray matter of the spinal cord, the central connexions 

 of the neurites of an afferent neuron may be of various kinds. The 

 telodendria may connect directly with the dendrites of a somatic motor 

 cell. This is the simplest relation. They may connect with a neuron 

 in Clark's column near the median line at the base of the dorsal column, 

 the nervous impulse being carried in the dorsal cerebellar tract; or they 

 may pass to a commissural neuron, the impulse being carried to the 

 opposite side of the spinal cord. There are also connexions with sym- 

 pathetic neurons. 



The neurites of receptor neurons, having entered the spinal cord, 

 immediately dichotomize to give off long ascending and short descending 

 branches and thus add to the fibers of the fasciculus cuneatus. As these 

 fibers pass towards the brain, they are displaced inwards by the fibers 

 which are added from higher levels. The result of this process is that, 

 in the neck region, the neurites which enter the cord in the lower trunk 

 region come to lie in a median fasciculus, the fasciculus gracilis. Most of 

 the descending short fibers end in the gray matter of the cord. From 

 both ascending and descending neurites, fine collateral branches pass 

 into the dorsal column of gray matter, and come into synaptic connexions 

 with dendrites of the same or of the opposite side. 



The fibers which enter the spinal cord by the dorsal root are somatic 

 afferent or visceral afferent, depending upon their peripheral connexions. 



