ASPECTS OF INHIBITORY PATHWAYS AND SYNAPSES 



37 



facts are only very crudely known. On the other hand the anatomic situation 

 is fairly clear and also conveniently accessible for degeneration experiments. 

 A diagram (Fig. 8) explains the situation. As generally known the cristae 

 ampullares of the labyrinth have strong three-neuron, i.e. bisynaptic, con- 

 nexions with the extraocular muscles (if the synapse between the receptor 

 epithelium and the primary sensory neuron is neglected ). There is a very sharp 

 almost "point to point" projection between the receptors and effectors, each 



Fig. 8. Diagram illustrating the pathway of reciprocal inhibition in the reflex arc 

 between labyrinth (cristae acusticae) and extraocular muscles. Secondary vesti- 

 bular neurons ascending in the medial longitudinal fasciculus (FLM) have only 

 excitatory synapses — terminal knobs of rather large size — with oculomotor 

 neurons. Secondary vestibular fibres ascending in the reticular formation (RET F) 

 terminate with end-feet synapses in the region of the Darkschewitsch nucleus on 

 a group of specific inhibitory interneurons (D nu). The mode of connection of these 

 specific neurons with motor ocular neurons (O nu) is not clear. No terminal 

 knob synapses are involved, only extremely fine fibres, the final termination of 

 which cannot be traced under the light microscope. 



