454 - Multicellular Animals, Especially Man 



STIMULUS -► 



INTERMEDIATE 

 NEURON 



FREE 

 NERVE 

 ENDING 



-EFFECTOR 

 (MUSCLE CELL) 



MOTOR 

 NEURON 



Fig. 25-8. Sensory-neuromotor connections in the vertebrate. Note that "long path" reflexes are 

 conducted through the brain; whereas intra- and intersegmented reflexes are restricted to the 

 spinal cord. 



of the pattern in the presynaptic nerve cell. ally involved. However, a few cases of purely 



The rate of firing (frequency of impulses) electrical synaptic excitations, not involving 



may be increased or decreased as a result of any chemical agency, have been demon- 



the relay. And if the postsynaptic neuron strated — particularly in arthropods. 



possesses more than one axon, the rate of 

 firing may be different in the different axons. 

 Also, neighboring synapses may mutually 

 exert an influence upon each other. In short, 

 the synapses are very important in determin- 



The first demonstration of a neurosecre- 

 tion, or neurohumor, was made by Otto 

 Loewi, in 1921, while studying the isolated 

 heart of the frog. Such a preparation will 

 keep beating for many hours. And if two 



ing the integrative functions of the nervous hearts are arranged in the fashion indicated 



system, as will be discussed more fully later in Figure 25-9, stimulation of the decelerator 



(p. 455). (vagus) nerve to the upper one not only 



Basic Nature of Synaptic Transmission. The causes this heart to slow down but also leads, 



question of whether synaptic transmission is a few seconds later, to a slowing of the beat 



chemical or electrical remained a matter of of the lower heart. Obviously these hearts are 



controversy for many years. Now, however, not connected by an)' nerves. Consequently 



most neurophysiologists agree that both the effect must be transmitted by some chem- 



chemical and electrical agencies are gener- ical agent, liberated into the saline fluid that 



