106 GENERAL CONCEPTS 



Experiments by the English physiologist Adrian, published in 1926, 

 provided the explanation as to how the nervous system transmits differ- 

 ences in intensity. By applying graded stimuli to an isolated sense organ 

 and amplifying and measuring the impulses in its nerve, Adrian showed 

 that variations in the intensity of a stimulus lead to variations in the 

 frequeticy with which impulses are transmitted: the stronger the stim- 

 ulus, the more impulses per second. This principle is true in both 

 vertebrate and invertebrate sensory nerves. In contrast, a single impulse 

 in a vertebrate motor nerve elicits a single twitch of all the muscle fibers 

 in the motor unit. The differences in the strength of contraction of the 

 muscle as a whole are due to variations in the total number of motor 

 units actively contracting at any given moment. In the motor nerves of 

 invertebrates, however, the frequency of the impulses does affect the 

 strength of contraction of the muscle innervated. A single nerve impulse 

 will, in general, not stimulate the muscle to contract. At least two suc- 

 cessive impulses are required, and the strength of contraction is inversely 

 proportional to the interval between the two. In many arthropods all 

 the muscle fibers in a given muscle are innervated by branches of a 

 single nerve fiber (axon). A single impulse in the axon will not produce 

 contraction but repeated impulses will; the tension in the muscle in- 

 creases with the frequency of the stimulation. It would appear that, 

 although the arrival of a single impulse at the nerve-muscle junction 

 is unable to bring about muscle contraction, it does affect the junction 

 in such a way as to make it possible for a second impulse to do this if it 

 arrives soon enough after the first. This phenomenon is known as 

 facilitation. 



The speed of propagation of the nerve impulse varies considerably 

 from one nerve to another, and even more from one animal to another. 

 Conduction is, in general, more rapid in those neurons with greater 

 diameters. A number of animals— squid, lobsters and earthworms— have 

 special giant axons which conduct impulses many times faster than the 

 adjacent small fibers. Conduction is more rapid in those nerves sur- 

 rounded by a thick myelin sheath. The speed of conduction is greater 

 in those nerves in which the myelin sheath is interrupted periodically 

 by nodes of Ranvier. 



Transmission at the Synapse. Where the tip of the axon of one 

 nerve comes close to the tip of the dendrite of the adjoining nerve is a 

 region, called the synapse, across which impulses travel from one nerve 

 to the other. Transmission across the synapse is slower than transmission 

 along a nerve fiber. The mechanism by which an impulse arriving at 

 the tip of one axon stimulates an impulse in the adjacent dendrite is 

 not clear. There are three hypotheses to explain synaptic transmission: 

 by the secretion of a neurohumor, acetylcholine or sympathin, by changes 

 in the concentration of cations in the synaptic region, or by the trans- 

 mission of an electric current. When a nerve impulse reaches the tip of 

 certain vertebrate nerves it stimulates the secretion of acetylcholine. This 

 diffuses across the synaptic junction and stimulates a nerve impulse in 

 the second neuron. Tissues contain a powerful cholinesterase, an en- 

 zyme which specifically splits acetylcholine to its constituents, which are 



