104 GENERAL CONCEPTS 



protoplasm of eggs and plant cells. Many of the ciliates have a network 

 of neurofibrils which connect the bases of the cilia, together with special 

 fibrils to the gullet and other special structures of the body. It would 

 appear that this net conducts impulses which coordinate the beating of 

 the cilia and the functioning of the special organelles, for coordination 

 is lost when the net is cut by a microneedle. There are no nerve cells in 

 sponges, but waves of excitation can be conducted from cell to cell, at 

 about 1 cm. per minute. There are spindle-shaped contractile cells 

 around the openings of the pores. These have been termed "independent 

 effectors" because they respond to touch by contracting and thus com- 

 bine sensory and motor functions. 



The simplest special coordinating system is the nerve net found in 

 coelenterates. The coelenterate nerve fibers are found all over the body 

 in a diffuse network; a few sea anemones and medusae have rudimentary 

 nerve trunks composed of aggregations of nerve fibers. Conduction in 

 the nerve net progresses in all directions; the fibers are not actually fused 

 together, but impulses pass from one fiber to an adjacent one in either 

 direction. 



The Nerve Impulse. Galvani, in the eighteenth century, first 

 showed that a muscle contracts when an electric shock is applied to the 

 nerve leading to it. DuBois-Reymond in the nineteenth century showed 

 that when a stimulus is applied to a sense organ electrical disturbances 

 in the efferent nerves can be detected. With the development of im- 

 proved instruments for detecting these weak currents, the electrical 

 disturbances in nerve fibers were shown to have a potential of about 0.05 

 volt, to last for a very short time, about 0.0005 second, and to travel 

 along the nerve at speeds as great as 100 yards per second. 



The transmission of a nerve impulse is not simply an electrical 

 phenomenon, like the passage of a current in a wire. It is a physico- 

 chemical process, which uses oxygen and produces carbon dioxide and 

 heat. The transmission of a nerve impulse obeys the "all-or-none law"; 

 The conduction of the impulse is independent of the nature or strength 

 of the stimulus starting it, provided that the stimulus is strong enough 

 to start any impulse. The energy for the conduction of the impulse 

 comes from the nerve, not from the stimulus, so that, although the speed 

 of the conducted impulse is independent of the strength of the stimulus, 

 it is affected by the state of the nerve fiber. Drugs or low temperature 

 can retard or prevent the transmission of an impulse. The impulses 

 transmitted by all types of neurons are believed to be essentially alike. 

 That one impulse results in a sensation of light, another in a sensation of 

 pain, and a third in the contraction of a muscle is a function of the way 

 the nerve fibers are connected, and not of any special property of the 

 impulses. 



According to the generally accepted theory of the nature of the 

 nerve impulse, the semipermeable membrane surrounding each nerve 

 fiber allows certain ions but not others to penetrate it. The metabolic 

 activities of the nerve cell keep the membrane polarized, with an excess 

 of cations on the outside and an excess of anions on the inside (Fig. 5.11). 

 The potential across the membrane due to the excess of positive ions out- 



