486 PRINCIPLES OF GENERAL PHYSIOLOGY 



ceases and the cells, now known as " neurones," although in functional continuity, 

 are separated from each other where contact takes place, the " synapse," by a 

 membrane, which plays a very important part in the mechanism of the reactions 

 which take place in nerve centres. 



The simplest of these mechanisms is that in which two neurones only are 

 concerned : the receptor neurone, whose cell body is outside the nerve centre, and 

 the motor neurone, whose cell body is within the nervous centre, but whose long 

 nerve fibre, or axone, passes out to some peripheral effector organ, such as a muscle. 

 This is a reflex arc, in which a sensory impression gives rise to a motor response. 

 It is the functional unit of the nervous system, as the neurone is its anatomical 

 one. 



Even as low as the earthworm, a new set of neurones is to be found, 

 association neurones, which lie entirely within the central nervous system. 

 These serve to connect the neurones of one segment with those of other segments ; 

 although, in this case, they rarely extend beyond two segments. 



The progress of the nervous centres in complexity and efficiency of integra- 

 tion depends essentially on the formation of longer and longer association 

 neurones. These form, as it were, loops, consisting often of several neurones, 

 which extend further and further from the original simple arc of two neurones, 

 so that the most highly developed parts of the system, such as the cerebral cortex 

 of the higher vertebrates, consist of association neurones only. 



The neurone itself, as a cell, possesses the general properties of protoplasm. 

 The cell body, containing the nucleus, consists of a viscous fluid, with numerous 

 very fine granules in suspension. In life, there is no evidence of the presence 

 of Nissl bodies or neuro-fibrils, and there is every reason to suppose that they 

 are artefacts, as we see them in fixed preparations, although the substance out 

 of which they are formed must have been present in the living cells. 



The mode of embryological growth of nerve fibres from cells suggests 

 aimeboid movement. Whether this is still present in the adult cell is uncertain, 

 although possible. 



The protoplasm itself does not seem to be essential for conduction of impulses 

 serving for reflexes, but probably acts as a means of reinforcing the strength 

 of disturbances and certainly, with its nucleus, acts as the nutritive centre of 

 the neurone. 



Owing to the protoplasmic nature of the cell bodies, nerve centres are very 

 sensitive to deprivation of oxygen. No metabolic process other than oxidation, 

 with evolution of carbon dioxide, has been shown to be present in the normal 

 activity of nerve centres. 



A true nerve network, with central functions, such as that of reflex action, 

 does not appear to exist outside of the very simplest types of nervous system. 

 The plexuses of distribution of the nerves to smooth muscle and related structures 

 serve only for conduction, not for initiation of impulses. 



The properties of the synaptic membrane are of great importance. Such 

 properties of membranes as those described in the earlier chapters of this book 

 must be shown by it. The phenomena of fatigue, summation, irreciprocal 

 conduction, excitation and inhibition are connected with this membrane. 



The use of one motor neurone by several receptors, brought about by the 

 existence of these modifiable synaptic membranes, enables great economy and 

 integrative efficiency to be obtained. This is the principle of the " final common 

 path " of Sherrington. 



Although, in rare instances, a reflex arc may consist of two neurones only, 

 receptor and motor, in the great majority of cases at least three are present, the 

 additional one being a longer or shorter association neurone. The whole 

 of the arc, with the exception of the motor neurone (final common path), may, 

 for convenience, be called the afferent arc. 



