214 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



The Detection of Nerve Impulses 



As the impulse moves along the nerve, it is possible to detect its pas- 

 sage by placing in parallel with a section of the nerve a sensitive electrical- 

 potential-recording instrument (see " Cathode-Ray Oscillograph "), 

 terminating at two Ag|AgCl| Isotonic NaCl non-polarizable electrodes in 

 contact with the surface of the nerve. For most purposes silver wires, 

 freshly coated by electrolysis, are placed in direct contact with the 

 tissues.* 



Structure of Nerve Fibers 



A nerve is a bundle of separate nerve fibers. Each is a thread of 

 protoplasm (Fig. VI-3), either myelinated or unmyelinated, which 

 functions as the conducting element of the nervous system. The myeli- 

 nated nerve consists of an axis cylinder surrounded by the myelin 



Node of Ranvier Nucleus Fibrillar sheath 



Myelin sheath Axis cylinder 



Fig. VI-3. Very much enlarged myelinated nerve fiber of frog, semi-diagram- 

 matic longitudinal section. Axis cylinder appears structureless. The neurilemma 

 (n) is visible chiefly at a node as a thin delicate membrane. 



sheath, which is a complex fatty substance of high specific resistance. 

 External to it lies the neurilemma sheath. At regular intervals along the 

 fiber, constrictions occur in the sheath, which are known as nodes of 

 Ranvier. The unmyelinated fibers have no apparent myelin sheath. 

 Those in the central nervous system are naked axis cylinders, but in the 

 periphery they are covered by a very thin neurilemma sheath. 



The axis cylinder is a soft, transparent, jelly like substance having 

 much the same composition as that found inside other living cells. Its 

 specific resistance is low. Its osmotic pressure is the same as that of 

 blood. The evidence seems to support the view that the axis cylinder is 

 limited by a plasma membrane. This limiting membrane is semi -per- 

 meable and electrically polarized, and it is probably the structure that 

 participates in the propagation of the energy known as the nerve 

 impulses. 



The living fiber maintains a difference of potential across its surface 

 which disappears when the nerve is deprived of oxygen. It maintains 



* Those designed by Adrian and Bronk [1929] are made of hypodermic needles 

 with an insulated silver or copper core, the end of which is flush with the bevel of 

 the needle. 



