Specialized Cells: Muscles, Nerves and Sense Organs 1 17 



Vitamin Ai. The retina can also resynthesize rhodopsin in two ways, 

 first from retinene, and relatively slowly from Vitamin Ai.' 



What happens when the protein of rhodopsin receives the liberated 

 electron is not clearly known, but the final result is to stimulate 

 the optic nerve, so that electrical disturbances pass down it to the 

 brain. This can be demonstrated by experiments, in which an elec- 

 trode is placed in contact with the optic nerve of the frog. Whenever 

 light falls on the retina an electrical disturbance passes down the 

 nerve, which can be detected by means of the electrode and if an 

 amplifier is put into the circuit with a loud speaker, the result of 

 the passage of the electric impulse is a click in the loud speaker. 

 It appears from these experiments that the frog responds to changes 

 in the intensity of light, i.e. when the light is turned on or off, but 

 steady illumination produces no further response. However in man 

 and probably other mammals steady illumination causes a con- 

 tinuous stimulation of the nerves. 



NERVES 



This brings us to the links between the sensory organs and the 

 muscles. Usually this is via the brain — a sort of clearing house for 

 the information coming from the senses; but this will require several 

 chapters by itself. For the moment we will look at the nerves them- 

 selves, white threads of many different sizes and lengths; sometimes 

 in bundles, sometimes singly, which traverse the body like a telephone 

 system. 



For many years scientists have wondered how these messages are 

 transmitted. Even Newton speculated on this and thought that it 

 was a purely mechanical transmission of a mechanical impulse. To 

 Descartes what was transmitted was 'like a very subtle wind . . . con- 

 tinually mounting in great abundance from the heart to the brain, 

 flows from there through the nerves to the muscles, and gives motion 

 to the members'. 



According to this the nerves were just tubes which permitted the 

 flow of 'animal spirits'. It was L. Galvani who showed that the flow 

 was electrical, and analagous to the electric discharges obtainable 

 from static electric machines which were common at this time. He 

 showed that frog's muscles could be made to contract by electric 

 currents applied to the nerve of the spinal column. 'Animal spirits' 

 thus became 'animal electricity' which was supposed to be secreted 

 by the brain and travelled along the nerves. 



Many years of research have confirmed the electrical nature of 

 the impulses which pass down the nerve fibres. This can easily be 



