428 - Multicellular Animals, Especially Man 



cell or associated neuron, small though these 

 may be (Fig. 23-10). Such techniques have 

 permitted investigators to detect and localize 

 small changes in the membrane potentials of 

 various sensory-neural couplets and to ana- 

 lyze the functional significance of these 

 changes. 



TO AMPLIFIER 



AND 



OSCILLOSCOPE 



ORIGIN OF 

 ACTION 



POTENTIALS 

 r 



SPREAD 



0F J 

 GENERATOR i 



POTENTIAL 



Fig. 23-10. Unicellular "stretch receptor" of a crusta- 

 cean muscle. One microelectrode (IE) is placed in the 

 cytoplasm of the body of the receptor cell while the 

 other electrode (EE) is moved from place to place 

 along the axon. Each time the muscle fiber is subjected 

 to a sudden stretch, a generator potential originates 

 in the dendrites and cell body, followed by a volley 

 of action potentials (nerve impulses) originating in the 

 axon. (From Edwards and Ottoson; modified, dia- 

 grammatic.) 



Whether a stimulus is chemical or me- 

 chanical, reception and transduction appear 

 to involve a change in the resting potential 

 (p. 191) of the receptor cell. This gives rise 

 to what is called a generator potential (Fig. 

 23-10). The generator potential differs from 

 an action potential. It is more sustained and 

 it is seldom propagated for any great dis- 

 tance. Moreover, it suffers diminution, or 

 decrement, during propagation. Essentially 

 the generator potential appears to represent 

 a partial depolarization (p. 191) of the mem- 

 brane of the receptor cell, resulting from an 



increase in the permeability to ions. For 

 mechanoreceptor cells, the increase in per- 

 meability results from a deformation of the 

 membrane — by stretch, pressure, etc. — where- 

 as for chemoreceptors, a chemical product, 

 or excitatory substance, appears to be in- 

 volved. Indeed, even in the case of mechano- 

 receptors it cannot be said with certainty that 

 no excitatory substance is involved in the 

 depolarization process. 



The generator potential of a receptor cell, 

 as the name implies, serves to trigger the 

 firing of not one but a voile) oi action po- 

 tentials, and these are conducted with great 

 speed and without decrement away from the 

 site of origin. 11 the receptor cell itself pos- 

 sesses an axon, the site ol the triggering ac- 

 tion of the generator potential appears to 

 be near the point where the axon originates 

 from the cell body (Fig. 23-10). Such sense 

 cells, accordingly, display two plainly differ- 

 entiated parts. One consists of the cell body 

 and dendrites, if present, for receiving the 

 stimulus and developing a generator poten- 

 tial. The other, an axon, serves to fire and 

 conduct nerve impulses (p. 447). When no 

 axon is present, the trigger point seems to be 

 the synapse (p. 453), that is, the junction be- 

 tween the sense cell and the associated nerve 

 cell (Fig. 23-3). In any event, it is important 

 to realize that the frequency of firing in each 

 volley is determined by the intensity of the 

 generator potential, which keeps rising as 

 the strength of the stimulus is increased (Fig. 

 23-11). 



The essential excitatory agency in virtually 

 all types of receptors appears to be either 

 mechanical or chemical. Plainly the touch 

 and pressure receptors are excited by me- 

 chanical deformations transmitted through 

 the surrounding capsules (Fig. 23-1); and it 

 is almost equally obvious that the hair cells 

 of the hearing and equilibrium organs are 

 excited mechanically, by disturbances in the 

 labyrinthine fluids (Fig. 23-8). The proprio- 

 ceptors are also excited by the strains of 

 stretch and pressure, while the muscles are 

 doing their work upon the tendons. 



