344 MECHANICAL AND ACOUSTICAL SENSES 



small currents that established potentials comparable in size to these re- 

 ceptor potentials. They also found, in contrast, that no change in the af- 

 ferent fibre discharge could be brought about by currents even 10 times as 

 large that were injected directly into the afferent fibre. 



In the lizard ear essentially similar potentials have also been recorded in 

 supporting cells, and on this basis Weiss et al. (1974) suggest that the sup- 

 porting cells and the hair cells might be coupled electrotonically. Such 

 coupling would not be totally unexpected, for Hama (1969a) has already 

 reported the presence of "gap junctions"— the accepted morphological basis 

 for electro tonic coupling— between neuromast cells in the fish macula 

 sacculi. 



Postsynaptic responses— The changes in the hair-cell resting po- 

 tential, seen as the receptor potential, presumably lead in some way to the 

 release of the transmitter at the hair-cell/afferent-nerve synapse, for intra- 

 fibre records close to the hair cell have provided confirmatory evidence that 

 these synapses function in the manner now well understood for chemical 

 synapses. Thus, spontaneous, small-amplitude, erratic potentials— supposedly 

 miniature potentials— have been seen in the afferent terminals that make 

 connection with Savi's vesicle (Szabo and Hagiwara 1966), the lateral-line 

 (Flock 1971), and saccular hair cells (Furukawa and Ishii 1967b; Furukawa, 

 Ishii, and Matsuura 19726). Mechanical stimulation promotes a massive in- 

 crease in the size of these potentials (Figure 6) which, on reaching a certain 

 level, initiate spike firing. These authors also report (Ishii, Matsuura, and 

 Furukawa 1971) that the amplitudes of the microphonic potentials and of 

 the excitatory postsynaptic potentials (EPSP's) are related in a sigmoid 

 fashion; these potentials are functioning therefore as generator potentials, 

 for increases in their size lead to enhanced firing of the afferent nerve fibres. 

 These postsynaptic potentials follow the microphonic with a delay of only 

 0.6-0.8 ms. 



In all cases so far studied the influence of the efferent innervation on the 

 hair cells has been found to be inhibitory, causing a decline or abolition of 

 afferent activity. The cellular basis of this inhibition has now been unravelled 

 by means of intracellular measurements from the hair cells and concurrent 

 recordings of microphonic potentials. Flock and Russell (1973a, 1976) have 

 successfully recorded from the lateral-line hair cells of the teleost Lota and 

 found that a train of stimuli applied to the efferent nerve results in the 

 development of a hyperpolarizing potential in the hair cell, up to 10 mV in 

 amplitude, which outlasts the stimulus by 150-200 ms (Figure 6). This 

 hyperpolarization of the cell should effect a decrease in transmitter release, 

 and recordings from afferent terminals during efferent-nerve stimulation 

 have shown a reduction in the EPSP's established by mechanical stimulation 

 (Figure 6). 



The inhibitory impact of the efferent terminals is registered by an extra- 

 cellular electrode placed close to the sense organ in two ways (Flock and 

 Russell 19736). First, as would be expected for an electrode sited in a 

 volume conductor some distance from an inhibitory synapse, a negative 



