484 



ELECTRICAL SENSES 



INTRODUCTION 



Ampullae of Lorenzini are found in all elasmobranchs and presumably are 

 homologous within the group. Although these receptors are sensitive to 

 thermal, mechanical, and even salinity changes, behavioral experiments 

 (described elsewhere in this volume by Kalmijn (1978); see also Murray 

 (1974)) make it clear that they are electroreceptors. Moreover, they are 

 extremely sensitive, responding to stimuli a few microvolts in amplitude. 

 Among electroreceptors ampullae of Lorenzini are classed as tonic, low- 

 frequency or ampullary receptors. They are called tonic and low-frequency 

 because there is a steady resting discharge in the afferent fibers and they 

 respond quasi-tonically to quite low frequency stimuli. They are called 

 ampullary because the receptor epithelium is an ampulla connected to the 

 exterior by a canal of variable length (Figure 1). 



Figure 1 Ampulla of Lorenzini 

 (after Waltman 1966). The ampulla 

 consists of a cluster of alveoli, one 

 of which is shown in cross section. 

 The receptor cells are innervated 

 by about five afferent nerve fibers 

 that ramify profusely over the sur- 

 face of the alveoli. The neck of the 

 ampulla, where the receptor cells 

 are no longer found, is called the 

 marginal zone (MZ). 



NERVE 



AMPULLA 



CANAL 



The ampulla contains the sensory and supporting cells of the sensory 

 epithelium; the jelly-filled canal provides a good electrical connection 

 between the ampullary lumen and the exterior. For purposes of physio- 

 logical analysis a few points may be noted here: (1) Between the receptor 

 cells, supporting cells, and cells of the wall of the ampullary canal are 

 extensive tight or occluding junctions (zonulae occludentes) that restrict the 

 flow of electric current through intercellular clefts (Figure 2); transepithelial 

 current passes through the cells rather than between them. (2) The receptor 

 cells have only a small portion of their total surface distal to the zonulae 

 occludentes and facing the lumen; by far the greatest part of their 

 membranes is basal face. (3) In the basal faces are the afferent synapses, 

 which have large presynaptic ribbons with vesicles lined up on their surfaces; 

 the morphology is typical of acoustico-lateralis receptors known to transmit 



