358 MECHANICAL AND ACOUSTICAL SENSES 



THE INNERVATION OF THE MECHANORECEPTORS 



The tactile endings of the skin are supplied by nerve fibres carried in cranial 

 nerve V and in the segmental spinal nerves. In contrast, the acoustico- 

 lateralis receptors are innervated by cranial nerves (VII, VIII, IX, X). This 

 important difference is illustrated in Figure 15, which emphasises that the 

 lateral-line and auditory nerves project to a specialized brain centre— the 

 acoustico-lateralis lobe— whereas the tactile input is widely distributed 

 throughout the brain and spinal cord. It is not surprising to find, therefore, 

 that the two systems generate quite different reflexes and behaviour pat- 

 terns. 



Figure 15 Diagram showing the pattern of innervation of the tactile sensory endings (top 

 half) and the acoustico-lateralis system in an idealised shark. 



Skin Receptors 



The tactile endings of the skin of the head are supplied by cranial nerve V, 

 which contains about 6000 sensory fibres in Scyliorhinus. Responses to 

 mechanical stimulation recorded from branches of nerve V in the head of 

 Torpedo (Piatt et al. 1974) have shown that the skin receptors are highly 

 sensitive to touch and respond well to "mild water flow." 



The composition of the elasmobranch spinal nerves has been discussed by 

 Roberts (196%), who showed that in the sharklike fishes the motor and 

 sensory fibres were held in separate bundles but that they were mixed to- 

 gether in a single bundle in the rays. Perhaps this is because in rays, as in 

 most vertebrates, sense organs lie among the muscle fibres. Each dorsal root 

 of a spinal nerve in Scyliorhinus contains around 500 fibres, not all of which 

 will supply tactile or Wunderer endings. If every segmental nerve contained 

 this number then there would be about 40,000 peripheral channels available 

 to the skin receptors in the body; most of these fibres are 6 jitm in diameter. 



