92 THE BIOLOGY OF MARINE ANIMALS 



Certain reflexes, initiated peripherally, affect cardiac activity and blood 

 pressure. If respiratory flow over the gills of a shark or skate is stopped, 

 cardiac inhibition ensues and blood pressure falls; mechanical handling 

 and a variety of other external stimuli produce the same effect. Electrical 

 stimulation of the central ends of cut vagus, hypobranchial and lateral 

 line nerves results in respiratory and cardiac inhibition. When branchial 

 arteries are perfused, raising the arterial pressure evokes sensory dis- 

 charge in the branchial nerves (glossopharyngeal, vagus), and a similar 

 rhythmic sensory discharge takes place in the normal animal at each 

 heart beat when pressure in the branchial arteries rises. Raising the pres- 

 sure in the first pair of afferent branchial arteries also produces cardio- 

 inhibition. It seems likely that there is a normal branchial depressor reflex 

 in fish: this reflex is stimulated by presso-receptors in the afferent branchial 

 arteries, and produces cardio-inhibition and fall of blood pressure. Its 

 function appears to be protection of the delicate branchial capillaries 

 lying near the heart (92). 



Afferent visceral impulses from many regions are fed into autonomic 

 medullary centres, from which efferent impulses proceed peripherally in 

 autonomic pathways. Efferent cardioregulatory routes available are 

 cardiac branches of the vagus (inhibitory fibres) and pre-ganglionic 

 sympathetic fibres to the suprarenal organs. There is no sympathetic supply 

 to the heart in fish. Vagus activity stops the heart and results in fall of 

 blood pressure. Experimentally, adrenaline and noradrenaline usually 

 produce acceleration of the heart and increased strength of beat, as well 

 as causing prolonged rise of blood pressure. It is possible that adrenaline 

 and noradrenaline, secreted by chromaffine tissue of the suprarenal bodies, 

 have pressor effects in the normal animal. 



Efferent pathways to blood vessels in fish require further study. Peri- 

 pheral somatic vessels in the trunk of elasmobranchs are innervated by 

 sympathetic fibres only; visceral (gut) vessels are within sympathetic and 

 parasympathetic fields: branchial and cephalic vessels receive parasympa- 

 thetic fibres. The following pharmacological observations are pertinent to 

 the morphological pattern. Injection of acetylcholine, although causing 

 cardiac inhibition, raises blood pressure owing to a vasoconstrictor action 

 on peripheral arteries. Adrenaline dilates branchial and constricts systemic 

 vessels, resulting in overall rise of blood pressure (Raja). This suggests that 

 sympathetic control of somatic vessels is mediated by adrenergic fibres; 

 parasympathetic fibres to the branchial vessels may be cholinergic. 



In teleosts the autonomic supply of heart and vessels is more complex: 

 noteworthy features are a demonstrated sympathetic innervation of 

 branchial and cephalic vessels, and sympathetic plexuses on splanchnic 

 vessels (Fig. 3-1). Vagus activity causes inhibition of the heart. Stimulation 

 of sympathetic trunks in the eel (Anguilla) causes vaso-constriction of 

 peripheral vessels. Adrenaline has a vaso-dilator effect on branchial 

 vessels and a constrictor effect on systemic vessels as in selachians (7, 19, 

 54, 65, 67, 68, 81, 82, 95, 97). 



