Branchial innervation.

Inspection of the dorsal end of fish gills reveals an impressive set of nerve trunks, connecting the gills to the brain. These trunks are branches of cranial nerves VII (the facial) and especially IX (the glossopharyngeal) and X (the vagus). The nerve trunks carry a variety of nervous pathways to and from the gills. A substantial fraction of the nerves running in the branchial trunks carry afferent (sensory) information from receptors within the gills. There are also efferent (motor) pathways, which control muscles within the gills, blood flow patterns and possibly secretory functions. Undertaking a more careful survey of the gills, it becomes evident that the arrangement of the microanatomy (particularly the blood vessels) and its innervation are strikingly complex. The complexity not only reflects the many functions of the gills but also illustrates that the control of blood flow patterns in the gills is of crucial importance in modifying the efficiency of its chief functions: gas transfer and salt balance. The "respiratory-osmoregulatory compromise" is maintained by minimizing the blood/water exchange (functional surface area of the gills) to a level where excessive water loss (marine teleosts) or gain (freshwater teleosts) is kept low while ensuring sufficient gas exchange. This review describes the arrangement and mechanisms of known nervous pathways, both afferent and efferent, of fish (notably teleosts) gills. Emphasis is placed primarily on the autonomic nervous system and mechanisms of blood flow control, together with an outline of the afferent (sensory) pathways of the gill arches. Copyright 2002 Wiley-Liss, Inc.