Presumptive sensory axons of the human urinary bladder: a fine structural study

The mucosa of the human urinary bladder possesses an extensive plexus of suburothelial nerve fibres which are believed to be sensory in nature. Many of these presumptive sensory nerves occur as single axons whose vesicle-packed varicose regions are totally devoid of neurilemmal cell covering and occasionally penetrate the urothelial basal lamina. The axonal vesicles are of two types, small agranular vesicles (median diameter 47 nm, range 39-54 nm) and large granulated vesicles (median diameter 105 nm, range 62.5-187.5 nm). When compared statistically with intramuscular axon varicosities the suburothelial varicosities are shown to possess a significantly greater packing density of axonal vesicles and to contain a significantly greater proportion of large granulated vesicles. The latter finding may reflect the presence of substance P, a neuropeptide known to occur in primary sensory nerves.