Receptive field characteristics of stretch-insensitive mechanosensitive units in the rat urinary bladder

The pelvic, hypogastric and pudendal nerves carry sensory information from the urinary bladder. The pelvic nerve is reported to be the most important one of these afferent nerves in producing urinary bladder sensation. The primary types of mechanoreceptors in the bladder can be divided into stretch-sensitive and stretch-insensitive units. The former is considered to be more important in producing bladder sensation. However, little is known about the precise receptive field properties of the stretch-insensitive mechanoreceptors in the urinary bladder. Therefore, in this study, we systematically investigated the receptive field characteristics of the pelvic single unit innervating the rat bladder wall.

Functional single unit recordings were made from the pelvic nerve afferent filaments of anesthetized rats. A von Frey device was used for quantitative mechanical stimulation of the bladder surface. In addition, electrical stimulation was used for estimating conduction velocities of the nerve fibers in the receptive field.

The threshold value for mechanical stimulation was statistically lower at the caudal portion of the bladder body (sites IV and V) than the other bladder sites. The bladder neck (site I) had the highest mechanical stimulation threshold value for the bladder stretch-insensitive mechanoreceptors. In most cases, the pelvic nerve had bilateral receptive fields. The majority of the pelvic nerve afferents had conduction velocities in the slow A-δ or C fiber range. Mechanical stimulation threshold values were higher in males than in females in a portion of the bladder sites.

The pelvic stretch-insensitive bladder mechanoreceptors are 1) higher threshold at the bladder base, 2) contain large bilateral receptive fields and 3) demonstrate relatively slow conduction velocities. These characteristics indicate a non-uniform distribution of stretch-insensitive mechanoreceptors in the rat urinary bladder wall.