(067) How Many Nerve Fibers Innervate the Human Glans Penis: A Histomorphometric Analysis of the Human Dorsal Nerve of the Penis
Authors: Ferrin PC , Uloko MI , Peters BR 
Published: February 2024

Abstract

Introduction
The mainstream media has frequently quoted that the penis has “half as many” nerve fibers as the human clitoris. Recent work has demonstrated that the number of nerve fibers in the dorsal nerve of clitoris was under-estimated, and no study has yet quantified the number of axons in the human dorsal nerve of the penis (DNP). Improving our understanding of penile innervation and sexual response has potential implications for fields of medical and surgical practice. Several nerve transfers have recently emerged targeting the DNP for re-innervation in order to restore penile sensation and/or erectile function, demonstrating variability in outcomes. Understanding the axon counts of the DNP at the anatomic level that these nerve transfers are performed may inform technical refinements and improve our understanding of current outcomes. Comparing DNP axon counts to the recently reported axon counts of the dorsal nerve of the clitoris may also inform techniques in transgender genital surgery, anticipate sensory outcomes in penile allotransplantation and further our understanding of sexual responses across genitalia.

Objective
The objective of this study is to quantify the number of axons in the human dorsal nerve of the penis.

Methods
Bilateral DNP samples were obtained from 5 fresh frozen cadavers. Samples were collected at the base of the penis at the level of the pubic symphysis, a location commonly targeted for coaptation in nerve transfers. Specimens were placed in 3% glutaraldehyde, post-fixed in 1% osmium tetroxide and serially dehydrated in ethanol and toluene. They were embedded in araldite, sectioned on ultramicrotome into 1 μm cross-sections and counter-stained with 1% toluidine blue. Histomorphometric evaluation was performed at 1000x magnification using Leitz Laborlux S microscope and image analysis software (Clemex Vision Professional, Longueuil, Quebec) to obtain nerve fiber counts from each specimen (Figure).

Results
10 DNP samples were analyzed. The mean number of axons in the human dorsal nerve of penis was 3,844 +/- 1,238 (range 1,915 to 5,494). Accounting for the bilaterality of the DNP, the mean total number of axons innervating the human glans penis was 7,688 +/- 1,762.

Conclusions
This study is the first to report the number of axons in the human dorsal nerve of the penis and approximate the number of axons innervating the human glans penis. At the level of the pubic symphysis, axon counts of the dorsal nerve of penis varied significantly and averaged 3,844. The variation in axon counts at this anatomic level was greater than seen in prior reports on the DNC and may be one factor contributing to variations in outcomes following nerves transfers for penile sensation and erection. This variation could also reflect the anatomic level of sample collection or differences in specimen origin (cadaver vs. live person). DNP axon counts were less than or equal to recently reported axon counts of the human glans clitoris, despite the much larger size of the glans penis. This finding may be important when considering clitoral construction in transgender genital surgery, penile allotransplantation and different in sexual responsiveness across genitalia.

Disclosure
No.