The neuroanatomical basis for the protopathic sensibility of the human glans penis

The human glans penis is covered by stratified squamous epithelium and a dense layer of connective tissue equivalent to the dermis of typical skin. Rete ridges of the epidermis are irregular and vary in height depending on location, age, and presence or absence of a foreskin. The papillary layer of the dermis blends into and is continuous with the dense connective tissue forming the tunica albuginea of the corpus spongiosum of the glans penis. The most numerous nerve terminals are free nerve endings (FNEs) present in almost every dermal papilla, as well as scattered throughout the deeper dermis. FNEs are characterized by an incomplete Schwann cell investment, and contain irregularly scattered neurofilaments and neurotubules, clusters of mitochondria, vesicles of variable size and various inclusions. The ratio of FNEs to corpuscular receptors is approximately 10:1 and a similar ratio of small to large axons is seen in dermal nerves. Genital end bulbs are present throughout the glans, but are most numerous in the corona and near the frenulum. The unique corpuscular receptor of the glans penis consists of axon terminals that at an ultrastructural level resemble a tangled skein of FNEs. Simple, Pacinian and Ruffini corpuscles were occasionally identified predominantly in the corona glandis. Epidermal Merkel nerve endings and other types of mechanoreceptors typically found in primate glabrous skin (lip or digit) are not present. Rarely, dermal Merkel cells have been identified associated with genital end bulbs. The abundance of FNEs in isolated as well as corpuscular form can be correlated with the embryogenesis and known neurophysiologic and psychophysical parameters of sensory function of the glans penis. Finally, the divergence in reported values for the threshold of tactile and painful stimuli when applied to glabrous skin of fingertip and glans penis can be considered as an example of dissociated sensibility. The anatomical basis for this dissociation is the abundance of FNEs and absence of Merkel terminals and typical Meissner corpuscles in the covering of the glans, and the converse in glabrous skin of the digit.