DISTRIBUTION AND PUTATIVE FUNCTION OF AUTONOMIC NERVE-FIBERS IN THE BILL SKIN OF THE PLATYPUS (ORNITHORHYNCHUS-ANATINUS)

The electroreceptors located in the bill skin of the platypus are modi fied secretory glands. The electroreceptive nerve terminals form bare endings in close proximity to the duct of these glands. In this study we describe the autonomic innervation of the glands and a separate spe cialized autonomic innervation of the epidermal portion of the glandul ar duct. A range of immunohistochemical labels showed that the gland c ells of the electroreceptors have a non-noradrenergic (putative parasy mpathetic) innervation. Phalloidin labelling revealed a 'sphincter' of epidermal luminal cells that labelled strongly for actin. These actin -dense keratinocytes were seen to have a noradrenergic (putative sympa thetic) innervation. Fine-diameter sensory fibres containing substance P (presumably C-fibre thermoreceptors or polymodal nociceptors) were observed to terminate in the superficial epidermis surrounding the por e of the gland. When the bill of the platypus is dry these pores were closed. However, when room temperature water was washed over the bill, the pores opened. It is proposed that this autonomic and sensory inne rvation, along with the actin sphincter, mediates the opening and clos ing of the pores. By doing this, the platypus prevents the desiccation of the bare electrosensory nerve terminals when it is out of the wate r, and it may also be a way to regulate the impedance of the internal electrical circuit presented to the water at the pores.