7.1. KIDNEY

In addition to the rich supply of efferent sympathetic nerves, the kidney also has abundant afferent sensory innervation, i.e., nerve fibers proceeding from the kidney to the neuraxis. In contrast to the widespread distribution of the efferent sympathetic nerve fibers in the kidney, the majority of the afferent renal sensory nerves are located in the renal pelvic area [139, 148, 149, 171] (Figure 7.1). The majority of the afferent renal nerves are unmyelinated [127]. The afferent renal sensory nerves contain substance P and calcitonin gene-related peptide (CGRP) as primary sensory neurotransmitters. The presence of afferent renal sensory nerves in the renal pelvic wall is ideal for sensing stretch of the renal pelvic wall.

FIGURE 7.1

The majority of the sensory nerve fibers, identified with an antibody against the neuropeptide calcitonin gene-related peptide (CGRP), are located in the muscle layer (arrows) with some fibers penetrating into the uroepithelium (arrow heads) in the renal (more...)

7.2. PROJECTION TO THE CENTRAL NERVOUS SYSTEM

The cell bodies of the afferent renal nerves are located in ipsilateral dorsal root ganglia (DRG) from T₆ to L₄, with predominance in T₁₂–L₃ [65, 246]. The distribution of the cell bodies varies slightly among various species. Within the spinal cord, the afferent renal nerves project to the ipsilateral dorsal horn in laminae I, III–V [41], where they synapse with interneurons projecting to sites within the central nervous system associated with cardiovascular regulation, including nucleus tractus solitarius, rostral ventrolateral medulla, subfornical organ, and paraventricular nucleus of hypothalamus [229]. There is also evidence for a monosynaptic projection of the afferent renal nerves to areas within the brainstem [254].

Evidence for supraspinal integration of the input from the afferent renal nerves involving neurons in the medulla is derived from studies in rabbits showing that decreases in ERSNA produced by electrical stimulation of the afferent renal nerves were blocked by renal denervation or spinal cord transection at C2 but not by transection of the brainstem at the pontine–medullary junction [215]. Electrical stimulation of the afferent renal nerves results in widespread decreases in ERSNA, including renal, cervical, and cardiac nerves, in association with decreases in arterial pressure [216]. A majority of the neurons in the ventral lateral medulla that decrease their activity in response to afferent renal nerve stimulation also respond to stimulation of the central portion of the aortic nerves with a decrease in ERSNA [216]. These neurons are also responsive to inputs from the carotid sinus nerves [238]. Activation of the afferent renal nerves has also been shown to alter the activity of vasopressin and oxytocin neurons in the paraventricular nucleus of the hypothalamus in rats [40] resulting in increased arterial pressure and plasma vasopressin and oxytocin concentrations. Because these effects were abolished by prior denervation of the stimulated kidney, these data are consistent with a neural circuit involving afferent renal nerves projecting to the neurohypophysis.

The convergence of the afferent signals from the renal and carotid sinus nerves on neurons in several brain areas involved in cardiovascular control [31, 77, 229] provides an anatomical basis for possible interactions among the afferent signals deriving from various organs, including the kidney.