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SUMMARY1. Certain afferent fibres in the renal nerves show an increased rate of discharge in response to small increases in renal vein pressure and to substantial increases of ureteral pressure. Such fibres enter the spinal cord largely through the upper lumbar dorsal roots.2. In spinal cats, stimuli exciting these afferent fibres evoke a reflex discharge in sympathetic nerves to the kidney and a fall in systemic arterial pressure. Change in flow or peripheral resistance, independent of arterial pressure, could not be demonstrated for the renal vascular bed. In spite of a lack of evidence for a renal vasomotor effect, the existence of the reflex fall in arterial pressure following excitation of receptors sensitive to venous pressure strongly implies that there is a form of circulatory regulation at the spinal cord level.

In the spinal cat, a single volley in afferent fibres of one segment evokes a response in some preganglionic axons emerging from the same and from nearby segments. The maagnitude and latency of the refle'x response can be varied by gradation of the afferent volley; however, the fact that certain preganglionic fibres exhibiting background activity do not respond to the afferent volley indicates that not all of the neurones forming a given preganglionic ramus participate in the reflex. The initial description of this reflex (Beacham & Perl, 1964) left unanswered many questions about its functional organization, such as the conduction velocity of the responsible afferent fibres, the temporal recovery of reflex excitability and the effects produced by conditioning and tetanic afferent stimulation. The present study provides information on these points. From the results it is evident that, while the sympathetic reflex under consideration can be depressed or inhibited, facilitation of the response to a large afferent volley is rare. METHODSAll experiments were done on adult-cats made decapitate under ether anaesthesia by a spinal section at C-1 and by complete occlusion of the circulation to the head. After spinal section, anaesthesia was stopped and the preparations were maintained by positive-pressure artificial respiration: the respiratory rate and pressure were controlled to maintain the endtidal CO2 close to 5% (measured by a Beckman LB-1 infrared CO2 detector). Deep body temperatures were maintained between 36 and 38°C. Repeated injections of Flaxedil (gallamine triethiodide, American Cyanamid Company) were routinely used to give continued, total skeletal muscle paralysis. Five to 18 hr elapsed between spinal transection and the reported observations. Afferent volleys were initiated by stimulation of dorsal root filaments or segmental spinal nerves with 0-1 msec rectangular pulses. Stimulation of spinal nerves was arranged so that the stimulating cathode lay at least 10 mm distal to separation of the preganglionic bundle. Afferent activity was monitored by recording between an electrode placed on the dorsal root entrance zone and an indifferent electrode on adjacent bone.Recordings of preganglionic activity were made from axons in the segmental rami communicantes after removal of the connective tissue sheath, and often after splitting of the preganglionic bundle with the aid of a binocular microscope. If the reflex discharge observed consisted of activity in more than one or two elements, its magnitude was estimated from the area under the potential produced by the evoked response. In some instances, this was obtained from enlargements of photographs of the response, using a planimeter. Since the

The microvascular bed of the rat mesocecum has been examined in vivo using differential interference (Nomarski) optics and subsequently by electron microscopy. The preferential channel, from terminal arteriole to collecting venule, has been examined. In the terminal arteriolar segment the endothelial layer is covered by a continuous layer of smooth muscle cells which, in turn, are surrounded by adventitia. In the metarteriolar segment the periendothelial cells still resemble smooth muscle cells but the tunica media is discontinuous. In the distal segment periendothelial cells are more scattered and have the appearance of pericytes. There appears to be a continuous transition of the periendothelial cell layer from terminal arteriole to distal segment. Nerve endings were seeen in both the terminal arteriolar and metarteriolar segments. During contraction smooth muscle cells, oriented circumferentially, shorten and thicken. Endothelial cells appear anchored by myoendothelial junctions. Endothelial cells have filaments which show increased banding during vasoconstriction, suggesting that such cells may contract. Capillary offshoots leave the preferential channel, usually at right angles. Smooth muscle cells are oriented to form a sphincter and there are many myoendothelial junctions at the branch point. Within a short distance the capillary branch loses its periendothelial coat.

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