Study of the reflex heart rate response in humans to apneic facial immersion (simulated diving) and its modifications showed that bradycardia caused by simple application of cold compresses to the face (cold face test) correlated well with that produced by the simulated diving reflex. Bilateral application of cold stimulus to the individual divisions of the trigeminal nerve revealed the ophthalmic division to be the most sensitive pathway for this reflex. The cold face test was standardized in 50 normal individuals and further validated in 10 patients by comparison with the simulated diving reflex, the Valsalva maneuver, and administration of atropine. Patients with diabetes mellitus, brainstem stroke, multiple sclerosis, or Shy-Drager syndrome developed less than normal bradycardia or minimal tachycardia in response to the cold facial stimulus. The cold face test is a novel, simple, safe, and economical test of the integrity of trigeminal-brainstem-vagal reflex pathways, can be utilized practically to assess vagal and brainstem dysfunctions, and has the special advantage of being applicable even in an uncooperative or comatose patient.

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Capillaries in the cat hypothalamus receive axon terminals which are comparable to neurovascular junctions in cerebral and systemic arteries and arterioles. The innervation of capillaries in the central nervous system may be derived from central neurons, in contrast to cerebral arterial vessels, which are supplied by the peripheral autonomic nervous system.

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Abstract• The scanning electron microscope (SEM) has been employed to study the effects of ischemia on the luminal surface of the common carotid artery. Fifteen adult rabbits were lightly anesthetized and the common carotid arteries surgically exposed. The right carotid artery was occluded with a single Heifetz clip for five minutes (five animals), 15 minutes (five animals), and 30 minutes (five animals). Following removal of the clip, the animals were immediately perfused with glutaraldehyde and the arteries excised and prepared for critical point drying. Four additional rabbits were perfused following the same method with no surgical procedures performed in the neck. Normal aortas were also examined. The nature and frequency of endothelial cell alterations were determined by analysis often randomly selected SEM fields. Examination of the endothelial surface of arterial segments distal to the occluding clip revealed the presence of numerous "crater-like" defects as well as outpouchings or "balloons." The numbers of craters and balloons were significantly increased in the ischemic (distal) arterial segment as compared to either proximal or sham-operated control segments (P < 0.001). These endothelial cell alterations were never observed in random micrographs of arterial segments taken from unoperated control animals, but were seen at the ostia of some intercostal arteries of the aorta. It is suggested that these craters and balloons could cause interference with blood flow and the formation of platelet thrombi by their protrusion into the lumen, as well as alteration of the permeability of the arterial intima.

The pericytes of mouse myocardium are extensively branched cells that form an incomplete layer around the endothelium of capillaries and postcapillary venules. The membranes of pericytes and endothelial cells are connected by specialized junctions. Microtubules, intermediate (10-nm) filaments and microfilaments are oriented within circumferentially-arranged cytoplasmic processes of pericytes so as partially to encircle the endothelial cylinder. The intracellular organization of these myocardial pericytes suggests that they are smooth muscle-like cells which may be capable of influencing microvascular dynamics in the heart.

The effects of temporary vascular occlusion with surgical clips on the underlying endothelial lining were studied with scanning (SEM) and transmission (TEM) electron microscopy. Twenty-five rabbits were anesthetized and both common carotid arteries exposed. A Heifetz clip was used to occlude the right carotid artery for 5, 15, and 30 minutes, and 2 hours in five animals each. The clips were removed and the vessels immediately perfused with glutaraldehyde. In five remaining animals, the right carotid arteries were occluded for 30 minutes followed by removal of the clip and resumption of blood flow for 30 minutes prior to fixation. Combined SEM and TEM examination of the endothelium of compressed segments revealed "craters" and "balloons", blebs and vacuoles, swollen mitochondria, dilated granular endoplasmic reticulum, and subendothelial edema. There were also areas of endothelial cell flattening, discontinuity, and desquamation exposing the subendothelial tissues. Following restoration of flow, platelets and fibrin were found adherent to altered endothelial cells and to exposed subendothelial tissues. Endothelial craters and balloons were also found distal and, significantly less frequently, proximal to the site of occlusion. It is suggested that antiplatelet aggregating agents may prove beneficial for the prevention of thrombus formation at the site of the clip as well as craters and balloons distal to the clip following procedures requiring temporary vascular occlusion.