METHODS

The results reported here have been obtained in 19 successful experiments on cats. Five animals were anesthetized with pentobarbital sodium (35 mg/kg intraperitoneally) and seven with chloralose-urethan(60 mg/kg and 250 mg/kg intraperitoneally, respectively). Two of the latter received additional intravenous doses of thiopental sodium (10 mg at a time). Four animals were decerebrated under transient ether anesthesia, and three others had the spinal cord sectioned at the first cervical level after receiving fully anesthetic doses of pentobarbital sodium. In the three spinal preparations the brain rostra1 to the section was left in tact. In every preparation, the trachea was cannulated, a thread was loosely passed around each vagus nerve, and polyethylene catheters were inserted into a femoral artery and a femoral vein. After these steps a paralyzing dose of gallamine triethiodide was injected and artificial ventilation maintained throughout the experiment. The thorax was then opened at the fourth left interspace and the pericardium incised longitudinally. A segment of the left coronary arterial tree (left main artery, left descending or circumflex ramus) was freed from the surrounding tissues, using a dissecting microscope and avoiding damage to pericoronary nerves. A thread slipped around the artery and passed through a polyethylene or glass tubing was pulled whenever the artery was to be occluded. To avoid excessive myocardial damage, each occlusion lasted from 20 to 90 set, and subsequent occlusions were spaced at intervals of at least 20 min. In a control experiment a similar occlusion was performed on the main left coronary vein.

A bilateral pneumothorax was produced by widely opening that portion of the pleura close to the heart. The preparation was placed with the left side up and firmly fixed by vertebral clamps. The stellate ganglion was exposed retropleurally by removing overlying muscles and ribs, covered by a pool of mineral oil maintained at body temperature by thermal radiation. The thoracic sympathetic rami communicantes entering the left stellate ganglion were identified and Ts was then dissected clear to its entrance into the ganglion. The external connective sheath was removed and filaments for recording were prepared by cutting portions of the ramus. All fibers from which recordings were to be made were distally disconnected from the ganglion and could carry only efferent impulses. In most experiments the ramus filaments were split until only discharges from a single active fiber were present. In some cases, recordings were obtained from filaments containing up to ten active fibers in order to assess the responsiveness of a small population of neurons. Recordings of nerve impulses were obtained by a bipolar silver electrode coupled with a Tektronix 122 preamplifier(band-pass filter, usually between 8 cycles/set and 10 kc/set); signals were then fed into one beam of a Tektronix 565 oscilloscope. A