Pressor reflex evoked by muscular contraction: contributions by neuraxis levels

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is one important model of cardiovascular adjustments during static exercise. The central nervous system (CNS) structures mediating this reflex have remained largely obscure. Therefore, we examined the contribution of selected levels of the neuraxis in mediating the pressor reflex evoked by muscular contraction from stimulation of ventral roots. Decerebrate cats exhibited larger pressor reflexes than those found in intact alpha-chloralose-anesthetized cats, a difference more apparent at low (5 Hz or repeated twitch) rather than at high (50 Hz or tetanic) stimulus frequencies. Although a depressor response to 5-Hz stimulation was observed in the intact anesthetized cats, it appeared to be primarily due to anesthetic level, since a depressor response was not observed in decerebrate animals (nonanesthetized). Cerebellectomy produced no changes in the reflexes of the decerebrate animal. Further transection of the neuraxis (caudal to the midcollicular level) attenuated the exercise pressor reflex. The spinal cat demonstrated slight evidence of exercise pressor reflex activity. These results provide clarification as to representation of this pressor reflex within the CNS and establish the reflex's characteristics at several levels of neuraxis integration.

Vagal inhibition of cardiovascular activity in the decerebrate cat

Cardiovascular responses after decerebration at three levels of the central neuraxis and subsequent vagotomy were studied in the cat. Midpontile decerebration produced a marked, transient depression in heart rate and arterial blood pressure, from which recovery to and sometimes above pretransection levels usually occurred within 30 min. Subsequent vagotomy in the midpontile decerebrate animals produced marked tachycardia and hypertension. This cardiovascular hyperactivity, particularly hypertension, was conspicuously diminished by stimulation of the central ends of the sectioned vagi. In distinct contrast to the dramatic augmentation of cardiovascular activity after vagotomy in the midpontile animals, vagotomy in midmesencephalic and medullary animals typically produced moderate and transient elevations in heart rate and blood pressure. These data support the concept of a cardiovascular-augmenter area in the caudal pons and a cardiovascular-depressor area in a more rostral portion of the brain stem. These data suggest that the vagal system antagonizes and masks a potential cardiovascular hyperactivity in the midpontile decerebrate animal.

Evidence for a Medullary Inspiratory Pacemaker

Experiments on central respiratory regulation were performed with unanesthetized cats which were both decerebrated and decerebellated. Electrical stimulation of a circumscribed area in the floor of the fourth ventricle in the medullary preparation consistently produced an increase in the rate of gasping. In the decerebrate animal, stimulation of this area converted the respiration into a gasping form of breathing which varied in rate and amplitude according to changes in stimulus parameters. Electrical stimulation of the medial reticular formation in the medullary animal revealed a functional separation of gasping and sustained inspiratory responses. Cauterization of the medullary tegmentum in the decerebrate preparation produced gasping. In the medullary preparation it was necessary to destroy the dorsal half of the medulla in order to abolish gasping. Surface cauterization of the medullary tegmentum prevented the rate-stimulating effect of cyanide which is regularly evocable in the animal with intact medulla. Because of the increasing need for revised terminology in central respiratory physiology, a new set of terms, namely, oscillator, modulator, integrator, and pacemaker, was incorporated in a functional concept of the central control of respiration.

On the nature of postural reflexes

The “reflex standing” seen characteristically in the spinal animal, and exaggerated in the decerebrate animal, when analysed myographically is found to be a reflex contraction which is caused by stretch of the muscle itself, the “ stretch reflex ” (31). In upright postures gravity provides the stretch which elicits the reflex, and the resulting antigravity stretch response is prominent in all limb extensors. This response is a typical extensor reflex and is found only in fractional form in the flexors (5) in the cat, dog, and other quadrupedal mammals. In such animals as the sloth where the typical postures involve stretch of flexor muscles, decerebrate rigidity involves the flexors (46).

The influence of various conditions on the time relations of tendon reflexes in the human subject

The following observations constitute a preliminary account of an investigation into the time relations of certain reflexes in the human subject. The knee-jerk was primarily selected for study. The variability of magnitude of the knee-jerk in response to equal stimuli has been commented on by the majority of observers, and, according to Seheven, such a variable response may occur even in the decerebrate animal. Whether variations of the irritability of the spinal reflex centre take place in the intact human subject there is no means of knowing, but study of the human responses has convinced us that by far the greatest factor in determining the variability of the knee-jerk is cortical activivity, and that, by taking suitable precautions, variability may be reduced to a negligible factor. In a previous publication, one of us (F. G.) has shown that these variations of the knee-jerk are secondary to alterations in the tonus of the quadriceps muscle. It was found that any stimulus of a nocuous nature, whether physical or psychical, caused an increase in the quadriceps tonus when the limb was flexed in the position in which the knee-jerk is elicitable, and that the magnitude of the knee-jerk was correspondingly increased. Similarly, pleasurable stimuli leading to an inhibition of effort were responded to by diminished quadriceps tonus and a diminished knee-jerk. It was found in our present experiments that, by using a trained subject and avoidance of all sources of psychical disturbance, it was possible to obtain responses to uniform stimuli which only rarely showed any irregularity. Neglect of the psychical factor has been the cause of the irregularity of response noted by so many observers. Uniformity of the stimulus was secured by the use of electrically released pendulums, which could be weighted to any desired extent. The relatively inefficient slow pushing stroke of the pendulum on the patellar tendon was converted into a more efficient stimulus of briefer duration by interposing a small piece of metal-covered wood, held in position by a stout springy steel rod. This anvil, for the reception and conversion of the pendulum blow, is adjusted in contact with the patellar tendon, and on the pendulum striking it the circuit of a Deprez signal is closed, and the moment of stimulation is photographed on the recording paper. The muscular response was recorded by photographing the excursions of the string of an Einthoven galvanometer, through which the muscle current of the quadriceps was led. For observation of more than one group of muscles simultaneously, a galvanometer with two strings was employed, and the muscle currents were explored by fine needle electrodes pushed into the substance of the muscle.