Spinal cord participation in the Cushing reflex in the dog

✓ The vasopressor response to increased intradural pressure (Cushing reflex) is caused by decreased arterial perfusion (hypoxia) and the mechanical effects of pressure acting in concert. Increased pressure in the range 50–200 mm Hg alters ongoing activity in all neurons concerned with the autonomic nervous system at multiple levels of the neuroaxis. Therefore, the response of an intact organism is highly variable and unpredictable. Conversely, the response to increased pressure exerted upon the spinal cord of a spinal animal is relatively uniform and predictable. Spinal neurons of both the somatic and autonomic nervous systems show facilitation followed by inhibition, but the time course and magnitude of these events are much different for the two systems. Autonomic neurons retain the ability to fire under very adverse conditions, thus enhancing the organism's chance for survival.

Spontaneous Rhythms in the Motoneurons of Spinal Dogfish (Scyliorhinus Canicula)

The swimming musculature of spinal dogfish, Scyliorhinus canicula (L.), was paralysed with curare while recordings were made of the motor activity in the abdominal spinal nerves. Spontaneous, periodic motor bursts of long duration and decreasing frequency were detected during the first 2 h or so of the experiment. The spinal neurons were incapable of sustaining motor activity for more than 1 or 2 h and were dependent on proprioceptive feedback to maintain their excitability; their discharge frequency could, however, be enhanced by un-patterned sensory stimulation.The spinal neurons on each side of the cord are sufficiently organized to dis-charge alternately but the longitudinal co-ordination of the locomotory wave is disrupted in the absence of phasic sensory excitation.INTRODUCTIONIt has been assumed that the locomotory movements of spinal dogfish, which persist with undiminished vigour for several hours, are co-ordinated in one of two ways. One possibility is that the entire periodic motor output is formulated by the nerve cells of the spinal cord (Gray & Sand, 1936 a, b; Le Mare, 1936); alternatively, it is conceivable that the motor pattern is triggered by sensory signals fed back to the spinal cord by proprioceptors stimulated during locomotory movements (ten Cate, 1933; Lissmann, 1946a, b).Lissmann attempted to distinguish between these two general hypotheses by cutting all the dorsal roots of spinal dogfish and thereby isolating the spinal neurons from sensory stimulation. His experiments showed that the spinal animal would swim normally even when a considerable number of dorsal roots had been sectioned but as complete de-afferentation produced im-mobile preparations he concluded'that the motor rhythm was ultimately dependent on proprioceptive activity.

THE CAUSE OF REFLEX AFTERDISCHARGE IN THE FROG'S SPINAL CORD

It is usually assumed that spinal reflex afterdischarge in the decerebrate or spinal animal is due to functional circuits of interneurons around which excitation can "chase its own tail" until fatigue brings the process to an end. This hypothesis has been tested in the frog. Reflex afterdischarge of motoneurons innervating the biceps femoris was produced by electrical stimulation of the ipsilateral foot. After the end of reflex stimulation, but during the afterdischarge, a direct single stimulus was applied to the animal's spinal cord. A strength of stimulus (of duration greater than five milliseconds) could always be found which would terminate the afterdischarge abruptly. This strong stimulus did not halt the afterdischarge by producing transient damage to the neurons of the cord, for when the stimulus was given during stimulation of the foot, there was no interruption of either reflex response or afterdischarge. Such experimental results are consistent with Forbes' hypothesis of reverberatory circuits.

THE CAUSE OF REFLEX AFTERDISCHARGE IN THE FROG'S SPINAL CORD

It is usually assumed that spinal reflex afterdischarge in the decerebrate or spinal animal is due to functional circuits of interneurons around which excitation can "chase its own tail" until fatigue brings the process to an end. This hypothesis has been tested in the frog. Reflex afterdischarge of motoneurons innervating the biceps femoris was produced by electrical stimulation of the ipsilateral foot. After the end of reflex stimulation, but during the afterdischarge, a direct single stimulus was applied to the animal's spinal cord. A strength of stimulus (of duration greater than five milliseconds) could always be found which would terminate the afterdischarge abruptly. This strong stimulus did not halt the afterdischarge by producing transient damage to the neurons of the cord, for when the stimulus was given during stimulation of the foot, there was no interruption of either reflex response or afterdischarge. Such experimental results are consistent with Forbes' hypothesis of reverberatory circuits.

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).