James B. Preston

Functional Distribution of Right and Left Stellate Innervation to the Ventricles

Changes of the electrocardiogram and of ventricular refractory period were measured following either unilateral stellate ganglion stimulation or ablation in the open chest dog preparation. Right stellate ganglionectomy or left stellate stimulation produces prolonged Q-T intervals and increased T-wave amplitude. Left stellate ganglionectomy or right stellate stimulation produces increased T-wave negativity without measurable change in the Q-T interval. The differing patterns of electrocardiographic wave form resulting from changes in sympathetic tone mediated by right and left stellate innervation could be correlated with changes in ventricular refractory period. Following right stellate ganglionectomy, refractory period prolongations were most marked over the anterior ventricular surface; left stellate ganglionectomy produced the greatest prolongation on the posterior surface. Although the right and left stellate innervations of the ventricles overlap, the left stellate influence is predominant over the posterior wall of the ventricles, while right stellate influence dominates the anterior ventricular walls. The electrocardiographic form changes observed following unilateral alteration of sympathetic tone paralleled those electrocardiographic abnormalities seen in patients with lesions of the central nervous system, suggesting a possible functional explanation for these clinical findings.

Physiologic Evidence for a Dual A-V Transmission System

A study of the transmission of early premature contractions between atria and ventricles and in the retrograde direction in the dog heart suggests the existence of two parallel A-V conduction pathways communicating with each other over one or more branches. The evidence is based on the excessive delay of very early premature responses in traversing the node, suggesting that a slowly conducting pathway recovers earlier than the normal “fast” pathway; on the echoing back to the chamber of origin of early premature responses; and on ventricular electrograms of “abnormal” configuration obtained during early premature responses. These observations and the hypothesis to which they lead provide a natural explanation for reciprocal rhythm and nodal paroxysmal tachycardia.

Durations of Transmembrane Action Potentials and Functional Refractory Periods of Canine False Tendon and Ventricular Myocardium:: Comparisons in Single Fibers

In vitro studies have shown that alterations in the ionic composition of the perfusion fluid, substitution of blood or plasma for Tyrodes solution, time lapse following removal of the cardiac preparation from the animal, and addition of varying amounts of acetylcholine and/or epinephrine, do not change differentially the functional refractory period and action potential duration of false tendon and papillary muscle fibers; false tendon values exceeded those of papillary muscle in all of the above conditions. Cycle length was the only variable found to influence differentially false tendon and papillary muscle functional refractory periods and action potential durations. At very short cycle lengths, produced by premature responses, the functional refractory period and action potential duration of false tendon and papillary muscle cells approached each other. In some experiments the functional refractory periods of premature false tendon responses were less than corresponding values for papillary muscle. The relationship between the action potential durations and functional refractory periods of canine epicardial and endocardial cells was investigated also. It was found that the epicardial cells had a shorter functional refractory period and action potential duration than endocardial units when studied at physiological rates.

Patterns of motor cortex effects on ankle flexor and extensor motoneurons in the “pyramidal” cat preparation

Abstract Descending volleys were initiated by single-pulse stimulation of the motor cortex (pericruciate region) in the “pyramidal” cat preparation. The effects of these cortical volleys were determined on the ventral root recorded monosynaptic reflex discharge evoked by stimulation of the several ankle flexor and extensor muscle nerves as well as on the segmental monosynaptic reflex initiated by dorsal root stimulation. The temporal pattern of cortical influence on the dorsal root evoked monosynaptic reflex discharge was similar to the results of Lloyds 1941 study. However, study of monosynaptic reflexes evoked for specific muscle nerves demonstrated that motor cortex volleys produced predominantly facilitation of ankle flexor monosynaptic reflexes and inhibition of ankle extensor monosynaptic reflexes. These results support the conclusion that ankle flexion is the principal movement represented on cat motor cortex. Ankle extensor muscles are also represented but the effect is predominantly inhibition rather than facilitation as originally proposed by Sherrington.

A comparison of motor cortex effects on slow and fast muscle innervations in the monkey.

Abstract Descending volleys were evoked by single-pulse stimulation of the precentral gyrus in the “pyramidal” monkey preparation. The effects of these volleys were determined on the soleus, the gastrocnemius, or the tibialis anterior monosynaptic reflex. Both cortical facilitation and inhibition were found in all motoneuron populations studied. Inhibition predominated in the soleus motoneuron population whereas facilitation was the principal motor cortex influence observed in gastrocnemius and flexor motoneuron nuclei. It was also shown that soleus motoneurons have a quantitatively greater response to tonic muscle stretch than was observed for the synergistic gastrocnemius and antagonistic flexor motoneurons. The results of these experiments suggest that the output from the motor cortex may be organized to curtail the activity of tonic postural muscles during cortically initiated movement.

Motor cortex-pyramidal effects on single ankle flexor and extensor motoneurons of the cat

Abstract Descending volleys were initiated by stimulation of the pericruciate cortex in the “pyramidal” cat preparation. Single motoneuron units were isolated by subdissection of selected ventral roots of the lumbosacral spinal segments. The identity of the isolated motoneuron unit was ascertained by conventional methods, utilizing stimulation of isolated peripheral nerves of the sciatic distribution. Only neurons identified as belonging to the populations innervating the medial and lateral heads of the gastrocnemius muscle, the soleus muscle and the ankle flexor muscles were studied. The results obtained confirmed and extended our previous observations which demonstrated that flexor motoneurons are predominantly facilitated by motor cortex volleys and extensor motoneurons are inhibited by the same volleys. In addition, it was possible to demonstrate cortical facilitation of some ankle extensor motoneurons. Such facilitation could not be demonstrated in earlier studies in which the monosynaptic reflex of the entire motoneuron population was employed for testing cortical effects. The temporal patterns of cortical effects on single motoneurons of a homonymous population differed for different units in the same preparation. Since systematic variation of cortical stimulation parameters and of site of stimulation of the motor cortex did not change these differences in cortically evoked unit patterns within the same population, it is assumed that these differences in cortically evoked patterns may be of functional significance.

Spontaneous Neural Activity

Single units which discharged with regular spontaneous rhythms without intentional stimulation were isolated from the ventral nerve cord of the crayfish by intracellular recording methods in the proximity of the sixth abdominal ganglion. These units were divided into two groups: group A units in which interspike intervals varied less than 10 msec; group B units in which interspike intervals varied within a ringe of 10–30 msec. Group A units maintained “constant” interspike intervals and could not be discharged by sensory inputs, while the group B units could be discharged by appropriate sensory-nerve stimulation. Both group A and B units discharged to direct stimulation of the impaled unit, and the evoked direct single-spike activity reset the spontaneous activity. In Group B units presynaptic volleys reset the spontaneous rhythm of some units and in other units synaptically evoked spikes were interpolated within the spontaneous rhythm without resetting the rhythms. In a few units it was possible to initiate ongoing spontaneous activity by direct stimulation of the impaled unit. Spontaneously active units demonstrated the property of enhancement when driven by repetitive direct stimulation. It is concluded that endogenous pacemaker activity is responsible for much of the ongoing regular spontaneous rhythms of the crayfish central neurons and that interaction of evoked responses with endogenous pacemaker sites is responsible for the observed results.