Carmine D. Clemente

REGENERATION IN THE VERTEBRATE CENTRAL NERVOUS SYSTEM.

Publisher Summary Through observations on the developing nervous system by the methods of experimental embryology and on nerve regeneration in the adult nervous system, three hypotheses have evolved regarding the factors involved in eventual guidance of a growing nerve fiber toward its goal. These have been termed, respectively, “the chemical,” “electrical,” and “mechanical theories” of nerve outgrowth orientation. The chemical theory states that particular specific chemical substances secreted by localized centers attract the growing nerve fiber. The third theory, which to date appears to have very convincing experimental data, emphasizes the importance of mechanical factors in the development of nerve patterns. The electrical theory proposed that differences in electrical potentials have an orienting effect on nerve fibers. A phylogenetic analysis of the problem of regeneration in the vertebrate central nervous system is introduced by pointing out the factors considered important in leading or guiding a developing nerve fiber to its destination during ontogenesis.

Forebrain inhibitory mechanisms: cortical synchronization induced by basal forebrain stimulation.

Abstract Cortical synchronization is of interest because of its similarity to the spontaneously occurring cortical spindle bursts associated with sleep and other states of central nervous system suppression. Although the diffuse thalamic projection system is primarily associated with this cortical response, other subcortical regions demonstrate involvement with the visceral, somatic, and endocrine correlates to these states. The preoptic region of the hypothalamus is of particular interest in this regard, and the present experiment attempted to determine whether a cortically directed influence from this region also exits. Stimulation and recording experiments were performed in the brains of acutely prepared, immobilized adult cats. Stimulating electrodes were lowered into anterior hypothalamic and basal forebrain regions. Electroencephalographic activity from several cortical areas was monitored during subcortical stimulation. An immediate and sustained cortical synchronization was observed upon stimulation of a specific preoptic-basal forebrain zone, which has been named the basal forebrain synchronizing area. A second focus inducing cortical synchronization was observed in the medial and basal amygdaloid complex. Stimulation interactions of the basal forebrain synchronizing area with the brainstem reticular activating system were assessed in other experiments, and the prevailing cortical influence appeared to depend on a factor of order and intensity of stimulation. It was also noted that basal forebrain induced cortical synchronization is completely abolished by the intravenous administration of 15 mg per kg of Nembutal. We believe that the basal forebrain synchronizing area is significant, in that it provides a stable anatomical starting point for the further exploration of forebrain suppressor mechanisms.

Afferent vagal stimulation: Neurographic correlates of induced eeg synchronization and desynchronization

Summary o 1. Cortical synchronization and desynchronization were induced by afferent cervical vagal stimulation in the cat. 2. By simultaneously recording the vagal neurogram and the induced EEG responses, it was determined that rapidly conducted vagal potentials were associated with EEG synchronization, while potentials conducted at approximately 15 m/sec were correlated with EEG desynchronization. 3. The specific factor which determined the nature of the EEG response was whether synchronogenic or desynchronogenic vagal afferent fibers were stimulated and not the frequency of stimulation. 4. Following supranodose transection of the vagus nerve, the sensory elements remaining in the vagus could be stimulated to induce a vagal neurogram which contained the potential complexes seen in the intact nerve and which had been correlated with the induced EEG responses. A histologic analysis of intact and transected vagi confirmed the neurographic findings. 5. The patterns of the induced synchronization which occur as a result of afferent vagal stimulation are similar to those which occur spontaneously or during quiet sleep in the unrestrained cat. It was concluded that the vagal afferent system is comprised of fiber groups which are able to initiate and maintain either synchronization or desynchronization of the EEG and that these fiber groups are both functionally and structurally discrete.

Post-reinforcement EEG synchronization during alimentary behavior

Abstract 1. 1. Cats were trained to manipulate a lever which delivered small amounts of liquid food (milk-broth) from a dispenser. Discrete periods of EEG synchronization were observed coincident to the development of this simple instrumental response. 2. 2. Slow wave synchronous activity in the EEG replaced low voltage fast activity when the animals received positive reinforcement. These “spindles” did not appear if the animals were not reinforced. They could be abolished by thalamic electrical stimulation at arousal parameters or by novel auditory stimuli, even though these internal and external stimuli were not intense enough to alter the behaving pattern of the cats in the instrumental situation. 3. 3. Water substituted for the milk-broth reward in a food deprived animal resulted in a disappearance of the post-reinforcement cortical synchronization. 4. 4. The EEG synchronization previously described during prolonged alimentary behavior was also observed in our animals under these circumstances. 5. 5. The presentation of a sensory stimulus in association with the delivery of larger amounts of food resulted in the classical conditioning of this synchronization. Extinction of the conditioned synchronization was also achieved. The prolonged presentation of a non-reinforced CS resulted in the reappearance of a synchronization, which, in this case, was associated with the extinction of an alimentary response.

Nature of suppression of the masseteric monosynaptic reflex induced by stimulation of the orbital gyrus of the cat

Abstract The nature of the suppression of the masseteric monosynaptic reflex due to stimulation of the orbital gyrus was investigated by intracellular recording from massetteric motoneurons. The following results were obtained: (1) Either single pulses or pulse trains delivered to the orbital gyrus usually exerted a prolonged suppression (up to around 40 msec) of the masseteric monosynaptic reflex recorded along the masseteric nerve in response to stimulation of the trigeminal mesencephalic nucleus or the masseteric nerve itself. (2) The antidromic focal potential, recorded within the trigeminal motor nucleus in response to the stimulation of the masseteric nerve, was remarkably suppressed by stimulation of the orbital gyrus. (3) For intracellular studies, masseteric motoneurons were identified by anti-dromic spike potentials induced by stimulation of the masseteric nerve. Monosynaptic EPSPs were recorded in these masseteric motoneurons in response to stimulation of the trigeminal mesencephalic nucleus, confirming the monosynaptic excitatory linkage between the mesencephalic and the motor nucleus of the trigeminal nerve. (4) Stimulation of the orbital gyrus induced a prolonged hyperpolarizing potentials in masseteric motoneurons, the time course of which closely resembled that of the cortically induced suppression of the masseteric monosynaptic reflex. Monosynaptic, as well as antidromic, spikes of masseteric motoneurons were inhibited coincident with this hyperpolarizing potential. Hyperpolarization was changed to depolarization by the intracellular diffusion of Cl−. (5) Simultaneous recording of monosynaptic spikes in masseteric motoneurons and the masseteric reflex evoked by stimulation of the mesencephalic nucleus of V revealed a close temporal correlation between inhibition of the monosynaptic spikes and suppression of masseteric monosynaptic reflex. (6) It was concluded that an active postsyna[tic inhibition is primarily responsible for the observed suppression of the massteric monosynaptic reflex by electrical stimulation of the orbital gyrus.

Cortical and subcortical patterns of response to afferent vagal stimulation

Cortical and subcortical patterns of activity were studied upon electrical stimulation of the cut central end of the cervical vagus nerve in acutely prepared adult cats. The effects of different frequencies and voltages of afferent vagal stimulation were studied while the duration of the stimulus pulses was kept constant (0.75 msec). High-frequency (> 70/sec) low voltage (< 3v) stimulation resulted in the induction of cortical EEG synchronization, while all other combinations of frequency and voltage gave rise to cortical activation. Subcortical structures responded in a manner consistent with the cortical observations. The cerebral cortex and thalamic nuclei responded to synchronogenic vagal afferent stimulus parameters only after an initial latent period, while the hippocampus responded immediately. All responses were bilaterally equivalent and could be induced by either vagus nerve. Severance of both sympathetic trunks and vagi and transection of the neuraxis at the spinomedullary junction indicated that the central responses were neurogenic in origin. These and other studies presently in progress offer convincing evidence that the different cortical responses (synchronization or desynchronization) observed by various investigators from stimulation of the central end of the cervical vagus nerve result from the differential stimulation of specific afferent fiber systems within that nerve.

The role of the lower brain stem in cortically induced inhibition of somatic reflexes in the cat.

Abstract The role of the lower brain stem in cortically induced alteration of somatic reflexes was investigated in the cat. The anterior portion of the orbital gyrus, previously implicated in the inhibition of monosynaptic and polysynaptic reflexes at all levels of the neuraxis, was shown to project directly to the ventromedial bulbar reticular formation (chiefly to the nucleus reticularis gigantocellularis) and to the pontine tegmentum (mainly to the nucleus reticularis pontis oralis). As a result of single pulse application to the anterior portion of the orbital gyrus, short latency (0.4–0.5 msec), direct responses were obtained in the ipsilateral and contralateral pontine and medullary reticular regions. The amplitude of these responses was approximately equal in the medullary inhibitory area, whereas, in the pontine facilitatory region, the size of contralaterally evoked responses was 40–65 % less than that of the ipsilaterally evoked potentials. The fact that the ventromedial bulbar reticular formation participates in the mechanism of orbital-cortically induced inhibition of the monosynaptic masseteric reflex, has been demonstrated in transection experiments. Transections of the medulla oblongata 2 mm caudal to the obex (P17) and lower had no effect on the efficiency of cortically induced reflex inhibition. More rostral transections (P15-P10) led to ineffectiveness and abolition of the cortically induced inhibitory influence. Moreover, transections at even more rostral levels (P8-P5) caused facilitation of the masseteric reflex following orbital-cortical stimulation. It is assumed that the descending orbito-medullary and orbito-pontine fibers synapse with neurons of the reticulo-spinal tract, and that axon collaterals of these neurons or axons of other reticular cells in the medullary or pontine reticular formation mediate inhibitory or excitatory influences, respectively, to the trigeminal and possibly also to other cranial nerve motor nuclei. A simple schematic drawing illustrates the role of the lower brain stem in orbital-cortically induced reflex alteration.

Inhibition of monosynaptic and polysynaptic reflexes and muscle tone by electrical stimulation of the cerebral cortex

Abstract Electrical stimulation of the rostral end of a forebrain inhibitory and EEG synchronizing system has resulted in suppression of motor behavior, induction of sleep, and inhibition of the monosynaptic masseteric reflex in cats and monkeys. The experiments reported here are concerned with a closer examination of the inhibitory effects exerted by the cortical end of this system on somatic reflexes at different levels of the neuraxis and in various stages of vigilance in the adult cat. Electrical stimulation, consisting of a short train of pulses, was applied to the rostral portion of the orbital gyrus. Following the train of impulses to the cortex, test reflexes were electrically elicited, and the effects of cortical stimulation on reflex discharges were studied. Electrical stimulation of the cortical inhibitory area resulted in immediate and consistent inhibition of a variety of monosynaptic and polysynaptic reflexes at different levels of the neuraxis. This inhibition was diffuse and nonreciprocal when the concurrently recorded EEG was synchronized. On the other hand, reflexes for antigravity muscles were inhibited and those for their antagonists facilitated, when the EEG was desynchronized. Additionally, diffuse inhibition of spontaneous muscle tone and synchronization of the EEG were obtained by electrical stimulation of this same cortical area. These findings provide some neurophysiological basis for an understanding of forebrain inhibitory mechanisms.

Developing blood brain barrier to trypan blue.

Summary Trypan blue injected into developing rat embryos (10 days to birth) fails to stain the central nervous system. This impermeability in the developing central nervous system exists at the time when blood vessels invade the brain. It cannot be assumed, however, that the mechanisms which prevent protein-bound dyes from penetrating the cerebral blood vessels are the same for all other substances.

Acceptance of high concentration saccharin solutions by cats after hypothalamic lesions.

Abstract Twenty cats with monopolar electrodes implanted bilaterally in the posterior, lateral or rostral hypothamus were deprived of water and trained to press a lever to obtain distilled water or to press another lever to obtain saccharin solution. It was found that the cats initially preferred saccharin solution over distilled water, but when concentration of saccharin in solution was higher than 2.0–3.0% most animals refused to accept it and drank only distilled water. Bilateral hypothalamic lesions made through the implanted electrodes resulted in acceptance of saccharin solutions of high concentrations (5–7%), although a considerable decrease in the intake of both distilled water and saccharin solution was observed during several weeks after lesions. The strongest effect was found in those cases in which lesions involved medial forebrain bundle, zona incerta, and fields of Forel.