Antidromic stimulation in the ponto-medullary reticular formation of local axon branches of contralateral vestibular neurons

The review demonstrates that control of posture and locomotion is provided by systems across the caudal-to-rostral extent of the neuraxis. A common feature of the neuroanatomic organization of the postural and locomotor control systems is the presence of key nodes for convergent input of multisensory feedback in conjunction with efferent copies of the motor command. These nodes include the vestibular and reticular nuclei and interneurons in the intermediate zone of the spinal cord (Rexed's laminae VI–VIII). This organization provides both spatial and temporal coordination of the various goals of the system and ensures that the large repertoire of voluntary movements is appropriately coupled to either anticipatory or reactive postural adjustments that ensure stability and provide the framework to support the intended action. Redundancies in the system allow adaptation and compensation when sensory modalities are impaired. These alterations in behavior are learned through reward- and error-based learning processes implemented through basal ganglia and cerebellar pathways respectively. However, neurodegenerative processes or lesions of these systems can greatly compromise the capacity to sufficiently adapt and sometimes leads to maladaptive changes that impair movement control. When these impairments occur, the risk of falls can be significantly increased and interventions are required to reduce morbidity.

The physiological properties of neurons in the second somatosensory cortex (SII), and the pattern of projection of these neurons to area 4γ of the motor cortex in cat were studied by using single unit recording and collision techniques. Antidromically activated neurons were recorded along the anterior and posterior regions of the lateral bank of the anterior suprasylvian sulcus (ASSS) and from the middle part of the anterior ectosylvian gyrus (AESG) following weak intracortical microstimulation (ICMS) to area 4γ. Stimulation of the region around the activated neurons failed to produce muscle contraction or movement with currents of 30 μA or less. The majority of antidromically identified neurons received somatotopically organized afferent inputs from the skin on the contralateral side of the body. A small number of SII neurons received bilateral input. In 91% of the cases receptive field information was available for both the antidromically activated SII neuron and for neurons around the stimulating electrode in area 4γ. In 71% of these cases, both cortical sites were activated by sensory input from the same or adjacent peripheral area of the body. Neurons in the rostrocaudal region of the lateral bank of ASSS and the upper part of AESG (forelimb area) projected to the lateral cruciate gyrus of the motor cortex (forelimb area), while neurons in the ventrocaudal region of the medial part of AESG (hindlimb area) projected to the medial part of the postcruciate subregion of the motor cortex (hindlimb area. Antidromically activated SII neurons were typically found in layer III. These results suggest a topographically organized pattern of projection to the motor cortex from SII.

Axons from neurons in the occipital cortex transiently extend to the pyramidal tract (PT) during the early postnatal development of rats. Normally, these axons are eliminated by the end of the third postnatal week. However, if a portion of fetal occipital cortex is transplanted to the parietofrontal region in newborn hosts then some neurons in the transplant will extend pyramidal tract axons and maintain them. Intracortical microstimulation and electrophysiological recording techniques were used to identify the physiological characteristics of the transplanted pyramidal tract cells and to determine if motor effects could be elicited from the occipital transplant. Microstimulation of the transplant did not reliably evoke movement but the low density and disarray of PT cells within the transplant might account for this. Recording from within the transplant revealed that the overall cell activity was depressed. We were able to identify neurons within the transplant which responded antidromically to stimulation of the pyramidal tract, indicating that their axons have the capacity to conduct impulses and are therefore likely to have developed some viable connections. The functional significance of such projections remains uncertain.

EMG responses were recorded from axial muscles transversospinalis, medial longissimus, and lateral longissimus in urethane-anesthetized rats during combined electrical stimulation of the reticular formation and midbrain central gray. Central gray stimulation facilitated reticular formation-evoked EMG activity in the back muscles of the rat. Electrical stimulation of the central gray lowered the threshold for reticulo-spinal activation of axial muscles and could maintain firing in these muscles after the end of a reticular formation train. Units were recruited in order of size from small to large. In only one case, central gray stimulation activated axial muscles directly without reticular formation stimulation. The central gray may be important in relaying hypothalamic influences to the reticular formation, which has direct access to the axial muscles responsible for lordosis behavior.

Respiratory neuron activity was recorded with microelectrodes in the vicinity of the nucleus tractus solitarius (NTS) and the rostral portion of nucleus retroambigualis (rNRA) in pentobarbital anesthetized cats with bilateral pneumothorax. Inspiratory NTS neurons receiving facilitatory volume inputs (inspiratory beta neurons) had short latency responses to threshold electrical stimulation in the ipsilateral vagus, but rarely to stimulation of the inspiratory inhibitory region of the rostral pons (pneumotaxic areas), suggesting that beta neurons do not appear to be the site of convergence of inspiratory inhibitory inputs. Inspiratory neurons in the rNRA did not have short latency facilitatory responses to vagal stimulation, but 15% responded to pneumotaxic area stimulation. One-third of the rNRA inspiratory neurons appeared to have spinal axons. None of those with spinal axons responded to pneumotaxic stimulation. Pneumotaxic area stimulation did consistently cause short latency evoked potentials in the rNRA. It is concluded that some rNRA-area propriobulbar inspiratory neurons may play a role in inspiratory inhibition by pneumotaxic mechanisms.

Inputs to medial longissimus (ML) and lateral longissimus (LL) motoneurons were studied in urethane or urethane-chloralose anesthetized rats by recording from ML and LL nerves while stimulating ipsilateral lumbosacral dorsal roots, medial medullary reticular formation (RF), vestibular nuclei (VN), dorsal midbrain (MDBR), or ventromedial hypothalamus (VMH).

Stimulation of appropriate dorsal roots produced short-latency responses (1.5–3.0 ms) in nerves to medial longissimus or lateral longissimus. The connections underlying these responses, which could be monosynaptic, are weak, since generally two or more stimuli were necessary for a response to occur. Short-latency LL nerve responses required more dorsal root stimuli than did ML nerve responses and stable LL responses sometimes could not be obtained, suggesting that segmental reflexes to a back muscle (LL) could be weaker than those to a proximal tail muscle (ML).

Trains of conditioning stimuli delivered to the RF, VN, and MDBR facilitated segmental responses in ML nerves or LL nerves. Temporal profiles of facilitation of ML differed for the three regions. On one extreme, the facilitation produced by RF conditioning required few stimuli (median, 3 shocks) and peak facilitation occurred at short condition-test intervals (median, 1.5 ms). On the other extreme, facilitation produced by MDBR conditioning required long trains (median, 14 stimuli) and peak facilitation occurred at longer condition-test intervals (median, 10ms).

Stimulation within the VMH never facilitated ML or LL nerve activity.

These results demonstrate excitatory connections from reticular formation, vestibular nuclei and the dorsal midbrain to medial longissimus and lateral longissimus. Such connections could be involved in behaviors mediated by midbrain, and in postural regulation through brain stem control of axial musculature.

Motoneuron cell bodies for LL, ML and lumbar transversospinalis (TS) muscles were localized by ejecting dye at sites where unitary antidromic responses to muscle nerve stimulation were recorded extracellularly. ML cells were found ventrolaterally in the L₆-S₁ ventral horn. LL and TS cells were found medially in the ventral horn of the lumbar enlargement.