Jack L. Feldman

In vitro brainstem-spinal cord preparations for study of motor systems for mammalian respiration and locomotion

Recently developed in vitro preparations of the brainstem-spinal cord from neonatal rat suitable for investigation of motor control systems for mammalian locomotion and respiration are described. The preparations remain viable for extended periods under standard in vitro conditions and generate rhythmic motor patterns for locomotion and respiration. The methodology of the preparations and characteristics of the motor output patterns are described. The preparations retain functional circuitry for major components of the motor control systems, including brainstem respiratory and spinal locomotor pattern generating networks, brainstem locomotor command regions, descending bulbospinal and ascending spinal pathways, and mechanosensory afferent input systems. They therefore offer potential for investigation of diverse aspects of the mammalian respiratory and locomotor control systems.

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves.

SummaryThe role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 μm thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 μA, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.

Pulmonary Afferent Influences on Respiratory Modulation of Sympathetic Discharge

Experiments were performed on decerebrate or urethane-anesthetized, gallamine-paralyzed cats with pneumothorax and intact vagi. Efferent splanchnic and cervical sympathetic nerve discharges were recorded monophasically (0.2–1000 HZ). Phrenic nerve discharge served as an indicator of respiratory center output. Lung inflation was applied co-incidentally with phrenic discharge during control cycles by means of a cycle-triggered pump. Changes in the timing of cycle-triggered pump inflations were used to evaluate effects of pulmonary afferent activity from lung stretch receptors on central respiratory modulation of sympathetic discharge. When inflation was not applied for one inspiratory phase, the Breuer-Hering inspiratory-inhibitory reflex did not occur: inspiratory prolongation with no change in slope of the integral of phrenic activity, as measured with an average-response computer. In contrast, this test produced an increase in slope of the integrated sympathetic discharge. This indicates that inflations during the control inspiratory phases inhibited sympathetic discharge. The striking difference in the slope of phrenic vs sympathetic activities implies that this inhibition of sympathetic activity was acting via circuits different from, but related to, those of the Breuer-Hering reflex. Lung inflation also inhibited sympathetic discharge during the expiratory phase, since inflations applied during the expiratory phase reduced sympathetic discharge, concomitantly with lengthening of the expiratory phase (Breuer-Hering expiratory-facilitatory reflex). The latency for sympathetic inhibition from the onset of inflation was of the order of 100–200 ms both for inspiratory and expiratory inflations. Vagotomy abolished these effects and resulted in an increased respiratory modulation of sympathetic discharge. These results indicate that pulmonary afferent activity exerts an important influence on the central respiratory modulation of sympathetic discharge.

A stereotaxic system for independent coordinated positioning of two or three microelectrodes.

This paper describes a straightforward, easily built and relatively inexpensive stereotaxic system that permits independent coordinated positioning of 2 or 3 microelectrodes. This system is ideally suited for situations where the ability to routinely place one electrode at a precise position relative to another is important. Resolution of relative position and interelectrode distance is +/- 50 micrometers. This system should prove useful for the study of local interactions in such neural aggregates as nuclei, motoneuron pools and within and between cortical columns.