Lower lumbar branching of caudal medullary expiratory neurons of the cat

Axonal projection of the medullary expiratory neurons in the feline thoracic spinal cord

Expiratory neurons in the caudal ventral respiratory group extend descending axons to the lumbar and sacral spinal cord, and they possess axon collaterals, the distribution of which has been well-documented. Likewise, these expiratory neurons extend axons to the thoracic spinal cord and innervate thoracic expiratory motoneurons. These axons also give rise to collaterals, and their distribution may influence the strength of synaptic connectivity between the axons and the thoracic expiratory motoneurons. We investigated the distribution of axon collaterals in the thoracic spinal cord using a microstimulation technique. This study was performed on cats; one cat was used to make an anatomical atlas and six were used in the experiment. Extracellular spikes of expiratory neurons were recorded in artificially ventilated cats. The thoracic spinal gray matter was microstimulated from dorsal to ventral sites at 100-μm intervals using a glass-insulated tungsten microelectrode with a current of 150-250 μA. The stimulation tracks were made at 1 mm intervals along the spinal cord in segments Th9 to Th13, and the effective stimulating sites of antidromic activation in axon collaterals were systematically mapped. The effective stimulating sites in the contralateral thoracic spinal cord with expiratory neurons in the caudal ventral respiratory group (cVRG) occupied 14.4% of the total length of the thoracic spinal cord examined. The mean percentage of effective stimulating tracks per unit was 18.6 ± 4.4%. The distribution of axon collaterals of expiratory neurons in the feline thoracic spinal cord indeed resembled that reported in the upper lumbar spinal cord. We propose that a single medullary expiratory neuron exerts excitatory effects across multiple segments of the thoracic spinal cord via its collaterals.

Firing properties of medullary expiratory neurons during fictive straining in cats

Expiratory (E) neurons in the caudal nucleus retroambigualis extend descending spinal axons to the lumbar and sacral spinal cord. Discharge rates of single E neurons were recorded to examine differences in activity of E neurons projecting to the lumbar or sacral spinal cord during fictive straining induced by distention of the colon with a balloon. Firing frequencies of E neurons with descending axons in the thoracic and lumbar spinal cord increased during the repetitive rise of rectum pressure, whereas those of E neurons with descending axons in the sacral spinal cord decreased. E neurons with descending axons in the thoracic/lumbar and sacral spinal cord exhibit different firing characteristics during the repetitive rise of rectum pressure when straining during defecation. The activity of abdominal nerves during fictive straining is in phase with changes in rectum pressure, but out of phase with the activity of E neurons.

Atoh1-dependent rhombic lip neurons are required for temporal delay between independent respiratory oscillators in embryonic mice

All motor behaviors require precise temporal coordination of different muscle groups. Breathing, for example, involves the sequential activation of numerous muscles hypothesized to be driven by a primary respiratory oscillator, the preBötzinger Complex, and at least one other as-yet unidentified rhythmogenic population. We tested the roles of Atoh1-, Phox2b-, and Dbx1-derived neurons (three groups that have known roles in respiration) in the generation and coordination of respiratory output. We found that Dbx1-derived neurons are necessary for all respiratory behaviors, whereas independent but coupled respiratory rhythms persist from at least three different motor pools after eliminating or silencing Phox2b- or Atoh1-expressing hindbrain neurons. Without Atoh1 neurons, however, the motor pools become temporally disorganized and coupling between independent respiratory oscillators decreases. We propose Atoh1 neurons tune the sequential activation of independent oscillators essential for the fine control of different muscles during breathing.DOI: http://dx.doi.org/10.7554/eLife.02265.001.

Connections between expiratory bulbospinal neurons and expiratory motoneurons in thoracic and upper lumbar segments of the spinal cord

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

Cough motor mechanisms

Cough is a defensive airway mechanisms which involves the sequential activation of several laryngeal and respiratory muscles in the generation of the typical four-phase motor pattern. Activation of such muscles can be considered to represent the "primary" cough motor mechanism, and its functional significance, although complex, appears to be fairly well established. Nonetheless, the outflows of cough are numerous, and may additionally involve the reflex or mechanical activation of other respiratory and non-respiratory motor systems. These additional, or "secondary", outflows of cough can mainly be regarded as being involved in either enhancing the defensive function of cough, or opposing the possible noxious effects exerted by the mechanical stresses of coughing. In addition, both the primary and secondary cough motor mechanisms are known to play multiple functional roles, thus considerably complicating the cough panorama. Finally, some of the secondary cough motor responses, such as the changes in the pattern of breathing, seem to be devoid of any favourable action and their functional meaning, if any, is not fully understood. Although it is well known that all patterns of cough can be produced voluntarily, the extent to which also volitional cough is accompanied by an array of motor responses similar to that of reflex cough remains to be elucidated.

Functional Heterogeneity Among Neurons in the Nucleus Retroambiguus With Lumbosacral Projections in Female Cats

Nucleus retroambiguus (NRA), in the caudal medulla, projects to all spinal levels. One physiological role is abdominal pressure control, evidenced by projections to intercostal and abdominal motoneurons from expiratory bulbospinal neurons (EBSNs) within NRA. The roles of NRA projections to the lumbosacral cord are less certain, although those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we physiologically characterized NRA projections to the lumbosacral cord. Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO2gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L6, L7, or sacral segments and to microstimulation in the region of semimembranosus motor nucleus or ON. All units were found at sites showing expiratory discharges. Units that showed collisions between antidromic and spontaneous spikes (all in late expiration) were identified as EBSNs. These were common from the ventrolateral funiculus (VLF) of L6(42.5%) or L7(32.9%), but rare from the sacral VLF or the motor nuclei. Antidromic latencies revealed a subthreshold respiratory drive in some non-EBSNs. This group had lower conduction velocities than the EBSNs. The remainder, with a negligible respiratory drive, had even lower conduction velocities. A new population of NRA neurons has thus been defined. They are not active even with a strong respiratory drive, but may provide most of the synaptic input from NRA to lower lumbar and sacral segments and could subserve functions related to mating behavior.

Localization and distribution patterns of nicotinamide adenine dinucleotide phosphate diaphorase exhibiting axons in the white matter of the spinal cord of the rabbit

The funicular distribution of nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-exhibiting axons was examined in the white matter of the rabbit spinal cord by using horizontal, parasaggital, and transverse sections. Four morphologically distinct kinds of NADPHd-exhibiting axons (2.5-3.5 microm in diameter) were identified in the sulcomarginal fasciculus as a part of the ventral column in the cervical and upper thoracic segments and in the long propriospinal bundle of the ventral column in Th3-L3 segments. Varicose NADPHd-exhibiting axons of the sympathetic preganglionic neurons, characterized by widely spaced varicosities, were found in the ventral column of Th2-L3 segments. A third kind of NADPHd-positive ultrafine axons, 0.3-0.5 microm in diameter with numerous varicosities mostly spherical in shape, was identified in large number within Lissauer's tract. The last group of NADPHd-exhibiting axons (1.0-1.5 microm in diameter) occurred in the Lissauer tract. Most of these axons were traceable for considerable distances and generated varicosities varying in shape from spherical to elliptical forms. The majority of NADPHd-exhibiting axons identified in the cuneate and gracile fascicles were concentrated in the deep portion of the dorsal column. An extremely reduced number of NADPHd-exhibiting axons, confirmed by a computer-assisted image-processing system, was found in the dorsal half of the gracile fascicle. Axonal NADPHd positivity could not be detected in a wide area of the lateral column consistent with the location of the dorsal spinoccrebellar tract. Numerous, mostly thin NADPHd-positive axonal profiles were detected in the dorsolateral funiculus in all the segments studied and in a juxtagriscal portion of the lateral column as far as the cervical and lumbar enlargements. A massive occurrence of axonal NADPHd positivity was detected in the juxtagriseal layer of the ventral column all along the rostrocaudal axis of the spinal cord. The prominent NADPHd-exhibiting bundles containing thick, smooth, nonvaricose axons were identified in the mediobasal and central portion of the ventral column. First, the sulcomarginal fasciculus was found in the basal and medial portion of the ventral column in all cervical and upper thoracic segments. Second, more caudally, a long propriospinal bundle displaying prominent NADPHd positivity was localized in the central portion of the ventral column throughout the Th3-L3 segments.

Spinal interneuronal systems: identification, multifunctional character and reconfigurations in mammals

This review focuses on the flexibility of operation of spinal interneuronal networks and their multifunctional character in mammals. It concerns, in particular, two ways in which spinal interneuronal networks may be functionally reorganised, namely by modulating the synaptic actions of primary afferents by monoamines and by GABAergic presynaptic inhibition. The evidence will be reviewed for topographical and target-related differences in modulatory effects in various interneuronal networks and these will be related to differences in the intrinsic properties of different functional types of interneurones in these networks and to the role played by them.

Localization of NADPHd-exhibiting neurons in the spinal cord of the rabbit

Segmental and laminar distributions of nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-exhibiting neurons were examined in the rabbit spinal cord by using horizontal, sagittal, and transverse sections. A large number of NADPHd-positive neurons in the spinal cord of rabbit appeared to fall into six categories (N1-N6), but others could not be classified. Major cell groups of NADPHd-exhibiting neurons were identified in the superficial dorsal horn and around the central canal at all spinal levels and in the intermediolateral cell column at thoracic and upper lumbar levels. NADPHd-exhibiting neurons of the pericentral region were divided into a thin subependymal cell column containing longitudinally arranged, small bipolar neurons with processes penetrating deeply into the intermediolateral cell column and/or running rostrocaudally in the subependymal layer. The second pericentral cell column located more laterally in lamina X contains large, intensely stained NADPHd-exhibiting neurons with long dendrites radiating in the transverse plane. In the pericentral region (lamina X), close association of NADPHd-exhibiting somata and fibers and mostly longitudinally oriented blood vessels were detected. Neurons of the sacral parasympathetic nucleus, seen in segments S1-S3, exhibited prominent NADPHd cellular staining accompanied by heavily stained fibers extending from Lissauer's tract through lamina I along the lateral edge of the dorsal horn to lamina V. A massive dorsal gray commissure, highly positive in NADPHd staining, was found in segments S1-S3. Scattered positive cells were also found in the deeper dorsal horn, ventral horn, and white matter. Fiberlike NADPHd staining was found in the superficial dorsal horn and pericentral region in all the segments studied. Dense, punctate, nonsomatic NADPHd staining was detected in the superficial dorsal horn, in the pericentral region all along the rostrocaudal axis, and in the nucleus phrenicus (segments C4-C5), nucleus dorsalis (segments Th2-L2), Onuf's nucleus (segments S1-S3), and the dorsal part of the dorsal gray commissure (S1-S3).

Spontaneous electromyographic activity in adult rat soleus muscle

Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16-25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20-30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.

Thyrotropin-releasing hormone immunoreactive boutons form close appositions with medullary expiratory neurons in the rat

The aim of the present study was to assess the size of the input from TRH immunoreactive varicosities to medullary respiratory neurons in the Bötzinger complex and caudal ventral respiratory group. Neurobiotin was intracellularly injected into seven neurons in the Bötzinger complex, between 0.4 and 0.9 mm caudal to the facial nucleus. Five of the seven Bötzinger neurons had extensive local axonal projections, with bouton-like varicosities clustered predominantly between their somata and the nucleus ambiguus. Seven neurons in the caudal ventral respiratory group, located between 1.6 and 2.4 mm caudal to the facial nucleus, were also labelled. All but one caudal respiratory neurons had no, or very few, medullary collaterals. TRH immunoreactive fibres were seen in many medullary nuclei, including the ventral reticular formation. Bötzinger neurons were closely apposed by an average of 29 +/- 8 TRH immunoreactive boutons/neuron (mean +/- S.D., n = 7). In contrast, caudal ventral respiratory group neurons were apposed by only 5 +/- 3 TRH immunoreactive boutons/neuron (n = 7). Bötzinger neurons form many intramedullary and bulbospinal inhibitory connections with premotoneurons and motoneurons that are important in the timing, amplitude and shape, of respiratory activity. Our findings suggest a role for endogenous TRH-containing neurons in modulating the activity of inhibitory Bötzinger neurons and neurons in the caudal ventral respiratory group. The significance of the apparent difference in size of this input remains to be determined.

An electrophysiological demonstration of axonal projections of single ventral inspiratory neurons to the phrenic nucleus of the cat

Axonal branching patterns of single inspiratory (I) neurons of the nucleus retroambigualis (NRA) were studied electrophysiologically in cat phrenic nucleus (C4-C6). Experiments were performed on Nembutal anesthetized, artificially ventilated cats, and extracellular spikes of I neurons were recorded. The cervical spinal gray matter was microstimulated from dorsal to ventral sites at 100 microns intervals with an intensity of 150-250 microA using a glass insulated tungsten microelectrode. The stimulations were made at 1 mm intervals rostrocaudally along the spinal cord, and effective stimulating sites of antidromic activation in axonal collaterals were systematically mapped. I neurons examined (n = 8) descending contralaterally distributed multiple collaterals in the phrenic nucleus. These collaterals were found throughout the rostrocaudal phrenic nucleus. An I neuron (n = 1) descending ipsilaterally also distributed collaterals in the ipsilateral phrenic nucleus. Axonal collaterals in the contralateral phrenic nucleus occupied 44.2% of the total length of the cervical spinal cord examined. To determine the detailed trajectory of collaterals in the cervical gray matter, microstimulation was performed in and around the collateral arborizations at the maximum intensity of 50 microA. The descending stem axons could be localized in the lateral funiculus in four I neurons and in the ventral funiculus in one I neuron. I neurons distributed axonal collaterals within the phrenic nucleus. Some part of the collaterals ran to the medial region of the gray matter, re-crossed the midline under the central canal and reached the phrenic nucleus ipsilateral to the I neuron. Re-crossed collaterals arborized in the phrenic nucleus, but did not extend to the gray matter more lateral than the phrenic nucleus. Rostrocaudal extension of the re-crossed collaterals was found to be narrow.

Multifunctional ventral respiratory group: bulbospinal expiratory neurons play a role in pudendal discharge during vomiting

Pudendal motoneurons are activated in phasic bursts during the retching and expulsion phases of vomiting. The resulting contraction of the anal and urethral sphincters serves to maintain continence during the large increase in abdominal pressure that occurs during vomiting. We evaluated the contribution of bulbospinal expiratory neurons located in the portion of the ventral respiratory group (VRG) caudal to the obex (nucleus retroambigualis) to the control of pudendal motoneurons during fictive vomiting in decerebrate, paralyzed cats. Pudendal nerve discharge is abolished by cutting the axons of caudal VRG expiratory neurons as they cross the midline between the obex and C1 before descending in the spinal cord. All caudal VRG expiratory neurons that were antidromically activated from the sacral spinal cord, where the pudendal motor pool (nucleus of Onuf) is located, discharged strongly during the end of the expulsion phase of vomiting. However, only a small proportion of these neurons was active in phase with pudendal discharge during the retching phase. The apparent involvement of caudal VRG expiratory neurons in the control of pudendal motoneurons during vomiting is another example of the multifunctional role that can be played by respiratory-related neurons in the mammalian nervous system.

Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: evidence for direct projections possibly representing the final common pathway for lordosis

The nucleus retroambiguus (NRA) projects to distinct brainstem and cervical and thoracic cord motoneuronal cell groups. The present paper describes NRA projections to distinct motoneuronal cell groups in the lumbar enlargement. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify the retrogradely labeled neurons in the caudal medullary lateral tegmentum. These injections were combined with spinal hemisections to distinguish between neurons having ipsi-or contralaterally descending axons. The NRA-lumbosacral fibers descend almost exclusively contralaterally, but neurons in areas surrounding the NRA project mainly ipsilaterally. In an anterograde tracing study, injections of WGA-HRP or tritiated leucine were made in the region of the NRA to determine the NRA targets in the lumbosarcral cord. Hemisections in C2 made it possible to distinguish between NRA projections and projections from neurons in the adjoining lateral tegmentum. The results show delicate NRA projections to distinct lumbosacral motoneuronal cell groups innervating specific hindlimb muscles (iliopsoas, adductors, and hamstrings) as well as axial muscles (medial longissimus and proximal tail muscles). The projection is bilateral, with a contralateral predominance. Ipsilaterally terminating fibers are derived from NRA neurons whose axons cross the midline at the level of the obex, descend through the contralateral spinal white matter, and recross at the level of termination. A conceptual description is presented in which the periaqueductal gray-NRA-lumbosacral projections form the final common pathway for lordosis in the cat.

Axon branching of medullary expiratory neurons in the lumbar and the sacral spinal cord of the cat

Intraspinal axon collaterals of expiratory (E) neurons in the caudal nucleus retroambigualis extending their desending spinal axons to the lower lumbar (L6-L7) and the sacral (S1-S3) segments were investigated in anesthetized cats. To search for axon collaterals of single E neurons in the lumbar segments, the spinal gray matter was microstimulated from the dorsal to the ventral sites at 100 microns intervals with an intensity of 150-250 microA at 1 mm intervals rostrocaudally along the spinal cord, and effective stimulating sites of antidromic activation in axon collaterals were systematically mapped. In addition, the detailed trajectory of collaterals in the upper lumbar (L1-L3), the middle lumbar (L4-L5), and the sacral (S1-S3) spinal cord was examined by microstimulation at a matrix of points 100-200 microns apart with a maximum stimulus intensity of 50 microA. The trajectory of axon collaterals was reconstructed on the basis of the location of low-threshold foci and the latency of antidromic spikes. Virtually all E neurons examined had 1-7 collaterals at widely separated segments of the lumbar cord. Many axon collaterals were found in the upper lumbar spinal cord as compared to the middle and the lower lumbar spinal cord. The locations of axon collaterals in the upper lumbar spinal cord overlapped with those of abdominal motoneurons. Axon collaterals in the sacral gray matter were found in 3 of 9 E neurons. Axon collaterals were found within the nucleus of Onuf, in the region dorsal to the nucleus of Onuf, and in the intermediate region. The functional significance of the divergent distribution of multiple axon collaterals of single E neurons in different spinal levels of the lumbar and the sacral spinal cord is discussed in relation to the respiratory function of E neurons and other spinal motor activities.