Functional relations between primate motor cortex cells and muscles: fixed and flexible

In behaving monkeys the effects of motor cortex cells on muscles are inferred from two quite different types of 'correlational' evidence: their coactivation and cross-correlation. Many precentral cells are coactivated with limb muscles, suggesting that they make a proportional contribution to muscle activity; however, such coactivation is typically quite flexible, and can be changed by operantly conditioning the dissociation of cell and muscle activity. Cross-correlating cells and muscles by spike-triggered averaging of the electromyogram (EMG) shows that certain cells produce short-latency post-spike facilitation of EMG; this correlational linkage is relatively fixed under different behavioural conditions and its time course suggests it is mediated by a corticomotoneuronal (CM) synaptic connection. CM cells typically facilitate a set of coactivated agonist muscles, and some also inhibit their antagonists. The firing patterns of CM cells can differ significantly from those of their target muscles. During ramp-and-hold wrist responses most CM cells discharge a phasic burst that precedes target muscle onset and that contributes to changes in muscle activity. At low force levels many CM cells are activated without their target motor units. Conversely, many CM cells are paradoxically inactive during rapid forceful movements that vigorously activate their target muscles; they appear to be preferentially active during finely controlled movements. Thus CM cells, with a fixed correlational linkage to their target muscles, may be recruited without their target muscles, and vice versa.

Some aspects of the organization of the output of the motor cortex

The precentral motor cortex in the macaque is defined here as that portion of the precentral motor-sensory areas which projects to the intermediate zone and motor neuronal cell groups in the spinal cord and their bulbar counterparts, i.e. the lateral reticular formation and motor nuclei of the lower brainstem. In this respect the precentral motor cortical areas differ from postcentral areas such that the descending projections from the latter are focused on the spinal dorsal horn and the spinal V complex. Differences in the distribution of the corticospinal fibres in different species are mentioned and differences in findings obtained by means of different tracing techniques are discussed. The projections from the precentral motor cortex to various brain-stem cell groups are also discussed and the areas of origin of these projections are delineated. The presence of branching neurons distributing collaterals to several of these areas is considered.

A cadaveric study of the anatomic variations of the recurrent motor branch of the median nerve

Anatomic dissections were performed in 72 cadaveric upper extremities from 36 cadavers to determine the incidence of anomalous variations in the course of the median nerve and its branches. The classic recurrent motor branch anatomy was demonstrated in 86% of the dissections (62/72). Of the 10 variations noted (14% of all upper extremities), all were transretinacular branches that pierced the transverse carpal ligament 2 to 6 mm proximal to the distal edge. Of the six cadavers with anomalous branching, four (67%) had bilateral anomalies and two (33%) had unilateral branching.

Expression of nerve-muscle topography during development

Previous studies have indicated that in 2 muscles of the adult rat, the anterior serratus and the diaphragm, the rostrocaudal axis of the motoneuron pool projects topographically onto the rostrocaudal axis of the muscle. In the present work we have asked whether this orderly topography emerges as a function of postnatal synaptic rearrangement or whether this pattern is already established at birth. The anterior serratus muscle was studied over the period ranging from embryonic day 17 through postnatal day 30. Using 2 criteria of topography, average segmental innervation and average target field of cervical roots C6 and C7, we found that a topographic distribution of the motoneuron pool is already present prior to birth and maintained throughout the postnatal period. Moreover, both C6 and C7 form an orderly map over the surface of the serratus in the embryo, and the topography is sharpened during postnatal periods. The diaphragm also is topographically innervated at birth and undergoes a comparable sharpening of the projection map postnatally. We conclude that the topographic projection of motoneurons is established prior to birth in these muscles, and postnatal synaptic rearrangement serves to sharpen the topographic map toward the adult pattern. These results also suggest that the pursuit of basic mechanisms underlying topography should be directed toward initial embryonic nerve-muscle contacts.

Anatomical and functional segregation in the stapedius motoneuron pool of the cat

1. Electromyographic activity (EMG) is detectable in the feline stapedius muscle 6-10 ms after the onset of an intense sound presented to either ear. Stapedius reflexes evoked by ipsilateral and contralateral sound were measured electromyographically before and after brain stem lesions were made. In some cases, stapedius motor axons were cut; in others, brain stem regions containing motoneuron cell bodies were destroyed electrolytically. 2. Electrolytic lesions that contacted an anatomically separate cluster of stapedius motoneurons (the ventromedial perifacial group) greatly reduced responses to contralateral sound without noticeably affecting responses to ipsilateral sound. 3. Electrolytic lesions in other brain stem areas had different effects; one appeared to reduce responses to ipsilateral sound selectively, whereas others reduced both responses or had little effect. 4. After subsets of stapedius motor axons were cut at the facial colliculus in the floor of the fourth ventricle, responses to contralateral sound were almost eliminated, while substantial responses to ipsilateral sound remained. 5. The results are consistent with the hypothesis that inputs from the two cochleas are distributed inhomogeneously across the stapedius motoneuron pool in such a way as to produce a segregation of function, with motoneurons in one brain stem region responding preferentially (or exclusively) to contralateral sound and motoneurons in other regions responding preferentially (or exclusively) to ipsilateral sound. This topographic organization of acoustic input to the stapedius motoneuron pool produces a "central partitioning" in the acoustic stapedius reflexes similar in some respects to the partitioning observed in proprioceptive spinal reflexes.

The somatotopy of the masticatory neurons in the rat trigeminal motor nucleus as revealed by HRP study

Young adult albino rats of Wistar strain were used for the present study. 0.5 to 15 microliters of 20-50% of horseradish peroxidase (HRP) were injected into each individual muscle of mastication to label neurons in the trigeminal motor nucleus (TMON) for light microscopic study. The results reveal that: (1) Many HRP-labeled, multipolar neurons are observed in the motor nucleus in each jaw-closing muscle (JCM) with less in each the jaw-opening muscle (JOM). (2) The motor neurons innervating each masticatory muscle in the motor nucleus show a somatotopic arrangement: (a) those innervating the temporalis muscle are located in the medial and dorsomedial parts; (b) those innervating the masseter muscle are located in the intermediate and lateral; (c) those innervating the medial and lateral pterygoid muscles are located in the lateral, ventrolateral and ventromedial parts, respectively; and (d) those innervating the mylohyoid and the anterior belly of the digastric muscles are located in the most ventromedial part of the caudal one-third of the nucleus. Axons of most masticatory motor neurons run ventrolaterally in between the motor and the chief sensory nuclei of the trigeminal nerve. However, those of the mylohyoid and anterior belly of the digastric muscles ascend dorsally to the dorsal aspect of the caudal nucleus and then turn ventrolaterally to join the motor root of the trigeminal nerve. Furthermore, the dendrites of the motor neuron of JCM converge dorsocaudally to the supratrigeminal region. The diameters of neurons of each JCM display a bimodal distribution. However, an unimodal distribution is present in the motor neurons from each JCM. It is suggested that the motor nucleus innervating the JCM is comprised of comprised of alpha- and gamma-motor neurons. It, thus, may provide a neural basis for the regulation of the muscle tone and biting force.

Anatomical parameters of the voice

Analysis of the anatomical parameters of the voice allows us to observe the relations--which exist not only within the cortical centres but also in the underlying centres--between structures governing phonation functions and those concerned with the functions of hearing and of language. Voice, articulation, speech, language and hearing are so many elements of one and the same function of communication.

The relationship between structural features of peripheral nerves and suture methods for nerve repair

Based on techniques for identifying and distinguishing motor, sensory, and mixed fasciculi in peripheral nerves, the authors propose guidelines for selecting suture methods for nerve repair. When many mixed fasciculi are known to exist at the nerve lesion, epineurial repair is preferable; fascicular (perineurial) repair is more suitable when pure motor and sensory fasciculi are clearly recognized. Generally, epineurial repair is indicated for more proximal injuries, with fascicular repair most appropriate for more distal sites. A greater ratio of epineurial connective tissue to intrafascicular nervous tissue implies an inclination toward fascicular repair.

Neural cartography: sensory and motor maps in the superior colliculus

The sudden onset of a novel or behaviorally significant stimulus usually triggers responses that orient the eyes, external ears, head and/or body toward the source of the stimulus. As a consequence, the reception of additional signals originating from the source and the sensory guidance of appropriate limb and body movements are facilitated. Converging lines of evidence, derived from anatomical, electrophysiological and lesion experiments, indicate that the superior colliculus is an important part of the neural substrate responsible for the generation of orienting responses. This paper briefly reviews the functional organization of the mammalian superior colliculus and discusses possible linkages between the sensory and motor maps observed in this structure. The hypothesis is advanced that the sensory maps are organized in motor (not sensory) coordinates and that the maps of sensory space are dynamic, shifting with relative movements of the eyes, head and body.

Sensorimotor mapping and oropharyngeal reflexes in goldfish, Carassius auratus

The vagal lobe of goldfish and some carps is a laminated, specialized lobe of the midmedulla containing both primary sensory terminals and primary motor neurons. Both the sensory and motor components are represented in the lobe in a matching, orotopic fashion, i.e. the oral cavity is mapped across the surface of the lobe. Anatomical tracing studies reveal that the circuitry exists for a point-to-point reflex system in which the superficial sensory layers are mapped directly onto the underlying motor layer. The utility of this relatively direct sensorimotor coupling appears to be in terms of sorting food within the mouth according to its gustatory properties. The direct coupling between the mapped sensory layer and the similarly mapped motor layer may be a useful model in which to study the evolutionary development of less tightly coupled sensorimotor systems.

Between the retinotectal projection and directed movement: topography of a sensorimotor interface

This article reviews some recent findings on the character of the neuronal organization lying between the optic tectum and motor pattern-generating circuitry in the case of orienting behaviors. It focuses on frogs but notes parallels to existing work on saccade control in mammals and suggests some additional ones for further exploration. In general, the map-like function of orienting does not appear to be subserved by a comparable map-like organization. It is argued that the current conceptual vocabulary for describing interface organization (sensory map, motor map, pattern-generating circuitry) is inadequate and that some additional concepts (activity-gated divergence, intermediate spatial representation) are necessary. Finally, some questions are raised about the appropriateness of the term 'motor map'.

Sensory, motor and sympathetic neurons forming the common peroneal and tibial nerves in the macaque monkey (Macaca fascicularis)

The motoneurons, dorsal root ganglion (DRG) and sympathetic ganglion (SG) cells forming the common peroneal (CPN) and tibial (TN) nerves of young and semiadult monkeys (Macaca fascicularis) were localised by the horseradish peroxidase method of tracing neuronal connections. The motoneurons forming the CPN occur in the L4-L6 segments, appearing as 1-3 groups and occupying the retroposterolateral (rpl), posterolateral (pl) and central (c) groups of motor nuclei. The motoneurons forming the TN occur in the L4-L7 segments, appearing as 1-4 groups and occupying the rpl, pl, c and anterolateral (al) groups. The motoneurons and DRG cells forming the CPN show peak frequencies at the L5 level, and the SG cells forming the same nerve, at the L6 level in most cases. The motoneurons and DRG cells forming the TN show peak frequencies at the L6 level and the SG cells forming the same nerve, also at the L6 level in most cases. The bulk of motoneurons, DRG and SG cells forming the CPN and TN are concentrated in two segmental levels. For CPN the motoneurons measure between 14-76 micron in their average somal diameters and for TN, 16-70 micron. The majority of them (65.5% for CPN motoneurons and 72% for TN motoneurons) have average somal diameters greater than 38 micron. The size spectrum of the DRG cells forming the CPN is similar to that of DRG cells forming the TN, being 12-78 micron for CPN and 10-76 micron for TN. The sympathetic neurons forming the CPN (measuring 10-44 micron) have a larger size spectrum than those forming the TN (measuring 6-33 micron). The diameter spectrum (3-20 micron for TN and 2-19 micron for CPN) and peak frequency distributions (10 micron for both TN and CPN) of the myelinated fibres present in the CPN and TN are also similar, with the CPN fibres skewing towards a slightly larger size. Many of the fibres in the young and semi-adult monkeys are not yet myelinated.

Organization of the facial nucleus and corticofacial projection in the monkey: a reconsideration of the upper motor neuron facial palsy

The somatotopic organization of the facial nucleus and the distribution of the corticofacial projection in the monkey were studied by the use of retrograde and anterograde transport of horseradish peroxidase. Facial motor neurons innervating lower facial muscles were primarily found in the lateral part of the nucleus, those supplying upper facial muscles in the dorsal part of the nucleus, and those innervating the platysma and posterior auricular muscles in the medial part of the nucleus. Descending corticofacial fibers innervated the lower facial motor nuclear region bilaterally, although with contralateral predominance. The upper facial motor nuclear regions received scant direct cortical innervation on either side of the brain. Our results indicate that upper facial movement, like that at the shoulder, is relatively preserved in upper motor neuron palsy because these motor neurons receive little direct cortical input. By contrast, the lower facial muscles, like those of the hand, are more severely affected because their motor neurons normally depend upon significant cortical innervation.

Distribution of motoneurons involved in the prey-catching behavior in the Japanese toad, Bufo japonicus

Coordinated activities of several muscles in the head region underlie the prey-catching behavior of anuran amphibians. As a step in elucidating the neural mechanisms generating these activity patterns in the Japanese toad, we labelled the motoneurons innervating 8 behaviorally relevant muscles using intramuscular (i.m.) injection technique of horseradish peroxidase (HRP), and examined their localization within the motor nuclei whose boundaries were determined by HRP application to the nerve trunk. All the motoneurons innervating the two jaw closer muscles (m. masseter major, m. temporalis) and m. submentalis were localized within the rostral subdivision of the trigeminal motor nucleus. The motoneurons innervating the only mouth opener muscle (m. depressor mandibulae) were scattered throughout the facial motor nucleus. The motoneurons innervating tongue (m. hypoglossus, m. genioglossus) and hyoid muscles (m. sternohyoideus, m. geniohyoideus) appeared within the hypoglossal nucleus with distribution patterns characteristic of the target muscles. Thus, we have revealed the neuroanatomical organization of the motoneurons relevant to the prey-catching behavior.

Development and axonal outgrowth of identified motoneurons in the zebrafish

We have observed the development of live, fluorescently labeled motoneurons in the spinal cord of embryonic and larval zebrafish. There are 2 classes of motoneurons: primary and secondary. On each side of each spinal segment there are 3 individually identifiable primary motoneurons, named CaP, MiP, and RoP. The motoneurons of the embryo and larva are similar in morphology and projection pattern to those of the adult. During initial development, axons of primary motoneurons make cell-specific, divergent pathway choices and grow without error to targets appropriate for their adult functions. We observed no period of cell death, and except for one consistently observed case, there was no remodeling of peripheral arbors. We have observed a consistent temporal sequence of axonal outgrowth within each spinal segment. The CaP motor axon is the first to leave the spinal cord, followed by the axons of the other primary motoneurons. The Mauthner growth cone enters the spinal cord after all the primary motoneurons of the trunk spinal cord have begun axonal outgrowth. Secondary motor growth cones appear only after the Mauthner growth cone has passed by. Our results suggest that this stereotyped temporal sequence of axonal outgrowth may play a role in defining the contacts between the Mauthner axon and the motoneurons; the behavior of growth cones in the periphery suggests that interactions with the environment, not timing, may determine path-finding and peripheral connectivity of the motoneurons.

Identified motoneurons and their innervation of axial muscles in the zebrafish

The organization of spinal cord motoneurons and their innervation of axial (white) muscles in the zebrafish were studied. Motoneurons can be divided into 2 classes, primary and secondary, on the basis of their cell-body sizes and positions. Each side of each spinal segment contains 3 primary motoneurons that are uniquely identifiable as individuals by their stereotyped cell-body positions and peripheral branching patterns. Moreover, these motoneurons precisely innervate cell-specific subsets of contiguous muscle fibers in mutually exclusive regions of their own body segment. Individual muscle fibers receive inputs from a single primary motoneuron and, in addition, from up to 3 secondary motoneurons. The results demonstrate that the precision of innervation previously described in invertebrates is also present in some vertebrates.

A histologic pattern approach to the anatomy of the face

Superficial facial anatomy is usually presented in texts by sequential, two-dimensional illustrations of dissections at various levels of depth. Three-dimensional structural relationships of nerves, vessels, and fasciae can be best understood, however, by recognizing their patterns of organization. These patterns are reviewed and illustrated by histologic sections from various anatomic sites.

Pathways of sympathetic innervation to the superior and inferior (Müller's) tarsal muscles

Fluorescent staining and fluorescent microscopy were used to evaluate the pathways which the sympathetic nerves followed to reach the smooth muscles of the eyelids. Selected structures from fresh orbits were removed, sectioned, and fluorescently stained utilizing glyoxylic acid. Photographic results from the fresh orbits showed fluorescence accompanying the oculomotor, trochlear, and abducent nerves and branches of the ophthalmic division of the trigeminal (lacrimal, supraorbital, and nasociliary) nerves. The eyelids exhibited intense fluorescence within the tarsus and throughout adjacent connective tissue. The principal paths of the sympathetic nerves are the sensory (first dimension of the trigeminal) and the motor nerves to the orbit. The arteries (in contrast to previous descriptions) do not appear to be a major pathway.

The accessory nerve nucleus in the baboon

In the savanna baboon, Papio cynocephalus, the accessory nerve nucleus was identified by using a mixture of 20% free horseradish peroxidase (HRP) and 2.5% HRP conjugated to wheat germ agglutinin (WGA) in a 5% aqueous detergent solution (Nonidet P-40). Following surgical exposure of the appropriate nerve branch to the sternocleidomastoid or trapezius muscle, the nerve was transected, placed in an Argyle tubing collar, and bathed in 5-10 microliter of the tracer. After a 48-hour survival time and vascular perfusion-fixation, 40-micron sections of the lower medulla oblongata and the cervical spinal cord were treated according to the tetramethyl benzidine (TMB)-HRP method of Mesulam (J. Histochem. Cytochem. 26: 106-117, 1978). The accessory nucleus extends as a distinct column of neurons from lower medullary levels into the rostral part of C5. One to ten labeled cells were present in each section, and all labeled neurons were located on the side of the bathed nerve. The rostral portion of the accessory nucleus occupies a central position, its intermediate portion occupies a lateral position, and its caudal portion occupies a central position within the ventral horn. All labeled neurons were confined to Rexed's lamina IX, ranged from 15 to 75 micron in diameter, and were either distinctly round (oval) or stellate in shape. Neurons within the baboon accessory nucleus supplying the sternocleidomastoid muscle were located from lower medullary to upper C2 spinal cord levels, while those supplying the trapezius muscle extended from C2 to C5.

Topographic organization of facial motoneurons to individual pinna muscles in rat (Rattus rattus) and bat (Rousettus aegyptiacus)

The location and number of motoneurons to individual pinna muscles were determined by retrograde transport of horseradish peroxidase in rat and flying fox. The degree of ear mobility differs considerably between these species in that rats perform simpler ear movements while flying foxes move their pinnae in a sophisticated way. Five pinna muscles were investigated in each species. Motoneurons lay within the medial subdivision of the facial motor nucleus extending over its entire rostrocaudal length. They were topographically organized; however, a somatotopic order could not be observed. With one exception homologous pinna muscles were represented in corresponding areas in both species, supporting the idea of a common representation of ear muscles in mammals. In rat, motoneuron pools overlapped considerably, whereas in flying fox overlap was minute. A total of 1,110 and 1,646 motoneurons were labeled in rat and flying fox, respectively. We conclude that the higher number of pinna motoneurons in the latter species in addition to the more clear-cut topography provide the structural substrates that underlie differences in the quality of ear movements as seen in bats vis-a-vis other mammals.

Plasticity in the cockroach neuromuscular system

The retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase extracellularly injected into a leg muscle was used to identify the regenerating cockroach motor neurons that have grown an axonal branch into that muscle. At least 66% of the animals with crushed nerve roots eventually reform the original innervation pattern of this muscle with no mistakes. In spite of this apparent specificity the cockroach neuromuscular system can express plasticity as evidenced by the correction of mistakes made at early stages of regeneration. These mistakes are corrected through elimination during the time interval between 40 and 60 days after nerve crush. In addition, when the distal segments of the leg are removed, thus depriving some motor neurons of their normal target muscles, many of them form stable inappropriate axonal branches in denervated as well as fully innervated muscles. These observations are discussed in terms of possible mechanisms responsible for the specificity of the cellular interactions and in terms of their relevance to understanding the development of vertebrate neuromuscular systems.

Projections from the ventral respiratory group to phrenic and intercostal motoneurons in cat: an autoradiographic study

Anterograde transport of tritiated amino acids (leucine, lysine, and proline) was used to examine the spinal projections of respiratory premotor neurons in the ventral respiratory group (VRG) of cats. This population of neurons corresponds anatomically with the nucleus ambiguus-retroambigualis. Small volumes (20 to 50 nl) of tritiated amino acids were pressure ejected into the middle of the VRG through a micropipette which permitted simultaneous recording of respiratory modulated activity. In two cats injections were made caudal to the obex in regions which contained expiratory modulated neurons. In five cats injections were made rostral to the obex in regions containing inspiratory neurons. After a 2-week survival period, cats were anesthetized and perfused. The entire neuraxis was removed and processed using standard autoradiographic techniques. Transport of tritiated amino acids revealed a marked bilateral projection to lamina IX of the spinal cord at the C4 to C6 level and a primarily contralateral projection to laminae VIII and IX in the thoracic spinal cord. Distinct descending pathways to the phrenic motor neurons were observed in the lateral funiculus and in the ventral funiculus; descending fibers to the intercostal motoneurons in the thoracic cord appeared to be restricted to the ventral funiculus. Labeling of axon terminals in both the cervical and thoracic cords was confined to ventral horn regions which contain motoneurons. These results suggest that monosynaptic projections from brainstem bulbo-spinal neurons to spinal motoneurons are important in controlling respiratory movements of the diaphragm and intercostal muscles.

Effects of changes in motor unit size on transmitter release at the frog neuromuscular junction

The dependence of transmitter release and synaptic effectiveness on the size of a neuron's peripheral field was studied using neuromuscular junctions in sartorius muscles of adult frogs (Rana pipiens). The size of the peripheral field (motor unit size) was reduced by crushing the sartorius nerve and surgically removing half of the muscle fibers. Synapses thus formed were compared with those formed when crushed nerves reinnervated intact whole muscles, as well as with synapses in normal unoperated muscles. Indirect observations suggested that all motor axons participated in reinnervation and that motor unit size was indeed smaller in half-muscles. Synaptic safety margins, as measured by the sensitivity of nerve stimulus-evoked twitching to low Ca2+, were substantially higher in muscles with reduced motor units. These higher safety margins were due to enhanced evoked transmitter release. In Ringer solution containing Mg2+ and lowered Ca2+, total evoked release and evoked release per unit nerve terminal length were approximately 2-fold higher in muscles with reduced motor units, when studied 7 to 18 weeks postoperatively. A similar difference was seen when unblocked release was measured in a normal physiological solution, after blocking excitation-contraction coupling and muscle fiber action potential generation with formamide. Miniature endplate potential frequency in half-muscles was 2 to 3 times higher than in controls when tested in normal physiological solution, but was not significantly different in low Ca2+, Mg2+-containing solution. By 34 weeks postoperatively, there was no longer a difference in evoked release, even though the difference in motor unit size persisted.(ABSTRACT TRUNCATED AT 250 WORDS)

The central morphology of the giant interneurons and their spatial relationship with the thoracic motorneurons in the cockroach, Periplaneta americana (Insecta)

Three groups of giant fibers are found in the cockroach ventral nerve cord. A latero-dorsal group (dorsal GIs), a latero-ventral group (ventral GIs) and a medio-ventral group. The morphology of all three groups of fibers within the thoracic ganglia is described. The morphology of the dorsal and ventral GI pathways in the abdominal and suboesophageal ganglia is also described. The projection patterns of the neurons in each ganglion are remarkably similar which suggests a common function. When motorneurons 5rl (depressor) and 6Br4 (levator) are stained simultaneously with the dorsal and ventral GI groups, some branches from both motor and giant neurons converge. The branching of the remaining medio-ventral group of fibers and their proximity to areas receiving motorneuronal input suggests that these are the small diameter axons described by Dagan and Parnas (1970).

Hormonal control of the anatomical specificity of motoneuron-to-muscle innervation in rats

Motoneurons of the spinal nucleus of the bulbocavernosus innervate bulbocavernosus muscles in male rats. Adult female rats normally lack both the spinal nucleus and its target muscles. Prenatal treatment of females with testosterone propionate resulted in adults having, like males, both the spinal nucleus and its target muscles. However, prenatal treatment with dihydrotestosterone propionate preserves the muscles but not the motoneurons. This paradoxical condition might result from (i) bulbocavernosus muscles without innervation; (ii) muscles innervated by morphologically unrecognizable motoneurons; (iii) muscles innervated by a very few spinal nucleus cells, each innervating many bulbocavernosus fibers; or (iv) muscles innervated by motoneurons outside their normal anatomical locus in the spinal nucleus. The results of retrograde marker injections into the bulbocavernosus muscles of females treated with androgen refute the first three possibilities and confirm the last: the different androgen treatments result in anatomically distinct spinal motor nuclei innervating bulbocavernosus muscles.

A horseradish peroxidase study of rat lingual motoneurons with axons passing through the cervical nerve

The peripheral course of axons of rat lingual motoneurons was studied by HRP injection into the hypoglossal nerve in combination with transecting of the hypoglossal and/or cervical nerve components of the hypoglossocervical plexus. Furthermore, soma sizes of labeled lingual motoneurons were compared in transverse section with those of labeled geniohyoid and thyrohyoid motoneurons, which are situated adjacent to the lingual motoneurons. We found that axons of the majority of lingual motoneurons lying in the main hypoglossal nucleus passed through the hypoglossal nerve throughout their course to the tongue. In a remaining small number of lingual motoneurons lying in a medial portion of the ventromedial subnucleus in the caudal fourth of the main hypoglossal nucleus, their axons passed through the first cervical nerve to the upper root of the ansa cervicalis to the hypoglossal nerve and then to its medial branch. The labeled lingual motoneurons with axons passing through the cervical nerve were intermingled with those whose axons passed through the hypoglossal nerve. The latter motoneurons, however, diminished in number while being traced caudally, and finally in the most caudal main hypoglossal nucleus the former motoneurons occupied a major part of this nucleus. The lingual motoneurons with axons passing through the cervical nerve were smaller in soma size than those with axons passing through the hypoglossal nerve. These two types of lingual motoneurons were both smaller in soma size than the geniohyoid and thyrohyoid motoneurons, and their soma shape was not as flat as that of the latter types of motoneurons.

The pattern of monosynaptic Ia-connections to hindlimb motor nuclei in the baboon: a comparison with the cat

The pattern of Ia-connections to motor nuclei of 17 hindlimb muscles (or groups of muscles) has been investigated in baboons by intracellular recording of Ia-e.ps.p.s evoked in motoneurons from different muscle nerves. The amplitudes are normalized to 70 mV resting potential and compared with similarly normalized Ia-e.ps.p.s in cats. As in the cat, Ia-excitation is drawn from a restricted number of muscles and the homonymous effect is usually dominating. Heteronymous connections to many motor nuclei are different in the two species. For example, hip extensors are generally more Ia-isolated from each other in baboons than in cats, and also knee flexors have fewer Ia-interconnections than in cats. A unidirectional Ia-synergism between some hip extensors and knee flexors in cats has changed to a bidirectional one in baboons, with a tendency to lateralization of the connections. Among ankle extensors, soleus has smaller heteronymous Ia-connections from its synergic ankle extensors than in cats. In baboons, plantaris is heteronymously Ia-excited from gastrocnemius-soleus but not from the intrinsic plantar muscles; whereas in cats there exists a considerable Ia-projection from the intrinsic plantar muscles but not from gastrocnemius-soleus. There is a corresponding difference in the insertion of the plantaris tendon, which shows that this muscle acts as toe extensor in cats but as ankle extensor in baboons. For most of the motor nuclei, the homonymous as well as the total aggregate of Ia-e.ps.p.s is smaller in the baboon than in the cat; but the amplitude range between different motor nuclei is larger in the baboon. Reciprocal Ia-i.ps.p.s are evoked only after spinal transection or when brain function is depressed. It is postulated that baboons, contrary to cats, have descending tonic inhibition of transmission in the reciprocal Ia-inhibitory pathway. The phylogenetic flexibility of Ia-connections is discussed and contrasted with their ontogenetic stability.

The absence of specific dye-coupling among frog spinal neurons

A double fluorescence labeling technique was developed to study the specificity of dye-coupling among frog spinal neurons. A pool of motoneurons known to be electrically coupled was prelabeled with a large molecule (rhodamine conjugated to horseradish peroxidase) that was not expected to pass through gap junctions. Then a single sensory or motor neuron within or outside this pool was injected with lucifer yellow to see if the dye spread specifically among neurons that are electrically coupled. We observed almost no examples of specific dye-coupling.

Afferent projections to the oral motor nuclei in the rat

Projections to the trigeminal, facial, ambiguus, and hypoglossal motor nuclei were determined by using horseradish peroxidase histochemistry. Most of the afferent projections to these motor nuclei were from the brainstem reticular formation, frequently in areas adjacent to other synergetic motor nuclei. The reticular formation lateral to the hypoglossal nucleus and reticular structures surrounding the trigeminal motor nucleus projected to each of these other brainstem motor nuclei involved in oral-facial function. Afferent projections to these motor nuclei also were organized along the rostrocaudal axis. Within the reticular formation most of the afferent projections to the trigeminal motor nucleus originated rostral to the majority of neurons projecting to the hypoglossal and ambiguus nuclei, which in turn were rostral to the primary source of reticular afferents to the facial nucleus. In comparison, projections from the sensory trigeminal nuclei and nucleus of the solitary tract were sparse. The interneuron pools that project to the orofacial motoneurons provide one further link in understanding the brainstem substrates for integrating oral and ingestive behaviors.

Tracing of sensory neurones and spinal motoneurones of the pigeon by injection of fluorescent dyes into peripheral nerves

The projection of peripheral sensory and motor nerves was investigated in the pigeon (Columba livia) by means of retrogradely transported fluorescent dyes. Two combinations of fluorescent tracers were used that could be identified within the same cell when excited by light of 405 nm: 1) Propidium iodide and Bisbenzimide, which label the cytoplasm orange and the nucleus blue, respectively; 2) Fast Blue, which labels the cytoplasm blue, and Nuclear Yellow, which labels the nucleus (especially the nucleolar ring) yellow. The presence of the tracers in a given cell was confirmed microspectrophotometrically. Following injection of the tracers into peripheral nerves, labelled sensory neurones were seen in the dorsal root ganglia and motoneurones of the spinal cord. The peroneal and tibial nerves projected to L2-L5 and L2-L7, respectively, whereas the median and ulnar nerves projected to C12-Th2 and C13-Th1. Double-labelled sensory neurones were observed when both peroneal and tibial, or median and ulnar nerves were injected with different tracers. This indicates that some sensory neurones possess peripheral processes that dichotomize to pass down two different peripheral nerves. Double labelling was never seen in motoneurones, or in sensory neurones after tracer injection into the sciatic and femoral nerves.

Topographic arrangement of motoneurons innervating the suprahyoid and infrahyoid muscles. A horseradish peroxidase study in cats

The topographic arrangement of motoneurons innervating the muscles which participate in the rostrocaudal movement of the larynx was studied in cats by means of the horseradish peroxidase (HRP) method. After HRP injection into these muscles, all labeled neurons were found in the nuclei of the brainstem and cervical nerves (C1, C2) ipsilaterally. The mylohyoid and anterior digastric motoneurons were seen in a cluster in the medial part of the rostrocaudal area of the motor trigeminal nucleus. Within the medial part of the nucleus, the mylohyoid motoneurons were located ventrally and the anterior digastric motoneurons dorsally. The posterior digastric neurons were situated in the accessory facial nucleus, the stylohyoid neurons in the ventral aspect of the facial nucleus, the geniohyoid motoneurons in the ventral aspect of the hypoglossal nucleus, and the thyrohyoid motoneurons in the lateral aspect of the hypoglossal nucleus and the dorsomedial part of the ventral horn cells of C1. The HRP-labeled neurons of the sternohyoid and sternothyroid muscles were observed in the central portion of the ventral horn cells of C1 and C2.

Principles of motor organization of the monkey cervical spinal cord

The organization of spinal cord motor columns innervating 18 selected macaque forelimb muscles was studied with the technique of retrograde transport of horseradish peroxidase. The reliability of the method was evaluated in the cat hindlimb. Motor columns innervating forearm muscles with similar actions on the hand appear to overlap in the anterior horn. Extensor motoneurons are generally positioned ventral and/or lateral to flexor motoneurons. Motoneurons controlling hand movement are located primarily in segments C8 and T1.

The localization of the motor neurons innervating the cricothyroid muscle in the adult dog by the fluorescent retrograde axonal labeling technique

Injection of 4'6-diamidino-2-phenylindol 2 HCl (DAPI) into the cricothyroid muscle of the adult dog resulted in blue fluorescence of cells found exclusively in the rostral part of the ipsilateral nucleus ambiguus. These labeled neurons on the average extended 1.8 mm from the level just caudal the facial nucleus to the caudal extension. In the rostral tip of the nucleus, labeled cells were located in a scattered ventral group of larger neurons of the nucleus.

Direct observations on the contacts made between Ia afferent fibres and alpha-motoneurones in the cat's lumbosacral spinal cord

1. The enzyme horseradish peroxidase was injected into identified lumbosacral alpha-motoneurones and Group Ia afferent fibres in cats anaesthetized with chloralose and paralysed with gallamine triethiodide. Subsequent histological examination allowed the determination of (a) the extent of the motoneuronal dendritic trees, (b) the number and location of Ia synapses upon the motoneurones. 2. alpha-motoneurones had seven to eighteen primary dendrites and each produced daughter branches up to the fourth to the sixth order. At dendritic bifurcations Rall's 3/2 Power Law was obeyed. There was little or no dendritic tapering up to about 800 micrometers from the soma. Beyond this distance, however, there was considerable tapering. 3. Horseradish peroxidase injections revealed that motoneuronal dendrites are much longer than previously thought. Individual dendrites could be traced for up to 1600 micrometers from the soma and dendritic trees were usually 2-3 mm from tip to tip. Nearly all the motoneurones had dendrites that entered the white matter of the cord. Dendrites could also reach as far dorsally as laminae V and VI. 4. Ia synapses upon motoneuronal somata were examined in cords counterstained with cresyl violet or methylene green. About 10% of Ia boutons in lamina IX were on somata and each Ia collateral terminated on 3.66 motoneuronal somata or the most proximal (30 micrometer) dendrites, with on average about two contacts per motoneurone. 5. Ten Ia afferent fibre-motoneurone pairs were injected with horseradish peroxidase. The following conclusions were drawn: (i) only one collateral of any given Ia axon makes contact with a motoneurone even though other collaterals from the same axon might pass through the dendritic tree, (ii) usually all contacts made between a Ia fibre and a motoneurone are at about the same geometrical distance from the soma, even when on different dendrites, (iii) between two and five contacts are made upon the dendritic tree (average 3.4) at distances of between 20 and 820 micrometers from the soma. 6. The results are discussed in relation to previous anatomical and electrophysiological work.

Facial nerve nucleus in the cat. Further study

The location of efferent neurons, supplying muscles innervated by facial nerve, was studied in the brain stem of the cat using horseradish peroxidase. One to four microliters of HRP was injected in the frontalis, posterior belly of digastric, platysma, and auricularis superior muscles of the kittens. Their representative locations were determined in the facial nucleus. No direct relation between the amount of HRP injected and the numer of labeled neurons was found. The number of positive cells was between 2 and 661. No labeled neurons were found in nucleus of Vth or any other brain stem nuclei.

Localization of spinal motor neurons supplying single ventral roots

The spinal cord location of motor neurons whose axons make up lumbar ventral roots L-5, L-6, and L-7 was investigated in the cat. Location was based on recording the extracellular action potentials from spinal motor neurons which had been activated antidromically by electrical stimulation of single ventral roots. The borders of the spinal segments (as defined by the rostral and caudal limits of the ventral root exit zone) and the sites recorded from were suitably marked, so that the territory occupied by the responding motor neurons could be determined. We showed that probably more certain localization of motor neurons could be achieved by using the positive polarity ("injury") antidromic responses which are recorded with needle electrodes of relatively large tip size. Negative polarity antidromic responses are recorded with fine-tipped electrodes. Results indicated that the origin of a single lumbar ventral root is from homolateral ventral horn cells and is mainly unisegmental, that is, from cells located within the segment of the root's attachment. There was, however, evidence of overlap of some motor cell bodies and/or dendritic processes into the caudal one-fourth of the adjacent more rostral segment. No difference was seen in the longitudinal extent of spinal motor cells supplying axons to the dorsal or ventral primary rami.