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.

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'.

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.

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.

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.

[The morphology and innervation of the levator muscles of the ribs in the dog, cat, horse, and pig]. / Morphologie und Innervation der Musculi levatores costarum bei Hund, Katze, Pferd und Schwein.

In the anatomical literature there are inconsistencies in the description of the levatores costarum muscles in man and in the domestic animals, and their innervation either by the dorsal or the ventral branches of the thoracic nerves. Therefore we studied the form, structure and, with the aid of the dissecting microscope, the innervation of these muscles in 7 dogs, 8 cats, 5 horses and 12 pigs. In the dog, cat and horse, mm. levatores costarum are present from the second to the last rib. In the pig, these muscles are present from the second to the 15th rib, even in individuals with 16 pairs of ribs. Mm. levatores costarum longi and a levator of the first rib could not be found in the domestic animals although these muscles are described in man. All mm. levatores costarum are innervated by branches of the lateral branch of the ramus dorsalis of the respective thoracic nerve. An additional branch of the r. muscularis proximalis of the intercostal nerves 1-3 innervates the lateral part of the levator muscles of the second to the fourth rib.

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.

Functional studies of motor cortex.

Defining functions for neural elements becomes more difficult the more remote they are, in synaptic linkages, from motor neurons. The precentral motor cortex contains a corticomotoneuronal projection system, only one synapse removed from motor neurons. Corticomotoneuronal fibres produce monosynaptic excitation of spinal motor neurons, which is more powerful for those acting distally, innervating extensor muscles of the fingers and intrinsic hand muscles. Latencies and time-course of corticomotoneuronal excitation are defined. Amplitudes of unitary corticomotoneuronal excitatory postsynaptic potentials are very small. Many corticomotoneuronal cells converge on a given motor neuron. Some motor neurons innervating proximally acting muscles appear to receive no corticomotoneuronal excitation. Disynaptic inhibitory actions are produced by corticospinal volleys via the common Ia inhibitory interneuron--possibly reciprocal actions produced over collaterals of corticomotoneuronal fibres. Anatomical divergence of projections of collaterals of an identified corticomotoneuronal fibre is extensive enough to provide both for delivery of synapses to a large number of motor neurons and for the dispersion of specific projections to inhibitory interneurons, to fusimotor neurons and to other interneurons. Functions of corticomotoneuronal elements in motor cortex whose targets have been identified and whose excitatory or inhibitory actions have been specified by cross-correlation have been studied by Fetz & Cheney (J Neurophysiol 1980; 44:751-772).

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.

The hypoglossal nucleus of the guinea pig, labeled with horseradish peroxidase (HRP).

H.R.P. technique has been used, and shown usefull in the retrograde tracing of the motor neurons from both main branches of the hypoglossal nerve. Traced motor neurons were observed at spinal levels and a portion of the brain stem slightly below the pyramidal decussation to a plane where the inferior olivary nucleus begins to disappear.