Ia Afferent input alters the recruitment thresholds and firing rates of single human motor units

Vibration of the patellar tendon recruits motor units in the knee extensors via excitation of muscle spindles and subsequent Ia afferent input to the α-motoneuron pool. Our first purpose was to determine if the recruitment threshold and firing rate of the same motor unit differed when recruited involuntarily via reflex or voluntarily via descending spinal pathways. Although Ia input is excitatory to the α-motoneuron pool, it has also been shown paradoxically to inhibit itself. Our second purpose was to determine if vibration of the patellar tendon during a voluntary knee extension causes a change in the firing rate of already recruited motor units. In the first protocol, 10 subjects voluntarily reproduced the same isometric force profile of the knee extensors that was elicited by vibration of the patellar tendon. Single motor unit recordings from the vastus lateralis (VL) were obtained with tungsten microelectrodes and unitary behaviour was examined during both reflex and voluntary knee extensions. Recordings from 135 single motor units showed that both recruitment thresholds and firing rates were lower during reflex contractions. In the second protocol, 7 subjects maintained a voluntary knee extension at 30 N for approximately 40–45 s. Three bursts of patellar tendon vibration were superimposed at regular intervals throughout the contraction and changes in the firing rate of already recruited motor units were examined. A total of 35 motor units were recorded and each burst of superimposed vibration caused a momentary reduction in the firing rates and recruitment of additional units. Our data provide evidence that Ia input modulates the recruitment thresholds and firing rates of motor units providing more flexibility within the neuromuscular system to grade force at low levels of force production. Electronic Publication     

Adaptive motor unit action potential clustering using shape and temporal information

An adaptive algorithm is described that groups motor unit action potentials (MUAPs), detected in a composite EMG signal during signal decomposition, and creates partial motor unit action potential trains (MUAPTs). Data-driven MUAP shape and motor unit firing-pattern based criteria are used to form the clusters. An algorithm for estimating MUAPT temporal parameter, which provides accurate estimates even for partially defined trains, is used to obtain firing-pattern information. No a priori knowledge is required regarding the number of clusters or the distribution of their template shapes. The clustering algorithm when applied to real concentric-needle detected MUAP data provides accurate and useful clustering results. Compared to a classical leader-based algorithm, it provides more robust performance, is better able to estimate the true number of motor units represented in a set of detected MUAPs, and obtains more complete and accurate MUAPTs.     

Report on two patients with Costello syndrome and sialuria

We report on 2 unrelated patients with Costello syndrome. The first is a 5-year-old girl with “coarse” face, nasal papillomata, redundant skin of feet and hands, hyperextensible hand and finger joints, curly hair, feeding problems due to oral motor apraxia, growth and psychomotor retardation. The second is a 3-year-old boy with “coarse” face, loose skin on hands and feet, curly hair, oral motor apraxia, severe growth and psychomotor retardation. In both patients urine sialic acid levels were found to be repeatedly high. The meaning of this biochemical abnormality is discussed. © 1993 Wiley-Liss, Inc.     

Behavioral phenotyping of mice in pharmacological and toxicological research

The evaluation of behavioral effects is an important component for the in vivo screening of drugs or potentially toxic compounds in mice. Ideally, such screening should be composed of monitoring general health, sensory functions, and motor abilities, right before specific behavioral domains are tested. A rational strategy in the design and procedure of testing as well as an effective composition of different well-established and reproducible behavioral tests can minimize the risk of false positive and false negative results in drug screening. In the present review we describe such basic considerations in planning experiments, selecting strains of mice, and propose groups of behavioral tasks suitable for a reliable detection of differences in specific behavioral domains in mice. Screening of general health and neurophysiologic functions (reflexes, sensory abilities) and motor function (pole test, wire hang test, beam walking, rotarod, accelerod, and footprint) as well as specific hypothesis-guided testing in the behavioral domains of learning and memory (water maze, radial maze, conditioned fear, and avoidance tasks), emotionality (open field, hole board, elevated plus maze, and object exploration), nociception (tail flick, hot plate), psychiatric-like conditions (porsolt swim test, acoustic startle response, and prepulse inhibition), and aggression (isolation-induced aggression, spontaneous aggression, and territorial aggression) are described in further detail. This review is designed to describe a general approach, which increases reliability of behavioral screening. Furthermore, it provides an overview on a selection of specific procedures suitable for but not limited to behavioral screening in pharmacology and toxicology.     

Distribution of corticospinal neurons with collaterals to the lower brain stem reticular formation in monkey (Macaca fascicularis)

Summary An earlier retrograde double-labeling study in cat showed that up to 30% of the corticospinal neurons in the medial and anterior parts of the precruciate motor area represent branching neurons which project to both the spinal cord and the reticular formation of the lower brain stem. These neurons were found to be concentrated in the rostral portion of the motor cortex, from where axial and proximal limb movements can be elicited. In the present study the findings in the macaque monkey are reported. The fluorescent retrograde tracer DY was injected unilaterally in the spinal cord at C2 and the fluorescent tracer FB was injected ipsilaterally in the medial tegmentum of the medulla oblongata. In the contralateral hemisphere large numbers of single DY-labeled corticospinal neurons and single FBlabeled corticobulbar neurons were present. A substantial number of DY-FB double-labeled corticospinal neurons were also found, which must represent branching neurons projecting to both the spinal cord and the bulbar reticular formation. These neurons were present in: 1. The anterior portion of the cingulate corticospinal area in the lower bank of the cingulate sulcus; 2. The supplementary motor area (SMA); 3. The rostral part of precentral corticospinal area; 4. The upper portion of the precentral face representation area; 5. The caudal bank of the inferior limb of the arcuate sulcus; 6. The posterior part of the insula. In these areas 10% to 30% of the labeled neurons were double-labeled. The functional implications of the presence of branching corticospinal neurons in these areas is discussed.Abbreviations A nucleus ambiguus - AS arcuate sulcus - C cuneate nucleus - Cing. S. cingulate sulcus - corp. call. corpus callosum - CS central sulcus - Cx external cuneate nucleus - DCN dorsal column nuclei - dl dorsolateral intermediate zone - IO inferior olive - IP intraparietal sulcus - Lat. Fis. lateral fissure - LR lateral reticular nucleus - LS lunate sulcus - ML medial lemniscus - MLF medial longitudinal fascicle - mn motoneuronal pool - MRF medial reticular formation - Occ. occipital pole - P pyramid - PG pontine grey - PS principle sulcus - RB restiforme body - RF reticular formation - S solitary nucleus - SPV spinal trigeminal complex - STS superior temporal sulcus - Sup. Col. superior colliculus - TB trapezoid body - VC vestibular complex - vm ventromedial intermediate zone - III nucleus oculomotorius - VI nucleus abducens - VII nucleus, n. facialis - X motor nucleus n. vagus - XII nucleus hypoglossus Supported in part by grant 13-46-96 of FUNGO/ZWO (Dutch organisation for fundamental research in medicine)     

Cortico-cortical mediation of short-latency (lemniscal) sensory input to the motor cortex in deeply pentobitone anaesthetized cats

In pentobarbitone-anaesthetized cats, responses were recorded as surface positive potentials in the motor cortex on forelimb and brachium conjunctivum stimulation. In such a preparation, the forelimb nerve responses are mediated via the spino-cervical tract and the dorsal column-lemniscal pathway. Lesions of the sensory cortex (sparing only the depth of the coronary sulcus) abolished or reduced short-latency peripheral responses, in the motor cortex, on both skin and muscle nerve stimulation to less than 10% of control, while brachium conjunctivum responses were unchanged. Lesions of the second somatosensory area alone reduced the motor cortex responses on peripheral nerve stimulation by 10–20%. When the sensory cortex were abolished before the spreading depression reached the recording point, as judged from the brachium conjunctivum response. The depth distribution of positive and negative field potentials, constituting the early componentsof a peripheral response in the motor cortex, closely resembled that of a cortico-cortical response evoked on stimulation in area 3. It differed from that of thalamo-cortical response evoked in brachium conjunctivum stimulation. These data suggest that most, if not all, sensory input through the dorsal column and spino-cervical tract to the motor cortex is mediatd via the sensory cortex.     

Modulation of stretch-evoked reflexes in single motor units in human masseter muscle by experimental pain

The interaction between muscle pain and motor function of the jaw has been examined in recent years, but the nature of the modulation of the short-latency stretch reflex by pain is not fully understood. In this study, the reflex responses to stretch were measured in single low-threshold motor units that were kept discharging at a constant frequency, before, during and after the induction of experimental pain in one masseter muscle by controlled infusion of hypertonic saline. The probability of evoking a reflex response in individual motor units in the painful muscle at near-monosynaptic latency was reduced by a mean of about 20%. However, the overall reflex response in the surface electromyogram of both the ipsi- and contralateral masseter muscles was greater during pain. This was apparently a secondary response to the pain-induced increase in pre-stimulus activity in the motoneurone pools of both muscles, because increased motoneurone excitability may facilitate stretch reflexes. It is concluded that the most likely explanation for the reduced reflex response of low-threshold masseter motor units during experimental pain is a tonic reduction in the fusimotor drive to the masseter spindles.     

Somatotopic organization of corticospinal and corticotrigeminal neurons in the rat

The method of retrograde axonal transport of horseradish peroxidase was employed to examine the topographic organization of corticospinal and corticotrigeminal neurons in the rat. In both the first somatic sensory (SI) area and the motor (MI) area of the cortex these labeled corticofugal neurons, all of which are found in layer V, are grouped in a well organized, somatotopic pattern. Corticospinal projections which extend to lumbar levels of the spinal cord originate only from neuronal somata located in the hindlimb representation of SI and MI. Those neurons projecting to the cervical enlargement have somata mainly in the forelimb representation of SI and MI and the ventrolateral part of the trunk representation within SI. Cortical projections to the rostral cervical spinal segments appear to originate mainly from the neck and posterior head representations of SI and MI, though this conclusion is clearest for SI. Finally, neurons located exclusively within the head, muzzle, and vibrissal representation of SI project to the spinal trigeminal complex. Corticofugal neurons near the frontal pole and in an area of cortex ventrolateral to SI also project to the spinal cord. The areas involved are probably homologous to the supplementary motor (MII) and second somatic sensory (SII) areas respectively. The corticospinal and corticotrigeminal projections from these areas also appear to be organized in a somatotopic manner.It is concluded that in the rat, as in other species, the corticospinal and corticotrigeminal neurons in the sensorimotor cortex are arranged somatotopically. The somatotopic pattern found correlates remarkably well with that determined by single unit, evoked potential and cortical stimulation techniques.     

Motor and cognitive deficits in apolipoprotein E-deficient mice after closed head injury

Previous studies suggest that traumatic brain injury is associated with increased risk factor for developing Alzheimer's disease. Furthermore, the extent of the risk seems to be most pronounced in Alzheimer's disease patients who carry the ε4 allele of apolipoprotein E, suggesting a connection between susceptibility to head trauma and the apolipoprotein E genotype. Apolipoprotein E-deficient mice provide a useful model for investigating the role of this lipoprotein in neuronal maintenance and repair. In the present study apolipoprotein E-deficient mice and a closed head injury experimental paradigm were used to examine the role of apolipoprotein E in brain susceptibility to head trauma and in neuronal repair. Apolipoprotein E-deficient mice were assessed up to 40 days after closed head injury for neurological and cognitive functions, as well as for histopathological changes in the hippocampus. A neurological severity score used for clinical assessment revealed more severe motor and behavioural deficits in the apolipoprotein E-deficient mice than in the controls, the impairment persisting for at least 40 days after injury. Performance in the Morris water maze, which tests spatial memory, showed a marked learning deficit of the apolipoprotein E-deficient mice when compared with injured controls, which was apparent for at least 40 days. At this time, histopathological examination revealed overt neuronal cell death bilaterally in the hippocampus of the injured apolipoprotein E-deficient mice.

The finding that apolipoprotein E-deficient mice exhibit an impaired ability to recover from closed head injury suggests that apolipoprotein E plays an important role in neuronal repair following injury and highlights the applicability of this mouse model to the study of the cellular and molecular mechanisms involved.     

Three hands: fragmentation of human bodily awareness

We describe patient E.P. who occasionally perceives a ‘ghost' hand which copies the previous positions of the left hand with a 0.5–1 min time lag, but follows the movement patterns of the right hand. The symptoms started after an operation of a ruptured aneurysm, followed by an infarction of the right frontal lobe; E.P. also has a previously lesioned corpus callosum. Neuromagnetic recordings revealed that activity of the left secondary somatosensory cortex was strongly suppressed during the ghost arm percept, thereby providing an objective correlate for E.P.'s sensations. We conclude that simultaneous mental contents about body scheme may be based on neural information extracted at considerably different times, resulting in fragmentation of bodily awareness.