Activity of fusimotor neurons during reflex muscle contraction

The influence of fusimotor activity via the gamma-loop on reflex responses of motoneurons to stretch or vibration stimulation of mm. triceps surae was studied in decerebrate cats. Action potentials of single fusimotor neurons were derived from thin filaments isolated from nerves innervating this muscle group, leaving their main nerve supply intact. Most fusimotor neurons tested were found to be coactivated with motor units during reflex muscle contraction. In the initial period of development of reflex muscle contraction a weak autogenetic inhibitory effect on discharge of fusimotor neurons was found. The results suggest that reduction of the reflex motor signal, leading to a "silent period," is partly the result of a transient decrease in the fusimotor output effect on contracting muscles. A study of changes in fusimotor discharge generation during the ascending phase of reflex muscle contraction may provide data useful for identification of autogenetic reflex influences on these motoneurons and for elucidating the conditions necessary for servoassistance of muscle contractions.Medical Research Institute, Belgrade, Yugoslavia. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 630–637, September–October, 1984.     

Development of muscle spindles deprived of fusimotor innervation

Summary Muscle spindles of limb muscles were deefferented in neonatal rats by sectioning ventral roots or by removal of the lumbosacral spinal cord.Ten to 56 days after the operation, muscle spindles were examined in the medial gastrocnemius, extensor digitorum longus and soleus muscles. The differentiation of muscle spindles was not affected by deefferentation. The number of spindles in the investigated muscles was not reduced. Intrafusal fibres increased in number from two at birth to four per spindle on the average, as in normal muscles. The characteristic ultrastructural distinctions of nuclear bag and nuclear chain fibres developed as under normal conditions. However, intrafusal fibres atrophied slowly after fusimotor denervation, their polar zones becoming reduced in diameter by about 25% in comparison with control fibre diameters. Spindle capsules, on the other hand, increased in size and attained diameters comparable with normal spindles, appearing even somewhat distended.As intrafusal fibres degenerate after complete denervation at birth (Zelená, 1957), but differentiate in the absence of fusimotor innervation, it can be concluded that sensory nerve terminals induce and support their development. It is assumed that the morphogenetic influence of sensory terminals is mediated by release and uptake of a trophic substance at the synaptic junction. The occurrence of light and dense core vesicles in the sensory terminals and of coated invaginations and vesicles at both the axonal and plasma membrane speak in favour of such a possibility.The authors wish to thank Mrs. M. Sobotková, Dr. Z. Liková, Mr. H. Kunz and Ing. M. Doubek for their skillful technical assistance.     

Model study of muscle spindles subjected to static fusimotor activation

An earlier proposed mechanical model of the de-efferented muscle spindle endings has been developed further in order to study whether the functional effects of activating the static fusimotor fibres may be accounted for by relatively simple mechanical events in the intrafusal muscle fibres. Particular attention has been paid to important controversial problems related to the processes involved in the fusimotor activation of the mammalian muscle spindles. In order to develop an adequate model, preliminary simulation studies of the functional effects of various reasonable modifications of the original “de-efferented” model were first carried out by means of a convenient direct electronic analogue of the mechanical system. The following results apparently reflect pertinent details about the peripheral receptor mechanisms associated with the activation of the static fusimotor fibres:

1. The γ _s-activation is most adequately accounted for by mechanical events occurring in the nuclear chain fibres.
2. Any uniform chain fibre contraction fails to account for the effects of the γ _s-activation.
3. The simulation of a local chain fibre contraction could apparently account for all significant effects of the γ _s-activation.
4. A reasonable increase of the position sensitivity and the vibration sensitivity of both the primary and the secondary endings could be accounted for by increasing the stiffness of about 1/4–1/2 of the length of the simulated nuclear chain fibre by a factor of about 2–10.
5. The powerful modulation of the background discharge of the receptor endings that is associated with the γ _s-activation could be accounted for by an independent contraction force related to the intensity of the static fusimotor activity.
6. The model predicts a decrease of the rate sensitivity of the primary endings (group Ia) and a slight increase of the (moderate) rate sensitivity of the secondary endings.
7. By an adequate selection of the parameters, the simulated γ _s-activation showed a negligible influence on the absolute level of the dynamic phase of the ramp response of the simulated primary endings, in spite of a considerable increase of the steady state (background) discharge before and after the termination of a simulated stretch.

     

Glial cell-line derived neurotrophic factor-dependent fusimotor neuron survival during development

Glial cell-line derived neurotrophic factor (GDNF) is a potent survival factor for motor neurons. Previous studies have shown that some motor neurons depend upon GDNF during development but this GDNF-dependent motor neuron subpopulation has not been characterized. We examined GDNF expression patterns in muscle and the impact of altered GDNF expression on the development of subtypes of motor neurons. In GDNF hemizygous mice, motor neuron innervation to muscle spindle stretch receptors (fusimotor neuron innervation) was decreased, whereas in transgenic mice that overexpress GDNF in muscle, fusimotor innervation to muscle spindles was increased. Facial motor neurons, which do not contain fusimotor neurons, were not changed in number when GDNF was over expressed by facial muscles during their development. Taken together, these data indicate that fusimotor neurons depend upon GDNF for survival during development. Since the fraction of cervical and lumbar motor neurons lost in GDNF-deficient mice at birth closely approximates the size of the fusimotor neuron pool, these data suggest that motor neuron loss in GDNF-deficient mice may be primarily of fusimotor neuron origin.     

Model study of muscle spindle primary endings subjected to dynamic fusimotor activation

An earlier proposed model of the de-efferented muscle spindle mechano-receptors has been developed further to simulate the effects of the dynamic fusimotor ( _D) activation of the group Ia afferents (primary endings).The rate sensitivity of the original second order receptor model might be increased simply by increasing the overall viscous damping of the simulated polar regions of the nuclear bag fibre. However, adequate simulation of the typical time course of the stretch response of the primary endings during _D-activation required a subdivision of the polar regions into an active and a passive part. A reasonable behaviour of the model was obtained by simulating a local contraction covering about 50–90% of the polar regions of the nuclear bag fibres.The ramp response of the model showed a quick rate response component that increased by increasing the rate of the simulated stretch. This component was not significantly influenced by the simulated _D-activation.A slow rate component appeared to increase approximately in the same proportion as the intensity of the simulated _D-activation.The behaviour of the model closely corresponds to that of the biological prototype. The study demonstrates that the electrophysiological effects of activating the dynamic fusimotor fibres are indeed compatible with peripheral mechanical events associated with contraction phenomena within the polar regions of the nuclear bag intrafusal muscle fibres.     

Descending influences from nucleus raphe magnus on fusimotor neurone activity in rats

1. 1.Single fusimotor fibres were isolated in the ventral roots of lumbosacral segments of urethane-anaesthetized rats, and effects of electrical stimulation of the nucleus raphe magnus (NRM) on their spontaneous activity were investigated. The experiments were carried out in rats whose bilateral preoptic and anterior hypothalamic regions (PO/AH) were electrolytically destroyed to eliminate the influences of these regions to fusimotor activity.

2. 2.Of 44 fusimotor fibres studied, 38 (86%) were found to be affected by NRM stimulation. The effects of NRM stimulation were classified according to their response pattern: primary depression (D-type, n = 24), facilitation followed by depression (F-D-type, n = 5) and primary facilitation (F-type, n = 8). The most predominant effect of NRM stimulation upon fusimotor activity was characterized by a strong depression followed by a complete cessation of firing lasting either for a short period or for more than 30 min (D- and F-D-type).

3. 3.In three fusimotor fibres studied in the different preparations, it was observed that a NRM-evoked depression response was blocked by an intraperitoneal administration of a serotonin antagonist, p-chlorophenylalamine (p-CPA) (10 mg/kg).

4. 4.The results indicate that the NRM exerts descending inhibitory or facilitatory influences on fusimotor neurones, and suggest that cold shivering is controlled by modulating fusimotor neurone activity via the serotonergic raphe-signal pathways.

End plate spikes in the human electromyogram. Revision of the fusimotor theory.

'End plate spike' (EPS) is a spontaneous action potential of a normal striated muscle. EPSs are found in local 'active spots' of the muscle. The prevailing hypothesis about the origin of EPSs states that when a needle electrode affects a motor nerve branch near the neuromuscular junction at the end plate zone, an increased leakage of acetylcholine to the synaptic cleft ensues. This elicits postsynaptic action potentials of the muscle fibre which can be recorded as EPSs with the same needle electrode. Thus EPSs are thought to be caused by needle injury or irritation of the motor axon. We suggest that EPSs are action potentials of intrafusal muscle fibres and that 'active spots' are in fact muscle spindles. Waveform analysis reveals three types of EPSs: small EPSs, not propagated outside the active spot either: i) with negative onset; or ii) with short positive initial deflection; and iii) large EPSs resembling propagated motor unit potentials (MUPs) but with a typical EPS firing pattern, distinctly different from that of the MUPs. Study of EPS activation in different manoeuvres associates small EPSs with intrafusal gamma motor units and large MUP-like EPSs with beta motor units.     

Differential sensitivity of skeletal and fusimotor neurons to Bcl-2-mediated apoptosis during neuromuscular development

Proper development of the nervous system requires that a carefully controlled balance be maintained between both proliferation and neuronal survival. The process of programmed cell death is believed to play a key role in regulating levels of neuronal survival, in large part through the action of antiapoptotic proteins, such as Bcl-2. Consistent with this, Bcl-2 has been shown to be a key regulator of apoptotic signaling in post-mitotic neurons. However, we still know remarkably little regarding the role that Bcl-2 plays in regulating the survival of specific motor neuron populations. In the present study, we have examined somatic motor neurons of the lumbar spinal cord, and branchiomotor neurons of the facial nucleus in bcl-2-null mice to determine the differential dependence among motor neuron populations with respect to Bcl-2-mediated survival. Examination of neuronal and axon number, axonal area, and the distribution of axonal loss in bcl-2-null mice demonstrates that, in contrast to the great majority of alpha motor neurons, gamma motor neurons exhibit a unique dependence upon bcl-2 for survival. These results demonstrate, for the first time, the connection between Bcl-2 expression, motor neuron survival, and the establishment of different motor populations.     

Biologically active fibers based on chitosan-coated lyocell fibers

The possibilities of obtaining biologically active cellulose–chitosan fibers were examined. An effective two-stage method was developed. The first stage involves the formation of dialdehyde cellulose by the potassium periodate oxidation of lyocell fibers, which is able to form Schiff’s base with chitosan. In the second stage, chitosan-coated lyocell fibers were prepared by subsequent treatment of oxidized lyocell fibers with a solution of chitosan in aqueous acetic acid. The impact of this two-stage protocol on the chemical and physical properties of lyocell fibers was evaluated by determining carbonyl group content, fineness and tensile strength of fibers, as well as chitosan content in the composite cellulose–chitosan fibers. Antibacterial activity of the chitosan-coated lyocell fibers against different pathogenens: Staphylococcus aureus and Escherichia coli, was confirmed in vitro experiments.     

Papermaking fibers

True paper, in the technical sense of the term, was first made in China in 105a.d. from the bast fibers of paper mulberry and probably also from bamboo. For many centuries old rags, principally cotton or linen, supplied the papermakers with raw material In modern times, sprucewood fibers have long been outstanding for papermaking. In recent decades, however, the spruces have been equalled, if not surpassed, in tonnage used, by various species of pine, especially the southern yellow pines. Of secondary importance have been the fibers of dozens of other species.     

Model-based prediction of fusimotor activity and its effect on muscle spindle activity during voluntary wrist movements

Muscle spindles provide critical information about movement position and velocity. They have been shown to act as stretch receptors in passive muscle, however, during active movements their behavior is less clear. In particular, spindle responses have been shown to be out-of-phase or phase advanced with respect to their expected muscle length-sensitivity. Whether this apparent discrepancy of spindle responses between passive and active movements is due to fusimotor (γ-drive) remains unresolved, since the activity of fusimotor neurons during voluntary non-locomotor movements are largely unknown. We developed a computational model to predict fusimotor activity and to investigate whether fusimotor activity could explain the empirically observed phase advance of spindle responses. The model links a biomechanical wrist model to length- and γ-drive-dependent transfer functions of type Ia and type II muscle spindle activity. Our simulations of two wrist-movement tasks suggest that (i) experimentally observed type Ia and type II activity profiles can to a large part be explained by appropriate, i.e. strongly modulated and task-dependent, γ-drive. That (ii) the empirically observed phase advance of type Ia or of type II profiles during active movement can be similarly explained by appropriate γ-drive. In summary, the simulation predicts that a highly task-modulated activation of the γ-system is instrumental in producing a large part of the empirically observed muscle spindle activity for voluntary wrist movements.     

Regenerating retinal fibers display error-free homing along undamaged normal fibers

After crushing one optic nerve in a bony fish, retinal fibers regenerate to both tecta. Anterograde labelling indicates that the ipsilaterally regenerating fibers have a rather straight growth, apparently along the undamaged fibers of the contralateral retina. In contrast, the contralaterally regenerating fibers deviate widely from a straight course. Retrograde labelling shows a mirror-symmetric distribution of regenerated ipsilateral and resident contralateral ganglion cells in a comparable annulus. In contrast, ganglion cells in the regenerated contralateral retina show no topological order after comparable small Dil applications to the ventrolateral tectum. These data suggest that regenerating fibers can orient on the undisturbed, contralateral fibers. © 1993 John Wiley & Sons, Inc.     

Olfactory nerve fibers

Cross sections of olfactory nerves present a unique appearance. They indicate the presence of large numbers of very small nerve fibers, with a modal diameter of about 0.2 µ and a narrow range for their size variation. From one side of the nasal septum of a pig the yield of fibers was estimated at 6,000,000; the number arising from the turbinates would be considerably larger. The fibers are attached to the membranes of the Schwann sheaths in large bundles through mesaxons longer and more branched than those that have been seen in other nerves. Continuity of the axons between the nerves and the bipolar cells was traced in an examination of the olfactory mucous membrane; and the indication of a one-to-one relationship between cells and axons was reinforced by a comparative count. After the axons leave the bipolar cells they become incased in the central projections of the sustentacular cells. Where the latter come into contact with the basal cells the axons emerge to push back the plasma membranes of the basal cells in the first step in acquiring their nerve sheaths. Later steps are described. When the axons are delivered by the basal cells to the collecting Schwann tubes, they are already aggregated into small bundles with sheaths fundamentally the same as those they will possess until they are delivered to the glia in the olfactory bulb. Some of the aspects of the cytology of the bipolar cells and adjoining sustentacular cells are described. A survey of the physiological properties of olfactory nerve fibers was made in some experiments on the olfactory nerve of the pike. Almost all of the action potential is encompassed within a single elevation, manifesting at its front a conduction velocity of 0.2 m./sec. For a comparison, the last elevation in the C action potential in the sciatic nerve of the frog is cited as an example of conduction at the same velocity. Though expressed through long time constants, the properties of the pike olfactory fibers conform to the generalized schema for properties of vertebrate nerve fibers. This conformity signalizes that they differ from the exceptional properties of the unmedullated fibers of dorsal root origin. An afferent function for unmedullated nerve fibers does not imply that the fibers concerned are alike in their physiological properties.