By W. Lukjan. Francis Marion University.
The type of fusimotor axon stimulated rones during different manoeuvres trusted 130mg malegra dxt ved erectile dysfunction treatment, a controversial will determine which intrafusal ﬁbre is activated order 130 mg malegra dxt with amex how erectile dysfunction pills work, topic discussed further below (pp. Studies that exploit the thixotropic properties Thixotropy in human investigations of intrafusal ﬁbres In human subjects, the activity associated with Underlying principle avoluntary contraction can induce long-lasting Thixotropy refers to the change in passive stiffness enhancements in spindle discharge, changes that of muscle, analogous to the behaviour of certain gels persistlongafterthecontraction(Fig. The discharge of the spindle primary to set into a gel again when allowed to stand. The discharge of the secondary because intrafusal thixotropy can dramatically alter endinginFig. In both Edin & Vallbo, 1990a), (ii) the after-effects of fusimo- instances,theafter-dischargeisnotevidenceofcon- toractivationonspindledischarge(Brown,Goodwin tinuing drive but of a long-lasting change in stiff- &Matthews, 1969), and (iii) stretch sensitisation of nessofintrafusalﬁbresthatcontractedunder drive spindle endings (Edin & Vallbo, 1988;Edin, 1991). As actin–myosin bonds been studied extensively by Proske and colleagues break down and re-form at the prevailing muscle in the cat and in human subjects (Proske, Morgan length,thedischargeslowlydeclines. They depend upon the formation, the enhanced spindle discharge is a lasting mem- breakdownandre-formationofactin-myosinbonds ory of past efferent activity, not evidence of the in the intrafusal ﬁbres, with consequent changes current level of fusimotor drive, and the enhanced in stiffness of the ﬁbres and an alteration in the discharge can be abolished by stretch sufﬁcient to stretch placed on spindle endings. Muscle spindle primary ending in tibialis anterior during and after a voluntary contraction illustrating the effects of thixotropy on spindle discharge. The traces are from top to bottom raw neurogram, force, and integrated EMG of tibialis anterior. The spindle was initially silent, maintained a discharge at ∼12 Hz throughout the 60-s contraction. There was a high-frequency burst of impulses on relaxation of the contraction, and the discharge continued at ∼8Hzinthe absence of EMG following the contraction. The lowest panel shows superimposed action potentials from the unit on a faster time base. A complicating factor is that some of the ﬁndings on / co-activation from that extrafusal muscle ﬁbres also display thixotropic human studies can be attributed to incorrect iden- behaviour. This serves as a warning that careful tiﬁcation of Ib afferents as Ia (Prochazka & Hulliger, controls are required even with this experimental 1983). A further lesson is that the thixotropic properties of intrafusal muscle can distort spin- Sensitivity to displacements dle discharge and must be considered when inter- preting unexpected changes in spindle discharge, Even small movements of the skin at the recording reﬂex behaviour or perception, especially when they site can disturb the microelectrode and disrupt the occurafteramusclecontraction. As a result the movement repertoire that the spindle was subjected to can inﬂuence how it can be studied with this technique is quite limited. This is possible in feline experiments but is rarely so in human sub- The technique is technically demanding jects, in whom spindle endings have been noted to It is not unusual to spend a couple of hours search- respond to mechanical stimuli that are not imme- ing in vain for a speciﬁc afferent type, having been diately obvious, such as respiration and the arterial unable to hold promising recordings for sufﬁciently pulse(Hagbarthetal. It is absolutely essen- tial that all possible perturbations to the highly sen- Thixotropy sitive spindle ending be controlled, whether they be Studies of intrafusal thixotropy can allow stronger external or internal. Organisation and pattern of connections 127 background spindle discharge or the response to Organisation and pattern stretch materially. The responses evidence is more cogent for static fusimotor ( s) to muscle stretch or to tendon percussion do not drive(seeVallboetal. This argues against signiﬁcant charge is similar to that of de-efferented feline end- drive to resting muscles, in agreement with the d ings (Burke, Skuse & Stuart, 1979; Nordh, Hulliger nerve block studies discussed above. Nevertheless, &Vallbo, 1983), and there is no evidence of a nega- there is some evidence to suggest that there may tive serial correlation between successive interspike be some background activity (Aniss et al. Thus, when the spindle There is extensive literature on the reﬂex effects of primary ending in Fig. These ﬁndings do not prove Evidence for reﬂex activation of motoneurones that there is no background activity in motoneu- by inputs from cutaneous mechanoreceptors has rones, but they suggest a very low level of activity, at been sought without success in lower limb muscles least in s motoneurones, insufﬁcient to affect the of reclining human subjects, who were at rest or 128 Muscle spindles and fusimotor drive (a) (b) (c) Fig. The effect of de-efferentation on the responses to stretch and shortening of a primary ending in the relaxed peroneus longus muscle. De-efferentation was achieved by complete nerve block with lignocaine proximal to recording site (horizontal double-headed dashed line in (a)). Responses of the ending are identical before (b ) and during (c ) the nerve block. How- (PSTH in the second trace), but produced no EMG ever, when subjects were performing an appropriate in the silent tibialis anterior.
Initial cocontractions vates hindlimb muscles purchase malegra dxt 130mg without a prescription erectile dysfunction venous leak, was anastomosed to tended to resolve over time quality malegra dxt 130 mg impotence 17 year old male. A reimplanted the S-1 ventral root, which innervates the blad- ventral root with intact Schwann cells proba- der. Motoneurons had innervated blad- been infused into the subarachnoid space after der parasympathetic ganglion cells, making experimental ventral root avulsion in rats. The remaining surviving motoneurons and a dramatic increase motoneurons in one model of a partial periph- in axons that extended from these cells to the eral nerve ligation were found to have a surface of the cord. Such changes may alter the functional proper- Brain-derived neurotrophic factor also re- ties of available motor units. The effects of stored some of the synaptic covering of the mo- lower motoneuron activity on sprouting adds toneurons, at least for inhibitory boutons. During the acute phase of Without reimplantation of the avulsed root, the sprouting after partial denervation in a rat effects of BDNF persist for about a month af- model, high levels of neuromuscular activity in- ter discontinuing it. The combination of reim- duced by electrical stimulation were detri- plantation and a neurotrophin infusion may mental to sprouting and reduced the enlarge- augment the benefits of each. Phys- Another surgical approach in rodents inter- ical activity that exceeds these physiologic lev- posed a peripheral nerve autograft between a els may be detrimental. Studies in humans Biologic Adaptations and Neural Repair 127 must determine whether limits for the inten- mechanisms of pain (see Chaper 8). In the sity and level of resistance exercise apply to pa- presence of central pain associated with a SCI, tients with reinnervating muscle. This approach may also prevent au- proach to reinstating proximal leg movement tonomic dysreflexia. For example, antibodies to and bowel and bladder control may be feasible NGF prevented small-diameter afferents from after a conus or cauda equina lesion. For ex- sprouting in the dorsal horn below a thoracic ample, at the time of surgery after an L-1 burst SCI in the rat. Em- studies are correct, the motoneurons will re- bryonic transplants of both spinal cord and generate axons into the implant and out to the brain tissue from rats, when placed between periphery to the hip and knee flexor and ex- the lumbar cord and a transected dorsal root tensor muscles and to the bladder. The pe- stump, provide cues that allow dorsal ganglia ripheral nerve and its Schwann cells are still in- axons to regenerate into the host gray mat- tact and will offer regenerative signals for these ter. The graft be reimplanted as well, placed just above the may supply neurotrophins and other molecules level of the SCI. Conceivably, reimplanting an L-4 or S-2 ven- Large myelinated dorsal horn neurons ex- tral root into the ventrolateral cord above the press trk C receptors. Neurotrophin-3 had a spinal lesion will sprout axons from the local greater effect than other neurotrophins in re- motoneurons and restore some supraspinal generating ascending fibers in one model. In this scenario, a conditioning lesion central axon from the ganglion enters the cord, of the motoneurons above the SCI, at the level but only if infused within 1 week of dorsal rhi- of the implant, will be necessary to help stim- zotomy. An autologous nerve graft or poly- duced considerable regeneration of axons into mer conduit from the torn lumbar or sacral root the dorsal horn 2 weeks after a dorsal root may also be needed to reach above the le- avulsion in adult rats. Tenascin and proteo- and preganglionic parasympathetic neurons of glycans are among the inhibitors at the dorsal the conus is to put autologous neural precur- root entry zone. Immuno- axons into existing or implanted ventral roots, philins also may increase root entry zone pen- and to make connections with regenerating etration by sensory axons. If lumbar root implants were shown to be efficacious in a clinical trial, SPINAL NEURONS AS TARGETS cervical implants of ventral roots into the cord above the lesion to muscles below the SCI may Some fundamental questions about the pools of be seen as worth the potential risks for patients. Which spinal neurons should neural repairists target with the new ax- DORSAL HORN NEURONS ons they coax down white matter columns of Most experiments relevant to the regeneration the cord? One of the remarkable chasms in of dorsal horn neurons with their inputs and knowledge about spinal cord anatomy and phys- outputs have been devoted to understanding iology is that very little is known about how and 128 Neuroscientific Foundations for Rehabilitation where descending inputs to the dorsal and ven- ermost in this tract at all spinal levels. Over what in representational maps were also found by in- expanse, rostrocaudal and mediolateral, do in- tracortical microstimulation of the leg area af- coming axons normally join interneurons and ter amputation of a hindlimb in monkeys. How many targets Cortical representations in M1 for the can one axon effectively reach and activate?
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