By Q. Cruz. Phillips University.
In some subjects buy discount prednisone 10 mg online allergy symptoms weakness, it has been possible to investigate Under these circumstances cheap 10 mg prednisone with mastercard allergy medicine 95a, the question arises the changes in reciprocal inhibition of the soleus about whether depression of reciprocal Ia inhibition Hreﬂex throughout the step cycle. In addition, the is merely the result of a subliminal co-contraction of modulation of reciprocal inhibition seen in the on- ankle ﬂexors and extensors or is related to the mech- goingrectiﬁedEMGofthesoleusandtibialisanterior anisms responsible for the generalised reﬂex rein- was explored during the stance and swing phases of forcement (cf. The amplitude of the conditioned H reﬂex (as a percentage of its unconditioned value) is plotted against the time after heel contact. The intensity of the posterior tibial nerve (PTN) stimulation was adjusted to maintain the control H reﬂex at ∼5% of Mmax. The lower part of (c) shows the Sol and TA EMG activity (average of 30 sweeps); abscissa time after heel contact. The amount of reciprocal inhibition of Sol EMG during the stance phase (e) and of TA EMG during the swing phase (f ) (expressed as a percentage of the amount of inhibition observed during a voluntary contraction at equivalent EMG) is plotted against the time after heel contact. This suggests that the pat- ternofafferentfeedbackcannotexplaintheobserved modulation. Modulation of reciprocal Ia inhibition In the subject illustrated in Fig. Around the onset of the swing phase, stance phase of gait, and that from plantar ﬂexors to reciprocal inhibition became greater than at rest, dorsiﬂexorsissmallinswing. This suggests that help ensure that antagonistic motoneurones are not transmission in the pathway of reciprocal Ia inhi- activated inappropriately during the walking cycle. Peroneal-induced inhibition of the on-going need to stabilise the ankle during the stance phase soleusEMGwasmuchsmalleratheelstrikethandur- of walking (see Chapter 11,p. It progres- sively increased through the stance phase, though always smaller than during the voluntary contrac- Studies in patients and clinical tion(Fig. Reciprocalinhibitionoftheon- implications going tibialis anterior EMG during the swing phase was similarly much smaller than during voluntary Methodology dorsiﬂexion (Fig. Because it is unusual reciprocal Ia inhibition for a sizeable H reﬂex to be recordable in tibialis (i) Presynaptic inhibition of Ia terminals on soleus anterior, peroneal-induced reciprocal Ia inhibition motoneurones is decreased during dynamic volun- ofthesoleusHreﬂexisusuallyexplored(however,see tary contractions of soleus but strongly increased p. Care is necessary to ensure that the condi- throughout the stance phase of walking (Chapter 8, tioning stimulus activates only the deep peroneal pp. Methodological reasons, in particular in patients with incomplete spinal cord injury who inadvertent stimulation of the superﬁcial peroneal had recovered sufﬁcient function to walk with some nerve,mayaccountforsomediscrepantﬁndings(see assistance than in healthy subjects. Tanaka & Ito (1976) found that a train of three shocks (ii) An early facilitation replacing the early inhi- totheperonealnervehadnoeffectin6of11patients bition was seen in two of four patients with incom- with hemiplegia, but produced an early inhibition in pletespinalcordinjuryandfourofthesevenpatients two patients and an early facilitation in the other withacompletespinallesionreportedbyCroneetal. Del- Perez&Field-Fote (2003)reported that, recipro- waide (1985) mentioned an early peroneal-induced cal inhibition tested at the 3-ms ISI was slightly facilitation in a few spastic patients, but gave no decreased. The absence lations show that the clear reciprocal Ia inhibi- of reciprocal inhibition on the unaffected side rep- tion observed in normal subjects was absent in the resents further evidence that spinal mechanisms patients (Fig. This is also illustrated in the are not normal on the clinically unaffected side of histogramsofFig. In patients in whom serial recordings The early facilitation that often replaces the early were obtained there was an increase in Ia inhibition inhibition could be due to Ib excitation duringtherecoveryperiodfollowingstroke,aﬁnding not conﬁrmed by Crone et al. It is possible that this facilitation in spastic patients could be due to the fact that a normal Ib excitation is moreeasilydisclosedbecauseofthedecreasedrecip- Patients with traumatic spinal cord injury rocal Ia inhibition. Studies in patients 231 (a) (b) Normal Spastic Corticospinal 120 100 Ia INs 80 TA Sol MN α MN Ia ISI (ms) Ia (c) Normal Spastic 40 TA Soleus 20 0 -60 -45 -30 -15 0 15 Difference between conditioned and control reflexes (% of control) Fig. Changes in reciprocal Ia inhibition of ankle muscles in patients with spasticity due to multiple sclerosis. The tonic corticospinal facilitation of tibialis anterior (TA)-coupled Ia interneurones (INs) is presumably interrupted in spastic patients (horizontal double-headed arrow). This produces both a reduction of the reciprocal Ia inhibition to soleus (Sol) motoneurones (MN), and a disinhibition of opposite soleus-coupled INs mediating reciprocal Ia inhibition to TA MNs. The size of the conditioned H reﬂex (expressed as a percentage of its unconditioned value) is plotted against the interstimulus interval (ISI). Average data from 74 normal subjects (●) and 39 patients with multiple sclerosis (❍). The number of subjects (expressed as a percentage of the total number of subjects in each population) is plotted against the difference between the size of the conditioned and control reﬂexes (expressed as a percentage of the control reﬂex size; negative values: inhibition, positive values: facilitation, at the 2 ms ISI). Changes during voluntary contraction soleus motoneurones (see Chapter 8,p. Themain ever, in functional terms, given the relatively weak abnormality in the patients was an absence of the sensitivity of the stretch reﬂex to presynaptic inhi- increase in peroneal-induced reciprocal Ia inhibi- bition of Ia terminals (see Chapter 8,pp. With the absence of modu- be a major factor in the unwanted stretch reﬂex lation of presynaptic inhibition of Ia terminals on activity triggered by the dynamic contraction of 232 Reciprocal Ia inhibition tibialis anterior in spastic patients (see Chapter 12, in normal subjects the excitabilities are similar (see pp.
Try to decrease their anxiety and provide positive re- take the medications generic prednisone 20mg free shipping allergy austin. Most drugs produce undesirable effects; breathing prednisone 20 mg with mastercard allergy treatment grand rapids, or other symptoms]) and a place with minimal noise some are minor, some are potentially serious. If re- • Proceed slowly, in small steps; emphasize essential informa- actions are reported, it may be possible to continue drug ther- tion; and provide ample opportunities to express concerns or apy by reducing dosage, changing the time of administration, ask questions. The occurrence of severe reactions indi- • Usually, a combination of verbal and written instructions is cates that the drug should be stopped and the prescribing more effective than either alone. Common client errors include taking incorrect doses, taking • When explaining a drug therapy regimen to a hospitalized doses at the wrong times, forgetting to take doses, and stop- client, describe the name, purpose, expected effects, and so on. Treatment failure can often be di- In many instances, the drug is familiar and can be described rectly traced to these errors. If the drug is unfamiliar, use avail- glaucoma medication can lead to optic nerve damage and able resources (eg, drug reference books, pharmacists) to learn blindness. With outpatients, pill counts may be done to compare doses remaining with INTEGRATING NURSING PROCESS, the number prescribed during a designated time. These CRITICAL PATHS AND DRUG THERAPY techniques may be used at every contact with a client, if appropriate. In many agencies, nursing responsibilities related to drug • General criteria include progress toward stated out- therapy are designated in critical paths (also called clinical path- comes, such as relief of symptoms, accurate adminis- ways or care maps). What side effects are likely and what do I do if they ✔ Keep all health care providers informed about all the occur? It is a good idea to carry a copy of this list at to medications as candy, to prevent accidental ingestion. Stopping a med- ✔ Inform health care providers if you have diabetes or kid- ication may cause a recurrence of the problem for which ney or liver disease. These conditions require special pre- it was given or withdrawal symptoms. Often, ✔ If breast-feeding, consult your obstetrician or pediatrician an adjustment in dosage or other aspect of administration before taking any medications prescribed by another may solve the problem. Some drugs require ✔ Develop a routine for taking medications (eg, at the same more frequent monitoring than others. A schedule that minimally dis- requires periodic checks with essentially all medications. Do not ✔ Take drugs in current use when seeing a physician for take medications if you are not alert or cannot see clearly. It may be helpful to remind ✔ Most tablets and capsules should be taken whole. If un- the physician periodically of the medications being able to take them whole, ask a health care provider before taken and ask if any can be discontinued or reduced in splitting, chewing, or crushing tablets or taking the med- dosage. Some long-acting preparations ✔ Get all prescriptions ﬁlled at the same pharmacy, when are dangerous if altered so that the entire dose is ab- possible. This is an important safety factor in helping to sorbed at the same time. CHAPTER 4 NURSING PROCESS IN DRUG THERAPY 53 CLIENT TEACHING GUIDELINES Safe and Effective Use of Prescription Medications (Continued ) ✔ Take most oral drugs at evenly spaced intervals around used to measure doses, for adults or children. For example, if ordered once daily, take about pecially important for young children because most of the same time every day. If ordered twice daily or morn- their medications are given in liquid form. Correct use of oral or nasal in- ications, or taking with ﬂuids other than water. Prescrip- halers, eye drops, and skin medications is essential for tion medications often include instructions to take on an therapeutic effects. If taking several medica- ✔ Report problems or new symptoms to a health care tions, ask a health care provider whether they may be provider.
Activation of various synergies 527 mechanisms capable of producing generic prednisone 5mg fast delivery allergy quiz diagnosis, as reﬂex results prednisone 20 mg overnight delivery allergy medicine cetirizine, Co-ordinated activation of co-ordinated movement altogether similar to those various synergies which are called forth by the will. Now it must be an economy to the body that the will should make No naturalmovementinvolvesjustonemuscle. Even use of these mechanisms already present, by acting planarmovementsatsinglejointsinvolvetheactiva- directly on their centres, rather than it should have tion of synergists operating at the same joint, and recourse to a special apparatus of its own of a similar relaxation (or disfacilitation) of antagonists. As discussed below, several spinal circuits may under contrived conditions, such as the artiﬁcial beusedbothinthecoordinationofmusclesynergies constraints of the laboratory, does a natural move- involvedincomplexmovementsandintheﬂexibility ment consist of action at just one hinge joint. Thus, for example, reaching withtheupperlimbinvolvesdisplacementsatshoul- Hierarchical control schema der, elbow, wrist and ﬁngers, while kicking a ball Spinal pathways and higher centres also contribute involves displacements at hip, knee and ankle. The to muscle synergies in complex co-ordinated move- correct spatial and temporal pattern of muscle acti- ments. According to this view, originally raised vation is crucial for smooth and coordinated move- by Bernstein (1967) and recently developed by ments, and the question is where motor synergies Macpherson (1991), motor control would be organ- are laid down in the central nervous system. This be higher level variables related to the goal of tar- led Beevor (1904, cited by Hultborn & Illert, 1991)to get acquisition. Speciﬁed at a lower level are the claim that the neuronal arrangements for relatively individualjointangulardisplacementsandthemus- stereotypedmovementswerelaiddowninthespinal cle activation patterns needed to achieve adequate cord. The problem of selecting complex activities such as scratching and locomo- the correct muscles to be activated is simpliﬁed by tion. Accordingly, Sherrington (1906) believed that certain basic rules of combination, rules that are the spatial and temporal patterning of muscle activ- probably formed during motor learning. One rule ity was driven by afferent input and linked by the could be to minimise expenditure or energy, and pathways subserving reﬂex arcs. This view had been another to make use of predictable forces such as proposed by Michael Forster in his Textbook of Phys- gravity or inertial perturbation among the body seg- iology (1879, cited by Hultborn, 2001). Heteronymous projections of different afferents to The fact that this pattern exists indicates that the avariety of motoneurones link muscles of the widespread Ia connections found in humans are of ipsilateral, and often contralateral, limbs in vari- functional importance, adapted to provide the reﬂex ous synergies, represented by different groups of assistance required for bipedal stance and gait (see interneurones or monosynaptic connections. Theymaybeexplained cal example is furnished by the muscles which con- in terms of the versatile synergisms required to tract and relax in the ﬂexion withdrawal reﬂex (see accomplish the various postural tasks that are more pp. Whether monosy- naptically or interneuronally mediated, synergies In the human upper limb, heteronymous monosy- transmitted through these spinal connections are naptic Ia projections are diffuse from distal mus- ﬂexible because of the control exerted on the cles to both the ﬂexors and extensors of proximal interneurones intercalated in their pathways and/or joints. Proximal muscles have a load-bearing func- PAD interneurones mediating presynaptic inhibi- tion during grasping and manipulatory movements tion of primary afferent terminals. Distal-to- proximal Ia connections might then be used to sta- Heteronymous monosynaptic Ia connections bilise the wrist and elbow to provide a ﬁrm support for the hand (see p. Speculation about their function is based on Relaxation of antagonistic muscles the assumption that, in movements, an adequate Co-ordinated synergies also imply relaxation of contribution of Ia afferent activity to motor output antagonistic muscles, whether by inhibition or depends not only on the fusimotor drive but also the disfacilitation. Discharge of remote motoneurones direct depolarisation by the central command of the through heteronymous Ia excitation presupposes relevant motoneurones (Hultborn & Illert, 1991). The relaxation of the antag- Lower limb onists at different joints during multi-joint move- Contractions of lower-limb muscles are usually ments may therefore be due to activation by the weight-bearing and often eccentric. These are cir- descending drive of the same spinal pathways as cumstances when the co-activated drive can rep- those ensuring the inhibition of the antagonists dur- resent a powerful input to muscle spindle endings ing single hinge (pp. Activation of various synergies 529 Flexible synergies role of the fusimotor system is now known to be sup- portive (Matthews, 1972). At times, an invariant diffuse pattern of monosynap- tic Ia connections in the human lower limb could be functionally inconvenient, because the activation of Ia afferents from a contracting muscle might then Cervical propriospinal system result in the automatic activation of unwanted mus- cle(s) linked in Ia synergism. In the cat, propriospinal neurones have also beenshowntoprojecttoIainhibitoryinterneurones, Activation of excitatory group II pathways. This creates a network ade- quate for translation of descending commands for The hypothesis regarding the role of group II path- multi-joint movements into the appropriate coordi- ways in supporting isometric contractions, pre- nated muscle synergies which underlie those move- sented above for one muscle (see p. The corticospinal projections onto speciﬁc and smaller FRA hypothesis suggests that a diffuse feedback sys- sets of motoneurones. The extensive convergence tem with a multisensory input, including group II ofdescendingexcitation,feedforwardinhibitionand afferents, could be used for the selective reinforce- feedback inhibition onto C3–C4 propriospinal neu- ment and prolongation of the descending command rones allows the cortical command to be updated at (see p. Because of the prewired limb, heteronymous group II excitatory projections connections of each subset of propriospinal neu- are widespread and strong (Chapter 7,Table 7. Available Reaching: an example of hierarchical control experimental data provide more evidence for an important role of group II pathways in posture and It is likely that, as in the cat, the human cervical pro- gait (see pp. Georgopoulos&Grillner(1989)have porting the contraction, not driving it, much as the proposed that, much as in locomotion, a signiﬁcant 530 Spinal pathways in different motor tasks part of such movements may be accomplished (see pp. Thus,speciﬁcationofthedirectionand Motor learning probably speed of the movement would be elabo- rated by supraspinal motor structures, especially the The motor performance of deafferented patients motor cortex (the higher level).
In general order prednisone 20 mg on line allergy forecast frisco tx, the but research suggests that the N-methyl D-aspartate (NMDA) pathogenesis of excessive CNS stimulation may involve one glutamate receptor subtype plays a role in memory cheap prednisone 5mg with mastercard allergy shots tallahassee. Excessive amounts of excitatory neurotransmitters mal neurotransmission, brief exposure of neurons to high (eg, norepinephrine, glutamate) concentrations can lead to neuronal cell death. Increased numbers or sensitivity of excitatory receptors ing to neuronal death are triggered by excessive activation of 3. Insufﬁcient amounts of inhibitory neurotransmitters NMDA receptors and movement of calcium ions into the (eg, GABA) neurons. Decreased numbers or sensitivity of inhibitory receptors 76 SECTION 2 DRUGS AFFECTING THE CENTRAL NERVOUS SYSTEM Cerebral Cortex molarity is low, less ADH is secreted, and more water is excreted in the urine. When body temperature as learning, memory, reasoning, verbalization, and voluntary is elevated, sweating and dilation of blood vessels in the body movements. When body temperature is nerve impulses and are called sensory areas; other parts send low, sweating ceases and vasoconstriction occurs. Assisting in regulation of arterial blood pressure by its areas, which occupy the greater portion of the cortex. The vasomotor cen- areas analyze the information received by the sensory areas ter in the medulla oblongata and pons maintains a state and decide on the appropriate response. In some instances, the of partial contraction in blood vessels (vasomotor response may be to store the perception in memory; in others, tone). The hypothalamus can exert excitatory or in- it may involve stimulation of motor centers to produce move- hibitory effects on the vasomotor center, depending on ment or speech. When nerve impulses from the hypothalamus excite the vasomotor center, vasomotor tone or vasocon- Thalamus striction is increased, and blood pressure is raised. When the impulses from the hypothalamus inhibit the The thalamus receives impulses carrying sensations such as vasomotor center, vasomotor tone or vasoconstriction heat, cold, pain, and muscle position sense. These sensations is decreased, with the overall effect of relative vasodi- produce only a crude awareness at the thalamic level. Regulating anterior pituitary hormones, including regarding location, quality, intensity, and signiﬁcance. The thyroid-stimulating hormone, ACTH, and growth hor- thalamus also relays motor impulses from the cortex to the mone. The hypothalamic factor called Hypothalamus prolactin-inhibiting factor inhibits secretion of prolactin, another anterior pituitary hormone. In the CNS, it is connected with the thalamus, thirst, appetite, hunger, and satiety centers. In the autonomic nervous system, it is the tions (eg, increased blood pressure and heart rate). In the endocrine system, the hypo- hypothalamus, thalamus, and cerebral cortex interact to thalamus controls the secretion of all pituitary hormones. It constantly collects information about the internal environment of the Medulla Oblongata body and helps maintain homeostasis by making continuous adjustments in water balance, body temperature, hormone The medulla oblongata contains groups of neurons that form levels, arterial blood pressure, heart rate, gastrointestinal the vital cardiac, respiratory, and vasomotor centers. The hypothalamus is ample, if the respiratory center is stimulated, respiratory rate stimulated or inhibited by nerve impulses from different por- and depth are increased. If the respiratory center is depressed, tions of the nervous system and by concentrations of nutri- respiratory rate and depth are decreased. Speciﬁc contains reﬂex centers for coughing, vomiting, sneezing, neuroendocrine functions include: swallowing, and salivating. Producing oxytocin and ADH, which are stored in the The medulla and pons varolii also contain groups of neu- posterior pituitary gland and released in response to rons from which originate cranial nerves 5 through 12. Oxytocin initi- gether with the midbrain, these structures form the brain stem. ADH helps maintain ﬂuid balance by control- Reticular Activating System ling water excretion. ADH secretion is controlled by the osmolarity of the extracellular ﬂuid. When osmolarity The reticular activating system is a network of neurons that is high, more ADH is secreted. This means that water is extends from the spinal cord through the medulla and pons to retained in the body to dilute the extracellular ﬂuid and the thalamus and hypothalamus. It receives impulses from all return it toward normal or homeostatic levels.
Thus buy 20 mg prednisone free shipping pollen allergy symptoms joint pain, suppression of the quadriceps Motor tasks and physiological Hreﬂex purchase prednisone 20mg fast delivery allergy shots fatigue, due to convergence of conditioning volleys implications inbothdeepandsuperﬁcialperonealnerveswiththe femoral test volley may be observed at rest. The brief Human tendon organs respond readily in isometric duration of the inhibition of the quadriceps H reﬂex voluntary contractions and usually discharge 268 Ib pathways strongly during shortening contractions, even in rest, but this inhibition is markedly depressed dur- the absence of an external load. The discharge ing a tonic contraction involving only the triceps increases during concentric contractions as EMG surae (Fournier, Katz & Pierrot-Deseilligny, 1983; builds up (Burke, Hagbarth & Lofstedt,¨ 1978). The stronger the force of the tonic Changes in transmission in oligosynaptic pathways contraction, the greater the suppression of the fed by Ib afferents during various motor tasks in Ib inhibition (Pierrot-Deseilligny & Fournier, 1986; humans have provided insight into the control of Fig. The controls so disclosed suggest that these pathways might serve several functions. Heteronymous Ib inhibition from quadriceps However, functional interpretations drawn from to soleus such experiments must be made with care because it cannot be taken for granted that the response During selective tonic contractions of triceps surae, of interneurones fed by Ib afferents to a natural Ib inhibition of the soleus H reﬂex produced by desynchronised input would be the same as to the stimulation of the femoral nerve is also suppressed, phasic synchronised input produced by artiﬁcial and this reveals the heteronymous monosynaptic Ia electrical volleys explored in the experiments below. This suppression probably reﬂects the con- vergence of group I afferents from quadriceps and Suppression of Ib inhibition to voluntarily triceps surae onto common Ib interneurones pro- activated motoneurones jecting onto soleus motoneurones (cf. ChangesintransmissionofIbinhibitiontovoluntar- Possible mechanisms underlying changes ily active motoneurones have been investigated for in transmission in Ib pathways the pathways from and to triceps surae during selec- tive voluntary contractions of this muscle (Fournier, Three questions arise about the mechanism(s) Katz & Pierrot-Deseilligny, 1983; Pierrot-Deseilligny responsible for the suppression of the group I inhi- &Fournier,1986;Stephens&Yang,1996). Theeffects bition of the soleus H reﬂex during triceps surae vol- ofconditioningcutaneousandarticularvolleysonIb untary contractions: (i) Does a decrease in Ia excita- interneurones have been investigated on the path- tion contribute to it? The changes elicited by the conditioning infer- Evidence for suppression of the inhibition to ior soleus volley are the net result of two effects voluntarily activated motoneurones (monosynaptic Ia excitation and Ib inhibition), and the suppression of the inhibition during contraction Homonymous Ib inhibition of soleus could therefore result from a decrease in presynap- motoneurones is suppressed during tonic tic inhibition of Ia terminals as well as a decrease contractions of gastrocnemius-soleus in Ib inhibition. However, in a tonic contraction, Thus, the early monosynaptic Ia facilitation of the there is no signiﬁcant change in presynaptic inhi- soleus H reﬂex produced by stimulation of the infer- bition of Ia terminals directed to involved motoneu- ior soleus nerve is followed by overt Ib inhibition at rones (see Chapter 8,p. The different descending actions possibly responsible for the depression of the transmission in Ib inhibitory pathways to soleus MNs are represented: facilitation of PAD interneurones (INs) mediating presynaptic inhibition of Ib terminals; mutual inhibition of Ib INs through facilitation of Ib INs mediating Ib inhibition of Q MNs; direct descending (reticulospinal) inhibition of Ib INs. Modiﬁed from Fournier, Katz & Pierrot-Deseilligny (1983)((b), (c)), and Pierrot-Deseilligny & Fournier (1986), (e) and unpublished, ((d ), (f )), with permission. This indicates that the to soleus, which is not contaminated by Ia excitation decreased inhibition is not due to a reduction of the (cf. The resulting gating Deseilligny & Fournier, 1986;Stephens & Yang, of the conditioning Ib volleys would be consistent 1996). On the other hand, PAD interneu- tion is provided by the ﬁnding that the Ib inhibition rones mediating presynaptic inhibition of Ib ter- of the soleus H reﬂex, whether evoked from the infe- minals receive corticospinal facilitation in the cat rior soleus or the gastrocnemius medialis nerves, is (see p. A reduction in Ib inhibition before depressed at the onset of a triceps surae contrac- the contraction-induced Ib feedback has reached tion or in the 50 ms preceding it (Fournier, Katz & the motoneurones could then be explained by a Pierrot-Deseilligny, 1983;E. Pierrot- focused corticospinal facilitation of PAD interneu- Deseilligny, unpublished data; Fig. A reduc- rones mediating presynaptic inhibition of Ib path- tion in Ib inhibition before the contraction-induced waystoactivemotoneurones. Thereissofarnoavail- group I discharge from the triceps surae has reached able method to investigate changes in presynaptic the spinal cord indicates a change in the descending inhibition of Ib terminals in human subjects. However, the ﬁnding Reduction of Ib inhibition by presynaptic inhibi- that Ib inhibition to both soleus and quadriceps was tion of Ib terminals feeding Ib pathways to triceps suppressed during co-contraction of these muscles suraebutnotquadricepsmotoneuronesmaybepro- (E. Pierrot-Deseilligny, unpublished duced in the cat by electrically-induced contrac- data) would be difﬁcult to explain solely on corti- tionsofgastrocnemiusmedialis. Motor tasks – physiological implications 271 Conclusions Ib inhibition is active at the beginning of the con- traction, and the gating mechanism still allows tran- There is strong evidence that the suppression of sient inhibitory potentials to appear in motoneu- group I inhibition to active motoneurones during rones when there are rapid increases in contraction contractions results from decreased transmission in force. These inhibitory potentials might help to limit Ib inhibitory pathways, not a change in presynap- the ﬁring frequency of motoneurones and/or the tic inhibition on Ia terminals. The suppression of Ib recruitment of new motoneurones in order to keep a inhibition is due to a descending control, which may smooth proﬁle of force development and avoid jerky be helped by the effects of the contraction-induced movements (Zytnicki & Jami, 1998). Facilitation of Ib inhibition by other afferent discharges Functional implications Ibinhibitionmayreappearwhenthetransmissionin Suppression of autogenetic group I inhibition Ib pathways is facilitated during appropriate phases Suppression of autogenetic group I inhibition to of movement or by other peripheral afferent inputs active motoneurones appears to be functionally which converge on the relevant Ib interneurones, as appropriate,becauseotherwiseIbinhibitionevoked demonstratedinthecatbyLundbergandcolleagues. This view is supported when the transmission of the latter inputs through by the ﬁnding that the suppression of autogenetic Ib ﬁrst-order interneurones receive descending facili- inhibition increases along with an increase in con- tation (see p. However,evenwithstrong have been shown to be able to produce such a facil- tonic contractions (∼30% of MVC), there is suppres- itation of transmission in Ib pathways. Cutaneous facilitation of transmission in Ib path- ways could help curtail an exploratory movement on meeting an obstacle (Lundberg, Malmgren & Possible functional role of Ib inhibition Schomburg, 1977). The resulting exteroceptive vol- If suppression of autogenetic Ib inhibition to active ley would facilitate transmission of Ib inhibitory motoneuronesisofvalue,questionsthenariseabout impulses to motoneurones of the contracting mus- the functional role of Ib inhibition during volun- cle (and its synergists), lessening contraction force. One from the skin might serve the same purpose, but answer is furnished by data from the anaesthetised increasing gain in the Ib force loop provides an ele- cat. In this preparation, prolonged electrically- gantsolutionsinceotherwisethisfeedbackmechan- induced contractions of the triceps surae produce ism would tend to maintain constant tension.
Space Cadets Corner featuring:
Ares: US space program - talk by Hall and Jenkins 9/06
Asteroid Dreams - Astra's story about asteroid exploration