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As with most meals discount imitrex 50mg with amex muscle relaxant walmart, avoidance of snacking order 25mg imitrex overnight delivery spasms when falling asleep, drinking water instead of other drugs discount imitrex 25 mg with mastercard muscle relaxant natural remedies, low doses decrease risks of adverse drug calorie-containing beverages order imitrex 50mg fast delivery spasms causes, and decreasing the time spent effects. Nutritional Support and • Maximum weight loss usually occurs during the first Obesity in Older Adults 6 months of drug therapy. With most available drugs, use for this long is an unlabeled use of the drug. Older adults are at risk for development of deﬁcits and ex- • After a weight loss regimen of a few months, some ex- cesses in ﬂuid volume. Inadequate intake is common and may perts recommend letting the body adjust to the lower result from numerous causes (eg, impaired thirst mechanism, weight before attempting additional losses. Thus, a impaired ability to obtain and drink ﬂuids, inadequate water weight maintenance program, possibly with continued with tube feedings). Fluid • the National Institutes of Health do not recommend volume excess is most likely to occur with large amounts or combining weight loss medications except in the con- rapid administration of IV ﬂuids, especially in older adults text of clinical trials. Inadequate intake may result from the inability to obtain and prepare food, as well as disease processes that interfere with the ability to digest and Nutritional Support and Obesity in Children use nutrients. When alternative feeding methods (tube feed- ings, IV ﬂuids) are used, careful assessment of nutritional sta- Children in general need increased amounts of water, protein, tus is required to avoid deﬁcits or excesses. Overweight and carbohydrate, and fat in proportion to their size to support obesity are also common among older adults. However, reports of needs are usually decreased, primarily because of slowed me- childhood obesity and inadequate exercise abound and are tabolism and decreased physical activity, most people continue steadily increasing. With the high incidence of atherosclero- is to meet needs without promoting obesity. Anorexiant drugs should the recommended rate of administration is no more than be used very cautiously, if at all, because older adults often 5 mL every 5 to 10 minutes for premature and small infants have cardiovascular, renal, or hepatic impairments that in- and 10 mL/minute for older infants and children. The use of orlistat in tion of formulas, positioning of children, and administration older adults has not been studied. Parenteral nutrition may be indicated in infants and children Use in Renal Impairment who cannot eat or be fed enterally. With newborns, especially preterm and low-birth-weight infants, parenteral nutrition is Because the kidneys excrete water and waste products of needed within approximately 3 days of birth because they have food metabolism, clients with renal impairment often have little nutritional reserve. However, lipid emulsions should be accumulation of water and urea nitrogen. As a result, these given cautiously in preterm infants because deaths have been clients have special needs in relation to nutritional support. With other infants and children, parenteral nutrition may renal failure (CRF). Overall, beneﬁts in- (eg, serious illness, sepsis, major surgery) that leads to meta- clude weight gain, increased height, increased liver synthesis bolic disorders. These disorders include glucose intolerance of plasma proteins, and improved healing and recovery. With CRF, clients are not usually as stressed as those work with the parents rather than the children themselves. However, they often have multiple metabolic and None of the available weight loss drugs is indicated for use in ﬂuid and electrolyte disorders. CHAPTER 30 NUTRITIONAL SUPPORT PRODUCTS AND DRUGS FOR OBESITY 447 Undernourished clients have increased morbidity and mortal- However, some inactive metabolites are also formed and these ity. Some considerations in nutritional support of clients with are excreted renally. The drug is contraindicated in clients with ARF and CRF are listed in the following sections. Acute Renal Failure Use in Hepatic Impairment • In early ARF, dietary protein is usually restricted to 20 the liver is extremely important in digestion and metabolism to 30 g/day to minimize urea nitrogen production. Thus, clients with impaired hepatic function are often ents with minimal sodium are needed. However, most impairment, these clients have special needs in relation to nu- clients are unable to tolerate enteral feedings because tritional support. Clients with ARF often have hyperkalemia, hyperphosphatemia, and severe hepatitis often have hypoglycemia because of hypermagnesemia, so that potassium, phosphorus, and impaired hepatic production of glucose and possibly magnesium should be omitted until serum levels return impaired hepatic metabolism of insulin. For clients able to tolerate enteral feedings (usually by Chronic Renal Failure GI tube), Hepatic Aid II is formulated for clients with liver failure. When peripheral or central parenteral nu- • Enteral nutritional support is usually indicated because trition is necessary for clients with hepatic failure and the GI tract is functional. Normal amounts of protein hepatic encephalopathy, HepatAmine, a special formu- (eg, 1 g/kg/day) may be given. Other amino acid dation that protein restriction delayed progression of preparations are contraindicated in clients with hepatic renal failure and the need for dialysis is not supported encephalopathy and coma. Nepro is a formulation for clients re- which are used to provide calories in other malnourished ceiving dialysis; Suplena, which is lower in protein and clients, may lead to coma in clients with advanced cir- some electrolytes than Nepro, may be used in clients rhosis. Clients who require parenteral nutrition may de- who are not receiving dialysis. If serum triglyc- carefully formulated according to nutritional status, the eride levels exceed 300 mg/dL, fat emulsions should be extent of metabolic disorders, and whether the client is used only to prevent a deﬁciency of essential fatty acids. Several amino acid solutions are for- • Sodium and ﬂuid restrictions are often needed to de- mulated for clients with renal failure (eg, Aminosyn- crease edema. In relation to drugs for weight loss and obesity, little infor- • Serum triglyceride levels should be measured before IV mation is available about their use in clients with hepatic im- fat emulsions are given. Because sibutramine is metabolized in the liver, it hypertriglyceridemia, which would be worsened by fat is contraindicated in clients with severe hepatic impairment. In relation to drugs for weight loss and obesity, little in- formation is available about their use in clients with renal Use in Critical Illness impairment. With sibutramine, dosage reductions are not rec- ommended with mild to moderate impairment because the Critically ill clients often have organ failures that alter their drug and its active metabolites are eliminated by the liver. Thus, their 448 SECTION 5 NUTRIENTS, FLUIDS, AND ELECTROLYTES nutritional needs vary with the type and extent of organ im- • With enteral nutrition, concentrated products (eg, 1. In addition to renal and hepatic impairments, which 2 kcal/mL) provide more calories and help with fluid were discussed, clients with pulmonary failure, cardiac fail- restrictions. Excessive amounts of sodium and ﬂuid or rapid administration may precipitate or worsen heart failure and should be avoided. Pulmonary Impairment If IV fat emulsions are used, they should be given over • In clients with chronic obstructive pulmonary disease 24 hours because faster infusion may depress myocardial (COPD), major concerns are weight loss and decreasing function. However, increasing caloric in- Multiple Organ Dysfunction Syndrome take in these clients must be done cautiously because overfeeding leads to increased carbon dioxide (CO2) pro- • Clients with MODS, who are usually in critical care duction, increased work of breathing, and perhaps respi- units, require nutritional support because they have ratory acidosis. Thus, excessive carbohydrate in enteral high rates of metabolism and tissue breakdown (ca- or parenteral feedings may cause respiratory failure. However, nutritional support is complex • Enteral nutrition is preferred if the GI tract is functional because a client may have a combination of renal, he- and accessible. Nutrivent and Pulmocare are enteral patic, pulmonary, and cardiac impairments. Thus, it products for clients with COPD or respiratory failure must be individualized according to the type and ex- and mechanical ventilation. Moderate amounts (1 to Clients with oliguric ARF or ARDS as part of their 1.
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The more and therefore constitutes the simplest example of widespread distribution of Ia connections found in reﬂex function buy generic imitrex 25mg on line spasms below middle rib cage. In the upper limb purchase imitrex with visa spasms 1983 movie, and here again investigations of the distribution of there is also a different organisation discount imitrex 50mg with amex muscle relaxant 8667, with absence monosynaptic Ia connections provides the easiest of the proximal-to-distal projections used in feline way to approach evolutionary changes order cheap imitrex on-line muscle relaxant use. Reﬂex attenuation (v) Reduction of post-activation depression at the Ia-motoneurone synapse could be one of the main A decrease in the amplitude (and an increase in spinal mechanisms underlying spasticity. Spasticity Background from animal experiments In spasticity the Hmax/Mmax ratio is increased in soleus, but is largely unchanged in FCR. Post- Ia afferents originate from the primary endings activation depression following previous activation of muscle spindles, and are sensitive to muscle of Ia ﬁbres is reduced in spastic patients. They have tion could be a consequence of altered use due to excitatory monosynaptic projections to homony- motor impairment, and may be an important spinal mous motoneurones, and this constitutes the exci- mechanism underlying spasticity. Projections are monosynapticIaexcitationinalllimbmusclestested stronger on motoneurones innervating slow-twitch (same latency as the H reﬂex after allowance for the units than on those of fast-twitch units. Heterony- trigger delay of the unit, low threshold, elicitation by mous projections are weaker than homonymous tendon taps). Transjoint con- Critique nections are more developed in the forelimb than in By analogy with the soleus H reﬂex, the H reﬂex and the hindlimb of the cat. Evidence for a two-neurone arc in the human soleus Heteronymous monosynaptic Ia excitation Stimulation of the posterior tibial nerve produces Heteronymous facilitation of the H reﬂex a synchronised response in the soleus muscle, and Hreﬂex studies do not allow reliable assessment of this has become known as the Hoffmann reﬂex or H the central delay of the resulting facilitation. Recordings of the action potentials from both dorsal and ventral roots intrathecally have demon- PSTHs of single motor units strated that the ﬁrst motoneurones discharging in the soleus H reﬂex may do so at a latency consistent the principle is to compare the latencies of the early with a monosynaptic pathway. Besides the mono- facilitation evoked in the same unit by stimulation synaptic latency, several arguments indicate that the of homonymous and heteronymous nerves. The dif- pathway is fed by Ia afferents: low electrical thresh- ference between the two latencies must reﬂect the old,similarexcitationelicitedbyatapontheAchilles differences in the afferent conduction times and tendon, facilitation by a homonymous volley to the in the central (synaptic) delay of the Ia effects of inferior branch of the soleus nerve, and simulta- the homonymous and heteronymous volleys. If, like neous blockade by ischaemia of the homonymous homonymous excitation, heteronymous excitation facilitation and of the Achilles tendon jerk. Afferent con- In routine studies there is a number of advantages ductiontimesfortheIahomonymousandheterony- of studying the H reﬂex during voluntary contrac- mous volleys can be estimated from the distance tion, because it is then possible to record the reﬂex from stimulation sites to the arrival of the afferent in virtually all accessible limb muscles, to reduce the volleys at the spinal cord, as measured on the skin, latency variability, to increase the stimulus rate, and andtheconductionvelocityinIaafferents. Thevalid- to direct the reﬂex response to the active motoneu- ity of the calculation depends on the reliability of rone pool. PSTH method Bidirectional connections Stimulation of the parent nerve evokes an early peak To eliminate uncertainties in estimates of peripheral in PSTHs with the characteristics of homonymous conduction times, studies have been performed on Resume´ ´ 103 Ia connections linking a pair of motor units in two Organisation and pattern of connections different muscles. Inthiscase,thehomonymousvolleyforonemember Homonymous monosynaptic Ia excitation of the pair is the heteronymous volley for the other, and vice versa. The absolute value of the difference In virtually all limb muscles, stimulation of the par- in afferent conduction times is the same for the two ent nerve can elicit an H reﬂex and a peak of mono- members of the pair. Such studies provide cogent synaptic Ia excitation in PSTHs of single motor units evidence for heteronymous monosynaptic connec- during voluntary contractions. At rest, H reﬂexes tions, independent of estimates of peripheral affer- can be recorded from the soleus, quadriceps and ent conduction times. The ease with which the Hreﬂex can be elicited at rest and the size of the peak of excitation elicited by stimulation subthresh- oldforthecompoundHreﬂexarecloselyrelated. Ia afferent origin Heteronymous monosynaptic Ia excitation Besides the monosynaptic connection, several fea- tures argue that the heteronymous pathway is fed by Lower limb Ia afferents: low electrical threshold, similar excita- In the human lower limb, in striking contrast with tion elicited by a tendon tap, increase in the thresh- dataforthecathindlimb,connectionsbetweensome oldoftheexcitationbylong-lastingvibrationapplied close synergists operating at the same joint. These transjoint connections can be strong, in cat and human experiments (2×Ia threshold e. They and 8×Ia threshold, respectively) is due to the often link a muscle or group of muscles to a pair fact that, in human experiments, the afferents are of antagonistic muscles operating at another joint, stimulated through surface electrodes at a distance e. Maturation of Ia connections during Walking development the spinal stretch reﬂex can produce a mechani- It has been reported that, in the normal newborn cally effective contraction and provides a pathway baby,atendontapmayelicitshort-latencyheterony- through which rapid automatic load compensation mous excitatory responses in antagonistic muscle to an unexpected disturbance can be generated. However, it gered by unexpected ankle joint displacement, con- would be prudent to retain reservations about these tribute signiﬁcant stabilisation of the supporting conclusions (see p. Motor tasks and physiological implications Perturbations of upright stance Muscle stretch elicits a reﬂex response from the Perturbationsoftheuprightstanceinsubjectsstand- corresponding motoneurone pool, and this has at ing on a rotating platform produce an early spinal least two separate components: the classical short- stretch reﬂex response (M1), which is prominent in latency spinal reﬂex (M1), the latency of which is soleus. After loss of large-diameter muscle spindle compatible with monosynaptic Ia excitation, and a afferents, M1 is absent but posture is quite stable, medium-latency component (M2) of more complex suggesting that M1 is not essential for equilibrium origin. The short-latency spinal stretch reﬂex during Spinal and transcortical stretch reﬂexes natural motor tasks the medium-latency (M2) response to stretch fol- the spinal stretch reﬂex utilises the simplest reﬂex lowing the early spinal (M1) response has a different pathway and interacts with pre-programmed and origininvariousmuscles:intheﬂexorpollicislongus other reﬂex mechanisms to compensate for distur- (and intrinsic muscles of the hand) the long latency bances during natural motor tasks. There is con- is due to a transcortical pathway fed by Ia afferents, siderable literature about the contribution of the whereas in foot and leg muscles the stretch response short-latencystretchreﬂexoftricepssuraetovarious is mediated through a spinal pathway fed by slowly natural movements, but few data for other muscles. Bothtranscorticaland spinal group II pathways could contribute to the M2 response in proximal upper limb muscles, such as Running, hopping and landing the biceps brachii. Duringthestancephaseofrunningandhoppingand after the impact of landing, the short-latency spinal Heteronymous monosynaptic Ia excitation stretch reﬂex of the triceps surae is superimposed on pre-programmed activity and contributes to the There are little experimental data on the functional musclecontractionresponsibleforthepushingoffof role of heteronymous Ia connections. However, the Resume´ ´ 105 different organisation of these connections in the and tibialis anterior when leaning backward). On cat and baboon hindlimb and the human lower theotherhand,diffuseIaconnectionscouldbecome limb suggests that the connections are functionally functionally inconvenient, because the activation of important, having adapted to provide the particular Ia afferentsfromacontractingmusclemightresultin reﬂex assistance required in each species. Suppressionofunwantedheterony- mousIadischargescanbeachievedthroughfocused Weak connections between ankle extensors corticospinal control of presynaptic inhibition of the weakness of the connections between ankle Ia terminals and of recurrent inhibition. The need extensors in human subjects may be related to the for this control suggests that the heteronymous Ia role of triceps surae in walking: it resists and brakes discharge does play a functional role, because it the passive ankle dorsiﬂexion produced by extrinsic must be suppressed in tasks for which it is not forces (kinetic force and gravity), but must be over- required. It would then be undesirable to have exces- Upper limb sive activity from the triceps surae stretch reﬂex, and weak Ia connections between the different heads of the diffuse distribution of the Ia projections from the muscle would help ensure this. Some of these connec- Studies in patients and tions are weak, but their strength has been under- clinical implications estimated in experimental studies and, in any case, this would not prevent them from modulating the In practice, assessing Ia connectivity involves meas- excitability of motoneurones that are already depo- urements of the H reﬂex. During the stance phase of running, hop- advantages of doing so during voluntary contrac- ping and landing, all extensors undergo a lengthen- tions (see above). Modulation of the on-going EMG ingcontractionthatevokesastrongIadischarge,and by a heteronymous volley may allow access to a it is probable that the extensive Ia connections link- motoneuronepoolbyafferentinputsthatdonottra- ing muscles across joints modulate the role played verse the same nerve or nerve root as homonymous by the different muscles in load compensation. Projections onto antagonists operating Peripheral neuropathies, mononeuropathies at another joint and nerve lesions Theseprojectionsaredesirablefunctionallybecause These may be accompanied by a decrease in the of the versatile synergisms required to accomplish amplitude and an increase in the latency of the the various tasks of the human lower limb. Reﬂex depression usually results from an co-contraction of quadriceps and gastrocnemius- afferent abnormality and will occur when there is soleus in running and hopping, but of quadriceps either a loss of conducting afferents or dispersion of 106 Monosynaptic Ia excitation the afferent volley. Tests of reﬂex function provide a sion is markedly attenuated, probably because the tooltodistinguishbetweenisolatedperipheralnerve enhanced Ia ﬁring during voluntary contraction lesions and lesions involving roots or plexus. Spasticity the ratio Hmax/Mmax is, on average, increased in soleus but not, or hardly so, in FCR in hemiplegics. Post-activation depression in spastic patients Whether measured as the depression induced by passive stretch of the test muscle or by high stimu- Post-activation depression at the lus rate, post-activation depression is signiﬁcantly Ia-motoneurone synapse decreased in spastic patients due to spinal cord injury and multiple sclerosis, and on the affected Background from animal experiments side of patients with hemiplegia. This reduction is It has long been known that the size of the mono- probably a consequence of the disuse due to motor synaptic reﬂex decreases when it is repeatedly impairment, and may be an important spinal mech- elicited. The results from a variety of preparations indicate the presence of an early facili- REFERENCES tationofrelativelyshortdurationsuperimposedona depression of much longer duration. Effectsofextensorandﬂexorgroup depression helps to maintain the synaptic efﬁcacy of I afferent volleys on the excitability of individual soleus the Ia-motoneurone synapse at a relatively low level motoneurones in man.
After the onset of contraction proven imitrex 50mg spasms 14 year old beagle, the group I feedback from the contracting muscle (and the Ia discharge from the antagonist stretched ECR) are channelled back into the reﬂex path already activated by corticospinal excitation cheap imitrex express muscle relaxant pregnancy safe, because transmission in the opposite pathway is inhibited purchase discount imitrex on line spasms back, and vice versa for voluntary wrist extension buy generic imitrex 25 mg online spasms toddler. Presynaptic inhibition (PAD INs) of FCR Ia terminals is depressed by a descending inhibition, possibly reticulospinal (RS) in origin. Hreﬂex are mediated through the pathway of non-reciprocal group I inhibition, not through Ia inhibitory interneurones transmitting reciprocal Ia Non-reciprocal group I inhibition during inhibition (see pp. In two exceptional subjects, in whom it was poss- (iv)Despitethisconvergence,theseinterneurones ible to evoke an ECR H reﬂex which did not disap- are arranged in subsets specialised with regard to pear during the contraction of the antagonistic FCR, their target motoneurones (see below). Lamy, unpub- I inhibition to FCR and ECR motoneurones, activa- lished data). In the other direction, the disynaptic tionofFCR-coupledgroupIinterneuronesproduces radial-induced non-reciprocal group I inhibition of inhibition of ECR-coupled group I interneurones, the FCR H reﬂex is suppressed prior to, and during and vice versa (see p. The ﬁnding while they depress it on Ia terminals on lower limb that the above effects occur before the contraction- motoneurones (see p. Given that cor- Non-reciprocal group I inhibition ticospinalvolleysproduceparallelfacilitationofFCR motoneuronesandcorrespondinggroupIinhibitory Non-reciprocal group I inhibition of wrist motoneu- interneurones (cf. This is in keeping with the ﬁnding FCR motoneurones, through mutual inhibition of that, in the cat, non-reciprocal group I inhibition is opposite group I inhibitory interneurones. Non-reciprocal group I inhibition during voluntary wrist extension Corticospinal control Is there descending facilitation of the pathway? Stimulation of the motor cortex produces potent disynaptic suppression of the radial-induced inhi- the difﬁculty in proving descending facilitation of bition of the FCR H reﬂex. This reveals a more potent corticospinal untary wrist extension, radial-induced inhibition of Movements involving ball joints 525 the FCR H reﬂex has been reported to be increased, reasons for these motoneurones receiving recip- suggesting descending facilitation of ECR-coupled rocal inhibition from the conditioning radial vol- group I inhibitory interneurones (Day, Rothwell & ley, unless there is an associated voluntary ﬁn- Marsden, 1983), but this result has not been con- ger extension. Dur- whether care was taken that the wrist exten- ing tonic wrist extension, radial-induced inhibition sion was accompanied or not by ﬁnger extension. However, when the natural periph- Functional implications eral feedback from the contracting wrist extensors Corticospinal excitation of the appropriate is interrupted by a block of the radial nerve using pathway lignocaine injected distal to the stimulation site, the radial-induced inhibition of the FCR H reﬂex is During FCR contractions, FCR-coupled group enhanced during attempted tonic wrist extension I inhibitory interneurones projecting to ECR (Day et al. This differential of an interaction between the peripheral feedback control indicates that interneurones mediating from the contracting muscle and the conditioning non-reciprocal group I inhibition to wrist motoneu- volley. Much as in the case of reciprocal Ia inhibi- ronesarearrangedinsubsetsspecialisedwithregard tion (see pp. The corticospinal on the natural group I discharge during voluntary facilitation of group I inhibitory interneurones contraction could contribute to the decreased efﬁ- precedes the parallel discharge of the corresponding cacy of the conditioning volley in activating group motoneurones, and its main function is prob- I inhibitory interneurones: occlusion at interneu- ably to select the appropriate subset of group I ronal level, presynaptic inhibition of the condition- interneurones, i. These interneurones also at the synapse between group I ﬁbres and inhibitory inhibit inhibitory interneurones projecting to active interneurones at wrist level. After the onset of contraction, the group I feedback from the contracting muscle is channelled back into the reﬂex path already Synergy between ECR and ﬁnger ﬂexors activated by corticospinal excitation, because trans- the functional synergy between wrist extensors and mission in the opposite pathway is inhibited (see ﬁnger ﬂexors in clenching and grasping (see Liv- Fig. The H reﬂex of fore- stretched antagonists from inhibiting the active arm ﬂexor muscles to stimulation of the median motoneurones. Intuitively, there are no good wrist muscles, the discharge of spindle endings 526 Spinal pathways in different motor tasks in the contracting muscle will decrease (see so far tested (see pp. However, inhibitory group I interneu- isometric nature of the contractions studied. Facili- rones to wrist motoneurones also receive Ib inputs, tation of PAD interneurones could be selected by the which are related to muscle contraction, not to motor cortex in order to turn off the monosynap- muscle length. Moreover, the Ia afferent discharge tic Ia assistance during rapid unloaded wrist move- from the stretched antagonists may also contribute ments, in which reﬂex feedback could have dele- to activation of the selected pathway because of terious effects (see p. Testing this hypothe- the convergence of group I volleys from ﬂexors sis would require further experiments using rapid and extensors onto common interneurones (see isotonic movements. Thus,theparticularorganisationofthepath- ways of non-reciprocal group I inhibition to wrist motoneurones ensures that the relevant interneu- Other spinal pathways possibly involved ronesreceiveperipheralfeedback,evenduringrapid in wrist movements movements. Mutual recurrent inhibition between FCR and ECR motoneurones Changes in presynaptic inhibition on Ia terminals on wrist motoneurones This particular pattern has been described during voluntary contractions of the target muscle in stud- Non-speciﬁc decrease in presynaptic iesofthemodulationofthedischargeofsinglemotor inhibition units and of the on-going EMG activity, but there are no data on how transmission in the relevant path- Presynaptic inhibition of Ia terminals on FCR ways is modulated during other types of contrac- motoneurones is moderately depressed at the onset tion. A functional explanation for these connections of and during isometric voluntary contractions of is proposed below (p. It has been speculated that this non-speciﬁc effect might be due to reticu- lospinal depression acting on the last-order PAD Cutaneomuscular responses interneurones (p. The isometric contractions resemble postural co-contractions of wrist muscles. Cutaneomuscularresponsescanbeelicitedinvolun- Assuming that there is the same descending con- tarily active wrist muscles by low-intensity stimuli to trol of PAD interneurones in postural contractions, theﬁngers. Innormaladultsubjects,theseresponses decreased presynaptic inhibition on Ia terminals aredominatedbythelong-latencytranscorticalexci- on wrist motoneurones would reinforce the servo- tatory response. So far, there are limited data on the assistance provided by the monosynaptic Ia stretch modulation of the responses in wrist muscles during reﬂex of wrist muscles and improve the reﬂex sup- different motor tasks (though such changes are well- port to manipulatory movements of the ﬁngers. Corticospinal facilitation of PAD Propriospinal pathways interneurones Propriospinal pathways transmit some of the cor- the corticospinal facilitation of presynaptic inhibi- ticospinal command for isometric contractions of tion on Ia terminals on wrist motoneurones, seen wrist muscles (pp. Activation of various synergies 527 mechanisms capable of producing, as reﬂex results, 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.