By T. Topork. Providence College.
Idiosyncrasy refers to an unexpected reaction to a ness and the need for endotracheal intubation and mechani- drug that occurs the ﬁrst time it is given discount lady era 100mg amex womens health connection. Serious cardiovascular effects mind-altering drugs lady era 100 mg low cost menstrual disorders symptoms, such as opioid analgesics effective lady era 100mg menstruation 2 weeks after birth, (eg, cardiac arrest, dysrhythmias, circulatory impairment) are sedative-hypnotic agents, antianxiety agents, and CNS also common and warrant admission to an ICU. Dependence may be physiologic or psy- The main goals of treatment for a poisoned patient are sup- chological. Physiologic dependence produces unpleas- porting and stabilizing vital functions (ie, airway, breathing, ant physical symptoms when the dose is reduced or circulation), preventing further damage from the toxic agent the drug is withdrawn. Psychological dependence by reducing additional absorption or increasing elimination, leads to excessive preoccupation with drugs and drug- and administering speciﬁc antidotes when available and in- seeking behavior. Carcinogenicity is the ability of a substance to cause antidotes are listed in Table 2–2; and speciﬁc aspects of care cancer. Several drugs are carcinogens, including are described in relevant chapters. For patient who are seriously ill on ﬁrst contact, enlist ity apparently results from drug-induced alterations in help for more rapid assessment and treatment. Teratogenicity is the ability of a substance to cause ingestion leads to better patient outcomes. Standard cardiopulmonary Toxic Effects of Drugs resuscitation (CPR) measures may be needed to maintain breathing and circulation. An intravenous (IV) line is Drug toxicity (also called poisoning, overdose, or intoxica- usually needed to administer ﬂuids and drugs, and inva- tion) results from excessive amounts of a drug and may sive treatment or monitoring devices may be inserted. It Endotracheal intubation and mechanical ventilation is a common problem in both adult and pediatric popula- are often required to maintain breathing (in uncon- tions. It may result from a single large dose or prolonged scious patients), correct hypoxemia, and protect the ingestion of smaller doses. Hypoxemia must be corrected quickly to avoid scription, over-the-counter, or illicit drugs. Poisoned pa- brain injury, myocardial ischemia, and cardiac dys- tients may be seen in essentially any setting (e. In some cases, the patient or someone accompanying the Serious cardiovascular manifestations often require patient may know the toxic agent (eg, accidental overdose of pharmacologic treatment. Hypotension and hypoperfu- a therapeutic drug, use of an illicit drug, a suicide attempt). Dysrhythmias are treated according to Advanced causative drug or drugs are unknown, and the circumstances Cardiac Life Support (ACLS) protocols. For unconscious patients, as soon as an IV line is es- and may indicate other disease processes. Because of the vari- tablished, some authorities recommend a dose of able presentation of drug intoxication, health care providers naloxone (2 mg IV) for possible narcotic overdose CHAPTER 2 BASIC CONCEPTS AND PROCESSES 25 TABLE 2–2 Antidotes for Overdoses of Selected Therapeutic Drugs Overdosed Drug (Poison) Antidote Route and Dosage Ranges Comments Acetaminophen (see Chap. Give IV slowly, over Infrequently used because of its (atropine; see Chap. If cardiac arrest seems immi- nent, may give the dose as a bolus injection. If amount seizures and correction of of INH unknown, give 5 g; may be acidosis repeated. Lead Succimer Children: PO 10 mg/kg q8h for 5 days Opioid analgesics (Chap. Maximum dose, 30 g/24h (continued) 26 SECTION 1 INTRODUCTION TO DRUG THERAPY TABLE 2–2 Antidotes for Overdoses of Selected Therapeutic Drugs (continued) Overdosed Drug (Poison) Antidote Route and Dosage Ranges Comments Tricyclic antidepressants Sodium bicarbonate IV 1–2 mEq/kg initially, then contin- To treat cardiac dysrhythmias, (see Chap. This group issued treatment guide- fingerstick blood glucose test should be done and, if lines that have also been endorsed by other toxicology hypoglycemia is indicated, 50% dextrose (50 ml IV) organizations. Once the patient is out of immediate danger, a thorough used routinely and that adequate data to support or ex- physical examination and efforts to determine the clude their use are often lacking. Opinions expressed drug(s), the amounts, and the time lapse since exposure by the consensus group and others are described are needed. If the patient is unable to supply needed in- below: formation, interview anyone else who may be able to do Ipecac. Ask about the use of prescription, over-the-counter, may delay administration or reduce effectiveness alcohol, and illicit substances. There are no standard laboratory tests for poisoned pa- bowel irrigation. Ipecac may be used to treat mild poi- although baseline tests of liver and kidney function are sonings in the home, especially in children. Specimens of blood, urine, or gastric should call a poison control center or a health care ﬂuids may be obtained for laboratory analysis. If used, it is most Screening tests for toxic substances are not very beneficial if administered within an hour after in- helpful because test results may be delayed, many sub- gestion of a toxic drug dose. It is contraindicated in less than alert pa- layed to obtain results of a toxicology screen. Identiﬁ- tients unless the patient has an endotracheal tube in cation of an unknown drug or poison is often based on place (to prevent aspiration). If the in- Serum drug levels are needed when acetaminophen, gested agent delays gastric emptying (eg, tricyclic alcohol, digoxin, lithium, aspirin, or theophylline is antidepressants and other drugs with anticholinergic known to be an ingested drug, to assist with treatment. For orally ingested drugs, gastrointestinal (GI) de- after ingestion of pills or capsules, the tube lumen contamination has become a controversial topic. For should be large enough to allow removal of pill frag- many years, standard techniques for removing drugs ments. Sometimes called the universal an- tients, to induce emesis; gastric lavage for patients tidote, it is useful in many poisoning situations. It is with decreased levels of consciousness; activated being used alone for mild or moderate overdoses and charcoal to adsorb the ingested drug in the GI tract; with gastric lavage in serious poisonings. It effec- and a cathartic (usually 70% sorbitol) to accelerate tively adsorbs many toxins and rarely causes compli- elimination of the adsorbed drug. It is most beneﬁcial when given within an whole bowel irrigation (WBI) was introduced as an hour of ingestion of a potentially toxic amount of a additional technique. Its effectiveness de- Currently, there are differences of opinion regard- creases with time and there are inadequate data to sup- ing whether and when these techniques are indicated. These differences led to the convening of a consensus Activated charcoal is usually mixed in water group of toxicologists from the American Academy (about 50 g or 10 heaping tablespoons in 8 oz. Adverse effects in- clude pulmonary aspiration and impaction of the Answer: Grapefruit juice interacts with many medications, including charcoal–drug complex. The drug level of felodipine increases because the If used with whole bowel irrigation, activated grapefruit juice inhibits the isozyme of cytochrome P450, which is important in the metabolism of felodipine. As the blood level in- charcoal should be given before the WBI solution is creases, serious toxic effects can occur. If given during WBI, the binding capacity of cytochrome P450 so it would be safe to have Mrs.
Te small town of Fairhope over the years has become a haven for artists lady era 100mg overnight delivery women's health clinic in rockford il, writers buy lady era 100mg women's health clinic riverside campus, and eccentrics of all sorts cheap lady era 100mg pregnancy apps. It is a community of intellectuals and thus was an ideal setting for the work of H. He had accepted the chair of the newly formed Department of Family Medicine at the University of South Alabama School of Medicine in Mobile. Family medicine in academic circles was still a new specialty, and Moon set out to build an outstanding depart- ment. Recalling my own days as a dean when I struggled with what to do about incorporating family medicine, I also wanted to ﬁnd out what the specialty was all about. Moon had practiced family medicine for more than twenty years and knew intuitively that medicine at its heart had to encom- pass human communication. As soon as he became chair of the department, he recruited an outstanding team of behavioral sci- entists. When I heard of his eﬀorts, I immediately made plans to spend my sabbatical with Moon and his team. In addition to gathering the team of behavioral scientists, Moon had built a near state-of-the-art audiovisual recording facil- ity in the clinic building in Fairhope. Tis teaching clinic was re- Looking Back on Fairhope 97 mote from the main campus in Mobile, but it provided a laboratory for more ordinary types of clinical problems. Tere were one-way mirrors, conference rooms, extensive audiovisual re- cording equipment, and everything one might need to observe and record interactions between doctors and patients. To supplement the stationary recording equipment in the clinic, we soon built portable enclosures, crude folding panels that we could ﬁt into the corners of any exam room, two to a room. Moon had recruited Stonewall Stickney, a psychiatrist and for- mer commissioner of the state mental health system for Alabama. He was analytically trained and extraordinarily well-read both in psychiatry and in world literature. Te other two members of the team, in addition to me, were Joseph and Susan Conley. Conley had completed work on her doctoral degree in animal behavior, specializing in observing foxes. Conley, her husband, did his graduate work in clinical psychiatric social work. He too was intuitive and could get to the heart of a clinical problem quickly. We spent months attempting to videotape full encounters be- tween patients and physicians in the clinic. Tis was in the early days of videotape recordings, and we had constant equipment fail- ures of all kinds. Te equipment was not yet remote, so we had two people in the exam room behind wooden blinds to run the 98 Symptoms of Unknown Origin cameras. In addition to the two cameras in the room, we had a third wide-angle camera in a cut-through in the wall to capture both the physician and the patient, feet to head, in the same recorded view. Te problems in the oﬃces of private physicians were even more formidable. Eventually, we ﬁnally got one full recording of one patient and one physician at the main clinic. Tis one recording became the focus of our eﬀorts for the next several months. Susan spent more than sixty hours mov- ing the video frames back and forth, noting every movement and verbalization of both the physician and the patient, along with the time interval of the movement. She then transcribed the sequence so we could read serially what the patient did and said and what the physician did and said. Susan was in eﬀect creating a dictionary of minute doctor and patient be- haviors. She then noted what she called utterances, classifying each utterance and noting its time intervals. Our intent was to catego- rize with no preconceptions what we saw and heard. Susan categorized every slight movement, tone of voice, inﬂection, and utterance. Her dictionary of behaviors ran well over ﬁfty pages, all from the 157-second tape. Even though Susan and Joe refused to speculate, Stone and I spent hours theorizing about what we observed on the tapes and about the clustered behaviors that Susan had teased out. Most of our conjectures came from our observations through the one- way mirrors. Stone might say, as we watched a doctor and patient, What do you make of that? Tey introduced me to the ideas of unspeciﬁed language and methods for establishing rapport, partic- ularly the notion of people having verbal, visual, or kinesthetic rep- resentational systems. I also learned to pay attention to the verbs patients used and to their facial expressions (Bandler and Grinder 1976a, 1976b, 1979). Grinder and Bandler modeled many of their ideas from careful observations of Milton Erickson, a psychiatrist and superb therapeutic hypnotist (Bandler and Grinder 1982; Haley 1986, 1987; Erickson and Rossi 1979). Te appeal of their ideas is that they are stated in terms that can be refuted by direct observa- tion—thus they are subject to scientiﬁc study. No one has yet done such a study, and the writings of both authors remain outside the mainstream medical literature. In many of the cases that follow in this book, I use techniques that came from the ideas of these au- thors. Stonewall Stickney and I made some tentative observations and speculations from our experiences. I say tentative because we did not conduct full-ﬂedged scientiﬁc experiments. We did not do ﬁeld experiments with the physicians, having them, for example, redirect their cued statements to test the notion of cuing. In one observation, we noticed a lot of movement (hands, arms, head, feet, breathing, eyeblinks) going on by and between patient and doctor. Sometimes these movements of the physician were copied by the patient and a synchrony developed. We speculated that when the patient copies a movement of the physician, the pa- tient is in a receptive state for instructions. Often the physician phrased the question with an aﬃrmative or negative direction. We speculated that the doctor was cuing the patient to answer questions along some preconceived lines of thought. Tere are many instances where cuing could be oc- curring between physician and patient, but none of these have been studied by direct observation. Unwittingly, negative out- comes and reactions can also be conveyed by cuing. According to Frank, cuing appears to occur beyond the awareness of the person being cued.
For each of them purchase lady era 100 mg mastercard menstrual bloating treatment, the structural hierarchy is given in terms of neurons (axon hillocks) buy lady era 100mg fast delivery menstruation meaning, synapses order 100 mg lady era otc menstrual distress questionnaire, and channels (ﬁgure 7. Functional interactions are for activity, the membrane po- tential c that propagates from one neuron at r0 to another at r and for synaptic mod- ulation, the postsynaptic potential F at s, or equivalently, synaptic e‰cacy m. Let the density of neurons at r be rðrÞ and the density connectivity between the neurons at r0 and the synapses at s0 be p 0 0. For synapses at s in neurons at r, the density connec- r s tivity prs is determined by the connectivity in the postsynaptic neuron between spines and soma where the membrane potential is measured. A similar hierarchical structure in the synapses in which the channels are distributed leads to a similar ﬁeld equation for the functional interaction at this level, say g, given the anatomy of the system. Operators are determined by the explicit analytical relationship between input and output: Pðr0Þ applies to c; that is, it transforms the action potential c into the postsynaptic potential f using the synaptic e‰cacy s in the activity time scale. The structure of the ﬁeld equation is such that these operators correspond to an input- output block model, that is, a nonlinear transfer function. PCðrÞ applied to post- synaptic potentials f, and then integrated over all the pathways gives rise to the membrane potential at r. The neural ﬁeld equations derived using the S-propagator formalism for the c-ﬁeld at ðr; tÞ in the time scale fTg, and with the unknown factor Kðs0; s; dÞ (in case of linearity for the propagators P) for the f-ﬁeld equation at ðs; tÞ in the time scale ftg, are given by Eq. Each of these equations corresponds to a level of functional organization. These two levels of functional organization are coupled by a relationship, for example: Et A ½ti; ti þ Dt: sðtÞ¼sðtiÞ¼mðtiÞ or hsðtÞiDtðtiÞ¼mðtiÞ; ð7:8Þ where Dt is the time unit deﬁned experimentally and hsðtÞi denotes the average value of sðtÞ taken over this time interval. Application of the Formalism The Cerebellum and the Coordination of Movement Clinical studies have established that the coordination of movement depends on speciﬁc circuits in the cerebellar cortex and on highly organized interactions among several nuclei in the brain (Thompson, 1986, 1990). Over the past few years, the adaptive control of movement has been extensively investigated through mathemati- cal studies of artiﬁcial as well as biological neural networks (Barto et al. Much e¤ort has gone into determining the mechanisms of pattern learn- ing and recall; in other words, toward deﬁning the conditions of stability in dynamic systems. Mathematical Modeling of Neuromimetic Circuits 143 The cerebellar cortex is a network of networks. An element of the cerebellar cor- tex, called the Purkinje unit, consists of ﬁve types of cell: the Purkinje cell, which has the largest number of dendrites; the granular cells; the Golgi cell; and the basket and stellar cells. The geometry of the cortex allows us to deﬁne (approximately) a Pur- kinje unit. Consider a granular cell (gc) belonging to the unit containing the nearest Purkinje cell it is in contact with. Then (gc) may be considered to belong to a speciﬁc unit labeled k if the following conditions are satisﬁed: (gc) synapses with at least one Purkinje cell of unit k, the distance between (gc) and the Purkinje cells is the smallest distance between (gc) and any Purkinje cell it is in contact with outside the unit, and (gc) synapses with at least one Golgi cell of unit k. The basket and stellar cells included in the unit are those that are in contact with the Purkinje cell of unit k. This unit may be divided into two subsystems, the granular cell subsystem (GCS), that is, the neural network composed of granular cells (ﬁgure 7. The Purkinje unit, which is the repeating unit of the cerebellar cortex, is thus the basic element of a hierarchical network. We know that the function of the cerebellar cortex is the learning and recall of spatiotemporal patterns (Thompson, 1994). Therefore, a satisfactory transformation of the cerebellum would Y F X1... The nonlinear transformation is F, and for signals before transformation, lower-case letters are used (e. Inputs are the outputs Xi of the granule cell subsys- tem (on the right). Berger require that the output of the system remain within physiological limits and that the modiﬁable synaptic weights be asymptotically stable to ensure the learning process. The conditions necessary for the stability of the observed function call for adequate values of geometric and physiological parameters, that is, the number of cells in- volved, the value of the synaptic weighting, and so on. These conditions thus con- tribute to the determination of the Purkinje unit. Using the earlier deﬁnition of a functional unit, the Purkinje unit associated with the deep cerebellar nuclei, that is, the local circuit composed of one Purkinje cell and its associated cells, can be considered as the functional unit of the cerebellar cortex. This is supported by the following arguments: The deﬁnition of a Purkinje unit is geometric as well as functional. A set of Pur- kinje units corresponds to a microzone (Ito, 1984), although it should be noted that the deﬁnition of the microzone is not based on mathematical criteria. The stability of the function, which takes into account the internal dynamics that are due to the time lag in the propagation within the unit and between two units (P. Chauvet and Chauvet, 1995), determines the conditions for the deﬁnition of the structural unit. Chauvet, 1995) deduced from neural learning rules apply to Purkinje units and govern the coordination of movement through excitatory and inhibitory interactions among the units. The hypothesis of synaptic plasticity, applied to granular cells, reveals a wide range of learning be- haviour. The same learning rules probably apply during the developmental period as well as in adult life to ensure the convergence of signals carried by the climbing ﬁbers of the cerebellar cortex. The coupling between units increases the overall stability of the system, in agree- ment with the general theory (G. Appendix B lists various structures of the cerebellum and the corresponding func- tions, with their mathematically derived properties. The Network of Purkinje Units Let us now consider the hierarchical network of Purkinje units in which each unit is itself a neural network as deﬁned earlier. The interactions among the Purkinje units lead to new learning rules governing the co- ordination of movement on the basis of the external context. Here we refer to the learning mechanisms associated with circuits adjacent to the local circuit correspond- ing to the individual Purkinje unit. Basically, the dynamics of the coordination may be explained by the hierarchy of the system of Purkinje units and by the granular Mathematical Modeling of Neuromimetic Circuits 145 cell subsystem associated with a Golgi cell. The learning rules then emerge at a higher level of Purkinje units, if certain conditions are satisﬁed [see Eqs. Applied to Purkinje units, these learning rules give the model a predictive value, at least from a qualitative point of view. For example, it is su‰cient to know the sense of the variation in cerebellar inputs to be able to determine the sense of the varia- tion in the synaptic e‰cacies and the outputs. In the learning phase, the outputs and the modiﬁable synaptic weights are given by the solutions of algebraic nonlinear equations coupled with integral-di¤erential nonlinear equations. The Purkinje network, because of its hierarchical nature, may thus be conveniently investigated on a mathematical basis.
Ithasbeenshown (ii) The absence of projections from Renshaw at ankle level that the increased stiffness during co- cells to inhibitory interneurones mediating non- contraction occurs too early after application of the reciprocal group I inhibition between wrist ﬂexors external load to be mediated through a reﬂex mech- and extensors enables a parallel increase in the out- anism (Nielsen et al purchase 100 mg lady era fast delivery menstrual migraine treatment. This is not of the greater stiffness during co-contraction with possible at hinge joints (see above) trusted 100mg lady era womens health group lafayette co. Control of the stretch reﬂex at hinge joints Stretch reﬂex Because co-contraction of the different muscles operating at the ankle will stabilise the joint in dif- The ﬁndings that increased recurrent inhibition is ﬁcult tasks (see above) discount lady era 100mg without prescription women's health center of york, there may be no need for not required and that presynaptic inhibition is not additional stiffness through an active stretch reﬂex. Under these conditions, itated (or at least unsuppressed) recurrent inhibi- several non-exclusive mechanisms might prevent tion (see above), will prevent oscillations and clonus the stretch reﬂex braking into oscillations. Control of the excitation at ball joints (ii) Tactile cutaneous receptors provide critical information for the control of grip force according to Renshaw cells the physical properties (weight, slipperiness, shape A different control of the motor output is likely and mass distribution) of the manipulated object. Unlike the quadrupedal increased during tonic co-contractions, this does digitigrade stance of the cat, humans balance on not imply that the strong corticospinal facilitation their skeleton as an inverted pendulum (see below). The control of body sway during quiet stance (iv) Similarly, the fact that recurrent inhibition and of responses to destabilising perturbations to may be suppressed during strong co-contraction at stance may involve different mechanisms and are wrist level does not mean that the ﬂexible control therefore treated separately. Normal quiet standing Because the centre of gravity is maintained over Conclusions arelatively small base of support, human stand- The main cause of the greater joint stiffness dur- ing posture is inherently unstable. Body instabil- ing co-contraction is simply that more muscles are ity, therefore, has a high potential energy, leading to then active, not an increase in stretch reﬂex activ- the priority of equilibrium control during almost all ity. The decoupling of motoneurones and group I motor tasks including quiet standing. This trasts with the linkage seen during simple ﬂexion- posture requires a background triceps surae activity, extension movements, and allows the simultaneous which is, however, not continuous, and little mus- activation of antagonistic motoneurone pools to be cular activity is needed to maintain balance (Bon- relatively unhindered by reciprocal inhibition. The main body sway occurs in the decoupling results from different drives for the two sagittalplane,wheretherearequasi-randomsponta- types of movements from higher centres. In addi- neous alternating movements of the centre of mass, tion, the different organisation of the connections which happen mostly at the ankle joint (e. Ithasthereforebecomecommontoregardthe increased recurrent inhibition and presynaptic inhi- body as an inverted pendulum pivoted at the ankle bition of Ia terminals. Smallersway movements also occur in the frontal plane, mostly at hip level, where they are stabilised by hip abductor– Maintenance of bipedal stance adductor activity (Deniskina & Levik, 2001). Only the maintenance of upright bipedal stance Multiple sources of feedback is considered here. Postural adjustments occur in Afferent cues from multiple sources many other situations, e. Many stud- tion about the movement of the centre of gravity ies have shown that, when various sensory systems with regard to the feet is necessary at all times, and are manipulated, body sway is affected: (i) absence (ii) balance has to be maintained during body of visual input increases the amplitude and speed conﬁgurations that may be continuously changing. A model has been pro- afferentsproduces1Hzsway(Mauritz&Dietz,1980), posed in which the intrinsic elastic properties of and postural responses are induced by vibration of the activated ankle musculature alone would be suf- ankle muscles (Eklund, 1972); (iv) a role of group ﬁcient to stabilise the upright posture. The stabil- II muscle afferents is suggested by balance abnor- isation of quiet standing would then be an essen- malities observed in patients with different types of tially passive process without any signiﬁcant active peripheral neuropathy (Chapter 7,p. Signals coming from these multiple sensory sources co-vary with every postural change. Because exclu- Contrary arguments sion of any of the above cues may be compensated for in normal subjects with a small (but signiﬁcant) However, attractive as it may be, this simple hypoth- increase in body sway, the question of redundancy esis does not explain a number of ﬁndings. However, once again, redundancy is more Other calculations have shown that ankle stiffness apparent than real. For instance, tactile afferents was overestimated in the above model and is actu- fromthesoleofthefootareinvolvedintheregulation ally insufﬁcient to stabilise the body (Morasso & of small-amplitude sway, whereas the muscle affer- Schieppati, 1999;Morasso & Sanguineti, 2002). Inaddition, ﬁbres appear to be the main source of ankle stiff- evidence for the necessity of an interaction between ness in quiet standing (Loram & Lakie, 2002b). Positive correlations, with time lags ness) but, rather, a sensory deﬁcit. In them, the in cross-correlations of 200–300 ms between triceps reduced efﬁcacy of predictive control due to unreli- surae EMG activity and antero-posterior motion of able sensory information is frequently compensated the centre of gravity have been interpreted simi- for by increased ankle stiffness resulting from co- larly as a feedforward modulation of muscle activ- contractionofanklemuscles(Morasso&Sanguineti, ity. Finally, the low intrinsic ankle stiffness found by Loram & Lakie Stretch reﬂex (2002a,b) implies the existence of an active neural It was initially assumed that shifts in the centre of control for modulating ankle torque, and they sug- gravity stimulated stretch afferents of postural mus- gest that this control is predictive, possibly origin- cles that contracted reﬂexively (Hellebrandt, 1938). Interestingly, changes This strategy was questioned because the angu- in voluntary set can minimise body sway when the lar motion at the ankle was less than necessary to subject attempts to be still (Fitzpatrick et al. It was but the predictive process is also operative when the then demonstrated repeatedly that spinal stretch subject is paying minimal attention (Loram & Lakie, reﬂexes are not relevant to the maintenance of quiet 2002a). Accordingly, Conclusions (i) quiet stance is only slightly destabilised by selec- tivesuppressionofthegroupIinputfromanklemus- In quiet standing, attenuation of body sway is due cles (see pp. Affer- ent input due to near-physiological perturbations ent cues from multiple sources evoked by previous have a low loop gain (∼1), which is insufﬁcient to swayingmovementsinteracttoorganiseapredictive explain stable standing as a feedback control task neural response producing the least ankle stiffness (Fitzpatrick, Burke & Gandevia, 1996). During quiet standing, because the knee joint is locked in extension and crossed by the gravitational action line, there may be little or no activity in Anticipatory control of the body sway thigh muscles (Kelton & Wright, 1949; Clemensen, The low loop gain of the soleus EMG response 1951;Joseph, 1962;deVries, 1965;Soames & Atha, evoked by small perturbations and the fact that they 1981). In contrast, when leaning backward or for- lead ankle movements with a phase advance that ward, co-contractions of quadriceps and tibialis increases with frequency are consistent with a feed- anterior or hamstrings and triceps surae, respec- forward process (Fitzpatrick et al. Inthisrespect,eventheveryweak Group II pathways also link one muscle to antago- tonic or phasic contractions occurring during quiet nistsoperatingatanotherjoint(Table7. Theselec- stance to maintain balance are accompanied by tion of the appropriate group II pathway for a given increased fusimotor drive sufﬁcient to affect spindle postural task might be ensured by the parallel acti- afferent discharge (see p. Theyalso favour the associated co-contraction of heterony- Conclusions mous muscle(s) operating at another joint, through During unstable upright stance, co-contractions of thestrongtransjointexcitatoryconnectionsthatlink ankle and knee muscles are required to maintain human lower-limb muscles, whether monosynap- posture. TransmissionofgroupIIexcitationisfacili- Group II excitation tated, possibly due to decreased monoaminergic ga- Transmission of heteronymous group II excitation ting. Ia and group II excitations can be focused by from tibialis anterior to quadriceps and from gas- descendingcontrolofRenshawcellsandofinterneu- trocnemius medialis to semitendinosus is facilitated rones controlling the transmission of spindle when leaning backwards and forwards, respectively. This facilitation is probably due to decreased activ- ity in the monoaminergic control of transmission in group II pathways from the locus coeruleus in Responses to fast transient perturbations the brainstem (see pp. Transmission of of stance the group II discharge from stretched leg muscles, tuned up by decreased activity in the monoamin- A sudden and unexpected perturbation to posture ergic control system, might thus contribute to the can occur when standing on unstable ground or on a co-contraction of leg and thigh muscles required to minimal area. The mechanisms involved in the con- maintain bipedal stance when leaning backwards or trol of quiet standing are then not sufﬁcient to sta- forwards. They are larger in soleus than in gastroc- antagonists and synergists operating at other joints. Reﬂex responses in ankle muscles The responses of ankle muscles have been investi- Medium-latency responses (M2) gated extensively, and conﬂicting results have been obtained due to different experimental approaches: M2 responses occur with a latency of ∼80 ms in the (i) perturbations produced by rotating the platform soleus and tibialis anterior (Fig. Several lines of evidence translating the platform (backward–forward), or by indicate that they are mediated through a muscle applying brisk acceleration impulses during stance group II spinal pathway without a signiﬁcant con- on a treadmill, all stimuli that are unusual in real tribution from cutaneous afferents from the foot life,butconvenienttoanalysethedifferentresponses (seepp. Thepictureis,however,complicated producedbytheperturbation;(ii)differentvelocities by three features, which may explain the discrep- of displacement; and (iii) responses in ankle exten- ancies between the different groups: (i) the larger sors recorded either in the gastrocnemius medialis the short-latency response the smaller the medium- or in the soleus. The issue is further complicated by latency response in the triceps surae, because of discrepancies in the deﬁnition of the various EMG an interaction between group Ia and group II exci- responses according to their latencies, and it is not tations at motoneuronal and interneuronal levels always clear whether latencies were measured to (see p. It is now agreed that, short- and medium-latency responses are slightly provided the ankle rotation is fast enough (>40◦ longer after translation than after rotation (Schiep- s−1,Diener et al. Long-latency responses in the stretched Usually, there is no short-latency response in the tib- muscle (M3) ialis anterior (see p. The ability of these responses to be mod- a monosynaptic response, and several other argu- iﬁed by will (Melvill Jones & Watt, 1971a, b) sug- ments indicate that it can be attributed to the short- gests that they are, in part, voluntary reactions. In further studies more rapid stretches were shown to produce gas- The different responses described above are not trocnemius medialis responses at earlier latencies modiﬁed signiﬁcantly after ischaemic blockade of (95–120 ms; Nashner, Woollacott & Tuma, 1979; 73– cutaneous afferents (Diener et al. This ﬁnd- 110ms,Horak&Nashner,1986),compatiblewiththe ing does not exclude a role for cutaneous informa- medium-latency responses described above.