Physio 18 - Postural Control PDF

PHYSIO 18 – Postural control Intro. Postural control I. Stabilization of upright posture II. Main neuronal component involved in the postural control III. Postural support of voluntary movement IV. Structures in the brain/brainstem controlling posture Posture Posture is a very complex issue: it r...

PHYSIO 18 – Postural control Intro. Postural control I. Stabilization of upright posture II. Main neuronal component involved in the postural control III. Postural support of voluntary movement IV. Structures in the brain/brainstem controlling posture Posture Posture is a very complex issue: it recalls a static situation but involves both voluntary movement and automatic rhythmic activities such as locomotion. It is part of the motor program since it consists in the maintenance of equilibrium both in static and dynamic phases. It’s indeed more challenging to maintain posture during movement, like in sports. Definition: Posture can be defined as an actively stabilized definite orientation of the body and its segment in space and in relation to each other maintained for a prolonged time. Posture is related to -​ our anatomy, the evolution of the skeleton from the quadrupedal locomotion to the bipedal locomotion, followed by the acquisition of the upright position; -​ it requires a cost in terms of muscle activation and distribution of muscle activity, so cannot rely only on the skeleton. Staying in an upright position can be considered an evolutionary gain since it has resulted in the independence of the upper limbs allwoong them to be used for other things, like dexterity. Posture is the synergic activity of muscles that keeps a stable position with respect to the gravity force. This position is kept by the different body parts by -​ egocentric coordinates: each part relative to the other parts – Proprioception information and vision. -​ exocentric coordinates: each part relative to the external environment – Monitor with visual system. -​ geocentric coordinates: each part relative to the gravity force – Monitor with the vestibule. The brain must refer to the body position in all the 3 coordinates to balance the limbs position. Postural control aims are: ​ Maintaining the appropriate alignment of body segments ​ Maintaining the appropriate relationship between the body and the environment ​ Establishing a vertical orientation to counteract the forces of gravity, when needed (during sleep is not needed as we lay down). ​ Creating a reference frame for perception and action with respect to the external environment Function: Postural control allows us (as humans) to: -​ Maintain a variety of positions/postures - possible to decide the position, there is a huge number of degrees of freedom. -​ Loose and regain midline displacements - response to perturbation, postural actions are used to not lose the posture. -​ Provide a reference frame and stability to move eyes, head, and or limbs  -​ Carry out dual tasks (cognitive tasks while moving). With ageing this is progressively loss, this is because there is a loss of avert attention which would normally be needed to walk and do something else at the same time (just talking on the phone for instance), this could lead them to fall. -​ Function independently within a changing environment Stability Postural stability consists in maintaining the body center of mass within boundaries of space referred to as “stability limits”, i.e. boundaries of an area of space in which the body can maintain its position without changing its base of support. If the body goes outside the area, the stability is lost, otherwise if it is maintained in the area the stability is maintained (for example oscillating and feeling to fall down, stability in this case can be lost). Stability limits are not fixed but may change according to the task, the individual and the environment – if on a horizontal plane or in an oblique one. The postural action are the motor skills that emerge from the interaction between the individual, the environment, and the task. I.​ Postural control 1.​ Stabilization of upright posture In a quadrupedal situation, the center of mass (CM) lies in the center of the rectangle that defines the stability area. When moving forward, backwards or laterally, if the center of mass falls within the stability area, nothing happens but if it lies out, then the quadrupedal loses stability. In the bipedal posture, the stability limits are way narrower since the CM (centre of mass) falls in a tiny area: between the lower limbs, depending on the distance between the feet (increasing/decreasing the base of support by adducting the limbs). (Left) Postural control system is keeping the projecting of the centre of mass within the limits of the supporting area. (Right) Postural stability in the frontal plane and in the sagittal plane is maintained by two different systems. The skeletal structure does not allow to keep the static upright position without muscular action. To avoid falling under the body weight the action of the force of gravity has to be counterbalanced by the action of the antigravitary muscles onto all the joints exposed to the force of gravity. Antigravitary muscles are generally extensors, with some exception referred preferentially to the upper limb and in the face (jaw elevators), in fact the antigravitary role depends on the position of the arm and forearm. -​ For the inferior limb o​ gluteus, o​ quadriceps. o​ Soleus-gastrocnemius, o​ flexor digitorum longus. -​ Superior limb o​ elbow flexor/extensors o​ wrist flexors. -​ Cervico-dorsal o​ dorsal muscles, o​ neck vertebral muscles o​ masticatory muscles (masseter). The lateral corticospinal tract innervates everyone, the ventral one goes mainly to the proximal and the epaxial, which happen to be mainly the proximal antigravitary muscles, being mainly the extensors. This is why professor Dellavia divided the groups in medial being the one for the extensors, this can only be applied when referring to this specific situation. 2.​ Muscle tone It takes a while to organise the synergy without even moving, indeed the babies cannot be in the upright position, the system takes approximately 1 year to mature. The control of the upright or sitting position is «low cost», because the segments are disposed along the vertical axis the body weight is released on the ground through the skeleton. There is a constant tonic effort in terms of muscle force, the vertical axis is the best to maintain the lowest cost of muscles. The role of the postural actions is to maintain the segments along the vertical axis in order to counterbalance the tangential component of the gravity force (while the radial, the major one, is counterbalanced by the bone). It is a low cost action only when an accurate control avoids great deviations from the vertical axis. The greater the variations of the vertical disposition, the greater is the effort needed by the muscles to correct them. The lower limbs are the first to be perturbed, that are also those mainly involved in the stabilisation of posture. 3.​ Oscillation in the upright position During the upright position the body undergoes slight oscillation on the sagittal and frontal plane. These are minimal, but during locomotion the perturbations are much greater. In postural control of locomotion, the latero-lateral perturbations are not under control of the neural locomotor centres as the nervous system has to control the centre of mass position which is continuously changing in the anteroposterior direction. Locomotor centre is not aware of the latero-lateral oscillation, as it is thought in order to have the walking stages of the steps, so in an anteroposterior direction. The brain takes into reference the lower limb to know the location of these with respect to the ground (to have an idea about the orientation). Taking into consideration two muscles, the tibialis anterior (TA) and the gastrocnemius (GC). The considered perfect alignment is the lower limbs being orthogonal to the ground. In this situation the TA and GC are equal in terms of tone. If there is a forward oscillation, the GC will be stretched: this activates the stretch reflex, in which the Ia fibres are activated and the muscle is immediately contracted to obtain the stabilisation, the opposite happens if the oscillation is in the posterior direction. The stretch of the muscle signals the direction of the oscillation. The default setting is more in the GC because the weight of the body must be maintained up. If the angle is not orthogonal (standing obliquely), the value of tone of the GC increases, the GC and the TA have to control the possible oscillations, this time in a different setting. The muscles of the lower limbs are the first ones to detect the oscillation. The actual situation involves all the muscles of the lower limbs, being concomitantly activated when standing. There is a different time course of activation of the muscles, there is not a simultaneous activation. Given an oscillation, a stretch reflex will be provoked to different muscles, depending on the type of oscillation. The stretch reflex is activated differently in multiple muscles depending on the orientation of the oscillation plus the stretch reflex receives an information from the muscle of a specific joint, as a response then acts on the muscles connected to the same joint -> there is an integration of all the information. Postural control: default muscle tone to maintain the upright position with the floor 90 degrees or tilted at different degrees, on top of this also the oscillations (which are constant and continuous) must be counterbalanced. A corrective motor response organized in the distal to proximal sequence When the oscillation leads to the movement of the CM away from the stability area, an activity is performed to re-establish the posture, hence a step, which is the only way to not fall under the activity of gravitational force.  II.​ Main neural component involved in the postural control Postural control in humans is a very complex function requiring the coordinate action of: sensory systems, skeletal muscles and central nervous structures. 1.​ Sensory systems The sensory information involved in the postural control are: ​ proprioception (muscle spindle in particular – stretch sensory fibres), ​ vestibular system ​ the visual system. Sometimes cutaneous afferents behave as proprioceptors (as those overlapping the joint, the skin stretches). They are the deep receptors in the skin overlapping the joint. The anterior portion of the foot can give some proprioceptive information, indeed the change in pressure is a good indicator to the change of the CM. Proprioception devoted to the control of posture: -​ pressure receptors on the basal surface of the foot -​ muscle spindle. These two are connected directly to the spinal cord. The other two (vision and vestibular system) are located in the head they need to reach the spinal cord through the brainstem, indeed using the descending system of the brainstem. Analysing the postural actions in light of the sensory stimuli, it is possible to state that three are the main components of the nervous system acting in postural control: ​ spinal, ​ vestibular ​ visual. It is important to know which ones are doing what and when (when are they activated, how are they modulated in the different situations, who is tonically active and who is activated only in some specific conditions). Aside these three main actors there are also: ​ straightening reactions ​ anticipatory postural adjustments These are postural actions that are part of the motor program related to the ongoing (final) movement, these two are organised with the contribution of the 3 components in a motor program (they are part of the motor program) involvement of the cortex. 2.​ Spinal component of the postural control The sensory inputs feeding the spinal component are: -​ Proprioceptors in o​ muscle (muscle spindles), o​ tendons (Golgi tendon organ) o​ joints (joint receptors) -​ Skin receptors o​ Meissner corpuscles o​ Merkel disks, o​ Ruffini and Pacini receptors. Somatic sensory inputs are for the egocentric component, mainly the proprioceptors but also the skin receptors. The spinal sensory inputs are very important for the regulation of the oscillation. Without these (in conditions such as ischemia), there would be an altered activation of the muscles (GC and TA for instance): in the case of ischemia, if the head would move then the CM would as well the ankle angular displacement would be increased the oscillations would result considerably. Different spinal reflexes act in muscular control, they help the system. The brain uses the reflexes as delegates. -​ Stretch reflex -​ Tonic neck reflexes -​ Crossed extensor reflex -​ Straightening reactions -​ Placing reactions a.​ Stretch reflex Increasing muscle stiffness causes the stretch reflex firing. The stretch reflex is the first degree of stabilization. In static posture: response to perturbation inducing muscle stretch. (Safe mechanism. After the correction the segments come back to the vertical line. No stretch, no Ia discharge, stop adjustment). Posterior muscles stretch reflex force exerted in opposite direction (backward) regain the vertical alignment. Stretch reflex in static posture During movement: the nervous system controls the gain of the stretch reflex depending on the motor act to be executed (One muscle can be used as postural in a given motor act or as a prime mover in another). Thinking about football players, they use the lower limb muscles sometimes as postural and other times as prime movers to kick the ball. Different types of fibers are active depending on the movement: in order to keep the erect positions, the γ static fibers are more active than the γ dynamic. The gain of the reflex depends on which is the movement that has to be performed: the muscle spindles in some specific muscles need sometimes to be tuned both for movement and posture. The postural tone is finely controlled by the cerebellum and by the brainstem. b.​ Tonic neck reflexes They can be studied by isolating the effect of the activation of proprioceptors of the neck by head movements. To study the sole effect of the neck receptors there is the need to inactivate the vestibular component. The labyrinth does not recognize if only the neck is moving or if it is moving with the body. The afference origin are the cervical paravertebral receptors induced by head louvements relative to the trunk. ​ Symmetric tonic reflexes: (symmetric because both limbs are used) the decerebrate animal below with a labyrinth lesion performs passive flexion posterior and anterior. -​ During posterior passive flexion the cat extends the forelimbs and flexes the hindlimbs. o​ Increase in extensor tone anterior limbs o​ Decrease in extensor tone posterior limbs -​ During anterior passive flexion the cat flexes the forelimb and extends the hindlimbs. o​ Decrease in extensor tone anterior limbs o​ Increase in extensor tone posterior limbs This means that the proprioceptors of the neck induce a change in the body asset which is not immediate. Asymmetric tonic reflexes: (asymmetric because the two sides of the body act in contrast) considering head rotation, -​ head rotation on the right activates the extensors on the right limb and flexion on the left limb. o​ Increase in extensor tone right limb o​ Increase in flexor tone left limb -​ The opposite happens in the head rotation on the left: activation the flexors of the right limbs and the extension on the left limb o​ Increase in flexor tone in right limb o​ Increase in extensor tone in left limb Remember that the body follows the head. These asymmetric reflexes can be observed in neonates and adults only when a strict control of posture is required Therefore, by getting rid of the vestibule, movements of the neck alone are able to induce complete modification of the whole body (like in the cat seen previously and the head rotation). Neck proprioceptors are able to change the whole body alignment if the head is moved in certain positions. c.​ Cross extensor reflex: It is paired with the withdrawal reflex. For example, when there is a withdrawal reflex on the right leg there will be a withdrawal of the whole limb but the other limb will have an increase in the extensors’ activity in order to not fall down. It maintains double body weight when the flexor reflex is activated by the nociceptors. d.​ Support positive reaction: There is an increased extensor tone in a limb following the plantar stimulation. There are cutaneous and muscular afferents coming from the interosseous muscles of the foot (digit opening) that increase the extensor tone. It is a sort of monitor of perturbation, an antigravitary stimulus. e.​ Placing reaction: The tactile stimulation of the dorsal surface of the foot and the limb triggers a flexion-extension of the limb in order to bypass the obstacle. It can be seen in the decerebrate cat below. Touch, flexion, extension The spinal component of posture control is fed by somatic stimuli (especially proprioception and cutaneous reception) and exerts a function in the posture control by triggering reflexes (the ones seen above). 3.​ Vestibular component of posture control The sensory inputs feeding the vestibular component are: ​ Maculae: are active when perturbations occur on postural stability. They are sensitive to the linear acceleration/velocity applied to the body. They move the otholites. Two forces can be distinguished: -​ Forces acting on same direction of gravity force (orthogonal): by increasing the action of macular receptors there is the homogenous distribution of muscular tone onto antigravitary muscles Looking at the monkey below, it is sitting on a chair that goes up and down orthogonal to the ground. Because there is this machine that pulls down the seat, the acceleration is increased and a reaction is induced in the soleus (antigravity). As can be seen below, the first trial remains the same and in the following trials there is an adaptation following a constant acceleration. The vestibule reacts more to changes rather than to something that is constant. -​ Forces acting on different directions of the force of gravity vectorial component: there is a modulation of the macular discharges according to the direction that leads to an action on muscle tone aimed at contrasting the force acting in perturbation. ​ Semicircular canals: they give the most relevant information to move the eyes. They give information that is used by the brain in order to coordinate head and eye movement rather than organizing the rest of the posture. The vestibular reflexes can be studied by isolating the effect of the activation of the maculae by head/body movements. To study the sole effect of the maculae there is the need to inactivate the proprioceptors of the neck. These reflexes are the opposite of the spinal reflexes: there is a flexion of the limb in the direction of the head rotation and an extension of the other limb. -​ Rotating the head clockwise (blocking the cervical vertebrae) induces a vestibular reflex resulting in the flexion of the right limb and the extension of the left limb. -​ Rotating the neck clockwise (blocking the head) induces the neck proprioceptive reflex resulting in the extension of the right limb and flexion of the left one. Neck and vestibular reflexes are antagonists only when the head is moving and the body is not moving. If the head for example turns right, none of the reflexes is elicited: the result is stability. The vestibular component is active both during rotation and linear movement. The head alone is able to apply a postural adjustment going down to the gastrocnemius muscle. The vestibular component of postural control can be compensated by other sources of information, such as proprioceptive and visual. 4.​ Visual component of postural control It comes from retinal information that goes to the occipital and the parietal cortexes. In the posterior parietal there is the body schema. Closing the eyes impairs the stability, and this can be seen in the graphs below: the person stands on a platform and the movements are recorded while his eyes are open and closed. This instability can also be seen when a person that wears glasses does not wear them (with eyes open or closed). In Parkinson patients the impaired locomotion is not only due to the fact that they have problems in organising the movement due to lack of dopamine but also because they cannot organize the proper posture. The locomotion of a neonate is similar. In the picture below, -​ the patient starts with eye closed: there are many oscillations -​ Then a visual target is placed near (2m) and the oscillations are much smaller. -​ With a visual target 200 m away the oscillations increase. -​ In the last scenario, a frame is placed 2 m away from the patient and the oscillations are less because the eye catches something that gives stability, such as the frame. The same concept can be applied when babies start walking: the parent put their hands in front of the baby without touching him in order to give him a point of stability. In the experiment below, the animal in the center sees a rotating image that creates the illusion of movement. The animal needs to organize the postural reactions following the changing environment. For example, after an impressive optic illusion there is the feeling of loss of stability.  III.​ Postural support of voluntary movement In order to have, for example, a lower limb flexion there is the need to reorganize the entire posture. The postural change is organized before the action, and indeed they are called anticipatory postural reactions. A person is pulling on a handle: there is an anticipatory activation of the gastrocnemius before the biceps because in order to pull the handle there is the need of stability. Pulling the handle is a potential risk to the stability. Learning how to perform a movement means learning the posture and using the prime mover program: the cortex together with the program adds the anticipatory postural reaction. It is a feedforward programming. There are two types of control: -​ Feedback control: done by vestibular, visual and somatosensory systems that process the information -​ Feedforward control: done by the muscular system, starting from the body position, the motor program firstly goes with the anticipatory postural reaction and then there is the voluntary movement. Experience and expectation have a role in posture: -​ a rapid postural response in the gastrocnemius muscle occurs progressively earlier with repeated trials -​ the large contraction of gastrocnemius evoked by an unexpectedly tilted platform is attenuated after a few trials. Postural control can be adapted to suit specific behaviours. The anticipatory adjustment adapts to the behavioral context. For example, in the image on the left there is the subject with just a handle and therefore a perturbation will need an action of the gastrocnemius. In the image on the right, there is a support for the handle and the reflex in the gastrocnemius is not needed. RECAP Regarding the spinal component (which refers to the somatic sensory system) included: reflexes and postural actions (stretch reflex, neck reflexes, symmetric and asymmetric responses, crossed extensor, placement response …) and the respective circuitry, fed by information coming from the somatosensory system: the circuits are fundamental units that build up complex postural control. The visual and vestibular systems’ involvement were analyzed too. The vestibular system provides information regarding geocentric coordinates (position of the body with respect to gravity) and on linear and angular acceleration. The vestibular system plays a role during movement, or when the perturbations are fast – this is because it adapts to stable conditions and promptly reacts in conditions characterized by a change in acceleration or velocity. In order to organize posture – static or dynamic (for movement) – the vestibular component must be added to the spinal one: in postural reactions, action is not only recorded in muscles that are directly involved in the stretch (activated by spinal cord stretch reflexes, which act on single joints), but also in muscles that are well above, thanks to the vestibular system’s contribution. Indeed, the postural response is a multi-joint response. Interaction between neck and vestibular reflexes is needed to set up the correct posture in conditions in which the head alone is being moved and in conditions in which the whole body is being moved. Taking all components into account, the following model is elaborated The elements involved in neural control of posture are summarized above: on the left, the sensory inputs and sensory systems. The comparator analyzes the designed posture and compares it to: -​ the actual, ongoing posture using feedback control, -​ the expected posture using feed-forward control. Thus, posture, in static but mostly dynamic conditions is part of the motor program: starting from the set up of the system via a feed-forward prediction, needed for anticipatory postural reactions; continuing with the monitoring of the center of mass during movement, together with incoming perturbations and ongoing errors using feedback control. IV.​ Structures in the brain/brainstem controlling posture The structures in the brain/brainstem that control posture are -​ the cortex, -​ the cerebellum, -​ the reticular formation -​ the extrapyramidal system. The integrity of the brainstem centers is necessary for postural control. Depending on the level of cut of the brainstem, there are different consequences in the distribution of the muscle tone. There are postures that are indicating immediately the possible level of lesion of the brainstem. These postures are the result of disconnections of the system from the structures (cerebellum and midbrain in particular) that control the postural control releasing a stereotyped posture. 1.​ Structures involved in the comparator system Apart from the vestibulospinal system and the spinal reflexes, the motor cortex and corticospinal tract, other structures involved in the comparator system must be investigated. To do this, systems descending onto the spinal cord are either lesioned or structures excluded to understand the relevancy of their role in posture: by analyzing deficits or altered parameters, conclusions regarding the excluded structure can be made. Three possible situations can be analyzed by disconnecting portions of CNS at different levels. 1.​ Despinalization - the spinal cord is disconnected from the brainstem and the brain (spinal model): The result is the animal model cannot maintain body weight. Thus, the spinal system is able to control, but cannot distribute the muscle tone needed to maintain weight. This is coherent the muscular effort needed to keep the upright position – bones’ anatomical disposition is not sufficient. If the spinal cord is not sufficient to maintain body weight, this must be achieved by the structures above. The brainstem is the passageway for the upper centers’ descending systems (corticospinal, corticobulbar), originates descending systems (medial and lateral) and is the entrance and exit to and from other structures (such as the cerebellum, considered the posterior enlargement of the brainstem given its relationship with the brainstem via the cerebellar peduncles). 2.​ The animal is decerebrated between the inferior and superior colliculi (decerebrated model): it is able to maintain body weight but is unable to perform postural adjustments. 3.​ The animal is decerebrated above the superior colliculi: correct reflexes are exhibited. Thus, centers controlling postural adjustments are either in the brainstem, pass through the brainstem, or both: indeed the vestibular system and its nuclei, involved in distributing tone, are in the brainstem and the medio-lateral vestibulospinal systems originate from the brainstem. The conclusions obtained from the decerebrated model confirm this involvement. Decorticate and decerebrate posture: a passive head movement induces different postures, mainly involving the upper limbs. The differences are due to different sites of lesion (upper midbrain vs upper pontine damage). In the case of intercollicular lesions, hypertonus is observed: ( muscle tone is distributed, but too extensively) an overextension, due to excessive activation of antigravitary muscles is observed. In this lesion the control defining the distribution of muscle tone is bypassed. By making a section caudal to the vestibular nuclei, hypertonus is abolished: The centers controlling the tone of the antigravitary miscles are situated in the brainstem. The center is not the one caudal to the vestibular but are: -​ bulbopontine section of the reticular formation: -​ vestibular nuclei Both structures act on α motor neurons, modulating their excitability and the reflexes. This is a model representing the levels of sections (in pink) and structures of the brainstem and brain. An intercollicular section (lesion below vestibular nuclei) results in hypertonus. Vestibular proprioceptive afferents act on the lateral vestibular nuclei, the nuclei of Deiters, (reaching the lumbar enlargement). The reticular formation is divided into two: -​ the portion that has an inhibitory effect goes down through the reticulospinal system -​ the portion that has an excitatory effect is aside the lateral vestibular nuclei and is fed by this nuclei. Thus, the excitatory system is made of the lateral vestibulo-spinal system and the exciting portion of the reticulospinal. A different part of the reticulospinal system has an inhibitory effect on motor neurons. The inhibitory portion is fed by upper centers (prosencephalon and basal ganglia) and by the cerebellum. An intercollicular section interrupts the connection between the prosencephalon and basal ganglia with the inhibitory branch of the reticular formation only. The excitatory components are completely released: vestibular and proprioceptive afferents only increase their excitatory effect: hypertonus is the default condition. The contribution of proprioceptive afferents is analyzed by cutting the posterior roots, in the intercollicular section. The reduction of the stiffness is irrelevant: the activity of the 1a fiber (proprioception) is therefore irrelevant in default muscle tone distribution: the spinal component of the vestibular system is responsible for the corrections of posture, rather than of setting up the adequate basic conditions. If the section is below the vestibular nuclei, the vestibulospinal and the reticulospinal inhibitory and excitatory portions are all cut and the tonus is abolished entirely (though even with more a more upper section, the intercollicular section, the inhibitory portion is abolished). Sections of the cerebellum result in a furtherly increased excitation: indeed, if a section between the colliculi is made and the cerebellum is excluded by cutting connections with the paleocerebellum (the spinal part of the cerebellum), the tonus is increased even more, resulting in a condition of opisthotonus. Figure A – decerebrate cat; figure B – decerebrate cat, cerebellum disconnected. RECAP When the spinal cord is disconnected, body weight cannot be maintained: basic postural muscle tone distribution must come from above and does not depend on internal circuitry at spinal cord level. To understand at which level in the brainstem muscle tone is controlled, cuts are performed at two levels: -​ in between inferior and superior colliculi -​ above the superior colliculi. When the cut is performed above the superior colliculi, nothing relevant happens: body weight is maintained as well as reflexes. Thus, the circuitry involved in muscle tone distribution for the maintenance of posture is in the brainstem – it is composed of: vestibular nuclei, reticular formation and cerebellum. The lateral vestibulospinal tract and the component of the reticulospinal system excited by the vestibulospinal system and excitatory on spinal cord elements (α motor neurons, gamma motor neurons, exciting antigravitary muscles) form the excitatory system arranging muscle tone. An intercollicular section results in hypertonus, interrupting the action of the reticulospinal inhibitory system on the motor neurons of the spinal cord: this system, given the connections, is controlled by upper centers (prosencephalon and basal ganglia). The excitatory system’s activity may depend on its connections with proprioceptive afferents: this aspect is investigated. The proprioceptive system is irrelevant in the maintenance of muscle tone, given that by cutting the dorsal roots, no relevant decrease in hypertonus’ stiffness is observed. The cerebellum also has connections with the brainstem: being fed by proprioceptive and vestibular inputs, it must be addressed. If the cerebellum is excluded, hypertonus is worsened, resulting in opisthotonus. Thus, in the hypertonus situation the action of the inhibitory reticular formation is decreased but not completely abolished: to completely abolish it the cerebellum must be excluded. The condition given by intercollicular cut and cerebellar exclusion is the worst one resulting in the most excessive muscle tone. Essentially, if the brainstem is left to exert its function on the spinal cord autonomously, the excitatory descending system is default. Normally, this default excitation is refined by the inhibitory system. 2.​ The cerebellum in motor refinement and motor learning Adaptive postural control requires an intact cerebellum, because the cerebellum is involved in movement control. Postural adjustments can be learned, and expectations refine a response which is much finer than that of unexpected perturbations. On the left: a rapid postural response in the gastrocnemius muscle occurs progressively earlier with repeated trials. On the right: the large contraction of gastrocnemius evoked by an unexpectedly tilted platform is attenuated after a few trials: atrophy of the anterior cerebellar lobe makes these improvements impossible. The cerebellum is involved in motor refinement and motor learning. Motor development of the infant and young child At birth, distribution of antigravitary muscle tone is not adequate. At around 2 months, the child starts keeping the head up At 4 months, the child is able to sit down, maintaining the trunk, head and neck upright. From 9 months on, the child keeps body weight. In order to walk properly, more than 15 months are needed. V.​ Summary Posture is an actively stabilized definite orientation of the body and its segments in space and in relation to each other. Postural control systems minimize deflections of the body from desirable orientation. Postural control systems are able to stabilize ≠ postures. Multimodal sensory inputs – somatosensory, visual and vestibular are used for postural control. To maintain a desirable posture, a family of adjustments is needed. Postural adjustments are necessary for all the motor tasks and need to be integrated with voluntary movements. Postural adjustments are achieved by 2 major mechnisms: -​ The compensortory or feedback mechanisms are artivated by the sensory events following a loss of a desirable posture (compensatory postural adjustments)  -​ Anticipatory or feed-forward mechanisms that predict disturbances and produce preprogramed responses that maintain stability (anticipatory postural adjustments) Some of the compensatory postural adjustments are innate, while others have to be acquired by motor learning. Anticipatory postural adjustments must be learned, and then they operate automatically. Postural control is adaptative, the shape of postural adjustment depends on behavioural context. All levels of the CNS are involved in postural control. The integrity of the: -​ brainstem centres are necessary for the generation of compensatory postural adjustments. -​ highest levels of the CNS, including the motor areas of the cerebral cortex is necessary for anticipatory postural adjustments. -​ cerebellum is necessary for adaptative postural control.