|by M. A. Fusco|
Living organisms are
constantly subjected to the force of gravity and to the
centrifugal force, acting clockwise in the southern hemisphere
and on the opposite way in the northern hemisphere. The whole
motory system of the living organisms is organized to counteract the
effects of this force, both when they stay and when they move. From a
merely physiologic point of view posture is a set of mechanisms such
as the minimizing of the energetic expenditure, the increasing of
movement's speed, force and precision that contribute all together
to carry out activities, that are effective from a biological point of
Therefore, posture is the specific somatic attitude of the living specie, it concerns the body as a single unit and under different conditions. Posture is the synthesis of complex mechanisms of correlation and neuromuscular integration of different impulses coming from the periphery of the body, elaborated and analyzed by the CNS and finally carried into effect by the whole body.
The expression "postural control" defines a set of static and dynamic mechanisms conditioning the spatial arrangement of the body and of its mobile segments and their mutual relationships, acting in order to maintain a specific orientation according to the gravity force.
Sir Charles Sherrington was one of the neurologists who most contributed to the understanding of the most important neuromuscular relationship that allows to maintain a correct posture.
"The upright posture- he wrote- is a wide and complex postural reaction allowed by the contraction of the anti gravitational muscles that in this way counterbalance the weight of the body, that otherwise bending its joints would cause a fall down".
Sherrington proved the hyper activity of the myotatic reflexes in the decerebellated animal. (Creed et al., 1932) The separation of the cranial cerebral structures through a surgical cut along a transversal plane passing through the culliculi, the so called decerebration, sets the facilitating activity of the brain stem free from descending inhibitory influences and gives rise to a condition of excessive contraction of the anti gravitational muscles causing an exasperation of the normal posture. Furthermore a selective surgical cut of non anti gravitational muscular and cutaneous nerves and of their dorsal roots demonstrates that the observed tonic contraction depends above all on afferent information coming from the anti gravitational muscles, able to trigger myotatic tonic reflexes.
On the other side, a limb deprived of its proprioceptive input and so of the corresponding myotatic reaction through a surgical cut of its dorsal roots is no more able to give rise to postural reactions, even if the muscular strength appears to be still able to support the weight of the body, when non myotatic reflexes such as a stretching crossed reflex are triggered. Even if segmentary reflex arches are still uninjured, this is not enough to support and allow the upright posture. In fact the postural tone of a spinal animal decreases dramatically after a cut of the spinal cord at C1 level, and increases too much in the decerebellated animal, whit a surgical cut's plane passing through the culliculi. This demonstrates that beside a spinal component an over spinal component is also needed to maintain the postural tone.
Spinal components of the postural tone:
Spinal components of the postural tone are divided into afferent and efferent systems.
Afferent systems consist of sensitive fibers involved in the indirect control of the posture and heaving following characteristics:
1. the suitable stimulus for the sensitive nerve endings should be the one coming from gravity force and/or from other forces acting on different body's parts;
2. when this stimulus acts the afferent triggering must continue;
3. central connections of these afferent pathways should give rise to a facilitation of the motory nuclei of anti gravitational muscles.
Many receptors fulfill the above mentioned characteristics, such as proprioceptors, exteroceptors and sometimes enteroceptors.
This component concerns above all neuromuscular spindles and Golgi's musculo-tendineous corpuscles.
The terms proprioceptive sensibility was coined by Sherrington in 1906 and it indicates the set of nervous signals originated within the organism during the movement. According to this definition suitable stimuli are conveyed from the organism to specific receptors located within the organism that's why they are called proprioreceptors ( that means own receptors ). Almost 100 years after this definition was coined, it is still sound, even if the concept of sensitive proprioceptivity gained new specific meanings. In particular, it concerns not only the two main proprioreceptors that can be observed in mammals, such as neuromuscular spindles and Golgi's musculo-tendineous corpuscles, but also receptors located within muscles and joints that collect tactile, pressure or sore stimuli. The gathered information is conveyed to the CNS, where it is processed in order to program and to control the statics and as consequence the movement. The main and most studied proprioceptors are the neuromuscular spindles and the Golgi's musculotendineous corpuscles. Both receptors belong to the class of stretching receptors because they are sensitive to physical variations caused by stretching muscles, but because of their different anatomic location within the muscular tissue, neuromuscular spindles provide to the CNS information on the length of the muscle, whereas the Golgi's musculo-tendineous corpuscles provide information about the degree of tension expressed by the muscle.
Neuromuscular spindles are little organs and are present within all skeletal muscles of crustaceans, amphibians, reptiles, birds and mammals. They are arranged parallelly with striated muscular fibers, there are no muscular spindles within the mimic facial muscles, within the stirrup-bone, within the tensor of the drum and within the under hyoid muscles.
The neuromuscular spindles function in the following way:
* the passive stretching of a skeletal muscle increases the frequency of the afferent triggering which is proportional to the variation of the lengthening; (Kuffler et al., 1951)
* the adjustment is very slow, so if the muscle through a passive stretching passes from an initial length to a new and bigger one, the afferent triggering gets, after a short phase of increasing of frequency enhanced by the dynamic component of the lengthening, a new level that doesn't change within the following hours; (Granit , 1955)
* the contraction of the skeletal muscle enhances a reduction or a stop of the neuromuscular spindles triggering. This can be observed above all during a quick contraction of a muscle enhanced by a synchronous stimulation of its motory nerve, while the stimulation is carried on it is possible to observe a pause of the afferent neuromuscular spindles triggering caused by the increasing of the tension; the increased tension is higher when the muscle can shorten (isotonic contraction);
* the contraction of a fiber of the neuromuscular spindles enhanced by activating the gamma efferent fibers gives rise to an acceleration of the triggering of the nerve endings which are connected to it. (Leksell, 1945)
Golgi's musculo-tendineous corpuscles are located in ligaments and musculo-tendineous joints, they are arranged in a set and their task is to record tension. Their adaptation's threshold and frequency are higher than the ones of neuromuscular spindles, even if in the case of active contractions, their triggering occurs with tension of some tenth of grams. (Houk e Henneman, 1967) They have not an efferent innervation.
Removed some doubts about the possibility to use the collected phasic data about the tonic postural condition, we can claim that the three classes of proprioceptors belonging to stretching muscles fulfill following functions:
1. the ring-like and spiral neuromuscular spindles afferences facilitate the contraction of the stretching muscles;
2. flower-like nerve endings inhibit ipsilateral stretching muscles;
3. the triggering of Golgi's musculo-tendineous corpuscles inhibits the motory nuclei of ipsilateral and synergic muscles;
4. every afference exerts reciprocal effects on controlateral muscles.
The ring-like and spiral nerve endings connected with fibers Ia represent the most important factor which supports stretching reflexes and postural tone. Nevertheless the total central effect of the self facilitation coming from the deefferented anti gravitational muscles that is muscles deprived of the spindles moving control, occurs only with low tension's level, in the gastrocnemius of a cat, it corresponds to 5-10% of the highest tension expressed by the muscle.
Articular, cutaneous and visceral components.
Other sensitive nerve endings located within or nearby joints consist of Pacini's corpuscles that are located also within muscles, of Ruffini's corpuscles and of some other free nerve endings. The stimulation of articular nerves of the spinal animal gives rise to polysynaptic triggers on the ventral roots according to a facilitating pattern on flexor muscles and to an inhibiting pattern on extensor muscles. (Beswick et al. 1955)
Their strategic location and the tone of their trigger entitle us to guess that they can influence the posture.
Furthermore cutaneous receptors can give rise to postural reactions, notwithstanding their prevalently phasic reaction, the central facilitator effects on flexor nuclei (Creed et al. 1932) and the proof that the decerebrated rigidity doesn't change after removing the cutis of the preparations (Sherrington , 1910). The stretching of the skin wrapping the extensor muscle gives rise to a contraction of the concerned muscle, both in cats and in dogs, and sometimes also in human beings. (Riddoch e Buzzard, 1921)
Finally some researchers such as Miller and Waud in 1925 and Moody and van Nuys in 1940 hypothesized that enteroceptors such as Pacini's corpuscles and some other tension and pressure visceral receptors, activated by the displacement of mesenteric and retro peritoneal structures caused by changes of the animal's posture, contribute to arrange the posture.
Even in a spastic man it is possible to observe a close relationship between the vesicle filling and the amount of flexor and extensor spasms.
Propriospinal component and component of Shiff-Sherrington.
The sensitive information coming in throughout the dorsal root produces indirect effects not only on its specific segment, but it also causes reactions at distance that are involved in the limbs coordination.
Among the segmental afferent pathways that can influence at distance the excitability of the motory anti gravitational nuclei, the most important role is played by the so called "neck's proprioceptors".
In fishes, whose head moves jointly with the trunk, labyrinth and organ of the lateral line suffice to provide suitable afferences for postural reflexes. But in living organisms, whose head moves independently from the trunk, available information about gravitational vector acting on the head is no more sufficient to inform the control centers about the body's spatial arrangement and for this reason it is also necessary to convey to these centers information about the position of the head compared with the one of the trunk. This afferent information coming from articular receptors and tension receptors localized within atlas-occipital and atlas-axial joints are responsible for postural indirect adaptations that can be observed in an animal deprived of its labyrinth, after a rotation or a forward and backward flexion of the head.
Propriospinal components can also inhibit the postural tone.
Efferent systems influence the final outcome of the postural interaction that is determined by combining activated motory muscles and pattern of motoneuronal information gathered within each nucleus. In quadruped animals the above mentioned process occurs through a contraction of the extending musculature of the limbs, through a curving of the dorsum and and upwards displacement of the head and of the tail. Abdominal muscles, classifiable as flexor muscles and masticator muscles, influence the posture, too. In human beings facial muscles and elevator muscle of lids have some anti gravitational tone, too.
The contraction of flexor muscles depends prevalently on upper limbs in monkeys, on wings in birds and posterior limbs in frogs; whereas in the arboreal sloth the balancing of postural integration is completely in favour of flexor muscles, so that the exaggeration of the postural arrangement in decerebellated animals causes a rolling up of the animal.
The solely feature characterizing active motory nuclei in the postural tone is represented by the innervation of anti gravitational muscles. Mechanisms through which the motoneuronal pools maintain a state of under maximal contraction of muscles connected with them, depend above all from the metabolic-functional properties of the involved motory units. Involved motoneurons, the so called tonic motoneurons, differ from other elements belonging to the same pool, because they are smaller and in preference they are accessible to the monosynaptic innervation from afferent pathways coming from ipsilateral muscles. Furthermore they are characterized by posthumous hyper polarization, low average frequency of triggering and lower speed of axonal conduction. Muscular fibers innervated by them are characterized by oxidizing metabolism, a great numbers of vessels, red fibers, low speed of contraction, high resistance against fatiguing, metabolizable at a low frequency. (Burke, 1974) So they can be considered as an effector able to produce a long lasting output, characterized by a low loss of power and perfectly compatible with the function of anti gravitational postural support.
UPPER SPINAL COMPONENTS OF POSTURAL TONE
The function of the CNS on the postural control in statics is above all to transform a set of inhomogeneous afferent impulses into a regular and long lasting activation of suitable motoneuronal nuclei. Spinal cord could achieve a similar integration by itself. For instance, a dog, some months after a thoracic interruption of the upper control on the spinal cord, can move from a sitting position to an upright position and stand on four legs for a while. As a matter of fact the postural performance of spinal animal is widely incomplete. In fact after some minutes or at most after half an hour the posterior part of the body of the spinal chronic dog breaks down gradually or suddenly. Furthermore because of plastic alterations concerning inter neuronal synaptic connections, the behavior of animals with a spinal cord chronic cut is certainly very different than the one of a healthy animal. Spinal dog or spinal cat, suddenly after a period of spinal shock, are no more able to stand, not even in the case of an increasing of the spinal cord activity through a supply of amphetamine.
Reflexes of spinal animals occur suddenly, but have a short lasting effect.
The additional activity needed in order to produce an efficient expression of potential postural patterns arises form several upper spinal sources. Upper spinal components of postural tone, vestibule, reticular and cerebellar, have been studied firstly in the decerebelated animal, where a surgical cut passing through collicoli leaves out pre encephalic influences, but doesn't injure the brain stem influences. At present these components are studied also in the healthy animal by means of micro stimulation and selective micro lesion of single structures. (Schwindt, 1981)
Vestibular apparatus is an important sensitive system for maintaining posture and body's balance during the performance of voluntary movements. Information comes from 5 different equal and symmetric receptive systems hold in a cavity of the petrous portion of the temporal bone:
1.the three semicircular canals passing through crista ampullaris sense angular or rotational motion and are arranged on three mutual perpendicular planes, so that each pair gets the greatest excitation from the rotation of head on one of these three perpendicular axes;
2. Utricule and saccule, through a sensitive epithelium, the macula, provide reactions to the linear acceleration, on the horizontal and vertical plane respectively.
The plain effect of the vestibular input consists of a tonic facilitation of anti gravitational muscles, mediated by the lateral nucleus of Deiters, through anterior and lateral vestibular-spinal pathways. It is important to bear in mind that only a part of relay's inter neurons hold within the nucleus of Deiters gets excitation connections from labyrinth's organs. Remaining cells originated from vestibular-spinal pathways and neurons hold within the medial and inferior vestibular nucleus are influenced by brain stem and pre encephalic systems, both directly and through a reticular relay.
(Hoddevik et al., 1975)
Vestibular-spinal connections with the segmentary effector system occur both with alpha motoneurons and with gamma motoneurons. The contribute of the labyrinth to postural control in animal, whose head moves independently form the trunk, is constantly related with additional information coming from neck's receptors. (Frederickson et al., 1966) Therefore the final reaction to a disturbing o destabilizing situation requires an integration of the set of information coming from these two sources.
Postural control exerted by the anteromedial facilitating reticular component occurs through a strict cooperation with the vestibular control and originates from pontine back lateral reticular substance. The inhibiting component of the anterolateral portion originates from bulbar caudoventral reticular substance. Lateral and medial vestibular nuclei represent a preferential relay of the efferences of the gigantocellularis and parvicellularis reticular nuclei, whereas the superior vestibular nucleus gets connections from the reticular nucleus pontis oralis. (Hoddevik et al., 1975)
The destruction of the facilitating reticular portions causes hypotone and akinesis. On the contrary a cat whose brain stem has been deprived of descending, labyrinth and proprioceptive influences of the neck, shows a rigidity with anterior limbs partly flexed, whereas posterior limbs are maintained in low extension.
In 1927 Pollock and Davis introduced the anemic decerebellation. It consists of the ligation of basilar artery and of enclosure of the two carotids in order to achieve an ischemic inactivation of all encephalic structures around the trunk. In this preparation extending anti gravitational hypertone is really intense and gives rise to the so called opisthotonos with a backwards flexion of head and tail. The direct proof of the cerebellar component of the postural tone of the decerebellated animal was provided in 1944 by Stella. The ablation of the paleocerebellum of a decerebellated animal with a deafferented limb causes a return to rigidity of the limb that was previously in hyper tone. This rigidity presents certainly the features of the alpha hyper tone, because the gamma circuit was broken off by the deafferentation of the limb.