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INTEGRATIVE |
1 Department of Neuroscience, Karolinska Institute, SE-171 77, Stockholm, Sweden
2 Barrow Neurological Institute, Phoenix, AZ 85013, USA
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Abstract |
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(Received 6 February 2005;
accepted after revision 6 March 2006;
first published online 9 March 2006)
Corresponding author T. G. Deliagina: The Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institute, SE-17177, Stockholm, Sweden. Email: tatiana.deliagina{at}neuro.ki.se
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Introduction |
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In standing quadrupeds, each of the four limbs participates in supporting the body weight. When the animal's posture is perturbed, each of the limbs contributes to the generation of a corrective motor response (Lacquaniti et al. 1984; Macpherson, 1988a,b; Jacobs & Macpherson, 1996; Macpherson et al. 1997a; Beloozerova et al. 2003b). To join the efforts of individual limbs, they must be accurately coordinated. One possible means for coordination was proposed by the hypothesis of a single regulated variable (see, e.g. Ghez, 1991; Massion, 1994; Massion et al. 1997). According to this idea, sensory information from individual limbs is processed and integrated to obtain a generalized characteristic of body posture, like a position of the centre of mass or a direction of the body axis. With a deviation of this regulated variable from its desired value, specific commands are sent to individual limbs to elicit their coordinated movements.
In contrast to the concept of one regulated variable, Beloozerova et al. (2003b) suggested that the postural system in quadrupeds consists of two subsystems, stabilizing the dorsal-side-up orientation of the fore- and hindquarters, respectively. Coordination of activities of the two subsystems is achieved due to the interactions of their control mechanisms. A similar functional organization was earlier suggested for the locomotor system (von Holst, 1938; Shik & Orlovsky, 1965; Orlovsky et al. 1999).
In the present study, we investigated interlimb coordination in the standing cat, and used the previously developed technique for perturbing the posture (Beloozerova et al. 2005). The cat was standing on the platform, and maintained equilibrium when the platform was periodically tilted in the frontal plane. Responses to tilts included extension of the fore- and hindlimbs on the side tilting downward, and flexion of the limbs on the opposite side. By suspending a part of the body, we unloaded one, two, or three limbs and thus excluded them from the maintenance of body posture. By recording responses to tilt in such a ‘reduced’ postural system, we have characterized (i) the role of reflex mechanisms of individual limbs in the generation of postural corrections; (ii) postural capacities of the shoulder and hip girdles; and (iii) the role of mutual influences between different components of the system for interlimb coordination. Results of this study support the hypothesis of independent control of posture of the anterior and posterior parts of the trunk.
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Methods |
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Implantation of EMG electrodes was performed under isofluorane anaesthesia using aseptic procedures. Bipolar electrodes (flexible Teflon-insulated stainless-steel wires) were implanted bilaterally into the elbow extensor triceps muscle (Tric), and into the knee extensor vastus muscle (Vast). The wires were positioned under the skin and soldered to the connector fixed on the skull.
A description of the basic arrangement for postural tests has been provided previously (Beloozerova et al. 2003a, 2005). The cats were trained to quietly stand on a split platform that consisted of two parts – the F-platform under the forelimbs and the H-platform under the hindlimbs (Fig. 1A and B). The cat was rewarded by a paste-like food ejected from the feeder. The feeder (plastic tube of 18 mm outer diameter and 6 mm inner diameter) was positioned in front of the cat at a height of 21–23 cm (Fig. 1A and B). To examine the motor responses to externally applied perturbations of posture, the platforms under the cat were periodically tilted in the frontal (roll) plane of the animal. A sine-like tilt trajectory was used, with a period of 1 s and amplitude of ±15 deg; the effectiveness of such stimulation was shown earlier (Beloozerova et al. 2005). The F- and H-platforms were tilted either in-phase (Fig. 1A and C), or in antiphase (Fig. 1D and E). All cats were easily engaged in this postural task and maintained equilibrium during tilts. They tended to compensate for the platform tilts by performing lateral displacements of the body in relation to the supporting platform, which allowed them to hold the mouth against the feeder and to keep licking food despite the platform tilts.
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Test 1 (control)
The cat was standing on the non-split platform with all four limbs, and compensated for the platform tilts producing corrective movements by all of the limbs (Fig. 1A–C).
In tests 2–4, we examined postural functions of a single (shoulder or hip) girdle, as well as interactions between the girdles.
Test 2
The posterior part of the trunk was suspended in a hammock and slightly lifted, so that the hindlimbs were hanging freely and did not touch the platform. In this test, postural function, i.e. compensation for the platform tilts was performed by only the forelimbs (Fig. 1F).
Test 3
The anterior part of the trunk was suspended in a hammock and slightly lifted, so that the forelimbs were hanging freely and did not touch the platform. In this test, postural function was performed by only the hindlimbs (Fig. 1G).
Test 4
In this test, the two platforms were uncoupled, and the cat was standing with its forelimbs on the F-platform, and with its hindlimbs on the H-platform. The platforms were tilted in antiphase (Fig. 1D and E).
In tests 5–8, we examined postural functions of a single (fore or hind) limb, as well as interactions of the limbs within a girdle. For this purpose, one of the limbs of the shoulder or hip girdle was lifted, while the other limb remained on the platform. Two methods for lifting the limb were used, passive and active. With passive lifting, the experimenter took the limb by hand (in the elbow or ankle region), lifted it for a few centimetres, and kept at this position during the test (about 10 s). With this method one could not exclude, however, that the lifted limb was still loaded by a small part of the body weight, and thus slightly ‘assisted’ the supporting limb. This problem was absent when using the method of active lifting. When standing on four limbs, the cats easily lifted one of them in response to tactile stimulation of its foot.
Test 5
One of the forelimbs was lifted (Fig. 1H), and postural corrective movements were performed by one forelimb and two hindlimbs.
Test 6
One of the hind limbs was lifted (Fig. 1I), and postural corrective movements were performed by two forelimbs and one hindlimb.
Test 7
Test 7 was a combination of tests 2 and 5 (Fig. 1F and H). The hindquarters of the cat were suspended in the hammock. In addition, one of the forelimbs was lifted. In this test, postural corrective movements were performed by only one limb.
Test 8
Test 8 was a combination of tests 3 and 6 (Fig. 1G and I). The forequarters of the cat were suspended in the hammock. In addition, one of the hindlimbs was lifted. In this test, postural corrective movements were performed by only one limb.
The following values were recorded during postural tests: (i) two mechanical sensors measured the tilt angles of the F-platform and H-platform (Tilt F and Tilt H in Fig. 1A); (ii) in the previous study (Beloozerova et al. 2005) it was shown that lateral displacements of the upper point of the trunk in relation to the platform well characterized the efficacy of stabilization of the trunk position. In the present study, the lateral displacements were measured separately for the fore and hind parts of the trunk by means of mechanical sensors (BdF and BdH in Fig. 1A–C); (iii) contact forces produced by each of the feet (normal component, FN) were measured by means of the four force plates positioned under the limbs (shown for only the left forelimb in Fig. 1C). A disappearance of the contact force under a limb during lifting (in Tests 2–3 and 5–8) indicated that this particular limb did not support the body; (iv) to be sure that the position of the head against the feeder was stabilized, a contact of the tongue and lips with the feeder was continuously monitored by measuring an electrical conductance between the feeder and the animal; (v) the EMG signals were rectified and smoothed (time constant, 50 ms).
A representative example of the data recording (tests 1 and 2) is shown in Fig. 2. In every experimental session, each of the tests with body lifting (tests 2–3 and 5–8) and antiphase tilting (test 4) alternated several times with test 1 (control). In each cat, the experimental sessions (trials) were performed repeatedly (up to 36 times) during the period of 1–2 months.
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A depth of the tilt-related modulation of EMG was calculated in the following way:
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Results |
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In test 1 (control) we examined postural performance in the cat standing with all its limbs on the tilting, non-split platform (Fig. 1A–C). Normally during tilting the cat positioned its limbs so that the distance between the feet was about 8 cm for the forelimbs and about 15 cm for the hindlimbs. A tilt of the platform elicited a stereotypic postural response – an extension of the fore- and hindlimbs on the side moving down, and a flexion on the opposite side, as shown schematically in Figs 1C and 5A and D (see also Beloozerova et al. 2005). The limb movements made the trunk and head displace laterally in relation to the platform, in the direction opposite to the tilt. These lateral body movements reduced deviations of the trunk and head from the dorsal-side-up position. By means of sensors BdF and BdH (Fig. 1A–C), we recorded lateral displacements of the upper points of the fore and hind parts of the trunk, respectively, in relation to the platform. These corrective trunk movements occurred in antiphase with the platform tilts (compare TiltF and BdF trajectories in Fig. 2, test 1). The antiphase relationships between the platform tilts and the corrective movements of the fore- and hindquarters are well seen in Figs 3A1 and 2 (average over 30 cycles). The peak-to-peak value of corrective movements was about 8 cm for the forequarters and about 6 cm for the hindquarters. Due to postural corrections, the cat stabilized its mouth position against the feeder, and continued licking food despite tilts of the platform. An almost continuous contact between the mouth and the feeder was demonstrated by measuring the electrical conductance between them (Lick trace in Fig. 2).
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The extension of limbs on the side tilting down and flexion of limbs on the opposite side was caused by a specific modulation of the activity of their extensor muscles (Beloozerova et al. 2005), including Tric and Vast (Fig. 2). Figure 3A5 and 6 show the average EMG responses in Tric and Vast on the two sides. The EMGs slightly led in phase the tilt movements, with a peak about 0.1 cycle before the maximal downward tilt on the ipsilateral side. The depth of modulation M (see Methods) was 50–70%.
Major characteristics of postural responses in test 1 are summarized in Table 1. One can see that the amplitudes of postural corrections, depth of modulation of muscular activity, and phases of activity of muscles in the tilt cycle were consistent across repeated trials and similar in all three subjects.
It should be noted that, in all cats, the force trajectories in the two symmetrical limbs (fore or hind) in most trials were not quite symmetrical as, e.g. in Fig 3A4, where the peak forces under the left and right hindlimbs were 1.4 and 1.1 kg, respectively, suggesting an uneven distribution of the body weight between the limbs. This asymmetry could vary in sequential trials, but the ratio between the forces on the two sides did not exceed 1.4 in all subjects.
Postural functions of the shoulder and hip girdles
Postural functions of the shoulder girdle were examined in test 2, with only forelimbs on the platform (Fig. 1F). When the hindquarters were lifted, postural corrections in the forequarters persisted. Figure 2 shows the recording of postural responses under two conditions, i.e. when the cat was standing on all four limbs (test 1), and when the hindquarters were lifted (test 2). Lifting the hindquarters was monitored by a disappearance of the contact forces under the hindlimbs. The activity of the remaining part of the postural system occurred sufficient to stabilize the mouth position against the feeder, and to continue licking food (see Lick trace in Fig. 2). Video recording of the cat at the lifted position has shown that, when the posterior part of the trunk was suspended in a hammock, the hindlimbs were strongly extended (this is shown schematically in Fig. 1F), and did not perform any tilt-related movements in the sagittal plane. Small (<1 cm) tilt-related lateral oscillations of the suspended hindlimbs (probably caused by trunk muscles) were sometimes observed.
The representative average characteristics of postural performance in test 2 (cat 1) are shown in Fig. 3B. We will compare them to control (Fig. 3A). At the lifted position, the efficacy of postural corrections in the forequarters was similar to that in control – the peak-to-peak value of corrective movements (about 8 cm) was the same (compare Fig. 3A2 and B2). The temporal patterns of the forelimb forces and the Tric EMGs were also close to normal ones (compare Fig. 3A3 and B3 as well as Fig. 3A5 and B5), but their amplitudes were higher at the lifted position, suggesting an increase of the part of the body weight supported by the forelimbs. In other test 2 trials in cat 1, as well as in other cats, the force and EMG amplitudes in the forelimbs could slightly increase, decrease, or remain unchanged.
Periodic responses to tilts were also observed in the suspended hindlimbs. Both the left and the right Vast were active, and their EMGs were modulated with the peak activity timed to approximately the maximal ipsilateral tilt (Fig. 3B6). This EMG pattern did not differ qualitatively from that observed in control (Fig. 3A6).
The major results of test 2 were as follows (Table 2, test 2): (1) responses to tilts in the forelimbs were similar to those in control. The ratio between the peak-to-peak values of corrective movements in test 2 and test 1 was close to unity; (2) the temporal pattern of tilt-related modulation of Vast EMG in the suspended hindlimbs was similar to that in control. The depth of modulation M in test 2 was considerable. The difference between the phases of maximal Vast EMG in test 1 and test 2 was very small; this difference was less than the precision of phase measurements. In only 4/28 (14%) of trials, the tilt-related modulation of the Vast EMG in the suspended limbs was not observed, probably because of a lower level of EMG activity in these trials. These findings suggest that (i) the postural mechanism of the forelimbs by itself well compensates for tilts of the anterior part of the body; and (ii) the hindlimbs receive rhythmical tilt-related influences from the forelimbs.
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The representative average characteristics of postural performance in test 3 (cat 1) are shown in Fig. 3C. We will compare them to control (Fig. 3A). At the lifted position, the efficacy of postural corrections in the hindquarters was similar to that in control (compare Fig. 3A2 and C2) – the peak-to-peak value of corrective movements (about 5 cm) was practically the same. The temporal patterns of the hindlimb forces and the Vast EMGs were also close to normal ones (compare Fig. 3A4 and C4 as well as Fig. 3A6 and C6), but the force amplitudes were smaller at the lifted position, suggesting a decrease of the part of the body weight supported by the hindlimbs. No periodic responses to tilts were observed in the suspended forelimbs: both left and right Tric EMGs were much smaller than in control, and were practically not modulated (Fig. 3C5).
The major results of test 3 were as follows (Table 2, test 3): (1) responses to tilts in the hind limbs were similar to those in control. The ratio between the peak-to-peak values of corrective movements in test 3 and test 1 was close to unity; (2) no tilt-related modulation of Tric EMG was observed in the suspended forequarters (27 trials). These findings suggest that (i) the postural mechanism of the hindlimbs by itself well compensates for tilts of the posterior part of the body, and (ii) the forelimbs receive very weak (if any) rhythmical tilt-related influences from the hindlimbs.
A degree of independence of the shoulder and hip girdles was examined in test 4, with antiphase tilts of the fore- and hindquarters. When the F-platform and H-platform were tilted in antiphase, the cats were easily engaged in this complex postural task. They compensated for tilts of a platform by extending the limb on the side moving downward, and flexing the limb on the side moving upward (Fig. 1D and E). Since the platforms were tilted in antiphase, the ipsilateral fore- and hindlimbs performed flexion–extension movements in antiphase to each other, whereas the diagonal limbs were moving in-phase.
The average characteristics of postural performance in test 4 are shown in Fig. 4B. All characteristics are presented in relation to the tilt cycle of the F-platform. One can compare them to control (Fig. 4A). The lateral displacements of the fore and hind parts of the trunk were in antiphase to the tilts of the corresponding platform (compare Fig. 4B1 and 2). The efficacy of postural corrections was similar to that in control – the peak-to-peak value of corrective movements in the forequarters was 7 cm in test 4 against 8 cm in control; in the hindquarters it was 4 cm against 5 cm in control (compare Fig. 4A2 and B2).
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The temporal pattern of the Tric activity was close to the normal one (compare Fig. 4A5 and B5). The Tric EMGs led in phase the tilt movements of the F-platform, with the peak about 0.1 cycle before the downward tilt on the ipsilateral side (the range of phase lead was 0.0–0.2 cycle).
The temporal pattern of the Vast activity in test 4 was also close to that in control, but in relation to the H-platform (Fig. 4B6). The Vast EMGs changed approximately in-phase with the tilt movements of the H-platform, with the peak at the maximal downward tilt on the ipsilateral side (the range of phase lead was 0–0.2 cycle). In relation to the F-platform, however, the Vast EMGs occurred in antiphase to the tilt movements.
The major results of test 4 were as follows (Table 2, test 4): (1) responses to tilts in the forelimbs were similar to those in control. The ratio between the peak-to-peak values of corrective movements in test 4 and test 1 was close to unity; (2) responses to tilts in the hindlimbs were similar to those in control. The ratio between the peak-to-peak values of corrective movements in test 4 and test 1 was close to unity; (3) the Tric and Vast activity switched from the in-phase pattern in control to the antiphase pattern in test 4. The peak EMG slightly led in phase the maximal ipsilateral tilt of the corresponding platform. These findings suggest that postural corrections in the fore- and hindquarters are generated by two different mechanisms; each of them is driven by tilts of the corresponding platform.
Postural functions of single limbs
Postural functions of a single limb were examined in the experiments with a single-limb support of a girdle whereas the contralateral limb was lifted (tests 5–8). No substantial difference was found between the effects of passive and active lifting; these results will be described together. Also, the activity of the supporting limb was similar when the other girdle was not suspended (tests 5 and 6) or was suspended (tests 7 and 8).
Compensatory changes of the effective length of the supporting limb
Figure 5A–C shows representative stick diagrams (based on video recordings) of the left forelimb (cat 1) for the two extreme (up and down) platform positions in control (A), in test 5 (the right forelimb was lifted, B), and in test 7 (all limbs except for the left forelimb were lifted, C). In all three tests, the limb joints were flexing when the platform under the limb was moving upward, and extending when it was moving downward. As a result, the effective length of the limb (the distance from the shoulder joint to the foot) was changing in antiphase with the support height, with its maximal value at the maximal tilt downward, and its minimal value at the maximal tilt upward. Due to these changes in length, the vertical excursions of the platform under the forelimb (2.3 cm peak-to-peak) were only partly transmitted to the trunk: the shoulder joint exhibited excursions of 0.7 cm peak-to-peak in control (A), 1.3 cm in test 5 (B), and 1.6 cm in test 7 (C). Thus, when the shoulder girdle was supported by only one forelimb, compensation for movements of the platform was present, but it was much less efficient than in control. The residual tilt-caused trunk displacements did not prevent the cat from licking food from the feeder, however. Similar results (i.e. smaller changes in length with a single-limb support than in control) were obtained in all three cats. On the average over 12 trials in all subjects, relative changes in the effective length of the forelimb (at the up and down platform positions) were 10 ± 3% (mean ± S.E.M.) in control (test 1) against 5 ± 2% with one-limb support of the shoulder girdle (tests 5 and 7 taken together). The difference was statistically significant (P < 0.05, t test).
Similar results were obtained for the hindlimb. Figure 5D–F shows stick diagrams for control (D), test 6 (the right hindlimb was actively lifted, E), and test 8 (all limbs except for the left hindlimb were suspended, F). The vertical excursions of the platform under the hindlimb (4.2 cm peak-to-peak) were larger than under the forelimb (2.3 cm, see Fig. 5A–C) because of a wider base of support (Fig. 1B). Due to compensatory changes of the effective length of the limb (the distance from the hip joint to the foot), the platform movements were only partly transmitted to the trunk, and the hip joint exhibited excursions of 1.2 cm peak-to-peak in control (D), 1.9 cm in test 6 (E), and 2.0 cm in test 8 (F). Thus, compensation for platform movements with a single supporting hindlimb was much less efficient than in control. Similar results (i.e. smaller changes in length with a single-limb support) were obtained in all three cats. On the average over 12 trials in all subjects, relative changes in the effective length of the hindlimb were 18 ± 2% in control against 8 ± 1% with a single-limb support of the hip girdle (tests 6 and 8 taken together). The difference was statistically significant.
Force and EMG responses with a single-limb support of a girdle
We examined the force and EMG responses to tilt in the supporting forelimb and in the lifted forelimb in tests 5 and 7. Because of a similarity of the results obtained in these tests, here we illustrate only the results of test 7. Figure 6B shows representative average responses in the forelimbs in test 7, when only the right forelimb was on the platform, whereas all other limbs were suspended (cat 1). One can compare these responses to control (Fig. 6A). The contact force under the supporting (right) limb increased considerably in test 7 as compared to control (see Fig. 6A2 and B2). The temporal pattern of modulation was similar to control, but the depth of modulation was smaller. The activity of Tric in the supporting limb was higher than in control. The temporal pattern of Tric EMG modulation was similar to that in control: the EMG changed approximately in-phase with the tilt movements, with its peak at the maximal downward tilt on the ipsilateral side (compare Fig. 6A3 and B3). In the suspended (left) forelimb, the Tric EMG was considerably reduced, but remained modulated. The temporal pattern of modulation was similar to control, and the left Tric was active in antiphase to the right Tric.
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Discussion |
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Independent control of posture of the anterior and posterior parts of the body
Our experiments with suspension of the anterior or the posterior part of the body have shown that postural responses to tilts in a given girdle (shoulder or hip) persist when the other girdle does not participate in the maintenance of posture (Fig. 3). When the hindquarters were suspended, the forelimbs compensated for the platform tilts as efficiently as in control, and the cat continued licking food from the feeder. These experiments clearly showed that postural activity in one of the girdles is not necessary for the normal postural functioning of the other girdle.
When the hindquarters were suspended and the hindlimbs did not support the body, these limbs were strongly extended in the knee and ankle joints due to a high level of activity of the corresponding muscles, as documented by recording the Vast EMG (Fig. 3B). A tilt-related modulation of the Vast EMG was also observed. As in control (Fig. 3A), Vast was coactivated with ipsilateral Tric. These experiments have shown that postural mechanisms of the shoulder girdle exert influences on the mechanisms of the hip girdle; these influences elicit modulation of the Vast activity with a normal temporal pattern in relation to the platform tilts. Thus, the motor responses in the suspended hindlimbs were based on the information about tilts obtained by the forelimbs' somatosensory system.
When the forequarters were suspended and the forelimbs did not support the body, these limbs were extended. Practically, no tilt-related modulation of the Tric EMG was observed (Fig. 3C). These experiments taken together suggest that the influences of postural mechanisms of the shoulder girdle upon those of the hip girdle are stronger than the influences in the opposite direction.
The experimental design with antiphase tilts of the two platforms allowed us to change the phase relationships between the postural responses in the forelimbs and in the hindlimbs, and to cause a mismatch between the local somatosensory input to the hindlimb mechanisms on one hand, and the influences they receive from the forelimbs on the other hand. In control (in-phase tilts of the two platforms), the motor responses in the ipsilateral limbs, as well as the ipsilateral Tric and Vast EMGs were in-phase with each other (Fig. 4A). By contrast, with antiphase tilts they appeared in antiphase (Fig. 4B), despite the forelimb influences that tended to cause the in-phase activity in the hindlimbs (Fig. 3B).
These results were obtained for a complex postural task – independent tilts of the two platforms. For this particular situation, they make unlikely the hypothesis of the united control of posture of the fore- and hindquarters in quadrupeds (see, e.g. Ghez, 1991; Massion, 1994; Massion et al. 1997). The more likely explanation for our findings is that the limbs of the hip and shoulder girdles are controlled by their own postural mechanisms driven primarily by somatosensory inputs from the corresponding limbs. These two postural subsystems tend to maintain the dorsal-side-up orientation of the corresponding (anterior and posterior) parts of the body even when the surfaces under the fore- and hindlimbs are tilted differently (Fig. 8). A model with united control of the fore and hindquarters (one regulated variable) seems to be applicable to simpler postural tasks, however.
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A number of studies have shown that, under certain conditions, the system controlling the head position can operate independently from that controlling the trunk posture (Berthoz & Pozzo, 1988; Barberini & Macpherson, 1998; Deliagina et al. 2000; Boyle, 2001). Given that the trunk posture in quadrupeds is maintained by two mechanisms, one can conclude that the whole body is stabilized due to a simultaneous activity of the three principal mechanisms.
Postural mechanisms of a single girdle
In the first part of the present study we have found that each pair of limbs (fore or hind) well compensates for the lateral tilts of the corresponding platform, with a response pattern similar to control (Fig. 3). What is the functional organization of postural mechanisms of each girdle? One extreme would be that the mechanisms of two limbs strongly interact when generating postural corrections, and each girdle thus represents a functional unit. The other extreme would be that postural responses in an individual limb are primarily driven by sensory input from that particular limb, and each limb thus represents a functional unit.
To address this issue, we compared the limb movements in control with those when only one limb was supporting the anterior or the posterior part of the body. In control, tilting the platform evokes an increase of the ‘effective length’ of the forelimb and the hindlimb on the side tilting downward, and shortening of the limbs on the opposite side (Fig. 5A and D). This leads to a reduction of the trunk tilt, that is, to stabilization of the dorsal-side-up trunk position (as shown schematically in Fig. 1B and C). Lengthening of the limb is due to the extension at joints, whereas shortening is due to the flexion at joints. These movements at joints are primarily caused by modulation of the activity of extensor muscles – the extensor activity increases on the side moving down, and decreases on the opposite side (Fig. 3A). Thus, the extensor muscles increase their activity when they are shortening, and decrease their activity when they are lengthening (see also Beloozerova et al. 2005). Similar results were obtained for the rabbit keeping balance on the tilting platform (Deliagina et al. 2000; Beloozerova et al. 2003b).
When examining the one-limb support of a girdle, we found a qualitatively similar motor pattern. A single limb (fore or hind) was able to support the corresponding part of the body, and to partly compensate for the platform tilts by changing its effective length (Fig. 5). The compensatory changes in length, however, were considerably smaller than in control and therefore the residual vertical excursions of the trunk were much larger. The extensor EMG in the supporting limb (fore or hind) was considerably larger than in control, which was necessary to counteract the additional load (Figs 6 and 7). The pattern of EMG modulation was qualitatively similar to control: an extensor activity increased when the platform under the limb was moving downward and the limb was extending, and decreased when the platform was moving upward and the limb was flexing (Figs 6 and 7). These findings suggest that the control mechanisms of an individual limb contribute to the generation of postural responses to tilt, but the contribution is rather limited. A detailed analysis of these mechanisms requires special studies; for discussion of possible sensory origin of postural reflexes in a limb, see Gossard et al. (1994), Pearson (1995), Pratt (1995), and Duysens et al. (2000).
In the contralateral limb, which did not contact the platform, the extensor activity was usually present and modulated in relation to the tilt cycle. The temporal pattern of modulation was similar to control, with the peak at the maximal ipsilateral tilt, that is, in antiphase to the extensor EMG in the supporting limb (Figs 6 and 7). The presence of responses to tilt in the lifted limb suggests that this limb receives tilt-related influences from the supporting limb. Under normal conditions, these crossed influences would produce the motor effects that are added to the postural responses elicited in the limb by its local somatosensory input (Fig. 8). The lack (or decrease) of crossed influences from the contralateral limb, when this limb is lifted, seems to be the main cause for the reduction of compensatory responses when a girdle is supported by a single limb. One can thus conclude that both local reflexes of an individual limb and crossed influences between symmetrical limbs are important for normal functioning of the postural mechanisms of the shoulder girdle and of the hip girdle.
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