Biweekly progress report

Monzurul Alam11 February 2011

Somatosensory Control of Balance During Locomotion in Decerebrated Cat

Authors

Pavel  Musienko, MD PhDPostdocUniversity of ZurichSwitzerland

V. Reggie Edgerton, Ph.D.,ProfessorUniversity of California Los Angeles (UCLA)

Existing knowledge (Background)

• The postural system is normally dependent on a number of interconnecting control loops that utilize visual, vestibular, and somatosensory inputs (Macpherson et al. 1997; Beloozerova et al. 2003; Deliagina et al. 2006).

• Post-mammillary decerebrated animals are not able to stand or step by themselves due to isolation from the neuronal structures above the lesion (Magnus 1924; Bard and Macht 1958; Musienko et al. 2008).

• Locomotion with partial weight bearing, however, can be initiated and well controlled by tonic brainstem (Shik et al. 1966; Kazennikov et al. 1988) or spinal cord (Gerasimenko et al. 2005) electrical stimulation.

Problem

Since the trunk of the animals was supported both vertically and

laterally in all previous studies, the ability to sustain equilibrium during

locomotion has not been investigated.

Addressed question

In the present study the authors addressed the question of whether the brainstem-spinal cord circuitry of post-mammillary cats can acquire and sustain dynamic balance during stepping after isolation from the forebrain, vestibular, and other supraspinal descending systems or not?

Experiment• The experiments were performed on 11 adult cats (2.5-3.0 kg body

weight).• The cats were anesthetized deeply with a mixture of xylazine (1 mg/kg,

i.m.) and ketamine (40 mg/kg, i.m.) and supplemented (30-50% the initial dose)

• A mid-dorsal skin incision was made, the paravertebral muscles reflected, and a partial laminectomy was performed between the L3-L7 vertebral levels.

• An electrode for monopolar ES was made by removing a small length (~2 mm notch) of Teflon coating to expose the stainless steel wire.

• The electrode was secured at the midline of the L5 spinal segment by suturing the wire to the dura mater above and below the electrode.

• An indifferent ground electrode was made by removing ~1 cm of Teflon at the distal end of a similar wire that then was inserted into the paravertebral muscles.

Experimental Setup

• The treadmill was equipped with two separate belts each instrumented with a force sensor placed underneath the belt for quantifying contact ground reaction forces (GRFs).

• The head and upper trunk (T5-T7) of the decerebrated cats were secured using a stabilizing apparatus.

Result

Results shows that tonic epidural spinal cord stimulation (5 Hz at L5) of decerebrated cats initiated and sustained unrestrained weight-bearing hindlimb stepping for extended periods.

Result

Result

A pre-collicular post-mammillary decerebration essentially precluded visual input and the fixation of the head and spine at the thoracic level eliminated vestibular and proprioceptive head-neck-trunk reflexes as a source of ongoing dynamic control. Therefore, somatosensory input arising from the hindlimbs and lower trunk were the only source of modulation for the control of stepping and balance in the experimental model.

Result• Weight-bearing unrestrained locomotion requires lateral stability.

• Although the decerebrated cats were able to step with full weight bearing of the unrestrained hindquarters during ES, it was not clear if this was due to some passive mechanical consequence of the experimental paradigm or to the decerebrated cats utilizing neural control mechanisms driven by somatosensory input to actively maintain balance as observed in intact animals.

• To test this the authors compared the kinematics of the pelvis and the hindlimb stepping patterns of intact and decerebrated cats.

Result

• The hindquarters were displaced in the sagittal plane and laterally and vertically in the frontal plane in both groups (Fig. A).

• There was a strong correlation between the left-right maximal lateral displacements of the pelvis in decerebrated and intact cats (Fig. B)

Result

The EMG patterns during unrestrained weight-bearing hindlimb stepping were similar to those observed during unrestrained locomotion of intact cats.

Adaptive postural responses to perturbations during gait

• As a next step, we evaluated whether the brainstem–spinal cord motor systems of decerebrated cats are able to control balance not only during unperturbed stepping, but also during a disturbing influence artificially applied during gait. We introduced a prolonged lateral force to the trunk during continuous stepping facilitated by ES (n = 4 cats, Fig. 4). Such a perturbation deviated the hindquarters to the side (7-10 cm, to the right on Fig. 4A) repeatable from trial to trial and constrained the trunk movements to the opposite side (to the left on Fig. 4A) for as long as 15-20 s.

• Results shows that decerebrated cats could still continuously step in this unstable condition and efficiently maintain their lateral stability due to specific and asymmetric adaptations of the motor patterns in the left and right hindlimbs.

Result

Sustained modulation of GRFs, EMG activity, and step cycle duration in both legs was observed and lasted for the entire duration of the perturbation.

ResultFinally, we tested the ability of decerebrated cats to re-establish an equilibrium state sufficient to sustain well-coordinated stepping during ES after a sudden collapse (n = 4 cats, Fig.5). The cat pelvis was initially restrained in a frame by an additional clamp. After recording the stepping pattern in the restrained condition (10-20 steps) the pelvis clamp was suddenly released and the hindquarters collapsed (3-5 cm down).

Result

• There was more co-activation of the flexor and extensor muscles (After collapse in Fig. 5D) that increased the stiffness of the limbs.

• Foot placement and step width were more variable in the unrestrained than restrained condition (Fig. 5G)

Conclusion• In summary, the present data demonstrate that when tonic ES of the

spinal cord is applied in a post-mammillary decerebrated cat deprived of vestibular and other supraspinal sensory input, weight-bearing stepping with active balance control can be performed.

• These results imply that the sources for regulation of equilibrium during walking can be attributed to the ensembles of sensory input from the hindquarters to the spinal cord-brainstem neuronal circuits.

• The strong facilitating effect of spinal cord activation by ES on maintaining equilibrium during locomotion further demonstrates an important role of the spinal circuits in postural control during stepping.

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