Excitability properties of motor axons in adults with cerebral palsy

Altered flexor carpi radialis motor axon excitability properties after cerebrovascular stroke

Background

Spinal motoneurons may become hyperexcitable after a stroke. Knowledge about motoneuron hyperexcitability remains clinically important as it may contribute to a number of phenomena including spasticity, flexion synergies, and abnormal limb postures. Hyperexcitability seems to occur more often in muscles that flex the wrist and fingers (forearm flexors) compared to other upper limb muscles. The cause of hyperexcitability remains uncertain but may involve plastic changes in motoneurons and their axons.

Aim

To characterize intrinsic membrane properties of flexor carpi radialis (FCR) motor axons after stroke using nerve excitability testing.

Methods

Nerve excitability testing using threshold tracking techniques was applied to characterize FCR motor axon properties in persons who suffered a first-time unilateral cortical/subcortical stroke 23 to 308  days earlier. The median nerve was stimulated at the elbow bilaterally in 16 male stroke subjects (51.4 ± 2.9 y) with compound muscle action potentials recorded from the FCR. Nineteen age-matched males (52.7 ± 2.4 y) were also tested to serve as controls.

Results

Axon parameters after stroke were consistent with bilateral hyperpolarization of the resting potential. Nonparetic and paretic side axons were modeled by a 2.6-fold increase in pump currents (IPumpNI) together with an increase (38%-33%) in internodal leak conductance (GLkI) and a decrease (23%-29%) in internodal H conductance (Ih) relative to control axons. A decrease (14%) in Na⁺ channel inactivation rate (Aah) was also needed to fit the paretic axon recovery cycle. "Fanning out" of threshold electrotonus and the resting I/V slope (stroke limbs combined) correlated with blood potassium [K⁺] (R = -0.61 to 0.62, p< 0.01) and disability (R = -0.58 to 0.55, p < 0.05), but not with spasticity, grip strength, or maximal FCR activity.

Conclusion

In contrast to our expectations, FCR axons were not hyperexcitable after stroke. Rather, FCR axons were found to be hyperpolarized bilaterally post stroke, and this was associated with disability and [K⁺]. Reduced FCR axon excitability may represent a kind of bilateral trans-synaptic homeostatic mechanism that acts to minimize motoneuron hyperexcitability.

Novel insights into cerebral palsy

Cerebral palsy (CP) is a neurodevelopmental disorder characterized by abnormalities of muscle tone, movement and motor skills, and is attributed to injury to the developing brain. CP affects about 1 in 500 neonates. CP shows clinical features which evolve with age, and these may over time lead to deterioration of motor function although the lesion to the developing brain is non-progressive. The underlying causes for CP remain unclear. Based on recent research we are able to give a physiological explanation on the appearance and development of the condition. The damage to the central nervous system causes a change in collagen structure, with a higher level of deposition of collagen around the muscles, increasing throughout life. Assuming this premise is correct, the question is, will it by any treatment be possible to delay or prevent this collagen accumulation in the CP muscles, thereby giving CP patients a better prognosis in the future.

Hyperexcitability of brain stem pathways in cerebral palsy

Individuals with cerebral palsy (CP) experience impairments in the control of head and neck movements, suggesting dysfunction in brain stem circuitry. To examine if brain stem circuitry is altered in CP, we compared reflexes evoked in the sternocleidomastoid (SCM) muscle by trigeminal nerve stimulation in adults with CP and in age/sex-matched controls. Increasing the intensity of trigeminal nerve stimulation produced progressive increases in the long-latency suppression of ongoing SCM electromyography in controls. In contrast, participants with CP showed progressively increased facilitation around the same reflex window, suggesting heightened excitability of brain stem pathways. We also examined if there was altered activation of cortico-brain stem pathways in response to prenatal injury of the brain. Motor-evoked potentials (MEPs) in the SCM that were conditioned by a prior trigeminal afferent stimulation were more facilitated in CP compared with controls, especially in ipsilateral MEPs that are likely mediated by corticoreticulospinal pathways. In some participants with CP, but not in controls, a combined trigeminal nerve and cortical stimulation near threshold intensities produced large, long-lasting responses in both the SCM and biceps brachii muscles. We propose that the enhanced excitatory responses evoked from trigeminal and cortical inputs in CP are produced by heightened excitability of brain stem circuits, resulting in the augmented activation of reticulospinal pathways. Enhanced activation of reticulospinal pathways in response to early injury of the corticospinal tract may provide a compensated activation of the spinal cord or, alternatively, contribute to impairments in the precise control of head and neck functions.

NEW & NOTEWORTHY This is the first study to show that in adults with spastic cerebral palsy, activation of brain stem circuits by cortical and/or trigeminal afferents produces excitatory responses in anterior neck muscles compared with inhibitory responses in age/sex-matched controls. This may reflect a more excitable reticulospinal tract in response to early brain injury to provide a compensated activation of postural muscles. On the other hand, a hyperexcitable brain stem may contribute to impairments in the precise control of head and neck functions.

Differences in excitability properties between medial gastrocnemius, tibialis anterior, and abductor pollicis brevis motor axons

INTRODUCTION

Excitability properties of motor nerves to different muscles are different, but the explanation is uncertain. We characterized motor axon excitability properties to the medial gastrocnemius (MG) in 27 adults, and made comparisons with the peroneal nerve to the tibialis anterior (TA) and median nerve to the abductor pollicis brevis (APB) in 10 subjects.

METHODS

Recordings of multiple excitability properties were made using threshold tracking, stimulating the nerves at the wrist or knee.

RESULTS

Threshold electrotonus and superexcitability differed between nerves (APB>MG>TA axons) that may reflect differences in fast K⁺ conductance. APB axons had larger S2 accommodation and undershoot than TA and MG axons, indicating greater slow K⁺ conductance. TA axons demonstrated greater accommodation during hyperpolarizing currents than MG and APB axons, suggestive of greater inwardly rectifying current.

DISCUSSION

Inherent differences in several conductances underlie nerve differences in excitability, presumably related to muscle or motoneuron properties. Muscle Nerve 57: E60-E69, 2018.