Human spinal cord injury: motor unit properties and behaviour

Abstract

Spinal cord injury (SCI) results in widespread variation in muscle function. Review of motor unit data shows that changes in the amount and balance of excitatory and inhibitory inputs after SCI alter management of motoneurons. Not only are units recruited up to higher than usual relative forces when SCI leaves few units under voluntary control, the force contribution from recruitment increases due to elevation of twitch/tetanic force ratios. Force gradation and precision are also coarser with reduced unit numbers. Maximal unit firing rates are low in hand muscles, limiting voluntary strength, but are low, normal or high in limb muscles. Unit firing rates during spasms can exceed voluntary rates, emphasizing that deficits in descending drive limit force production. SCI also changes muscle properties. Motor unit weakness and fatigability seem universal across muscles and species, increasing the muscle weakness that arises from paralysis of units, motoneuron death and sensory impairment. Motor axon conduction velocity decreases after human SCI. Muscle contractile speed is also reduced, which lowers the stimulation frequencies needed to grade force when paralysed muscles are activated with patterned electrical stimulation. This slowing does not necessarily occur in hind limb muscles after cord transection in cats and rats. The nature, duration and level of SCI underlie some of these species differences, as do variations in muscle function, daily usage, tract control and fibre-type composition. Exploring this diversity is important to promote recovery of the hand, bowel, bladder and locomotor function most wanted by people with SCI.

Alterations in Localized Electrical Impedance Myography of Biceps Brachii Muscles Paralyzed by Spinal Cord Injury

“…Abnormal electromyography (EMG) from paralyzed muscles of SCI subjects commonly include fibrillation and sharp waves (8, 9), spontaneous motor unit activity, motor unit loss, and enlarged motor unit action potentials (10–12), as a result of degeneration of spinal motoneurons and peripheral neuromuscular deterioration (13). These findings also provide evidence of alterations in muscle architecture and components after SCI.…”

Contributions to muscle force and EMG by combined neural excitation and electrical stimulation

“…Disuse atrophy, particularly of large, fast motor units would similarly predict lower force augmentation by stimulation. Motor unit changes such as the above are also reported following SCI (Thomas et al, 2014), and would have similar effects on occlusion.…”

Electrical stimulation—a mapping system for hand dysfunction in tetraplegia

“…Anterior horn and cell body damage in the peripheral neuron is present within 8 days after injury. [7,18]. Damages in the distal cell body cannot immediately be detected.…”

“…The UMN and LMN are usually intact above the lesion. Below the lesion, there is an UMN lesion with intact reflex arcs and an intact LMN, whereas at the level of lesion the LMN may be partially or completely damaged [7]. Mulcahey et al [8] evaluated the LMN integrity for the muscles for grasp and release as well as for the wrist extensor.…”