Reticulospinal Contributions to Gross Hand Function after Human Spinal Cord Injury

Abstract

Multiple descending motor pathways likely contribute to the recovery of hand motor function following spinal cord injury (SCI). Reticulospinal neurons project to spinal motor neurons controlling hand muscles and extensively sprout into gray matter structures after SCI; therefore, it has been proposed that the reticulospinal tract is one of the descending motor pathways involved in recovery of hand function after injury. To test this hypothesis, we examined the StartReact response, an involuntary release of a planned movement via a startling stimulus that engages the reticulospinal tract, by measuring reaction times from electromyographic activity in an intrinsic finger muscle during three motor tasks requiring different degrees of hand dexterity: index finger abduction, a precision grip, and a power grip. Males and females with and without incomplete chronic cervical SCI were tested. We found that although SCI participants voluntarily responded to all tasks, reaction times were shorter during a startle cue while performing a power grip but not index finger abduction or precision grip. Control subjects had similarly shorter reaction times during a startle cue in all motor tasks. These results provide the first evidence for a contribution of the reticulospinal tract to hand control in humans with SCI during gross finger manipulations and suggest that this contribution is less pronounced during fine dexterous finger movements. It has been long proposed that brainstem pathways contribute to the recovery of hand function in humans with spinal cord injury (SCI). Here, we show that individuals with anatomically incomplete chronic cervical SCI responded to a startle stimulus, a test that engages the reticulospinal tract, while performing a power grip but not during index finger abduction or precision grip. Control subjects responded to a startle stimulus similarly across tasks. These observations suggest that reticulospinal outputs after SCI contribute to hand motor tasks involving gross finger movements. Interestingly, this contribution is less pronounced during fine dexterous finger movements.

Imbalanced Corticospinal and Reticulospinal Contributions to Spasticity in Humans with Spinal Cord Injury

“…StartReact response. The StartReact response was tested using a previously described paradigm in humans with SCI (Baker and Perez, 2017). Here, participants were asked to observe a light-emitting diode (LED) located ϳ1 m in front of the participants' head.…”

“…The VART and VSRT are both mediated via the cochlear nuclei, but only the high intensity sound of the VSRT activate the reticulospinal pathway (Davis et al, 1982;Brown et al, 1991;Valls-Solé et al, 1999). To estimate changes in the gain of reticulospinal output, we normalized the data as follows (Baker and Perez, 2017):…”

“…A number of individuals with SCI have weaker muscles and small corticospinal responses (Bunday et al, 2014), and those not taking a medication to reduce spasticity are unable to modulate corticospinal responses to a similar extent as control subjects (Barry et al, 2013;Bunday et al, 2014). In addition, participants with SCI showed an exaggerated response to a startle stimulus in certain conditions compared with control subjects (Jankelowitz and Colebatch, 2004;Kumru et al, 2008Kumru et al, , 2009Baker and Perez, 2017), suggesting a strong contribution from the reticulospinal tract after the injury. We hypothesized that individuals with incomplete SCI with spasticity show weak corticospinal and strong reticulospinal outputs compared with participants with no or low spasticity and control subjects.…”

Startle evokes nearly identical movements in multi-jointed, two-dimensional reaching tasks

“…While the classic startle response results in brief, synchronous activity in the body, startReact responses are more sophisticated and release muscle activity patterns and kinematics that are tightly regulated and scaled with the intended action (Valls-Solé et al 1999; Carlsen et al 2003, 2004a; Rothwell 2006). Because of these characteristics, the startReact phenomenon has been utilized to investigate movements extensively in both healthy and impaired populations (Carlsen et al 2003, 2004a, b, 2008, 2011, 2012, Maslovat et al 2009, 2011; Honeycutt and Perreault 2012; MacKinnon et al 2013; Fisher et al 2013; Nonnekes et al 2014, 2015; Tresch et al 2014; Honeycutt et al 2017, 2015; Baker and Perez 2017). …”

Limited Capacity for Ipsilateral Secondary Motor Areas to Support Hand Function Post-Stroke

“…The findings may be different for hand closing compared to opening. Ipsilateral corticoreticulospinal tract makes substantial innervations to the flexor muscles of the wrist and fingers, and thus may enable sufficient hand closing control, at least for power grasps (Lawrence & Kuypers, 1968;Baker & Perez, 2017). This could explain why extensor weakness of the fingers is usually a more significant problem than flexor weakness post-hemiparetic stroke (Kamper et al 2006;Conrad & Kamper, 2012;Miller & Dewald, 2012).…”