Signals to & from the brain are very different
The nerves that take signals from the brain to the legs and feet do so normally in HSPers, but different nerves that bring the signal to the brain from the feet are where the abnormal delay happens due to slow conduction times.
Editor: Here is a “plain English” interpretation of this research study from the Netherlands that examined posture and balance issues with HSP.
1. They put HSPers and non-HSPers on a platform that would tilt forwards or backwards under researcher control.
2. To maintain balance when the platform tips forward, it is necessary to contract the calf muscles to maintain balance (image on the left); when it tips backward, you have to contract your shin muscles (image on the right).
3. HSPers could not maintain balance as well, and had slower reaction times of the order of 30-40 milliseconds, compared to the non-HSPers.
The mechanism in operation is this:
* The feet detect a change in orientation of the surface with which they are in contact.
* Information is sent by an impulse along nerves to the brain where it is processed. The nerves that carry these impulses from the feet to the brain are called afferent nerves.
* A response is then sent by an impulse along different nerves down to various structures in the lower limbs such as muscles, which then respond so as to maintain balance. The nerves that carry these impulses down from the brain are called efferent nerves.
4. Next, the researchers introduced a sound into 25% of the trials to coincide with the start of the platform tipping backwards.
5. When there was a sound, both HSP and control groups responded faster, and they both responded in the same time.
The mechanism now in operation is this:
* The ear picks up the sound, which is transmitted to the brain for processing exactly the same for both HSPers and non-HSPers.
* This is faster and makes the (afferent) nerve response from the feet to the brain redundant as the brain already has the information it needs from the sound.
* A response is then sent by a nerve impulse along the efferent nerves down to the lower limbs.
So it is reasonable to conclude that the efferent nerves that take signals from the brain to the lower limbs do so normally in HSPers, and that the afferent nerves bringing the signal to the brain from the feet (when there is no sound) are where the abnormal delay happens due to slow conduction times.
Research Abstract
Hereditary spastic paraplegia (HSP) is characterized by progressive lower extremity spasticity and weakness, due to retrograde axonal degeneration of the corticospinal tract and posterior spinal columns. HSP patients fall frequently.
We hypothesized that delayed postural responses contribute to their balance impairments. To distinguish between a delay in afferent and efferent signals, we combined postural responses with a startling acoustic stimulus (SAS). The SAS triggers a postural response directly, bypassing afferent proprioceptive input.
We performed two experiments. First, 18 HSP patients and nine healthy controls stood on a balance platform and were instructed to counteract forward and backward balance perturbations, without taking a step or grabbing a handrail. Second, 12 HSP patients and nine controls received backward perturbations, while a SAS accompanied onset of platform motion in 25% of trials.
HSP patients were less successful than controls in maintaining balance following backward and forward perturbations. Furthermore, latencies of postural responses were significantly delayed in HSP-patients, by 34ms in gastrocnemius following forward, and by 38ms in tibialis anterior following backward perturbations. A SAS accelerated postural responses in all participants, but more so in HSP patients whose latencies were normalized.
Our results suggest that delayed postural responses in HSP patients contribute to their balance problems. Combining balance perturbations with a SAS restored normal latencies, suggesting that conduction of efferent signals (presumably by the reticulospinal tract) is normal. We therefore suggest that the delayed postural responses in HSP are caused by slowed conduction time via the posterior spinal columns.
J Neurol. 2013 Jun 20. [Epub ahead of print]
Mechanisms of postural instability in hereditary spastic paraplegia.
Nonnekes J, de Niet M, Nijhuis LB, de Bot ST, van de Warrenburg BP, Bloem BR, Geurts AC, Weerdesteyn V.
Department of Rehabilitation, Nijmegen Centre for Evidence Based Practice, Radboud University Medical Centre, Nijmegen, The Netherlands.