[i] when a doctor strikes your tendon just below the knee cap. But instead of this monosynaptic reflex occuring in a region that we can see, I am proposing this is a reflex that happens at the spinal level.
To give you a little background, intervertebral discs can lose up to 20% of their height over the course of the day. Think of them as a hydraulic shock that slowly lose their height over the course of the day as we are vertical. It is with sleep, or recumbancy, where this loss recovers. Although a different topic, I believe it is in REM sleep where the height loss is recovered the most.
Imagine the spine like a hydraulic accordian and within the first few minutes upon lying down (and relaxation of the muscles around the spine) the discs fill and lengthen. If the filling and stretch lengthening is quick enough…just like one has to strike the knee tendon fast enough to elicit a reflex in the quadraceps, a monsynaptic reflex is thought to occur. It is often a big reflex because of the proximity and number of nerves in the reflex.
Researchers have seen this reflex related to spinal position [ii] which one could imagine that spinal positions could allow a greater stretch when the muscles relax. If a body position enables a quicker “refilling” of the intervertebral discs, it is reasonable to think that the reflex could be elicited easier. To add to the supporting evidence, some actually do believe that the cause of a hypnic reflex is propriospinal.
Many other theories have been proposed for this commonly observed “jerk” most often experienced when just falling asleep. But with the reading I have done, I believe this spontaneous myoclonic muscle twitching experienced during atonia [i] is a local reflex occurring at the spine. Interestingly, some researchers believed a peripheral mechanism in 1986 to be relevant whereby they saw myoclonic limb movements associated with changes in spinal position.[ii] More recent research suspect a spinal generator as the cause of this propriospinal myoclonus reflex.[iii] [iv] If atonia (or muscle shut-off) during sleep creates an unloading of the buttressing effects of spinal compression, could it be possible that the muscle spindles be stretched quick enough (from the outward expansion of discs) when the muscles relax to cause a local spinal myoclonic reflex…just as is seen with the patellar reflex for example? More research is needed in order to definitively evaluate the changes of muscle spindles around intervertebral discs while they lengthen and reset in sleep and what that means from a proprioceptive neurological perspective. Interestingly, this hypnic jerk usually presents when people are over-tired and just falling asleep…..likely in those that have already have significant disc height loss related to their fatigue and require more height to recover.
[i] Karlsson KA, Blumberg MS The union of the state: myoclonic twitching is couples with nuchal muscle atonia in infant rats. 2002 Behav Neurosci Oct;116(5):912-7.
[ii] Dzvonik ML, Kripke DF, Klauber M, Ancoli-Israel S Body position changes and periodic movements in sleep. 1986 Sleep Dec;9(4):484-91.
[iii] Brown P, Thompson PD, Rothwell JC, Day BL, Marsden CD Axial myoclonus of propriospinal origin. 1991 Brain Feb;114(Pt1A):197-214.
[iv] Montagna P, Provivni F, Vetrugno R Propriospinal myoclonus at sleep onset. 2006 Neurophysiol Clin. Sept-Dec;32(5-6):351-355.Epub 2007Jan