When invited to submit an article to Canadian Chiropractor on the anatomy of the spine, I was excited to be able to communicate with many of my former students, and others, in this way. I was also daunted by the task of coming up with a topic on a familiar subject that would be both exciting to read and novel in some way.
At the risk of reminding former CMCC students of the long hours spent in the anatomy lab at the college preparing for my exams, let us review a few details of spinal anatomy. Then, some insight will be offered into what I feel is a major discovery that will surely change the way we think about the subject, having major implications for our profession. What could possibly be new in anatomy, let alone alter the direction of chiropractic science? Well, as Professor Crooksie always says, “I’m glad you asked!”
The discovery I refer to resulted from work in Dr. Stuart McGill’s lab at the University of Waterloo, which was published in the mid-1990s. McGill and his co-workers demonstrated that the tiny intersegmental muscles found between adjacent vertebrae in the deepest layers of the lumbar spine contain huge numbers of muscle spindles, the density of which far exceeds that seen in other skeletal muscles. (Wouldn’t you know it … histology changes the direction of chiropractic!) They hypothesized that muscles like rotatores brevis, intertransversarii and interspinalis do not function as primary movers of the spine, as thought previously. However, the fact remains that some of these muscles are named according to what was long thought to be their function. Even now, the functions of rotatores brevis and levator costae brevis are described in Moore’s Clinically Oriented Anatomy as rotation of the vertebra above toward the opposite side and elevation of a rib, respectively.
The fact that intersegmental muscles have a small and weak extrafusal component and an extremely large intrafusal component suggests that these muscles function as extraordinarily large sensory transducers and do not, in fact, act to move the spine. Also, since these muscles span only single vertebral segments, they are ideally located to monitor and measure even the slightest movements that may occur between adjacent vertebrae.
Rotatores brevis, for example, which attaches from the posterior aspect of a transverse process to the base of the spinous process above, will elongate when the spinous process rotates toward the opposite side. Even the slightest rotation of the spine will therefore result in a volley of proprioceptive signals from the muscle into the spinal cord, not only from the rotatores muscle being stretched but also from the muscle on the opposite side that is shortening.
Intertransversarii muscles become elongated when the spine flexes laterally away from the side of the muscle, causing the transverse processes to which it is attached to separate. Similarly, interspinalis muscles elongate during forward flexion and shorten during extension of the spine. In each instance, elongation of the intersegmental muscle will initiate increased neuronal firing in the spindles, and shortening of the muscle will result in decreased neuronal firing. You will recall that the intersegmental muscles previously mentioned are present between adjacent vertebrae throughout the length of the vertebral column. This means that we have an amazing neurophysiological mechanism that functions to monitor all movements of the spine, and this mechanism is ideally suited for fine postural control.
At the risk of stimulating chiropractic college flashback nightmares, let us try to remember where these neuronal signals are going in the spinal cord and brain. Incoming proprioceptive signals enter the posterior columns of the spinal cord, travel up into the medulla oblongata, relay into the medial lemniscus and again at the thalamus and traverse the internal capsule to finally terminate in the postcentral gyrus of the cerebral cortex, where we become consciously aware of our spinal movements.
Also, with each spinal movement, incoming proprioceptive signals synapse on neurons in nucleus proprius and nucleus dorsalis at the spinal cord level where they enter the cord. These nuclei are responsible for sending these proprioceptive signals via posterior and anterior spinocerebellar tracts to the cerebellum where this important sensory information is integrated with other sensory signals at an unconscious level and is then used to regulate the activities of the larger back muscles that are acting to move the vertebral column in the first place.
Are you still with me? Let us recap. We now know that there exists an exquisitely sensitive neurophysiological system, which functions to monitor and regulate postural changes and movements of the vertebral column. Let us say, for example, that multifidus, a nice large primary mover of the lumbar spine, is acutely hypertonic on the right side, as it so often is in acute low back pain. The right muscle is so tight that it overpowers its partner on the left and causes an antalgic posture. Consider the proprioceptive signals originating from intersegmental muscles on both sides of the spine. What do you suppose would happen to these signals if spinal manipulative therapy were to be applied to the L5-S1 facet joints? How would proprioceptive signals resulting from the spinal adjustment itself modulate motor signals to the primary mover muscles, like multifidus? How would low back pain be affected as a result of altered motor signalling? These are just a few of the many questions that if answered will help to elucidate the mechanism of the therapeutic effects produced by spinal manipulative therapy. It seems we are finally starting to make some inroads into this complex problem.
The next time you are sitting frustrated in the 401 traffic … or, perhaps, as in the case of Dr. Jean-François Latour in the Yukon, canoeing home from the clinic (Canadian Chiropractor, February 2007), and your neck muscles are getting tighter by the minute, think about the traffic of proprioceptive signals from every muscle in your body taking the off-ramp at the inferior cerebellar peduncle. Imagine the plethora of sensory signals entering and leaving the cerebellar cortex coming from and going to all parts of the central nervous system. The amazing thing is, even though a single body movement changes the pattern of neuronal signalling (but when do we ever see an isolated movement?), and so the pattern of signalling coming into and leaving the cerebellum is constantly changing, the appropriate neuronal signals get to the appropriate target neurons at the appropriate time, the cerebellum integrates all incoming signals and sends out appropriate signals to modulate all motor activity, all in a matter of milliseconds and without fail. I continue to stand in awe of and have the utmost respect for our functional anatomy.
What’s in a spinal adjustment, anyway?•
What’s New in Spinal Anatomy?
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