The Neck - A Sensory Organ for Balance
06/05/10 12:38 Filed in: Clinical Pearls | From the Research
When we operate as clinicians in the mechanical realm there is a tendency to view the neck simply as a series of linkages that enable the head to move around on top of the trunk. But it’s really so much more than that. The extraordinary repertoire of movements available to the human neck is only made possible by an exquisite neurology that ensures precise neuromuscular control over the joint segments.
To really appreciate this level of neural control, just focus your attention on your fingers for a moment. Feel how finely you can control their movements and how accurately you can perceive their position. Much of this sensory-motor prowess over your fingers is due to the sheer number of muscle spindles in the musculature - all providing a rich stream of proprioceptive information. And 16 muscle spindles per gram of muscle in the lumbricals certainly sounds like a lot...
But consider this: the obliquus capitus inferior muscle in the upper neck is endowed with a whopping 242 spindles per gram of muscle(1). That’s over 15 times the spindle density of finger muscles. And where does that data stream go? Well, much of it projects directly to nuclei in the brainstem for the control of orientation and equilibrium. In short - balance and postural stability.
In fact, we know that the visual, vestibular and proprioceptive input all converge in the brainstem and cerebellum, where it’s fused together to produce an internal frame of reference for your spatial position. So why is the neck so important?
Well, two of the three major sensory systems for balance and orientation are anchored in the head – the retinal sheet and vestibular apparatus. So they tell us about movement and position of the head. It’s the neck that tells us where the head actually is relative to the body…
So the neck could really be viewed as a massive sensory link between the head and trunk. It must continually report its position with pinpoint accuracy in order for the central nervous system to activate the correct postural reflexes to maintain stability of the entire body. And it seems that this is even more important as we age…
An interesting article in Experimental Brain Research(2) reports that the neck compensates for age-related deterioration of the vestibular system. Basically, vestibular hair cells, the little biological strain gauges in our inner ear that detect gravity and self-motion, are lost as we age. So the central nervous system looks to other sources of balance input to fill the gap. The neck would seem to be the natural choice, given its incredible sensory capabilities.
So with declining vestibular signalling, the elderly appear to rely more and more upon a proprioceptively ‘alive’ neck for balance and protection against falls. If the neck is rigid and degenerated then an enormous reliance is unknowingly placed upon vision. Such an over-reliance is often easily unmasked with a simple Romberg’s test. However, a problem arises when the visual environment is poor, as the nervous system has little else to fall back on and sudden disorientation and a fall commonly results.
As they conclude in this article:
“We hold that the mechanism by which humans combine vestibular and proprioceptive inputs compensates not only for the vestibular deterioration which normally occurs at low rotational frequencies, but also for that associated with disease (e.g. vestibular loss due to basal meningitis) and with age. The reason is that humans reference their self-motion perception by means of reliable proprioceptive input primarily to the body support surface and use the vestibular input to take into account the support kinematics in space”.
So next time you’re examining a patient’s neck, try and appreciate the ‘dizzying’ level of neurological control that lies underneath those small segmental movements. By gently enriching the proprioceptive sensibility of the neck in an elderly patient you may just be helping to prevent a major cause of death and disability.
Dr Anthony D. Nicholson - BSc. M.Chiro DACNB
References:
1. Boyd Clark L, Briggs C, Galea M. Muscle spindle distribution, morphology and density in the longus colli and multifidus muscles of the cervical spine. SPINE 2002; 27: 694-701
2. Schweigart G, Chien RD, Mergner T. Neck proprioception compensates for age-related deterioration of vestibular self-motion perception. Exp Brain Res 2002; 147: 89-97
To really appreciate this level of neural control, just focus your attention on your fingers for a moment. Feel how finely you can control their movements and how accurately you can perceive their position. Much of this sensory-motor prowess over your fingers is due to the sheer number of muscle spindles in the musculature - all providing a rich stream of proprioceptive information. And 16 muscle spindles per gram of muscle in the lumbricals certainly sounds like a lot...
But consider this: the obliquus capitus inferior muscle in the upper neck is endowed with a whopping 242 spindles per gram of muscle(1). That’s over 15 times the spindle density of finger muscles. And where does that data stream go? Well, much of it projects directly to nuclei in the brainstem for the control of orientation and equilibrium. In short - balance and postural stability.
In fact, we know that the visual, vestibular and proprioceptive input all converge in the brainstem and cerebellum, where it’s fused together to produce an internal frame of reference for your spatial position. So why is the neck so important?
Well, two of the three major sensory systems for balance and orientation are anchored in the head – the retinal sheet and vestibular apparatus. So they tell us about movement and position of the head. It’s the neck that tells us where the head actually is relative to the body…
So the neck could really be viewed as a massive sensory link between the head and trunk. It must continually report its position with pinpoint accuracy in order for the central nervous system to activate the correct postural reflexes to maintain stability of the entire body. And it seems that this is even more important as we age…
An interesting article in Experimental Brain Research(2) reports that the neck compensates for age-related deterioration of the vestibular system. Basically, vestibular hair cells, the little biological strain gauges in our inner ear that detect gravity and self-motion, are lost as we age. So the central nervous system looks to other sources of balance input to fill the gap. The neck would seem to be the natural choice, given its incredible sensory capabilities.
So with declining vestibular signalling, the elderly appear to rely more and more upon a proprioceptively ‘alive’ neck for balance and protection against falls. If the neck is rigid and degenerated then an enormous reliance is unknowingly placed upon vision. Such an over-reliance is often easily unmasked with a simple Romberg’s test. However, a problem arises when the visual environment is poor, as the nervous system has little else to fall back on and sudden disorientation and a fall commonly results.
As they conclude in this article:
“We hold that the mechanism by which humans combine vestibular and proprioceptive inputs compensates not only for the vestibular deterioration which normally occurs at low rotational frequencies, but also for that associated with disease (e.g. vestibular loss due to basal meningitis) and with age. The reason is that humans reference their self-motion perception by means of reliable proprioceptive input primarily to the body support surface and use the vestibular input to take into account the support kinematics in space”.
So next time you’re examining a patient’s neck, try and appreciate the ‘dizzying’ level of neurological control that lies underneath those small segmental movements. By gently enriching the proprioceptive sensibility of the neck in an elderly patient you may just be helping to prevent a major cause of death and disability.
Dr Anthony D. Nicholson - BSc. M.Chiro DACNB
References:
1. Boyd Clark L, Briggs C, Galea M. Muscle spindle distribution, morphology and density in the longus colli and multifidus muscles of the cervical spine. SPINE 2002; 27: 694-701
2. Schweigart G, Chien RD, Mergner T. Neck proprioception compensates for age-related deterioration of vestibular self-motion perception. Exp Brain Res 2002; 147: 89-97
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