Suboccipital Musculature - Morphology, Functions and Variants - An Update- Juniper Publishers
Juniper Publishers- Journal of Complementary Medicine
Introduction
The suboccipital muscle plays an important role in
the clinical reasoning of osteopaths, but is also examined and treated
by masseurs, chiropractors, manual therapists and in physiotherapy in
the context of various complaints. It lies in the depth of the
craniocervical junction and connects and moves the head joints. However,
the SOM is involved in many other functions of the human body and is
much more complex than the first glance in the anatomy book makes it
appear. The craniocervical junction is the most mobile part of the spine
and at the same time it hosts important vital structures such as the
brainstem or the transversal artery. In addition to the numerous and
tensile band structures that secure this region, the SOM play an
important role in terms of several functional tasks. This makes them the
target of numerous therapeutic considerations for various dysfunctions
and clinical pictures. For example, in the treatment of tension-type
headaches [1], after craniocerebral trauma - as the atrophy of the
Rectus capitis posterior minor (RCPmi) significantly correlates with the
post-traumatic complaints [2], or in the treatment of neurodynamic
dysfunction of the Nevus medianus [3-4]. Since the first description of
the mycural bridge of the RCPmi by Hack et al. [5], this connective
tissue bridge between the SOM and the highly cervical dura mater is
thought to play a significant role in the pathogenesis of headache,
changes in sensomotor function and cerebrospinal fluid flow [5-8]. In
order to effectively investigate and treat the complex anatomy and
functioning of SOM and the structures involved, such as myo-dural
bridges, cervical joints or neurological connectivity using
non-invasive, functional methods, this mini-review provides some recent
research on SOM and hopefully encourages to integrate this
multi-functional tissue into clinical reasoning for various
dysfunctions.
Sensomotoric, Coordination and Perception in the Room
The SOM has a high density of muscle spindle per
gram. Muscle spindles per gram are found here between 98 (Rectus capitis
posterior major - RCPma) and 242 (M. obliquus inferior), which is
immense compared to the already well-structured hand muscles (M.
opponens pollicis - 17 spindles / gram) [9]. As in other parts of the
body, the SOM also shows a higher density of rotational muscles, in this
case the obliquus inferior (OCI) and the superior (OCS). The high
number of muscle spindles of the SOM seems to be a prerequisite both for
the function as a receptor, as well as an effector and for the
interaction with various equilibrium and orientation systems. The
extraorbital eye muscles appear to have very comparable densities of
muscle spindles [10], which seems to be the basis for the oculo-cervical
and optokinetic reflexes arising from both organs. In addition to the
optokinetic and the oculocervical control circuits, the said density of
muscle spindles continues to be the basis for the cervico-vestibular
control circuits [11]. In addition to this knowledge and various
empirical experiments, the long discussed controversial cervicogenic
cervicogenic dizziness was recently accepted by the mass of evidence
[12].
With regard to the above-described link between the
vestibule and the SOM, a pain-free, sufficiently mobile and easily
recruited suboccipital region seems to be the basis for these control
circuits. Complementary and alternative medical therapies should define
these as treatment goals in the treatment of cervicogenic dizziness and,
if necessary, re-train them once these basics have been restored. In
addition to balance exercises, numerous oculo-cervical exercises can be
used [13]. Through this complex linkage of SOM to other sensory organs
and the
high number of muscle spindles, the SOM plays a significant role
in orientation in space.
Morphological and Anatomical Aspects
In addition to motor innervation of the SOM, the C1 spinal
nerve also appears to deliver sensory fibers to the lateral atlantooccipital
articular capsule, sharing the sensory innervation of
this structure with the hypoglus nerve [14]. Irritations of the
aforementioned capsule parts could, in addition to the sensory
irritation of the tongue in neck-tongue syndrome, also lead to
hypertension or altered sensorimotor function in the area of
SOM due to nociceptive afferents. The fiber distribution of the
SOM is very homogeneous, allowing both postural control and
dynamic functions [15]. As with many other structures, the
SOM facilities seem to vary quite a bit. Thus, Yamauchi et al.
Show that 2.3-4.5% of a population can have, for example, the
a bilateral aplastic RCPmi -replaced by adipose tissue: or the
RCPma has two to three instead of a single muscle belly [16].
As described above, morphological RCPmi shows the clinically
most significant changes in various pathomechanisms. Thus,
it is atrophied in craniocerebral trauma [2] and hypertrophied
in chronic headache [1]. Both of these changes seem to have a
functionally negative influence on the respective symptoms,
which has to be considered clinically. Thus, in addition to the
reduction of nociceptive inputs and inhibition in headache
patients, facilitation and advanced training in atrophy may be in
the foreground. It should be noted that the SOM must be palpated
very deeply for manual intervention, as the SOM is the third and
most profound layer of the high-cervical musculature under the
trapezius muscle and the mm. splenius capitis and semispinalis
capites represents. Due to their complex, three-dimensional
position, the motion functions of the SOM are not always to
be classified at first glance. The RPCma and RCPmi lengthen
each other by up to 30% in craniocervical flexion, whereas in
a heterolateral rotation an extension of up to 40% in the area
of the RCPma and the OCI occurs [17]. The SOM is actively
involved in both protraction [18] and head retraction [19] after
recent electromyographic measurements, helping to maintain
joint conformation during sagittal movement. Due to the clearly
different mechanics between the craniocervical transition and
the lower cervical spine, as well as the complex neurological
and vascular interconnections of the SOM, the knowledge of
embryological development is very useful [20]. Thus, the three
upper segments C0-C2 develop together from four occipital
and three cervical somites, which explains the networking of
diverse functional and anatomical structures, including possible
developmental disorders [21].
Myodural Bridges
Myodural bridges (MDB) denote fibrous connective tissue
that pull from the SOM towards the spinal canal and insert at
the dura mater. In the meantime, MDBs have been detected in
the RCPmi, RCPma and the OCI [6,22-23]. These have a common
approach with the so called “to be named” ligament fibers of the
ligamentum nuchae [24] and extend through both the atlantoaxial
and atlanto-occipital spaces [25]. The fibers of the MDB
consist of collagen type-1 and are thus resistant to tensile stress
and can directly transfer tension to the high-cervical dura [26].
On the one hand the function of the MDB seems to be on the
one hand the posterior stabilization of the dura mater spinalis
[6-8] and on the other hand the drive of the cerebrospinal fluid
transport in the spinal canal [7,27-28]. Of further interest,
the sites of MDB appear to be in the atlanto-axial and atlantooccipital
spaces. Membrane-free zones enriched with fatty
tissue exist here, which ensures the MDB frictionless [29]. This
transition zone between the SOM and the spinal canal could
be a potential location for dysfunction and lead to various
symptoms due to friction, which could be treated by local
manual techniques, for example [20]. In particular, the mobility
of the head joints, but also symmetric stress patterns of the
MDB-forming structures and intraspinal tension vectors could
be the target of manual interventions. However, in addition to
a potential source of headache and cervical symptoms, further
limitations in movement and disbalances in whole-body
biomechanics may arise. Thus, first empirical studies carried
out multiple effects after manual treatment regarding the SOM,
such as mobility improvements of the N. medianus [3,4], the
mouth opening [30] and also far from the intervention area
improvements of mobility of the lower extremities [31-32],
questionable is whether the effects are due to the superfiscial
myofascial chains discussed by the authors. The effects could
also be due to an intraspinal mobility or tension regulation by
influencing the SOM and MDB. The effects of manual techniques
in the area of SOM on the biochemistry of the blood seemed to be
little researched. Fernández-Pérez et al. show that, for example,
there is a significant increase in CD-19-encoded B lymphocytes
after application of manual techniques in the field of SOM
[33]. However, the clinical relevance and breadth of treatment
should be further underpinned by further investigation and
other SOM techniques, such as muscle energy techniques, joint
manipulation, or defined technique combinations.
Conclusion
SOM plays a major role in many dysfunctional processes
in the human body and need not only be studied and treated
in the context of local nociception or movement deficits. Thus,
this highly complex muscle group can further influence the
balance and coordination mechanisms, the mobility of the
temporomandibular joints and neurodynamic situations down
to the lower extremity. This should be noted by all SOM treating
and investigating disciplines to exploit the potential of this
region therapeutically. The role of MDB in these effects needs
to be further explored in order to be able to perform functional
interventions as effectively as possible.
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