Chapter 40: Muscles, vessels, nerves and joints of the back
Muscles
The prevertebral muscles are those that are situated on the front of the column in the neck and abdomen and supplied by ventral rami. The muscles of the back include (1) superficially the trapezius and latissimus dorsi; (2) then the levator scapulae, rhomboids, and serrati posteriores; and (3) the deep muscles of the back, which are mostly supplied by dorsal rami. The fascia of the back is attached to the spines of the vertebrae, and it proceeds laterally, as the thoracolumbar fascia, to ensheathe such muscles as the latissimus dorsi. Two small muscles, the serratus posterior superior and serratus posterior inferior, extend from the thoracic spines to the ribs and are supplied by ventral rami. The serrati and the thoracolumbar fascia serve as retinacula that retain the underlying muscles.
The deep muscles of the back (1) occupy a "gutter" on each side of the vertebral column; (2) are practically all innervated by dorsal rami; (3) comprise longer and more vertical bundles superficially and shorter and more oblique components deeply; (4) produce extension and lateral flexion of the vertebral column by their longitudinal bundles and rotation by their oblique components; and (5) may be considered, in terms of attachments, as a spinotransverse system superficially and a transversospinal group deeply (fig. 40-1).
The "spinotransverse system."
The "spinotransverse system" consists of the erector spinae and the splenius muscles (fig. 40-2). The erector spinae (or sacrospinalis) is the chief extensor of the back. This muscle group ascends from a very large common origin on the iliac crest, dorsal part of the sacrum and lumbar spinous processes. This makes up the large muscle bulge that can be seen lateral to the lumbar spinous processes. The muscles segregate into three longitudinal columns: (1) the spinalis medially; (2) the longissimus; and (3) the iliocostalis laterally. These columns of muscles appear in overlapping bands, with muscle insertions interdigitating with origins of more supeior portions of the muscle column. The spinalis muscles arise and insert on the spinous processes, with the most superior muscle fibers reaching the occiput. The longissimus muscle group forms the bulk of the erector and inserts along the transverse processes in the thoracic and cervical regions. It also has an insertion more laterally on the occiput. The iliocostalis muscle column inserts on the ribs, with the most superior attachment to the lower cervical transverse processes.
The vertical muscles on the posterior aspect of the neck are bandaged by the splenius muscles, which ascend obliquely from the thoracic spinous processes and insert into (1) the mastoid part of the temporal bone and the superior nuchal line (splenius capitis) and (2) the cervical transverse processes (splenius cervicis). The splenius rotates the head toward the side of contraction. The splenii of the two sides, acting together, extend the head.
The "transversospinalis system."
The "transversospinalis system" consists of the semispinalis and a number of small underlying muscles (multifidus and rotatores), deep to which are interspinous and intertransverse muscles. This name derives from the origin of these muscles from transverse processes (mamillary processes in the lumbar region) and insertion on the spinous processes. The more superficial muscles are more oblique, inserting on vertebrae up to six levels above their origin. The deeper muscles are more transverse in orientation, skipping fewer veertebrae between their origin and insertion.
The semispinalis, so called because it extends chiefly along the upper half of the vertebral column, is responsible for the longitudinal bulges on the posterior side of the neck on either side of the median plane. This part (semispinalis capitis) ascends from cervical and thoracic transverse processes to reach the occipital bone. This muscle is necessary to hold the head up and when it contracts bilaterally, it is the chief extensor of the neck. The semispinalis capitus muscle covers the suboccipital triangle and a deeper muscle, the semispinalis cervicis, that is attaching heavily to cervical spinous processes (fig. 40-2). The splenius, semispinalis capitis, and sternomastoid are the chief rotators of the head.
The multifidi are prominent in the lumbar region, arising from the mamillary processes and inserting on spinous processes several segements superior to the origin. Multifidi are quite poorly developed in thoracic and cervical regions. The rotators are found at all spinal levels, arising from transverse processes and inserting on spinous process of the immediately suprajacent (rotator brevis) or skipping one vertebra before inserting on the spinous (rotator longus).
There are also small intersegmental muscles between spinous processes (interspinous) or between the transverse processes (intertransverse). These small muscles and the rotators are very small and contribute little to directly moving the spine. However, they are heavily endowed with receptors that contribute important sensory feedback for control of spine motions.
The suboccipital triangle.
The suboccipital triangle (fig. 40-3) is delimited by three points: the spinous process of the axis, the transverse process of the atlas, and the lateral part of the occipital bone. The boundaries of the triangle are the muscles attached to these three points: the obliquus capitis inferior, obliquus capitis superior, and rectus capitis posterior major. The muscles, supplied chiefly by the suboccipital nerve, are mainly postural. They also have receptors that detect head position. The triangle is roofed by the semispinalis capitis, and its floor is the posterior atlanto-occipital membrane. The suboccipital triangle contains the vertebral artery and the suboccipital nerve, both lying in a groove on the upper surface of the posterior arch of the atlas. The subarachnoid space can be tapped by inserting a needle at the back of the neck and piercing the posterior atlanto-occipital membrane. This rarely performed procedure is termed cisternal puncture (see fig. 41-1).
Blood vessels
The vertebral artery.
The vertebral artery is one of the main vessels to the brain. It arises from the subclavian artery, and its course comprises four parts: cervical, vertebral, suboccipital, and intracranial. The vertebral portion courses through the transverse foramina of the upper six cervical vertebrae. After passing the transverse formen of the axis, the artery follows a tortuous course to reach the laterally-positioned transverse formen of the atlas. The suboccipital part courses from lateral to medial, passing posterior to the lateral mass of the atlas and lying in a groove on the superior side of the posterior arch of the atlas (see fig. 39-3). Within the suboccipital triangle, it enters the vertebral canal by passing a hiatus in the posterior atlantoaxial membrane. It then perforates the dura and arachnoid and traverses the foramen magnum to become intracranial.
The vertebral venous system.
The vertebral venous system is a valveless network that extends between the cranial dural sinuses and the pelvic veins and is connected with the azygos and caval systems (see fig. 24-2A). It allows a flow of blood in either direction, depending on intrathoracic and intra-abdominal pressure, and it permits the spread of carcinoma, emboli, and infections. The divisions of the network (see fig. 41-3) are as follows: (1) the internal vertebral plexus, which surrounds the dura and is drained by veins in the intervertebral foramina; (2) the basivertebral veins on the back of the vertebral bodies, which drain a network in the marrow spaces of the vertebrae; and (3) the external vertebral plexus, which lies on the anterior and lateral sides of the vertebral bodies and on the vertebral arches. The suboccipital plexus is an extensive and complicated part of the external plexus. It lies on and in the suboccipital triangle and drains the scalp. The suboccipital plexus receives the occipital vein. and sends tributaries to the vertebral vein, which descends through the foramina transversaria to end in the brachiocephalic vein.
Nerves
The innervation of the back is from the meningeal branches and dorsal rami of the spinal nerves.
Each spinal nerve gives off a meningeal branch (or sinuvertebral nerve), which reenters the vertebral canal and supplies vasomotor and sensory fibers to dura, ligaments, periosteum, and blood vessels.
The dorsal primary rami of the spinal nerves contain motor, sensory, and sympathetic fibers. They supply the muscles, bones, joints, and skin of the back. Most dorsal rami divide into medial and lateral branches.
The dorsal primary ramus of the C1 nerve root, known as the suboccipital nerve, usually has no cutaneous distribution. The medial branch of the dorsal primary ramus of C2 nerve root, termed the greater occipital nerve, supplies a large region of the scalp, usually extending at least to the vertex. The cutaneous division of the medial branch of the dorsal primary ramus of C3 is known as the third occipital nerve. The dorsal primary rami of the C1 and C6 to 8 nerve roots usually give no cutaneous branches, so that the C5 and T1 dermatomes are adjacent over the posterior neck. Through most of the trunk, the sensory distribution of dorsal primary rami appear in regular bands. However, the lumbar and sacral dorsal primary rami overlap the skin of the buttocks as a series of clunial nerves.
Joints
The joints of the vertebral column are (1) those between the bodies of adjacent vertebrae (intervertebral discs), (2) those of the vertebral arches (between articular processes), (3) the atlanto-occipital and atlanto-axial joints, (4) the costovertebral joints, and (5) the sacro-iliac joint.
Joints Between Bodies of Adjacent Vertebrae.
The bodies of adjacent vertebrae are united by longitudinal ligaments and intervertebral discs. The anterior longitudinal ligament extends from the anterior tubercle of the atlas to the sacrum and is attached to the anterior aspect of the vertebral bodies and discs. The posterior longitudinal ligament is a continuation of the tectorial membrane, extending from the occipital bone to the sacrum. It lies within the vertebral canal, and is loosely attached to the back of the vertebral bodies while expanding and attaching firmly to the posteiror part of the discs (fig. 40-4).
The intervertebral discs.
The intervertebral discs account for about a quarter of the length of the vertebral column. They are shock-absorbing, fibrocartilaginous joints between adjacent vertebrae. In young adults, each disc consists of a semigelatinous nucleus pulposus surrounded peripherally by an anulus fibrosus. Each disc is separated from the bone above and below by two growth plates of hyaline cartilage (fig. 40-4). The anulus fibrosis consists of fibrocartilage containing concentric layers of dense, regular connective tissue. The collagen in these layers is not vertically arranged. Instead, each layer is obliquely oriented, with adjacent layers having an opposite obliquity. Therefore, the collagenous fibers in an anular layer is oriented nearly at right angles to the adjacent layers. The anulus fibrosis surrounds and contains the nucleus.
The nucleus develops from the notochord in the embryo. The discs contain much water, diminution of which (temporarily during the day and permanently in advanced age) results in a slight decrease in stature. The fluid nature of the nucleus acts as a hydraulic spacer to maintain the height of disc. With advancing age, the entire disc tends to become fibrocartilaginous, and the distinction between nucleus and anulus becomes lost. The forces of body weight are then transmitted more directly to the anular fibers which can degenerate. Additionally, as the discs wear out, more forces can be transmitted to the posterior joints, which can also degenerate. Pathological change in an intervertebral disc may be followed by herniation of the nucleus pulposus, which then compresses adjacent nerves. In the lumbar region, herniated discs usually compress the nerve exiting at the level below the herniation. This is because the lumbar pedicles attach to the superior part of the vertebral bodies, meaning that discs are at the inferior aspect of the intervertebral formen. Since nerve roots tend to occupy the supeiror part of the intervertebral formen, disc herniation does not usually affect the nerve exiting at that spinal level. However, the subjacent nerve root must pass the herniated disc and can be involved (see fig. 41-4). For example, herniation of the disc between the L4 and 5 vertebra often irritates the L5 nerve root (which exits at the intervertebral formen between the L5 vertebra and the sacrum).
The cervical discs may develop fissures at the lateral edges that have been dubiously called uncovertebral joints. They are situated laterally between lips on the adjacent upper and lower surfaces of the vertebral bodies and can undergo degenerative changes similar to other joints.
Joints of Vertebral Arches.
The atlanto-occipital and atlanto-axial joints are special cases that are discussed below. The articular processes of adjacent vertebrae are united by plane synovial joints often called facet joints. The facet joints are rather typical joints with hyaline cartilage covering the joint surface and a fibrous articular capsule. *
Facet joints guide the types of movement that can occur at the segment. The articular surfaces have different orientations in the various vertebral regions (see fig. 39-5). In the cervical region below the axis, the joint surface on the superior articular process faces more superior than posterior. This joint permits some rotation, but this must be coupled with with lateral flexion. This is due to the fact that, when the suprajacent vertebra moves anterior on one side, that side is also forced superior by the obliquity of the facet joint. Therfore, the lower cervical spine with laterally flex toward the side of rotation. In the thoracic region, the superior articular process faces mostly posterior, with a slight supeior and lateral direction. This pattern permits the greatest degree of rotation of the spine, with the axis of rotation centered on the disc. Of course, this rotation is limited by the attachemnt of ribs. In the lumbar region, the superior articular processes face posterior and medial and are concave. This orientation permits flexion and extension, but little rotation. Of course, since there are large numbers of vertebral segments, small motions between any two segements can add up to significant overall mobility when summed over the entire spinal column.
The vertebral arches are connected by ligaments, particularly strong in the lumbar region, e.g., the ligamenta flava between the laminae. The spinous processes are united by the interspinous and supraspinous ligaments, which merges in the neck with the ligamentum nuchae. This latter ligament is a median partition between the muscles of the two sides of the neck and is attached to the occipital bone.
Atlanto-occipital and Atlanto-axial Joints.
The atlanto-occipital joints are between the concave articular surface on the supereior side of the lateral masses of the atlas and the convex occipital condyles. Synovial in type, the right and left joints allow nodding of the head around a transverse axis and sideways tilting of the head around an anteroposterior axis. Anterior and posterior atlanto-occipital membranes connect the respective arches of the atlas to the margins of the foramen magnum (figs. 40-5 and 40-6).
The atlanto-axial joints, synovial in type, unite the first two vertebrae. The two lateral joints are between the articular processes. These joints are in a mostly horizontal plane, although the lateral part of the joint is inferrior to the medial part. This joint permits anterior and posterior motion of the atlas on the axis. The median atlanto-axial joint is between (1) the anterior arch and transverse ligament of the atlas and (2) the pivot formed by the dens of the axis (fig. 40-5).
The transverse ligament of the atlas unites the medial aspects of the lateral masses (fig. 40-6) and, together with longitudinal bands attaching to the foramen magnum and the body of the axis, constitutes the cruciform ligament of the atlas (fig. 40-6B). This ligament holds the anterior part of the dens against the posterior side of the anterior arch of the atlas and is designed to prevent posterior motion of the dens (which would damage the upper spinal cord). Since the lateral joints between the atlas and axis permit anterior and posterior motion, the dens provides a pivot around which the atlas can rotate. In fact, there is nearly 45 degrees of rotation permitted to each side between these two segments, an enormous contribution to the overall rotation of the spine. The apex of the dens is connected to the occipital bone by a median apical ligament and two lateral alar ligaments. The alar ligaments are the principal restriction to rotation since they become tight on the side that moves anterior during rotation. Finally, the tectorial membrane, the upward continuation of the posterior longitudinal ligament, is anchored to the basilar part of the occipital bone (fig. 40-5).
Costovertebral Joints.
The costovertebral joints (see fig. 20-6) are those between 1) the heads of the ribs and the vertebral bodies and (2) the tubercles of the ribs and the transverse processes (costotransverse joints).
Sacro-iliac Joint.
The sacro-iliac joint (see fig. 31-7) is a plane synovial joint formed by the union of the auricular surfaces of the sacrum and ilium. There are ligaments surrounding this joint on all sides, however, the strongest of these are located posteriorly since the body weight is transmitted through the posterosuperior part of the sacrum. The ilium is connected to the transverse process of the L5 vertebra by various strong bands collectively known as the iliolumbar ligaments. The weight of the head, upper limbs, and trunk is transmitted through the sacrum and ilia to the femora when one is standing and to the ischial tuberosities when one is sitting.
Movements.
The movements of the vertebral column are flexion (forward bending), extension (backward bending), lateral flexion to the right or left (bending to the side), and rotation (around a longitudinal axis). The axis of each type of movement runs through the nucleus pulposus. The cervical and lumbar regions are the most mobile and are frequent sites of degeneration and of aches and pains. Flexion and extension of the head occur mainly at the atlanto-occipital and atlanto-axial joints. The skull and the atlas rotate on the axis at the three atlanto-axial joints, pivoting on the dens like a ball-and-socket joint.
The chief flexors of the vertebral column are the prevertebral muscles, recti abdominis, iliopsoas, scaleni, and stemomastoids. Gravity may also be important, and the movement is controlled by the erector spinae muscles. The chief extensors are the erector spinae. Lateral flexion is carried out mainly by the oblique muscles of one side of the abdominal wall. The muscles of the back are relatively inactive when one is standing at ease. Injury or inflammation may easily result in reflex spasm of the muscles of the back.
Additional reading
De Palma, A. F., and Rothman, R. R., The Intervertebral Disc, W. B. Saunders Company, Philadelphia, 1970. An account of disc disease.
Questions
40-1 What are the prevertebral muscles?
40-2 Which muscles of the back can be most readily identified in vivo?
40-3 What are the main actions of the deep muscles of the back?
40-4 Which muscles are included in the "spinotransverse system"?
40-5 Which is the most prominent muscle of the "transversospinalis system"?
40-6 What is the most important structure in the suboccipital triangle?
40-7 How is the vertebral venous system arranged?
40-8 How many intervertebral discs are present in the body?
40-9 What is an intervertebral disc?
40-10 What is a "slipped disc"?
40-11 On which nerve would herniation of the disc between the L4 and L5 vertebra be likely to press?
40-12 Which joints are involved in (a) nodding in approval and (b) shaking the head in disapproval?
40-13 Which are the most mobile regions of the vertebral column?
Figure legends
Figure 40-1 Horizontal section through the muscles of the back, showing the arrangement of the spinotransverse and transversospinal systems. The posterior (P) layer of the thoracolumbar fascia encloses the latissimus dorsi. The middle (M) and anterior (A) layers of the thoracolumbar fascia enclose the quadratus lumborum. See also fig. 29-5. L, "lumbar intermuscular aponeurosis" (N. Bogduk, J. Anat., 131:525-540, 1980).
Figure 40-2 the erector spinae, splenius and transversospinalis. (after Winkler.)
Figure 40-3 The suboccipital triangle. Most of the semispinalis capitis has been removed. Note the greater occipital nerve emerging at the lower border of the inferior oblique muscle. The vertebral artery and the suboccipital nerve are seen in the triangle. The massive suboccipital venous plexus has been omitted. On the left side, lines indicate the directions and attachments of the muscles that bound the triangle.
Figure 40-4 Intervertebral discs in median and horizontal section.
Figure 40-5 Median section of the atlas and axis. (After Poirier and Charpy.)
Figure 40-6 The ligaments of the atlas and axis, posterior view. A shows the vertebral arteries. B shows the interior of the vertebral canal after removal of portions of the skull and vertebrae.
*Yu SW, Sether L, Haughton VM. Facet joint menisci of the cervical spine: correlative MR imaging and cryomicrotomy study. Radiology. 164:79-82, 1987. This combined radiographic and anatomical study defined 3 types of intraarticular extensions from the joint capsule that they termed "menisci" in the cervical spine. One arrangement appears to be mainly in childhood and one appears only associated with joint degeneration. The function of these "menisci" is not known.