DEEP BACK MUSCLES

MAJOR COMMENTS: The true (deep or intrinsic) muscles of the back constitute the EPAXIAL MUSCULATURE and thus are innervated solely by muscular branches of DORSAL PRIMARY RAMI. Collectively, they form the largest muscular mass in the body. Developmentally, they arise from the myotomic portions of adjacent somites ranging from the sacrum to the back of the head along the entire length of the vertebral column. The more superficial of these segmental (or rather intersegmental muscles) later undergo multiple fusions producing multifasciculated longitudinal columns whereas the deeper layers tend to maintain their embryonic intersegmental relations. In the adult this muscular mass is thickest in the lumbar region producing easily palpable fleshy columns which gradually thin out over the thorax. Additional muscles increase the thickness of the neck.

CLASSIFICATION: There is no universal agreement on grouping the intrinsic back muscles. For ease of comprehension I suggest the following scheme:

I. LONG SYSTEM

A. Splenius

B. Erector spinae

1. Iliocostalis

2. Longissimus

3. Spinalis

II. INTERMEDIATE SYSTEM (TRANSVERSOSPINALIS)

A. Semispinalis

B. Multifidus

C. Rotatores

III. SHORT SYSTEM

A. Suboccipitales

B. Interspinales

C. Intertransversarii

D. Levatores costarum

STUDY SUGGESTIONS: You may be disquieted to learn that the above classification is only a rudimentary outline in that each of the listed muscles may be subdivided into two or more named fascicles. Please don't panic. You are not expected to learn the intricate details of these myriad slips nor are you expected to define them by dissection. And the reason is -- they work together in functional groups. Any effort to memorize their precious details would be a waste of study time and neuronal circuitry.

• EVERYTHING YOU NEED TO KNOW ABOUT THE THORACOLUMBAR FASCIA:

The deep back muscles are enveloped by a fascial sheath known as the thoracolumbar fascia. The cervical and thoracic portions of this sheath are quite thin but the lumbar part is well developed being reinforced by the aponeurosis of the latissimus dorsi dorsally, and the posterior fascial attachments of the internal abdominal oblique and transversus abdominis laterally and ventrally. After splitting around the lumbar epaxial mass the thoracolumbar fascia is attached to the spinous processes of the lumbar and sacral vertebrae, the lumbar transverse processes, the iliac crest and the 12th rib.

• THE MOST IMPORTANT THINGS YOU NEED TO KNOW ABOUT THE LONG SYSTEM:

SPLENIUS: The splenius cervicis is a spino-transverse muscle running superolaterally from the spinous processes of the upper thoracic vertebrae to the transverse processes of the cervical vertebrae. The larger, more superficial splenius capitis (= "bandage") is a spino-occipital muscle arising from the ligamentum nuchae and the upper thoracic spinous processes and inserting to the occipital bone just below the superior nuchal line and to the mastoid process of the temporal bone (the mastoid area is believed to represent the cranial homologue of vertebral transverse processes).

ERECTOR SPINAE: This is a large tripartite, musculotendinous column (and layer) which extends from the sacrum and ilium to the occiput. It is thickest in the lumbar region and fills the vertebral groove (between the spinous processes and angles of the ribs) as it ascends cranially. Its three main divisions are the iliocostalis, longissimus and spinalis.

1. ILIOCOSTALIS: The lateral column arises from the sacrum and iliac crest and is attached by multiple tendinous slips to the angles of the ribs and to the cervical transverse processes. It is subdivided into lumbar, thoracic and cervical parts.

2. LONGISSIMUS: The intermediate column arises in common with the iliocostalis and is attached to the ribs, thoracic and cervical transverse processes, and to the posterior aspect of the mastoid process. It is subdivided into thoracic, cervical and capital parts.

3. SPINALIS: The medial (smallest) column arises from the spinous processes of T11-L2 and skipping T10 (the "center of motion") is attached to the spinous processes of the upper thoracic vertebrae. Its cervical and capital parts are inconstant.

• THE MOST IMPORTANT THINGS YOU NEED TO KNOW ABOUT THE INTERMEDIATE SYSTEM:

TRANSVERSOSPINALIS: This complex layer lies deep to the erector spinae and consists of fascicles running between transverse processes to more superior (cranial) spinous processes. Three divisions are recognized depending on their relative positions and fascicle lengths: semispinalis, multifidus and rotatores.

1. SEMISPINALIS: The most superficial layer occupies approximately half of the vertebral column ranging from the upper thoracic vertebrae to the occiput. Its fascicles span 5-6 vertebrae. It is subdivided into thoracic, cervical and capital parts. The semispinalis capitis is the largest muscle in the back of the neck, lies deep to the splenius capitis and forms the roof of the suboccipital triangle. It is attached to the occipital bone between the superior and inferior nuchal lines.

2. MULTIFIDUS: The intermediate layer ranges from S4-C2. Its fascicles span 3-4 vertebrae.

3. ROTATORES: The deepest layer also ranges from S4-C2. Its fascicles span one (short) or two (long) vertebrae. NOTE: The fascicles of the transversospinalis overlap in length, e.g. the shortest bundles of the semispinalis are the same length as the longest bundles of the multifidus which makes them difficult to separate.

• THE MOST IMPORTANT THINGS YOU NEED TO KNOW ABOUT THE SHORT SYSTEM (excluding the suboccipital muscles):

SHORT SYSTEM: The deepest group of the deep back muscles consists of the suboccipitales (described in the next section), the intertransversarii, the interspinales, and the levatores costarum.

INTERTRANSVERSARII: These muscles pass between adjacent transverse processes. They are best developed in the cervical and lumbar regions and are frequently paired into anterior-posterior and medial-lateral sets.

INTERSPINALES: These paired muscles pass between adjacent spinous processes. They are best developed in the cervical and lumbar regions and are basically absent in the thoracic region.

LEVATORES COSTARUM: These muscles arise from the transverse processes of C7-T11 and are attached to all twelve ribs just medial to their angles. They are subdivided into long (spanning two intercostal spaces) and short (spanning one intercostal space) bundles.

• THE BIG 5 GROUP FUNCTIONS: The synergistic actions of these muscles are enormously complex and are still undergoing investigation. It is thus impractical (if not impossible) to assign specific functions to individual fascicles since any activity ultimately involves the entire mass. Their major group actions, however, can be defined as follows:

1. EXTENSION of the vertebral column (including the atlanto-occipital joint)

2. LATERAL BENDING (= "lateral flexion") of the vertebral column

3. ROTATION of the vertebral column (including the atlanto-occipital joint)

4. ANTIFLEXION of the vertebral column (i.e., acting as antigravity muscles, they control flexion)

5. STABILIZATION of the vertebral column (by maintaining constant "tone").

• 4 MAJOR FUNCTIONAL CONCEPTS:

1. Extension involves bilateral activity.