The back forms the posterior aspect of the trunk and functions for posture, gait, support of weight and protection of the spinal cord. The bones of the back form the vertebral column that serves as a pillar of the body and particularly vulnerable to acute and chronic pain. Thus, a comprehensive knowledge of the back and its morphological components is critical for both primary care and specialized medical interventions. The purpose of this section to characterize important features of the back comprising the vertebral column as well as the three regions of the back.
Back pain is widely prevalent and costly resulting in substantial socioeconomic burden, with patients incurring health care expenditures approximately 60% higher than those without back pain. Lumbar spinal stenosis is a primary cause of lower back pain and affects more than 200 000 adults in the United States, resulting in substantial pain and disability. Lumbosacral radiculopathy represents one distinct presentation of low back-related leg pain, which constitutes between 23% – 57% of back pain cases. It is the most common reason for spinal surgery in patients over 65 years old. Thus an understanding of back musculature and nerve innervation is central to the study of the back.
The back receives nerve innervation from the spinal nerves. A typical spinal nerve consists of both sensory (afferent) and motor (efferent) components. Entering the dorsal horn of the spinal cord are the sensory axons that form the dorsal roots of the spinal nerve. Associated with the dorsal roots are the spinal sensory ganglia (aka, dorsal root ganglia) comprising pseudounipolar sensory neurons with one fiber directed peripherally while the other is directed centrally toward the dorsal horn. Exiting the ventral horn of the spinal cord are the motor axons forming the ventral roots of the spinal nerve. The spinal nerve is considered to begin where these dorsal and ventral roots unite. Shortly after the formation of the spinal nerve it divides into a dorsal primary ramus which innervates the epaxial musculature and associated skin and into a ventral primary ramus that innervates the hypaxial muscle and associated cutaneous structures. These spinal nerves carry voluntary motor, general sensory and sympathetic fibers.
A spinal nerve in the embryo innervates a corresponding embryonic dermatome, which eventually differentiates and forms skin. Therefore, the dermis is innervated segmentally by spinal nerves. In the postnatal individual, a dermatome refers to the area of skin which receives sensory information from a single spinal segment. These areas are distributed over the body in a uniform manner, with some overlap at their adjacent margins. This is especially obvious in the trunk. It is important to recall that a population of embryonic cells retain their original innervation regardless of where those cells subsequently migrate. Therefore, growth of the limbs and shoulder girdle modifies their dermatome pattern somewhat. The distribution of this segmental arrangement will become important when attempting to identify spinal nerve damage.
Muscles of the Back include superficial, intermediate and deep groups. The superficial group are associated with shoulder and upper limb movements. The intermediate group concerns thoracic movements, particularly rib position. The deep group, or intrinsic back muscles, concerns movements of the vertebral column and head. These are epiaxial ‘true back muscles’ since they are supplied by dorsal spinal nerve rami. (Note: Ventral rami supply the hypaxial muscles). They can be further subdivided into superficial (spinotransverales), intermediate (erector spinae) or deep (transversospinalis) groups.
The superficial back comprises the skin, fascia and a group of dorsal muscles that connects the limb to the trunk as well as the nerves, arteries and veins that support these structures. Although the muscles of the superficial back are located on the dorsal (posterior) aspect of the the back, they receive innervation from the ventral rami of cervical nerves indicating that these muscles of migrated from the anterior region of the trunk.
Review the Vertebral Column (Section 1.12) prior to this dissection.
TRAPEZIUS. The trapezius is a large triangular shaped muscle that extends superficially from the occipital region inferiorly to the thoracic region and laterally to the scapula. Bilaterally, the two trapezius muscles form a trapezoidal shape. The trapezius attaches to the spinous processes of the thoracic vertebrae and lower cervical vertebrae as well as the nuchal ligament and superior nuchal line of the occiput. A external occipital protuberance may be present. The muscle extends laterally to attach to the spine of the scapula and acromion as well as the lateral aspect of the clavicle. It is innervated by the accessory nerve (CN XI) and typically the cervical 3rd and 4th ventral rami. The upper portion of the trapezius functions to elevate the shoulder while in the absence of function the shoulders cannot be elevated against resistance. The middle and lower fibers work to retract the scapula and rotate it either laterally or medially.
LATISSIMUS DORSI. The latissimus dorsi is a large, triangular muscle that arises from the spines of the lower 6 thoracic vertebrae and indirectly from the spinous processes of the lumbar and and sacral vertebrae though its attachment to the posterior layer of the thoracolumbar fascia and finally the iliac crest. The muscle passes superolaterally receiving some attachments from the lower ribs and to some extent the inferior angle of the scapula. The muscle spirals around the inferior border of the teres major and inserts into the floor of the intertubercular (bicipital groove). This muscle is supplied by the thoracodorsal nerve (from the brachial plexus). It is a powerful adductor, medial rotator and extensor of the upper extremity and it commonly referred to as the ‘swimmers muscle’ owing to forceful downstroke movement of the arm during this activity.
TRIANGLE OF AUSCULTATION. The borders of the triangle of auscultation include the upper border of the latissimus dorsi, trapezius and inferomedial border of the scapula while the rhomboid major forms the floor. Owing to the absence of significant overlying tissue, the diaphgram of the stethoscope is placed here over the 6th intercostal space to listen to sounds of the thorax particularly during forced inspiration and expiration.
LUMBAR TRIANGLE. This triangle is formed by the medial aspect of the latissimus dorsi, the external oblique of the abdomen, and the iliac crest. In rare cases, the intestine can herniate through this triangle due to the relative thinness of the overlying tissues.
THORACOLUMBAR FASCIA. The back is covered with a superficial fascia directly underneath the skin. A deep fasical layer includes thoracolumbar fascia forming a thick and glistening retaining layer for the underlying back muscles. The posterior layer extends from the vertebral spinous processes laterally to the angles of the lower ribs in the thoracic region. In the lumbar region, the layering is more complicated. The posterior layer extends laterally and splits around the latissimus dorsi. It then joins middle layer and anterior layer that take origin from the from intertransverse ligaments of the vertebral column and all layers combine thus enveloping the deep back muscles. These conjoined ligament provides attachment for the oblique muscles of the anterior abdominal wall.
LEVATOR SCAPULA AND RHOMBOIDS. The levator scapula is thin strap-like muscle that attaches to the transverse processes of the first 4 cervical vertebrae and extends laterally to insert into the superior angle of the scapula. It is supplied by the dorsal scapular nerve as well as ventral rami of cervical spinal nerves 3 & 4 and act to elevate the scapula. The rhomboid major is a thin muscle that extends from the spinous processess and supraspinous ligaments of T2-5 and inserts into the medial border of the scapula. The rhomboid minor is commonly continuous with the major and it arises from the spinous processes of C7 and T1 and inserts into the medial border of the scapula near to the origin of the spinous process of the scapula. These two muscles are supplied by dorsal scapular nerve and function to retract the scapula.
ARTERIES & NERVES. Running superiorly in the superfical fasica is the greater occipital nerve and occipital artery. The occipital artery is a branch of the external carotid artery (covered in the Head and Neck section), that continues superior to supply the occipital region. The greater occipital nerve is a branch of the dorsal ramus of the C2 spinal nerve and provides cutaneous sensory innervation to the back of head and contributes to the C2 dermatome (note there is no dorsal C1 dermatome; see suboccipital nerve in the Deep Back section). Cutaneous branches of the dorsal primary rami can be easily identified in the superior fascia and piercing the trapezius. These cutaneous nerves are accompanied by small arteries and veins.
The transverse scapular artery arises from the thyrocervical trunk, a branch of the subclavian artery. It runs through the neck and passes deep to the trapezius within the subtrapezial plexus of veins where it can be seen. The dorsal scapular artery typically arises directly from the subclavian artery, progresses posteriorly and runs along the medial border of the scapula in the company of the dorsal scapular nerve that supplies the rhomboid muscles.
Answers: 6, 1, all are correct, 2