The Big Picture

The skin of the back is thick, with increasing thickness toward the nape of the neck. The cutaneous innervation of the back is segmentally innervated through the dorsal rami of spinal nerves. The spinous processess of the vertebrae and other osteologic landmarks are palpable, which enable localization of spinal levels through surface landmarks.

Cutaneous Innervation

The skin of the back is segmentally innervated by cutaneous nerves that originate from the dorsal rami (Figure 1-1A). Dorsal rami contain both motor and sensory neurons as they branch from each level of the spinal cord and course posteriorly in the trunk. The motor neurons terminate in the deep back muscles (e.g., erector spinae muscles), where they cause muscle contraction. The sensory neurons, however, continue on and terminate in the skin where they provide cutaneous sensations such as pain, touch, and temperature at each dermatomal level of the back (Figure 1-1B). There is some segmental overlap of the peripheral sensory fields from adjacent dermatomes.

It should be noted that

• The dorsal ramus of C1 carries only motor neurons to the suboccipital muscles.
  
• The dorsal ramus of C2 carries only sensory neurons to the back of the scalp.
  
• The lateral aspects of the back are innervated by lateral cutaneous nerves, which are derived from the ventral rami segments of spinal nerves at each level (Figure 1-1A).
  
• S5 and Co 1 only carry sensory neurans.

Osteology and Surface Anatomy

The following structures are easily palpated through the skin (Figure 1-1A):

• Cervical vertebrae. The first prominent spinous process that is palpable is C7 (vertebra prominens). Cervical spines 1 to 6 are covered by the ligamentum nuchae, a large ligament that courses down the back of the neck and connects the skull to the spinous processes of the cervical vertebrae.
  
• Thoracic vertebrae. The most prominent spine is the T1 vertebra; other vertebrae can be easily recognized when the trunk is flexed anteriorly. Thoracic vertebrae have long spines that point downward so that each spinous process is level with the body of the inferior vertebra. The spines can be counted downward from C7 and T1, or from a line joining the iliac crests at the L4 vertebral level and then counting upward from that site.
  
• Lumbar vertebrae. The body of L3 is approximately at the center of the body from superior to inferior and medial to lateral. When a person is in an upright posture, most vertebral spines are not obvious because they are covered by the erector spinae muscles in the cervical and lumbar regions.
  
• Sacrum. The spines of the sacrum are fused together in the midline to form the median sacral crest. The crest can be felt beneath the skin in the uppermost part of the gluteal cleft between the buttocks. The sacral hiatus is situated on the posteroinferior aspect of the sacrum and is the location where the extradural space terminates. The hiatus lies approximately 5 cm above the tip of the coccyx and beneath the skin of the natal cleft.
  
• Coccyx. The tip of the coccyx is located in the upper part of the natal cleft and can be palpated approximately 2.5 cm posterior to the anus. The anterior surface of the coccyx can be palpated via the anal canal.
  
• Ilium. The crest of the ilium is located at the L4 vertebral level and is easily palpated. Regardless of the size or weight of a person, the posterior iliac spines can be palpated because they lie beneath a skin dimple at the S2 vertebral level and the middle of the sacroiliac joint.
  
• Scapulae. The scapulae overlie the posterior portion of the thoracic wall, covering the upper seven ribs. The superior angle can be palpated at the T1 vertebral level, the spine of the scapula at T3, and the inferior angel at T7. The acromion forms the lateral end of the scapular spine and is easily palpated.

Vertebral Curvatures

In utero, the entire vertebral column has a primary or kyphotic curvature (concave anteriorly) (Figure 1-1C). After birth, with the demands of walking, weightbearing, and gravity, the cervical and lumbar regions form secondary or lordotic curvatures (concave posteriorly) (Figure 1-1D). Primary (kyphotic) curvatures occur in the thoracic and sacral regions, where the vertebrae curve posteriorly. This allows increased space for the heart and lungs in the thorax and birth canal in the sacral region. Lordotic or secondary curvatures occur in the cervical and lumbar regions, where the vertebrae curve anteriorly.

Abnormal primary curvatures are referred to as kyphosis (excessive kyphosis), whereas abnormal secondary curvatures are referred to as lordosis (excessive lordosis). Patients may present with abnormal lateral curvatures (scoliosis), which may be due to muscular dominance of one side over the other or to poor posture or congenital problems. To diagnose scoliosis, the physician may ask the patient to bend forward to determine if one side of the thorax is higher than the other due to asymmetry of the spine.

The Big Picture

The superficial back muscles consist of the trapezius, levator scapulae, rhomboid major, rhomboid minor, and latissimus dorsi muscles (Figure 1-2A; Table 1-1). Although these muscles are located in the back, they are considered to be muscles of the upper limbs because they connect the upper limbs to the trunk and assist in upper limb movements directly or indirectly via the scapula or humerus. The superficial back muscles are located in the back region and receive most of their nerve supply from the ventral rami of spinal nerves (primarily the brachial plexus) and act on the upper limbs. These muscles are discussed in much greater detail in Section VI, Upper Limb, but are included here because a brief discussion must precede the primary discussion of the deeper muscles and structures of the back.

Trapezius Muscle

The trapezius muscle is the most superficial back muscle. It attaches to the occipital bone, nuchal ligament, spinous processes of C7–T12, scapular spine, acromion, and clavicle. The trapezius muscle has a triangular shape and has the following muscle fiber orientations:

• Superior fibers. Course obliquely from the occipital bone and upper nuchal ligament to the scapula, causing scapular elevation and upward rotation.
  
• Middle fibers. Course horizontally from the lower nuchal ligament and thoracic vertebrae to the scapula, causing scapular retraction.
  
• Inferior fibers. Course superiorly from the lower thoracic vertebrae to the scapula, causing scapular depression and upward rotation.

In addition, the multiple fiber orientation of the trapezius muscle fixes the scapula to the posterior wall of the thorax during upper limb movement. It is innervated by the spinal accessory nerve (CN XI). The superficial branch of the transverse cervical artery supplies the trapezius muscle, whereas the deep branch of the transverse cervical artery supplies the levator scapulae and rhomboid muscles. In some instances, the dorsal scapular artery will replace the deep branch of the transverse cervical artery.

Levator Scapulae Muscle

The levator scapula muscle is located deep to the trapezius muscle and superior to the rhomboids. The levator scapula muscle attaches to the cervical vertebrae and the superior angle of the scapula, causing elevation and downward rotation of the scapula. The nerve supply is from branches of ventral rami from spinal nerves C3–C4 and occasionally from C5, via the dorsal scapular nerve, and vascular supply is from the deep branch of the transverse cervical artery.

Rhomboid Major and Minor Muscles

The rhomboid minor is superior to the rhomboid major, with both positioned deep to the trapezius muscle. The rhomboid muscles attach to the spinous processes of C7–T5 and the medial border of the scapula, resulting in scapular retraction. They are innervated by the dorsal scapular nerve (i.e., the ventral ramus of C5) and the vascular supply from the deep branch of the transverse cervical artery.

Latissimus Dorsi Muscle

The latissimus dorsi is a broad, flat muscle of the lower region of the back. It attaches to the spinous processes of T7, inferior to the sacrum via the thoracolumbar fascia, and inserts laterally into the intertubercular groove of the humerus. The latissimus dorsi acts on the humerus (arm) causing powerful adduction, extension, and medial rotation of the arm. It is innervated by the thoracodorsal nerve (ventral rami of C6–C8) and receives its blood supply from the thoracodorsal artery (branch off the axillary artery).

Scapular Movements

The scapula glides over the thoracic wall, but there are no distinct anatomic joints (Figure 1-2B). Instead, muscles pull the scapula forward and backward (protraction and retraction, respectively) and up and down (elevation and depression, respectively) and rotate it so that the glenoid fossa moves superiorly (upward rotation) or inferiorly (downward rotation). Rotation of the scapula is defined by the direction that the glenoid fossa faces. For more details describing the upper limb and scapular muscles, see Section VI.

The Big Picture

The deep back muscles consist of the splenius, erector spinae, transversospinalis, and suboccipital muscles (Table 1-2). These deep back muscles are segmentally innervated by the dorsal rami of spinal nerves at each vertebral level where they attach. It is not important to know every detailed attachment for the deep back muscles; however, you should realize that these muscles are responsible for maintaining posture and are in constant use during locomotion. The erector spinae muscles course medially to superolaterally, and they extend the vertebral column and rotate the trunk ipsilaterally. The transversospinalis group course laterally to superomedially and extend the vertebral column and rotate the trunk contralaterally.