Back - Summary

The Back - Summary

Skeleton

• The axial skeleton forms the long axis of the body, including the skull, vertebral column, and bony thorax. The appendicular skeleton consists of the pectoral and pelvic girdles and the long bones of the limbs.

Vertebral Column

• The vertebral column consists of 33 vertebrae, including the 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccyx. The atlas and axis are atypical vertebrae. The ring-like atlas supports the skull and helps make (anterior/posterior) nodding movements possible. The axis has a dens that helps the head to rotate. The intervertebral disks, with their nucleus pulposus cores and annulus fibrosus rings, act as shock absorbers. Herniated disks usually involve rupture of the annulus followed by protrusion of the nucleus. The primary curvatures are located in the thoracic and sacral regions and develop during embryonic and fetal periods, whereas the secondary curvatures are in the cervical and lumbar regions. The cervical curvature appears at birth (when a baby starts to lift its head), and the lumbar curvature becomes pronounced during infancy (when a toddler starts to walk). In old age, the intervertebral disks thin. This, along with osteoporosis, leads to a gradual decrease in height.

Spinal Cord

• Occupies approximately the upper two thirds of the vertebral canal, is enveloped by three meninges, and has cervical and lumbar enlargements for nerve supply of the upper and lower limbs, respectively. It has a conical end known as the conus medullaris, which terminates at the level of L2 vertebra.
  
• Consists of 31 pairs of nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). The spinal nerves are mixed nerves, containing sensory (GSA and GVA) fibers and somatic motor (GSE) and visceral motor (sympathetic and parasympathetic GVE) fibers.
• The cervical spinal nerves exit above the correspondingly numbered vertebrae except the eighth cervical nerves, which emerge below the seventh cervical vertebra; the remaining spinal nerves exit below the correspondingly numbered vertebrae.
• The meninges consist of a pia mater (innermost layer), arachnoid mater (transparent spidery layer), and dura mater (tough fibrous outermost layer). The subarachnoid space between the pia and arachnoid maters contains CSF, the subdural space between the arachnoid and dura mater contains moistening fluid, and the epidural space external to the dura mater contains the internal vertebral venous plexus.
• The denticulate ligaments are 21 pairs of lateral extensions of the pia mater; the filum terminale is an inferior extension of the pia mater; CSF is formed by vascular choroid plexuses in the ventricles of the brain and is contained in the subarachnoid space; and the cauda equina (horse's tail) is formed by dorsal and ventral roots of the lumbar and sacral spinal nerves.

Venous System

• The vertebral artery arises from the subclavian artery and ascends through the transverse foramina of the upper six cervical vertebrae.
• The vertebral veins are formed in the suboccipital triangle by tributaries from the venous plexus around the foramen magnum and the suboccipital venous plexus and descend through the transverse foramina.
• The internal vertebral venous plexus lies in the epidural space and communicates superiorly with the cranial dural sinuses and inferiorly with the pelvic veins and with both the azygos and caval systems in the thoracic and abdominal regions. This venous plexus is the route of early metastasis of carcinoma from the lung, breast, and prostate gland or uterus to bones and the CNS.
• The external vertebral venous plexus lies in front of the vertebral column and on the vertebral arch and communicates with the internal vertebral venous plexus.

Muscles

• The superficial muscles of the back are involved in moving the shoulder and arm and are innervated by ventral primary rami of the spinal nerves.
• The deep muscles of the back are responsible for extension of the spine and head and are innervated by dorsal primary rami of the spinal nerves. Flexion and rotation of the head and neck are brought about by the sternocleidomastoid and scalene muscles in the anterior and lateral neck.
• The triangle of auscultation is bounded by the latissimus dorsi, trapezius, and scapula (medial border) and is the site where breathing sounds can be heard most clearly. The lumbar triangle is formed by the iliac crest, latissimus dorsi, and external oblique abdominal muscles. It may be the site of an abdominal hernia.

Nerves

• The accessory nerve consists of a cranial portion, which joins the vagus nerve, and a spinal portion, which supplies the sternocleidomastoid and trapezius muscles.
• The dorsal scapular nerve (C5) supplies the rhomboid major and minor and levator scapulae muscles.
• The suboccipital nerve (C1) supplies the muscles of the suboccipital region. The greater occipital nerve (C2) is derived from the dorsal primary ramus and communicates with the suboccipital and third occipital nerves and may supply the semispinalis capitis.

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