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THE BACK

  1. Which of the following is a structure innervated by a dorsal primary ramus from the spinal cord?

    1. Rhomboid major—Rhomboid major is innervated by the dorsal scapular nerve from the C3 to C4 spinal cord levels and arises from the brachial plexus; thus, the nerve is from the ventral primary rami, and the muscle is considered a hypaxial muscle, not an epaxial, true, or intrinsic muscle of the back.

    2. Levator scapulae—This muscle is primarily innervated by the dorsal scapular nerve as well, and the muscle has a hypaxial embryological origin, making it a superficial, not an intrinsic muscle of the back.

    3. Serratus posterior inferior—Branches from T9 to T12 anterior primary rami innervate this muscle close to its proximal, medial attachments.

    4. Latissimus dorsi—The thoracodorsal nerve arising from the posterior cord of the brachial plexus innervates the latissimus dorsi. As all branches of the brachial plexus are anterior primary rami, this is also not the correct answer. Latissimus dorsi is also embryologically derived from hypaxial mesoderm, making it an anteriorly innervated structure.

    5. Iliocostalis thoracis—This is the correct answer. Iliocostalis thoracis, along with all of the erector spinae muscles, are “true” or intrinsic muscles of the back. This muscle was embryologically derived from epaxial mesoderm, or undifferentiated tissue above the axis of the spine, which will become muscle tissue. Epaxial muscles uniquely receive their innervation from dorsal, not ventral, primary rami of the spinal cord.

  2. The primary goal of this question was to evaluate your ability to categorize muscle tissue as epaxial or hypaxial in origin. Alternatively, the classification of a back muscle as a “superficial” or extrinsic muscle of the back contrasted with “deep,” “true,” or intrinsic muscles of the back, also follows the embryological origin categories. Answering this question correctly was most quickly and easily achieved by understanding these broad concepts as opposed to remembering the individual innervation patterns.

  3. Which of the following is the structure that most limits extension of the vertebral column?

    1. Ligamentum flava—Ligamentum flava is located between the vertebral lamina and becomes slack during extension of the vertebral column.

    2. Nuchal ligament—The nuchal ligament is the midline concentration of fascia over the vertebral spinous processes and is the most posterior of the listed structures. It also becomes slack during extension, but serves as a powerful limiting structure in flexion. When flexing your neck so your chin approaches your sternum, you can palpate your nuchal ligament over your cervical spinous processes as it becomes taut.

    3. Anterior longitudinal ligament—This is correct. The anterior longitudinal ligament is the only one of these listed structures that is anterior to the axis of the vertebral column; thus, it becomes taut during extension, limiting excess extension. Physiologically, other structures, such as hypaxial musculature and epaxial bony formations, will also limit extension of the vertebral column to varying degrees.

    4. Posterior longitudinal ligament—The posterior longitudinal ligament associates with the posterior surface of the body of the vertebrae and checks flexion.

    5. Intervertebral disc—The intervertebral disc rests between the ...

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