Dalley, 10th Edition

Chapter 2 ■ Back

130

as straight as possible, and uses the muscles of the buttocks (nates) and lower limbs to assist with the lifting. Loads should be carried as close to the trunk as possible (Fig. B2.22).

As a protective mechanism, the back muscles go into spasm after an injury, including a herniated/ruptured disc, or in response to in ammation (e.g., of ligaments or arthritis). A spasm is a sudden involuntary contraction of one or more muscle groups. Spasms are attended by cramps, pain, and interference with function, producing involuntary move ment and distortion, and are sometimes relieved by chang ing or avoiding particular positions (Fig. B2.23).

NO

YES

Upper body weight

Upper body weight

Muscle forces

Muscle forces

Disc load

Object weight

Object weight

Disc load

(A) Improper lifting

(B) Proper lifting

FIGURE B2.22. Load on IV discs created by improper and proper lifting techniques. A. Ergonomics of improper lifting technique. B. Ergonomics of proper lifting technique. In A , the body weight is a greater distance away from the fulcrum (IV disc center) than in B . The load on the discs is dependent on the weight of the object, upper body weight, forces of back muscles, and their respective lever arms relative to the center of the disc. The lever balances below each gure demonstrate that smaller muscles forces and disc loads occur when the object is carried close to the body, that is, closer to the IV disc center.

(A) Posterior view view

(B) Posterior view

FIGURE B2.23. Muscle spasm following protrusion of an IV disc on left side. Protrusions are shown passing lateral ( A ) and medial ( B ) to the nerve root. Leaning in the directions shown compresses the nerve against the protrusion and increases pain; leaning in the opposite direction (not shown) reduces nerve compression, relieving pain.

The Bottom Line: Muscles of Back

Extrinsic back muscles: The super cial extrinsic back muscles are axio-appendicular muscles that serve the upper limb. ■ Except for the trapezius—innervated by CN XI—the extrinsic back muscles are innervated by the anterior rami of spinal nerves. Intrinsic back muscles: The deep intrinsic back muscles connect elements of the axial skeleton, are mostly innervated by posterior rami of spinal nerves, and are arranged in three layers: super cial (splenius muscles), intermediate (erector spinae), and deep (transversospinalis muscles).

■ The intrinsic muscles provide primarily extension and proprioception for posture and work synergistically with the muscles of the anterolateral abdominal wall to stabilize and produce movements of the trunk. Suboccipital muscles: Suboccipital muscles extend between vertebrae C1 (atlas) and C2 (axis) and the occipital bone and produce—and/or provide proprioceptive information about—movements at the craniovertebral joints. by the vertebrae, their associated ligaments and muscles, the spinal meninges, and the cerebrospinal uid (CSF). The spinal cord begins as a continuation of the medulla oblongata (often called the medulla ), the caudal part of the brainstem (see Fig. 9.36 in Chapter 9, Head). In adults, the spinal cord is 42–45 cm long and extends from the foramen magnum in the occipital bone to the level of the L1 or L2 vertebra (Fig. 2.45B). However, its tapering inferior end, the conus medullaris , may terminate as high as T12 vertebra or as low as L3 vertebra. Thus, the spinal cord occupies only the superior two thirds of the vertebral canal.

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CONTENTS OF VERTEBRAL CANAL

The spinal cord, spinal nerve roots, spinal meninges, and the neurovascular structures that supply them are located in the vertebral canal (see Fig. 2.30). Spinal Cord The spinal cord is the major re ex center and conduction pathway between the body and brain. This cylindrical struc ture, slightly attened anteriorly and posteriorly, is protected