Section 1.1: Anatomy of the Spinal Cord

CERVICAL SPINAL CORD 

The cervical spinal cord begins as the medulla oblongata emerges from the foramen magnum, at the level of the atlanto-occipital joint.

It gives off 8 paired spinal nerves. The superiormost seven nerves--called C1-C7--(one on each left and right) exit the vertebral canal through intervertebral foramina above their respective vertebrae. The eighth cervical nerve--called C8--exits the vertebral canal through the intervertebral foramen created by the articulation of the seventh cervical and first thoracic vertebrae.

In cross-section, the cervical cord is the largest of the four divisions of the spinal cord. It is somewhat oblong and has a large, well-defined ventral horn, especially in segments C5 - C8, which contribute to the brachial plexus to control the upper extremities.

Figure 1.1: Cross-section of the cervical spinal cord, with Rexed Laminae shown

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The cervical cord is subject to injury at the C2 level by Hangman's Fractures, which are bilateral fractures of the C2 pars interarticularis.

 Additionally, as the phrenic n. is composed of lower motor neurons from segments C3-C5, severing the spinal cord above C3 level will cause respiratory insufficiency. Severing the cord above T1 level will cause motor deficits in the upper extremity, the most significant of which entail loss of manual dexterity in the hands.

Pain in the upper extremity can be caused by disc herniation, usually at the C5-6 and C6-7 levels, along with weakness and potential upper motor neuron signs. Thus it is important from the perspective of spinal cord stimulation for pain to know very well the anatomy of the cord at C5, C6, and C7 vertebral levels (which correspond roughly to those same spinal cord segments. This is in contrast to the thoracic, and particularly upper lumbar, segments, at which spinal cord segments are located above their corresponding vertebral levels).

THORACIC SPINAL CORD

With the exception of the T1 level, the thoracic spinal cord is a much smaller caliber than the cervical cord, as it responsible almost exclusively for axial musculature, which is not as densely innervated as the upper extremities. However, in contrast to the cervical cord, it contains an additional and very important element: cell bodies for preganglionic sympathetics, which extend from spinal cord segments T1 to L2.

Figure 1.2: Cross-section of thoracic spinal cord

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Of particular surgical importance is the fact that thoracic vertebrae are the only vertebrae to articulate with ribs. This makes anterolateral access to the thoracic cord more complicated than in lumbar and cervical levels. (On each side, the first thoracic vertebra articulates with the entire head of the first rib and the upper half of the second rib. Vertebrae T2 to T9 articulate with the lower half of the rib corresponding to that vertebral level, as well as the upper half of the rib below. T10 articulates with something, and T11 and T12 each articulate with the entire head of their respective vertebrae.)

Figure 1.3: Sagittal MRI of the thoracic spine and surrounding structures


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However, it is somewhat fortunate that disc herniations and mechanical problems with the thoracic spinal cord are relatively rare.