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INTRODUCTION

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Anatomy of the spinal cord is often broken down by associated vertebral level. The various functions of the spinal cord depend on whether it is cervical, thoracic, lumbar, or sacral. The spinal cord is generally divided into a posterior sensory portion and an anterior motor portion.

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VERTEBRAL AND SPINAL CORD ANATOMY

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The spinal cord is housed within the vertebral canal. The anatomic boundaries of the canal are:

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  • Superior—Foramen magnum

  • Inferior—Coccyx

  • Lateral—Neural foramen

  • Posterior—Ligamentum flavum

  • Anterior—Posterior spinal ligament and vertebral bodies

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The spinal cord is made up of gray and white matter. The internal gray matter is made up of cell bodies, whereas the surrounding white matter consists of axons organized into various spinal tracts. Tracts are continuations of cell bodies and axons that originate in the brainstem and cerebral cortex. Each tract has specific functions, ranging from controlling motor and sensory inputs and outputs to transmitting temperature and pain signals from periphery to brain.

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The spinal cord is organized and named according to its corresponding vertebral level and consists of the cervical, thoracic, lumbar, and sacral levels. Each vertebral body has an associated spinal nerve consisting of both sensory and motor nerve roots which exit through its corresponding neural foramen. There are 7 cervical, 12 thoracic, 5 lumbar, and 5 sacral vertebral bodies and 31 spinal nerves, including the coccygeal nerve. Spinal nerves from C1 to C7 emerge above the corresponding vertebral level, and spinal nerves from C8 to S5 emerge below the corresponding vertebral level. Consequently, there are eight cervical nerves, but only seven cervical vertebrae. C8 emerges below C7 and from there, on all nerves exit below their corresponding level.

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SPINAL CORD MOTOR AND SENSORY DISTRIBUTION

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C1-C4 form nerves that provide both sensory and motor innervation to the head and neck. The phrenic nerve (C3-C5) supplies the diaphragm. C5-T1 supply upper extremity innervation. T1-T12 provide motor control to the thoracoabdominal musculature. L2-S2 provide lower extremity motor control.

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The clinically important dermatome levels include: C5, shoulder; C6, thumb; C7, index and middle fingers; C7–C8, ring finger; C8, little finger; T1, medial forearm; T2, medial, upper arm; L1, anterior, upper, medial thigh; L2, anterior, upper thigh; L3, knee; L4, medial malleolus; L5, dorsum of foot and toes 1-3; S1, toes 4-5 and lateral malleolus; S3-C1, anus; T4, nipple; and T10, umbilicus.

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The spinal cord extends from the transition between the upper cervical cord and lower medulla to its terminal end, which in the adult is the L1-L2 vertebral body. In the infant, the spinal cord extends to as low as L3-L4, but rises until adulthood due to vertebral growth with development. As the spinal cord terminates at the L1-L2 level, spinal nerves continue caudally and exit with their corresponding vertebral level. This collection of spinal nerves is referred to as the cauda ...

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