It leaves between the C7 vertebra and the first thoracic vertebra. C1-C7 nerves exit above their vertebrae whereas the C8 nerve exits below the C7 vertebra. In the cervical segments, there are 7 cervical vertebrae and 8 cervical nerves (Figure 3.2). This growth of the nerve roots occurring within the vertebral canal, results in the lumbar, sacral, and coccygeal roots extending to their appropriate vertebral levels (Figure 3.2).ДЄll spinal nerves, except the first, exit below their corresponding vertebrae. The outcome of this uneven growth is that the adult spinal cord extends to the level of the first or second lumbar vertebrae, and the nerves grow to exit through the same intervertebral foramina as they did during embryonic development. This results in displacement of the lower parts of the spinal cord with relation to the vertebrae column. As development continues, the body and the vertebral column continue to grow at a much greater rate than the spinal cord proper. The denticulate ligament, the dorsal root ganglion (A), and an enlarged drawing of the meninges (B).Дuring the initial third month of embryonic development, the spinal cord extends the entire length of the vertebral canal and both grow at about the same rate. Dorsal and ventral roots enter and leave the vertebral column respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment. These nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve (Figure 3.2). There are 31 segments, defined by 31 pairs of nerves exiting the cord. Two enlargements of the spinal cord can be visualized: The cervical enlargement, which extends between C3 to T1 and the lumbar enlargements which extends between L1 to S2 (Figure 3.1). The different cord regions can be visually distinguished from one another. The spinal cord is divided into four different regions: the cervical, thoracic, lumbar and sacral regions (Figure 3.1). Knowledge of spinal cord functional anatomy makes it possible to diagnose the nature and location of cord damage and many cord diseases.ДЈ.3 Segmental and Longitudinal Organization It is of great clinical importance because it is a major site of traumatic injury and the locus for many disease processes.ДЄlthough the spinal cord constitutes only about 2% of the central nervous system (CNS), its functions are vital.It contains neurons whose descending axons mediate autonomic control for most of the visceral functions.Motor neurons in the ventral horn project their axons into the periphery to innervate skeletal and smooth muscles that mediate voluntary and involuntary reflexes.It carries sensory information (sensations) from the body and some from the head to the central nervous system (CNS) via afferent fibers, and it performs the initial processing of this information.Similar cross-sectional structures at all spinal cord levels (Figure 3.1).Each spinal cord segment innervates a dermatome (see below and Figure 3.5). The spinal nerve contains motor and sensory nerve fibers to and from all parts of the body.

The spinal cord is a cylindrical structure of nervous tissue composed of white and gray matter, is uniformly organized and is divided into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), (Figure 3.1), each of which is comprised of several segments. These nerve roots join distally to form 31 pairs of spinal nerves. Two consecutive rows of nerve roots emerge on each of its sides. The spinal cord is 40 to 50 cm long and 1 cm to 1.5 cm in diameter. It is a vital link between the brain and the body, and from the body to the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. The spinal cord is the most important structure between the body and the brain. Schematic dorsal and lateral view of the spinal cord and four cross sections from cervical, thoracic, lumbar and sacral levels, respectively.

Spine diagram